Throughout 2004, our organization received applications for the development of technical proposals for boiler houses for heat supply to residential and public buildings, in which the loads on hot water supply were very different (to a lesser extent) from those previously requested for identical consumers. This was the reason for analyzing the methods for determining loads on hot water supply (DHW), which are given in the current SNiPs, and possible errors arising when they are used in practice.
E.O. SIBIRKO
Currently, the procedure for determining heat loads on hot water supply is regulated normative document SNiP 2.04.01–85* " Internal water supply and building drainage."
Methodology for determining estimated costs hot water(maximum second, maximum hourly and average hourly) and heat flows (heat power) during the hour at average and maximum water consumption in accordance with section 3 of SNiP 2.04.01–85* is based on the calculation of the corresponding flow rates through water taps (or groups of the same type devices with subsequent averaging) and determining the probability of their simultaneous use.
All service tables with data on various specific consumption rates, etc., given in SNiP, are used only for calculating consumption through individual devices and the likelihood of their action. They are not applicable for determining costs based on the number of consumers, by multiplying the number of consumers by specific consumption! This is precisely the main mistake made by many calculators when determining the heat load on the hot water supply.
The presentation of the calculation methodology in section 3 of SNiP 2.04.01–85* is not simple. Introduction of numerous superscript and subscript Latin indices (derived from the corresponding terms in English) makes it even more difficult to understand the meaning of the calculation. It is not entirely clear why this was done in the Russian SNiP - after all, not everyone speaks English and easily associates the index “ h"(from English hot- hot), index " c"(from English cold- cold) and " tot"(from English total- result) with corresponding Russian concepts.
To illustrate the standard error encountered in calculations of heat and fuel needs, I will give a simple example. It is necessary to determine the DHW load for a 45-apartment residential building with a population of 114 people. Supply water temperature DHW pipeline- 55°C, temperature cold water V winter period-5°С. For clarity, let’s assume that each apartment has two similar water points (sink in the kitchen and washbasin in the bathroom).
Option I of calculation is incorrect (we have repeatedly encountered this method of calculation):
According to the table “Rates of water consumption by consumers” of the mandatory Appendix 3 of SNiP 2.04.01–85* we determine for “Residential buildings apartment type: with baths from 1500 to 1700 mm long, equipped with showers,” the hot water consumption per inhabitant per hour of greatest water consumption is equal to q hhr, u = 10 l/h. Then everything seems to be quite simple. The total consumption of hot water per house at the hour of greatest water consumption based on the number of inhabitants of 114 people: 10. 114 = 1140 l/h.
Then, the heat consumption per hour of greatest water consumption will be equal to:
Where U- number of residents in the house; g - density of water, 1 kg/l; With- heat capacity of water, 1 kcal/(kg °C); t h - hot water temperature, 55°C; t c - cold water temperature, 5°C.
The boiler room, actually built on the basis of this calculation, clearly could not cope with the load of hot water supply at the moments of peak hot water supply, as evidenced by numerous complaints from the residents of this house. Where is the mistake here? It lies in the fact that if you carefully read section 3 of SNiP 2.04.01–85*, it turns out that the indicator q hhr, u, given in Appendix 3, is used in the calculation method only to determine the probability of operation of sanitary fixtures, and the maximum hourly flow of hot water is determined completely differently.
Calculation option II - in strict accordance with the SNiP methodology:
1. Determine the probability of the device operating.
,
Where q hhr,u = 10 l - according to Appendix 3 for this type of water consumer; U= 114 people - the number of residents in the house; q h0 = 0.2 l/s - in accordance with clause 3.2 for residential and public buildings, it is allowed to take this value in the absence of technical characteristics of the devices; N- number of sanitary fixtures with hot water, based on the two water points we have adopted in each apartment:
N= 45 . 2 = 90 devices.
Thus we get:
R= (10 x 114)/(0.2 x 90 x 3600) = 0.017.
2. Now let’s determine the probability of using sanitary appliances (the ability of the appliance to supply normalized hourly water flow) during the estimated hour:
,
Where P- the probability of the device action determined in the previous paragraph, - P= 0,017; q h0 = 0.2 l/s - second water flow rate related to one device (also already used in the previous paragraph); q h0,hr - hourly water consumption by the device, in accordance with clause 3.6, in the absence of technical characteristics of specific devices, it is allowed to take q h0,hr = 200 l/h, then:
.
3. Since P h is less than 0.1, we further use the table. 2 of Appendix 4, according to which we determine:
at 
.
4. Now we can determine the maximum hourly hot water flow:
.
