How to choose a buffer tank for a solid fuel boiler

The greatest disadvantage of solid fuel boilers is their cyclical nature: at maximum load and combustion, a peak (often excessive) thermal power is reached, which constantly decreases to 0 (complete attenuation) and is resumed with a new addition of fuel. Such cycling does not allow for a stable, quickly and accurately regulated heating system.

The uneven heat transfer of TT boilers can be smoothed out by a buffer tank (also known as a heat accumulator), which accumulates excess heat during peak operation of the boiler unit. However, there are many nuances in choosing and calculating the required volume of a heat accumulator.

What is a buffer tank for a solid fuel boiler?

A buffer tank (also a heat accumulator) is a tank of a certain volume filled with coolant, the purpose of which is to accumulate excess thermal power and subsequently distribute it more rationally for the purpose of heating a house or providing hot water supply (DHW).

What is it for and how effective is it?

Most often, a buffer tank is used for solid fuel boilers that have a certain cyclicity, and this also applies to long-burning TT boilers. After ignition, the heat transfer of the fuel in the combustion chamber quickly increases and reaches peak values, after which the generation of thermal energy fades, and when it fades, when a new batch of fuel is not added, it stops altogether.

The only exceptions are bunker boilers with automatic supply, where due to the regular uniform supply of fuel, combustion occurs with the same heat transfer.

With such cyclicality, during the period of cooling or attenuation, thermal energy may not be enough to maintain a comfortable temperature in the house. At the same time, during the period of peak thermal power, the temperature in the house is much higher than comfortable, and part of the excess heat from the combustion chamber simply flies out into the chimney, which is not the most efficient and economical use of fuel.

A visual diagram of connecting a buffer tank, showing the principle of its operation.

The effectiveness of a buffer tank is most clearly understood using a specific example. One m3 of water (1000 l) when cooled by 1°C releases 1-1.16 kW of heat. Let's take as an example an average house with a conventional 2-brick masonry with an area of ​​100 m2, the heat loss of which is approximately 10 kW. A 750 liter heat accumulator, heated by several loads to 80°C and cooled to 40°C, will release about 30 kW of heat to the heating system. For the above-mentioned house, this equates to 3 additional hours of radiator heat.

Sometimes a buffer tank is used in combination with an electric boiler; this is justified when heating at night: at reduced electricity rates. However, such a scheme is rarely justified, since in order to accumulate a sufficient amount of heat overnight for daytime heating, a tank of not 2 or even 3 thousand liters is needed.

Design and principle of operation

A heat accumulator is a sealed, usually vertical, cylindrical tank, sometimes additionally thermally insulated. He is an intermediary between the boiler and heating appliances. Standard models are equipped with an insert of 2 pairs of pipes: the first pair is the boiler supply and return (small circuit); the second pair is the supply and return of the heating circuit distributed throughout the house. The small circuit and the heating circuit do not intersect with each other.

The principle of operation of a heat accumulator in conjunction with a solid fuel boiler is simple:

1. After the boiler is fired up, the circulation pump constantly pumps the coolant in a small circuit (between the boiler heat exchanger and the tank). The boiler supply is connected to the top pipe of the heat accumulator, and the return pipe to the bottom. Thanks to this, the entire buffer tank is smoothly filled with heated water, without pronounced vertical movement of warm water.
2. On the other hand, the supply to the heating radiators is connected from above to the buffer tank, and the return is connected from below. The coolant can circulate both without a pump (if the heating system is designed for natural circulation) and forcibly. Again, this connection scheme minimizes vertical mixing, so the buffer tank releases accumulated heat to the batteries gradually and more evenly.

If you correctly select the volume and other characteristics of the buffer tank for a solid fuel boiler, heat loss can be minimized, which will affect not only fuel economy, but also the comfort of the firebox. The accumulated heat in a well-insulated heat accumulator is retained for 30-40 hours or more.

Moreover, due to a sufficient volume, much larger than in the heating system, absolutely all the generated heat is accumulated (in accordance with the efficiency of the boiler). After only 1-3 hours of heating, even with complete extinguishing, a fully “charged” heat accumulator is available.

Types of structures

Any of the above-described types of buffer tanks can have a larger number of pairs of pipes, which allows you to differentiate the parameters of the heating system into zones, additionally connect a water-heated floor, etc.

Battery buffer tank volume

Let's figure out what volume the heat accumulator should be. There are differing opinions that are based on calculations based on:

• room area;
• boiler power.

Let's look at each of them. Based on the area of ​​the room, there can be no exact recommendations. Since there are many factors that influence the battery life of a system without a boiler, the main one is the heat loss of the room. The better the house is insulated, the longer the buffer tank will be able to provide the home with heat.