5. And finally, we determine the maximum thermal load DHW (heat flow during the period of maximum water consumption during the hour of maximum consumption):
,
Where Q ht- heat losses.
Let's take into account heat losses, taking them as 5% of the design load.
.
We got a result more than twice the result of the first calculation! As shown practical experience, this result is much closer to real needs in hot water for a 45-apartment residential building.
You can give for comparison the result of the calculation using the old method, which is given in most reference literature.
Option III. Calculation using the old method. Maximum hourly heat consumption for hot water supply needs for residential buildings, hotels and hospitals general type by the number of consumers (in accordance with SNiP IIG.8–62) was determined as follows:
,
Where k h - coefficient of hourly unevenness of hot water consumption, taken, for example, according to table. 1.14 reference book “Adjustment and operation of water heating networks” (see Table 1); n 1 - estimated number consumers; b - the rate of hot water consumption per consumer, adopted according to the relevant tables of SNiPa IIG.8–62 and for apartment-type residential buildings equipped with bathrooms from 1500 to 1700 mm in length, is 110–130 l/day; 65 - hot water temperature, °C; t x - cold water temperature, °C, we accept t x = 5°C.
Thus, the maximum hourly heat consumption for hot water supply will be equal to:
.
It's easy to see that this result almost coincides with the result obtained using the current method.
Application of the hot water consumption rate per inhabitant per hour of greatest water consumption (for example, for “Apartment-type residential buildings with bathtubs from 1500 to 1700 mm long” q hhr == 10 l/h), given in the mandatory Appendix 3 SNiP 2.04.01–85* “Internal water supply and sewerage of buildings”, is illegal for determining heat consumption for DHW needs by multiplying it by the number of inhabitants and the difference in temperature (enthalpy) of hot and cold water. This conclusion is confirmed both by the given calculation example and by a direct indication of this in the educational literature. For example, in the textbook for universities “Heat supply”, ed. A.A. Ionin (M.: Stroyizdat, 1982) on page 14 we read: “...Maximum hourly water consumption G h. max cannot be mixed with the water consumption given in the standards at the hour of greatest water consumption G i.ch. The latter, as a certain limit, is used to determine the probability of operation of water-folding devices and becomes equal to G h. max only at infinitely large number water taps." Calculation using the old method gives a much more accurate result, provided that daily hot water consumption rates are applied at the lower limit of the ranges given in the corresponding tables of the old SNiP, than the “simplified” calculation that many calculators perform using the current SNiP.
The data from the table in Appendix 3SNiP 2.04.01–85* must be used specifically to calculate the probability of operation of water-folding devices, as required by the methodology outlined in Section 3 of this SNiP, and then determine bhr and calculate the heat consumption for the needs of hot water supply. In accordance with the note in paragraph 3.8 of SNiP 2.04.01–85*, for auxiliary buildings of industrial enterprises the value q hr can be determined as the sum of water costs for using a shower and household and drinking needs, taken according to the mandatory Appendix 3 according to the number of water consumers in the most numerous shift.
Main parameters residential buildings is water supply, sewer system and delivery electrical energy. Regardless of the number of residents ( private house or multi-story), the calculation of the main networks must be carried out according to certain rules, using the appropriate formulas. It does not take much time to create the correct electrical circuit; it is much more difficult to decide on the water supply. A particular difficulty is the design and calculation of hot water supply. In order to carry out all operations correctly, you need to know not only the technical side of the issue, but also the regulatory framework.
The most commonly chosen type of network is the circulation type. The principle of operation of such a system is the constant circulation of liquid. The only drawback circulation system hot water supply is too expensive. The costs are only justified when the maximum number of users for a residential building is reached.
Also, in addition to high pricing policy, constant circulation of water leads to significant heat losses, which entails additional costs. If there is a circulation system, designers try to reduce the length of the pipeline as much as possible. This option allows for additional savings on liquid transportation.
What is the waiting period and how is it calculated?
The waiting period is the time period that passes from the time the user opens the tap until hot water is supplied. This time they try to reduce it as much as possible; for this purpose, the hot water supply system is optimized, adjustments are made, and if the indicators are poor, they are modernized.
To set the waiting period, generally accepted standards are used. To calculate it correctly, you should know the following:
- To reduce the waiting period, you should create high blood pressure water in the system. But setting too high pressure parameters can damage the pipeline.
- To reduce the waiting period, increase throughput device through which the user receives liquid.
- The waiting period increases in direct proportion to the internal diameter of the pipeline, as well as if there is a circuit at a large distance from the consumer.
The correct sequence for calculating the waiting period is:
- Determination of the number of consumers. After the exact figure, you should make a small reserve, since there are peak hot water consumption.
- Determination of the characteristics of the pipeline: length, internal diameter of the pipes, as well as the material from which they are made.
- Multiplying the length of the pipeline and its internal diameter by the specific volume of water, which is measured in l/s.
- Determination of the shortest and most convenient fluid path. This parameter also includes sections of the circuit located farthest from the water tap. All volumes of water are also added.
- The amount of liquid is divided by the water flow per second. When obtaining this parameter, it is also taken into account total pressure liquids in the system.
To achieve the most accurate results, you should correctly calculate the specific volume of the pipeline. The following formula is used for this:
Cs = 10 (F/100)2 3.14/4, where F is the internal diameter of the pipeline.
When determining the specific volume, you cannot use the value of both the external and nominal diameter of the pipes. This will significantly reduce the accuracy of calculations. There are tables in which the specific volume value is pre-calculated for certain materials (copper and steel).
Calculation of hot water consumption per day
The amount of hot water that the user needs per day is a parameter calculated in advance. Typically, such data is taken from tables, where they are divided by type of room and its square footage. European parameters should not be confused with those of other countries; they are strikingly different from each other.
On average, hot water consumption per person per day ranges from 25 to 50 liters. Compiling and calculating the amount of hot water per person is possible only after the status of the room or building is known.
How to calculate a pipeline
For long-term operation of a hot liquid transportation system, the pipeline should be calculated under peak load conditions. This allows you to make a certain reserve, which will eliminate the occurrence of malfunctions in the system with a sharp increase in pressure.
To calculate a pipeline, most often, ready-made diagrams and tables with relevant data are used. The material most often used is copper or galvanized steel. You should know that important parameter calculation is an equivalent device Fixture Unit. This device called a conditional element for a certain type of water folding mechanisms.
Pipeline calculation sequence:
- The calculation begins with determining the Fixture Unit parameter, which is mandatory for each water intake point.
- The main hot water transportation network is divided into separate sections (nodes). The principle is based on the design of the heating system.
- Find the total number of Fixture Units that will be located at different sites.
- Based on the total Fixture Unit amount and the type of building, the estimated flow rate for each section of the system is found.
- Design flow, also referred to as throughput volume, is an important component in determining the diameter of the pipeline. The internal diameter of the pipes is determined under the condition that the final figures will not exceed generally established limits.
When calculating the circulation network, you can use general position, that for each Fixture Unit element there is 3 l/s. A separate point is the calculation recirculation pump, which has a certain throughput capacity. To determine this parameter, it is necessary to know the exact number of water points.
To provide the circulation network with additional savings, a thermostat is installed on the pump. The thermostat ensures that the device turns on when the temperature of the transported liquid drops. When the water temperature on the return circuit reaches a value less than the nominal value by 5 degrees, the pump turns off.
What you need to have to start calculating hot water supply
It is impossible to start calculating a hot water supply system without having technical and project documentation to the house. At the same time, the size of the house is not important; a private plot requires the same plan as a multi-story building.
The calculation begins with a certified architectural plan, on which the selected correct location buildings, as well as the placement of sanitary fixtures. The location of the house will help you choose the water supply system along the shortest route.
It is necessary to know the number of people who will live in the building. Naturally, it is impossible to find out the exact number of residents, so it is better to carry out the calculation using the maximum data. These numbers will allow us to calculate right time peak loads.
Determine the location where the hot water supply equipment will be placed. This area, in mandatory must be indicated on the diagram.
Water consumption for hot water supply needs should be determined according to hot water consumption standards, taking into account the likelihood of using water taps. The load on the hot water system is determined by the maximum flow of hot water and taken into account when choosing a heat source. Hello, dear friends! We are used to using hot water every day and can hardly imagine comfortable life, if you cannot take a warm bath or you have to wash dishes under a tap from which a cold stream flows. Water at the desired temperature and the right amount- this is what the owner of every private house dreams of. Today we will determine the estimated consumption of water and heat for hot water supply to our home. You must understand that at this stage It doesn’t really matter to us where we get this heat. Perhaps we will take it into account when choosing the power of the heat supply source and will heat water for the needs of hot water supply in the boiler. Perhaps we will heat water in a separate electric boiler or a gas water heater, or perhaps they will bring it to us.