Of course, the larger the tank, the better, but to heat a larger amount of coolant you will need more heater power. Calculation of boiler power is done based on the heated area. One kilowatt warms up ten meters. You can install a five-ton tank, but if the boiler cannot handle such volumes, there will be no point in installing such a large heat accumulator. This means that adjustments need to be made to the calculation of the power of the boiler itself.

It turns out that it may be more correct to make the calculation based on the power of the boiler. Let’s take for example the same house of 200 sq. m. An approximate calculation of the volume of the buffer tank is as follows: one kilowatt of energy heats 25 liters of coolant. That is, if there is a 20 W heater, then the volume of the heater should be about 500 liters, which is clearly not enough for such housing.

Based on the results of the calculations, we can conclude that if you are planning to install a heat accumulator, then you need to take this into account when selecting the boiler power and take not one, but two kilowatts per ten meters of heated area. Only then will the system be balanced. The volume of TA also affects the calculation of the capacity of the expanzomat. An expansion tank is an expansion tank that compensates for the thermal expansion of the coolant. To calculate its volume, you need to take the total volume of coolant in the circuit, including the capacity of the buffer tank, and divide by ten.

Reviews of household heat accumulators for boilers: advantages and disadvantages

What is a heat accumulator?

A heat accumulator is a buffer tank designed to accumulate excess heat from a heating system boiler. The accumulated resource is subsequently used for various needs: long-term heating of the house, hot heating, and so on.

If you correctly connect a heat accumulator to the CO, you can reduce fuel costs by almost 2 times, be it gas, firewood or even electricity.

In addition to accumulating excess heat, this container protects the boiler from cracking, which occurs during a sharp temperature change.

If such a system is equipped with various sensors and valves for multi-level savings, then heat transfer will increase and the cost of purchasing fuel will be significantly reduced.

How to choose a buffer capacity

Calculation of the minimum required volume

The most important parameter that you should decide on right away is the volume of the container. It should be as large as possible to maximize efficiency, but up to a certain threshold so that the boiler has enough power to “charge” it.

The volume of the buffer tank for a solid fuel boiler is calculated using the formula:

m = Q / (k*c*Δt)

• where m is the mass of the coolant, after calculation it is not difficult to convert it into liters (1 kg of water ~ 1 dm3);
• Q – required amount of heat, calculated as: boiler power * period of its activity - heat loss at home * period of boiler activity;
• k – boiler efficiency;
• c – specific heat capacity of the coolant (for water this is a known value - 4.19 kJ/kg*°C = 1.16 kW/m3*°C);
• Δt – temperature difference in the boiler supply and return pipes; indicators are taken during steady operation of the system.

For example, for an average house with 2 bricks and an area of ​​100 m2, the heat loss is roughly 10 kW/hour. Accordingly, the required amount of heat (Q) to maintain balance = 10 kW. The house is heated by a boiler with a power of 14 kW and an efficiency of 88%, the firewood in which burns out in 3 hours (the period of boiler activity). The temperature in the supply pipe is 85°C, and in the return pipe – 50°C.

First you need to calculate the required amount of heat.

Q = 14*3-10*3 = 12 kW.

As a result, m = 12 / 0.88 * 1.16 * (85-50) = 0.336 t = 0.336 cubic meters or 336 liters . This is the minimum required volume of buffer capacity. With such a capacity, after burning out the fill (3 hours), the heat accumulator will accumulate and distribute further 12 kW of heat. For the example house, this is more than 1 additional hour of warm radiators on one tab.

Accordingly, the indicators depend on the quality of the fuel, the purity of the coolant, and the accuracy of the initial data, so in practice the result may differ by 10-15%.

Calculator for calculating the minimum required capacity of a heat accumulator

Number of heat exchangers

Copper internal heat exchangers of the storage tank.
After selecting the volume, the second thing you should pay attention to is the presence of heat exchangers and their number. The choice depends on desires, requirements for CO and the tank connection diagram. For the simplest heating system, an empty model without heat exchangers is sufficient.

However, if natural circulation is planned in the heating circuit, an additional heat exchanger is needed, since the small boiler circuit can only function with forced circulation. The pressure in this case is higher than in a heating circuit with natural circulation. To provide hot water supply or connect underfloor heating, additional heat exchangers will also be required.

Maximum allowable pressure

When choosing a buffer tank with an additional heat exchanger, you should pay attention to the maximum permissible operating pressure, which should not be lower than in any of the heating circuits. Tank models without heat exchangers are in most cases designed for an internal pressure of up to 6 bar, which is more than enough for average CO.