Well, if there are no technical capabilities to implement a hot water system at home, then we will go to our own or the village bathhouse. Our parents mostly went to the city baths, and now the mobile Russian bathhouse under your window has rung. Of course, life does not stand still and having a bathtub and shower in the house today is no longer a luxury, but simple necessity. Therefore, we will provide a hot water supply system in the house. The correct calculation of the hot water supply will determine the load on the domestic hot water system and, ultimately, the choice of the power of the heat source. Therefore, this calculation must be approached very seriously. Before choosing the design and equipment of a domestic hot water system, we need to calculate the main parameter of any system - the maximum consumption of hot water per hour of maximum water consumption (Q g.v max, kg/h).
In practice, using a stopwatch and a measuring container, we determine the consumption of hot water, l/min when filling the bathtub
Calculation of the hourly maximum flow of hot water at the hour of its maximum water consumption
To calculate this consumption, let's turn to the hot water consumption standards (according to chapter SNiP 2-34-76), see Table 1.
Hot water consumption standards (according to chapter SNiP 2-34-76)
Table 1
g и.с – average for heating season, l/day;
g and – maximum water consumption, l/day;
g i.h – highest water consumption, l/h.
Dear friends, I want to warn you against one common mistake. Many developers, and even young inexperienced designers, perform hourly calculations maximum flow hot water according to the formula
G max =g i.h *U, kg/h
g i.h – rate of hot water consumption, l/h, maximum water consumption, taken according to table 1; U – number of hot water consumers, U=4 people.
G max = 10 * 4 = 40 kg/h or 0.67 l/min
Q year max = 40 * 1 * (55 – 5) = 2000 kcal/h or 2.326 kW
Having calculated the water flow in this way and selected the power of the heat source to heat this flow, you have calmed down. But when you get into the shower, you will be surprised to find that only 3 drops of water per second are dripping onto your dirty and sweaty bald head. Neither washing your hands, nor rinsing the dishes, not to mention taking a bath is out of the question. So what's the matter? And the mistake is that the maximum hourly water consumption for the day of greatest water consumption was not correctly determined. It turns out that all hot water consumption rates according to Table 1 should be used only to calculate the flow rate through individual devices and the likelihood of using their action. These standards are not applicable to determine costs based on the number of consumers, by multiplying the number of consumers by specific consumption! This is precisely the main mistake made by many calculators when determining the heat load on a hot water supply system.
If we need to determine the performance of heat generators (boilers) or heaters when subscribers do not have hot water storage tanks (our case), then design load for the DHW system should be determined by the maximum hourly consumption of hot water (heat) for the day of greatest water consumption according to the formula
Q g.v max =G max * s * (t g.wed –t x), kcal/h
G max – maximum hourly consumption of hot water, kg/h. The maximum hourly consumption of hot water, G max, taking into account the likelihood of using water taps, should be determined by the formula
G max = 18 *g * K and * α h * 10 3, kg/h
g – hot water consumption rate, l/with water taps. In our case: for a washbasin g y = 0.07 l/s; for washing g m = 0.14 l/s; for shower g d = 0.1 l/s; for a bath g in = 0.2 l/s. Choose higher value, that is, g = g in = 0.2 l/s; K and – dimensionless coefficient of use of a water-folding device for 1 hour of maximum water consumption. For a bathtub with a characteristic (highest) hot water flow g x = 200 l/h, this coefficient will be equal to K u = 0.28; α h is a dimensionless value determined depending on the total number N of water-folding devices and the probability of using them R h for 1 hour of greatest water consumption. In turn, the probability of using water-folding devices can be determined by the formula
R h =g i.h *U/3600*K and*g*N
g i.h – rate of hot water consumption per hour of greatest water consumption, l/h. It is taken according to table 1, g and.h = 10 l/h; N – total number of water taps installed in the house, N = 4.
R h = 10 * 4 / 3600 * 0.28 * 0.2 * 4 = 0.0496. At R h< 0,1 и любом N по таблице (N * Р ч = 0,198) определяем α ч = 0,44
G max = 18 * 0.2 * 0.28 * 0.44 * 10 3 = 444 kg/h or 7.4 l/min.
Q year max = 444 * 1 * (55 – 5) = 22200 kcal/h or 25.8 kW
No, neither the desired temperature nor the proper flow of hot water - discomfort
As you can see, dear friends, the consumption of water and, accordingly, heat has increased approximately 10 times. In addition, the heat consumption for hot water supply (25.8 kW) is 2 times greater than the total heat consumption for heating and ventilation of the house (11.85 + 1.46 = 13.31 kW). If this data is presented to the “Customer”, then his hair will stand on end and he will demand that they explain to him - what’s the matter? So let's help him. Tables 2 and 3 below will help us with this. Now let's turn to table 2 and calculate the hourly highest consumption water when loading all water consumers simultaneously. Adding up all the typical costs, we get 530 l/h. As you can see, the total characteristic consumption turned out to be 86 l/h more than the calculated one (444 l/h). And this is not surprising, since the likelihood that all water taps will work at the same time is very small. Our maximum requirement for hot water is already 84%. In reality, this value is even less – about 50%. Let's try to get the real value, for this we use table 3. Do not forget that hot water consumption standards are developed for consumers at t g.av = 55 o C, but from the table we will find costs at t g. av = 40 o C.
The minimum total consumption of hot water, with an average water temperature equal to t gw = 40 o C and the simultaneous operation of all water intake devices with a probability of this consumption of 84%, will be equal to G min =[ (5 * 1.5) + (20 * 5) + (30 * 6) +(120 * 10) ] * 0.84 = 342.3 l/h (239.6 l/h at t g.v = 55 o C)
The maximum total consumption of hot water, with an average water temperature of 40 o C and the simultaneous operation of all water intake devices with a probability of this consumption of 84%, will be equal to G max = [ (15 * 3) + (30 * 5) + (90 * 6 ) +(200 * 15) ] * 0.84 = 869.4 l/h (608.6 l/h at t g.v = 55 o C)
The average flow rate at t g.v = 55 o C will be equal to G avg = (G min + G max)/2 = (239.6 + 608.6)/2 = 424.1 l./h. So we got what we were looking for - 424.1 l/h instead of 444 l/h according to calculations.
Hot water consumption standards for water taps (chapter SNiP 2-34-76)
Table 2
Hot water consumption standards for various water intake devices
Table 3
|
Collection point |
Sink | kitchen sink | Economical shower | Shower standard | Shower comfort. | Bath |
| DHW temperature, o C | 35-40 | 55 | 40 | 40 | 40 | 40 |
| Consumption time, min | 1,5-3 | 5 | 6 | 6 | 6 | 10-15 |
| Hot water consumption for domestic needs, l | 5-15 | 20-30 | 30 | 50 | 90 | 120-200 |
Thus, when calculating hot water supply, it is imperative to take into account the following nuances: the number of residents; frequency of use of the bath, shower; number of bathrooms where hot water is used; technical specifications plumbing elements(for example, the volume of the bathroom); the expected temperature of the heated water, as well as the likelihood of using water taps at the same time. In the following posts, we will take a closer look at three common hot water supply systems. Depending on the method of heating water, these systems, for private country house, subdivided: DHW with storage water heater(boiler); DHW with instantaneous water heater; DHW with double-circuit boiler.
What do you think I’m doing?!!!
The obtained values of water and heat consumption for DHW needs are: G max = 444 kg/h or 7.4 l/min and Q g.v max = 22200 kcal/h or 25.8 kW we accept, with subsequent clarification, when choosing a heat source. Today we completed the 4th point of our home plan - we calculated the maximum hourly hot water consumption for a private house. Who hasn't joined yet, join us!
Best regards, Gregory
Introduction:
The topic of calculating fees for utilities is one of the most complex. For those who have not encountered the problem before, it is difficult to figure it out right away, and there seems to be no time for it.
However, let's try.
For calculations, RF PP No. 354 is used (procedure and methods for all occasions), RF PP No. 307 (only for heating and only until July 1, 2016, then RF PP No. 354 is in effect), RF PP No. 306 (standards).
The text of the documents is complex and practically inaccessible to mass payers. There is no clear system in notation physical quantities, which may confuse the reader, there are no names of physical quantities used in the calculation formulas and explanations. It was as if they were writing for themselves. Like we know it ourselves, but others don’t need to know.
And one more initial note. Gentlemen from the management company and from the developer often show great joy regarding the “energy efficiency” of new buildings, in particular our area.
The essence of energy efficiency is strict accounting of all utility resources and measures to save them. Let's see in the course of the discussion how justified such “joy” is.
Because we have a system DHW closed, that is, non-centralized, then the corresponding section of the RF PP No. 354 (Appendix 2 section IV) is used for calculations, when production public services, V in this case DHW is carried out by the contractor (MC) on our ITP equipment from the common property.
Regarding this very concept of “production” of hot water supply by the contractor, we will not go into details for now. This is a separate rather “murky” and controversial topic, who actually produces what and how.
Let us only note that according to RF PP No. 354, clause 54 of the Rules, it is clearly stated that fees for the maintenance of common property (ITP equipment, where the service provider heats water for hot water supply) are charged separately. That is, “production” - operating costs for this common property are included in the payment for the maintenance and repair of common property and are not included in the calculation of payment for hot water supply.
So, what needs to be taken into account to calculate the DHW fee?
Total cold consumption drinking water(via the cold water line) supplied to heating for DHW.
The total consumption of thermal energy taken in boilers from the coolant from the centralized supply of thermal energy (heating).
Everything seemed simple. I divided the total heat consumption (heating) by the total volume of cold water that was used for hot water supply and the order. I obtained the specific heat consumption per cubic meter of hot water.
However, our receipts do not account for the total volume of cold water and hot water separately.
And individual consumption data for DHW and cold water cannot be used due to systematic measurement error apartment meters. Therefore, the concept of ODN was introduced to eliminate this systematic error and accurately total water consumption for the entire house using a common house meter.
In this sense, RF PP No. 354 is not presented entirely correctly and has long since become outdated in some places, when it is proposed to base calculations on the total readings of the IPU if there is no common house meter, but at the same time, the authors of the regulatory text completely forgot about the systematic error of apartment IPU (dead zone IPU at low water flows).
According to the law “On Energy Saving...”, the first thing that must be done is to install common household meters, and where there is no technical possibility due to the design of the house, technical feasibility must be created by reconstructing (extension) the premises for the installation of utility metering units.
General household metering of utility resources is not beneficial to utility workers, which is why they sabotage the process. IN " muddy water“It’s easier to cheat.
Also, in our ITP, we do not have a separate accounting of the consumption of thermal energy, which is spent on heating the hot water supply. At least this is not clear from the content of the information given in the receipt.
But what about the super duper energy efficient ITP? Isn’t this too simple for a super duper energy-efficient ITP with “space technologies”?
Have you installed one common cold water meter and one common thermal energy meter for the entire block and are as happy as elephants?
And according to the Law, every single house must be equipped with metering devices.
How then does our ITP differ from the usual heating unit of an old Soviet house?
Why are we being told over and over again about energy efficiency?
I want some rogue - a “money pump” under an energy service agreement to “authoritatively” declare that we need to install metering devices to increase energy efficiency.
It is already clear to us that we need a comprehensive accounting of utility resources.
Who stopped you from installing a two-channel heat energy meter? Was it difficult to install a meter to account for the flow of make-up water for the hot water system?
And if they do exist, then why are their readings not used in calculations and not indicated on receipts?
Calculations of hot water supply, BKN. We find the volume, power of the hot water supply, power of the BKN (snake), warm-up time, etc.
In this article we will consider practical problems for finding the volume of hot water accumulation and DHW heating power. Heating equipment power. Hot water readiness time for various equipment and the like.
Let's look at examples of tasks:
Task 1. Find power instantaneous water heater
Instantaneous water heater- This is a water heater, the volume of water in which can be so small that its existence is useless for storing water. Therefore, it is believed that an instantaneous water heater is not intended to accumulate hot water. And we do not take this into account in our calculations.
Given: Water consumption is 0.2 l/sec. Cold water temperature 15 degrees Celsius.
Find: The power of an instantaneous water heater, provided that it heats the water to 45 degrees.
Solution
Answer: The power of the instantaneous water heater will be 25120 W = 25 kW.
It is practically not advisable to consume large number electricity. Therefore, it is necessary to accumulate (accumulate hot water) and reduce the load on electrical wires.
Instantaneous water heaters have unstable heating of hot water. The hot water temperature will depend on the water flow through the instantaneous water heater. Power or temperature switching sensors do not allow for good temperature stabilization.
If you want to find the output temperature of an existing instantaneous water heater at a certain flow rate.
Task 2. Electric water heater (boiler) heating time
We have an electric water heater with a capacity of 200 liters. The power of electric heating elements is 3 kW. It is necessary to find the time for heating water from 10 degrees to 90 degrees Celsius.
Given:
Wt = 3 kW = 3000 W.
Find: The time it takes for the volume of water in the water heater tank to heat up from 10 to 90 degrees.
Solution
The power consumption of heating elements does not change depending on the temperature of the water in the tank. (We will consider how power changes in heat exchangers in another problem.)
It is necessary to find the power of heating elements, as for an instantaneous water heater. And this power will be enough to heat water in 1 hour.
If it is known that with a heating element power of 18.6 kW, the tank will heat water in 1 hour, then it is not difficult to calculate the time with a heating element power of 3 kW.
Answer: The time for heating water from 10 to 90 degrees with a capacity of 200 liters will be 6 hours 12 minutes.
Task 3. Boiler heating time indirect heating
Let's take an example of an indirect heating boiler: Buderus Logalux SU200
Rated power: 31.5 kW. It is not clear for what reasons this was found. But look at the table below.
Volume 200 liters
The snake is made from steel pipe DN25. Inner diameter 25 mm. Outer 32 mm.
Hydraulic losses in the snake pipe indicate 190 mbar at a flow rate of 2 m3/hour. Which corresponds to 4.6.
Of course, this resistance is high for water and new pipe. Most likely, there were risks associated with pipeline overgrowth, high-viscosity coolant and resistance at connections. It is better to indicate obviously large losses so that someone does not miscalculate.
Heat exchange area 0.9 m2.
Fits 6 liters of water in a snake pipe.
The length of this snake pipe is approximately 12 meters.
Warm-up time is written as 25 minutes. It is not clear how this was calculated. Let's look at the table.
BKN snake power table
Consider the table for determining the power of the snake
Consider the SU200 snake heat dissipation power of 32.8 kW
At the same time, in the circuit DHW consumption 805 l/hour. Flows in 10 degrees comes out 45 degrees
Another option
Consider the SU200 snake heat dissipation power of 27.5 kW
A coolant with a temperature of 80 degrees flows into the snake at a flow rate of 2 m3/hour.
At the same time, the flow rate in the DHW circuit is 475 l/hour. Flows in 10 degrees comes out 60 degrees
Other characteristics
Unfortunately, I will not provide you with a calculation of the heating time for an indirect heating boiler. Because this is not one formula. There are many intertwined values here: Starting from heat transfer coefficient formulas, correction factors for different heat exchangers (since water convection also introduces its own deviations), and this ends with iteration of calculations based on changed temperatures over time. Here, most likely in the future I will make a calculation calculator.
You will have to be content with what the manufacturer of the BKN (Indirect Heating Boiler) tells us.
And the manufacturer tells us the following:
That the water will be ready in 25 minutes. Provided that the flow into the snake will be 80 degrees with a flow rate of 2 m3/hour. The power of the boiler producing heated coolant should not be lower than 31.5 kW. Ready-to-drink water is considered to be 45-60 degrees. 45 degrees wash in the shower. 60 is very hot water, for example for washing dishes.
Task 4. How much hot water does it take to take a 30-minute shower?
Let's calculate for example with electric water heater. Since the electric heating element has a constant output of thermal energy. The power of the heating elements is 3 kW.
Given:
Cold water 10 degrees
Minimum tap temperature 45 degrees
The maximum temperature of water heating in the tank is 80 degrees
Comfortable flow rate of flowing water from the tap is 0.25 l/sec.
Solution
First, let's find the power that will provide this water flow
Answer: 0.45 m3 = 450 liters of water will be needed to wash with the accumulated hot water. Provided that the heating elements do not heat the water at the time of hot water consumption.
It may seem to many that there is no accounting for the entry of cold water into the tank. How to calculate the loss of thermal energy when water temperature of 10 degrees enters water of 80 degrees. There will obviously be a loss of thermal energy.
This is proven as follows:
Energy spent on heating the tank from 10 to 80:
That is, a tank with a volume of 450 liters and a temperature of 80 degrees already contains 36 kW of thermal energy.
From this tank we take energy: 450 liters of water with a temperature of 45 degrees (through the tap). Thermal energy of water with a volume of 450 liters at a temperature of 45 degrees = 18 kW.
This is proven by the law of conservation of energy. Initially, there was 36 kW of energy in the tank, they took 18 kW, leaving 18 kW. This 18 kW of energy contains water at a temperature of 45 degrees. That is, 70 degrees divided in half gives 35 degrees. 35 degrees + 10 degrees cold water we get a temperature of 45 degrees.
The main thing here is to understand what the law of conservation of energy is. This energy from the tank cannot escape to no one knows where! We know that 18 kW came out of the tap, and there was initially 36 kW in the tank. Taking 18 kW from the tank, we will lower the temperature in the tank to 45 degrees (up to average temperature (80+10)/2=45).
Let's now try to find the volume of the tank when the boiler is heated to 90 degrees.
Used energy consumption of hot water at the outlet of the tap 18317 W
Answer: Tank volume 350 liters. An increase of just 10 degrees reduced the tank volume by 100 liters.
This may seem unrealistic to many. This can be explained as follows: 100/450 = 0.22 is not that much. Stored temperature difference (80-45)
Let's prove that this is a valid formula in another way:
Of course this is a rough theoretical calculation! In the theoretical calculation, we take into account that the temperature in the tank between the upper and lower layers is instantly mixed. If we take into account the fact that the water is hotter at the top and colder at the bottom, then the volume of the tank can be reduced by the temperature difference. It is not for nothing that vertical tanks are considered more efficient in storing thermal energy. Since than more height tank, the higher the temperature difference between the top and bottom layers. When hot water is consumed quickly, this temperature difference is higher. When there is no water flow, very slowly the temperature in the tank becomes uniform.
We will simply lower 45 degrees to 10 degrees lower. For place 45 it will be 35 degrees.
Answer: Due to the temperature shift, we reduced the volume of the tank by another 0.35-0.286 = 64 liters.
We calculated on the condition that at the time of hot water consumption the heating elements were not working and did not heat the water.
Let's now calculate under the condition that the tank begins to heat the water at the moment of hot water consumption.
Let's add another power of 3 kW.
In 30 minutes of operation we will get half the power of 1.5 kW.
Then you need to subtract this power.
Answer: The tank volume will be 410 liters.
Task 5. Calculation of additional power for hot water supply
Consider a private house with an area of 200 m2. The maximum power consumption for heating the house is 15 kW.
4 people live in the house.
Find: Extra power for DHW
That is, we need to find the boiler power taking into account: House heating power + hot water heating.
For this purpose it is better to use scheme No. 4:
Solution
It is necessary to find how many liters of hot water a person consumes per day:
SNiP 2.04.01-85* states that, according to statistics, 300 liters per day are consumed per person. Of these, 120 liters are for hot water at a temperature of 60 degrees. These city statistics are mixed with people who are not used to using so much water per day. I can offer my consumption statistics: If you like to take hot baths every day, you can spend 300-500 liters of hot water per day for just one person.
Volume of water per day for 4 people:
That is, to the heating power of a house of 15 kW, you need to add 930 W = 15930 W.
But if you take into account the fact that at night (from 23:00 to 7:00) you do not consume hot water, you get 16 hours when you consume hot water:
Answer: Boiler power = 15 kW + 1.4 kW for hot water supply. = 16.4 kW.
But in this calculation there is a risk that at the moment of high consumption of hot water at certain hours you will stop heating the house for a long time.
If you want to have a good flow of hot water for a private home, then choose a BKN of at least 30 kW. This will allow you to have an unlimited flow rate of 0.22 l/sec. with a temperature of at least 45 degrees. The boiler power should not be less than 30 kW.
In general, the objectives of this article were focused on energy conservation. We did not consider what was happening at a particular moment, but took a different route to calculate. We followed the undisputed method of energy conservation. The energy expended at the outlet of the tap will then be equal to the energy coming from the boiler equipment. Knowing the powers in two different places you can find the time spent.
Once we discussed the calculation of hot water supply on the forum: http://santeh-baza.ru/viewtopic.php?f=7&t=78
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A series of video tutorials on a private home
Part 1. Where to drill a well?
Part 2. Construction of a water well
Part 3. Laying a pipeline from the well to the house
Part 4. Automatic water supply
Water supply
Water supply for a private house. Operating principle. Connection diagram
Self-priming surface pumps. Operating principle. Connection diagram
Self-priming pump calculation
Calculation of diameters from central water supply
Water supply pumping station
How to choose a pump for a well?
Setting up the pressure switch
Pressure switch electrical diagram
Operating principle of a hydraulic accumulator
Sewage slope per 1 meter SNIP
Heating schemes
Hydraulic calculation of a two-pipe heating system
Hydraulic calculation of a two-pipe associated heating system Tichelman loop
Hydraulic calculation of a single-pipe heating system
Hydraulic calculation of radial distribution of a heating system
Scheme with a heat pump and solid fuel boiler - operating logic
Three-way valve from valtec + thermal head with remote sensor
Why the heating radiator in an apartment building does not heat well
How to connect a boiler to a boiler? Connection options and diagrams
DHW recirculation. Operating principle and calculation
You are not calculating the hydraulic arrow and collectors correctly
Manual hydraulic heating calculation
Calculation of warm water floors and mixing units
Three-way valve with servo drive for domestic hot water
Calculations of hot water supply, BKN. We find the volume, power of the snake, warm-up time, etc.
Water supply and heating designer
Bernoulli's equation
Calculation of water supply for apartment buildings
Automation
How servos and three-way valves work
Three-way valve to redirect the flow of coolant
Heating
Calculation of thermal power of heating radiators
Radiator section