Inner container material

At the moment, there are 2 options for manufacturing an internal tank:

• mild carbon steel – coated with a waterproof anti-corrosion coating, has a lower cost, and is used in inexpensive models;
• Stainless steel is more expensive, but more reliable and durable.

Some manufacturers also install additional wall protection in the container. Most often, this is, for example, a magnesium anoid rod in the center of the tank, which protects the tank walls and heat exchangers from the build-up of a layer of solid salts. However, such elements require periodic cleaning.

Other selection criteria

After determining the basic technical criteria, you can pay attention to additional parameters that increase efficiency and comfort of use:

• the ability to connect a heating element for additional heating from the electrical network, as well as additional control and measuring instruments that are mounted with a threaded or coupling (but in no case welded) connection;
• the presence of a layer of thermal insulation - in more expensive models of heat accumulators, between the inner tank and the outer shell there is a layer of thermal insulation material that promotes even longer heat retention (up to 4-5 days);
• weight and dimensions - all of the above parameters affect the weight and dimensions of the buffer tank, so it is worth deciding in advance how it will be brought into the boiler room.

How to calculate the volume of TA

In order for a heat accumulator for heating to perform its functions, its volume must be selected correctly. There are several methods:

• by heated area;
• by boiler power;
• according to the time available.

Most of the methods are based on experience. For this reason, there is a “fork” in the recommendations. For example, from 35 to 50 liters per square meter of heated area

How exactly to determine the number? It is worth taking into account the region of residence and the degree of insulation of the house. If you live in a region with not the most severe winters or the house is well insulated, it is better to take the lower limit or so

Otherwise - on the top.

It is possible to power not only radiators, but also heated floors from the TA, and you can also install a heat exchanger for hot water

When choosing the volume of a heat accumulator for heating, two points must also be taken into account. First, a large amount of water will allow you to heat it much less often.

Due to the stored heat, the temperature can be maintained for a long time. But, on the other hand, the time it takes to “accelerate” this volume to the required temperature greatly increases (heating to 85-88°C is considered normal). In this case, the system becomes very inertial. You can, of course, take a more powerful boiler, but paired with a buffer tank, this will cost a considerable amount. Therefore, you have to maneuver, finding the optimal solution.

By heated area

You can select the volume of the heat accumulator for the heating system according to the area of ​​the room. It is believed that 35 to 50 liters are needed per ten square meters. The selected value is multiplied by the quadrature divided by ten to obtain the desired volume.

For example, in the heating system of a house with an area of ​​120 m² with average insulation, it is better to install a heat accumulator for heating 120 m² / 10 * 45 l = 12 * 45 = 540 liters. This will not be enough for the Middle Zone, so you should look at containers with a volume of approximately 800 liters.

The greater the system performance, the larger the TA should be

In general, to make it easier to navigate, for a house with an area of ​​160-200 square meters, located in the middle zone, with average insulation, the optimal tank volume is 1000-1200 liters. Yes, with such a volume, you will have to heat it more often in cold weather. But this will not undermine your budget too much, and will allow you to live quite comfortably almost all winter.

By boiler power

Since the boiler will have to work on heating the water in the tank, it makes sense to calculate the volume based on its capabilities. In this case, 50 liters of capacity are taken per 1 kW of power.

You can make it even simpler - use the table (the values ​​that are optimal in terms of cost and performance are shaded in yellow)

The calculation is simple. For a 20 kW boiler, a TA of 1000 liters is suitable. With such a volume of heat storage for heating, you will have to heat it twice a day.

According to the desired duration of downtime and heat loss

This method is more accurate, as it allows you to select dimensions specifically for the parameters of your home (heat loss) and your wishes (duration of downtime).

Let's calculate the volume of a heat accumulator for a house with a heat loss of 10 kW/hour and an idle time of 8 hours. We will heat the water to 88 °C, and it will cool down to 40 °C. The calculation is as follows:

• To maintain normal temperature, the container must accumulate 80 kW of heat (10 kW * 8 hours).
• Temperature delta: 88°C - 40°C = 48°C.
• Next, we calculate using the formula for determining the heat capacity of water.
• Substituting the values ​​we get: 80000 W / 1.163 W/kg°C * 48°C = 80000 / 55.824 = 1433 liters.

For these conditions, the required capacity of the heat accumulator for heating is 1500 liters. This is because heat loss of 10 kW/hour is too much. This is a house with virtually no heating.

The best known manufacturers and models: characteristics and prices

Sunsystem PS 200

A standard inexpensive heat accumulator, perfect for a solid fuel boiler in a small private house with an area of ​​up to 100-120 m2. By design, this is a regular tank, without heat exchangers. The capacity of the container is 200 l at a maximum permissible pressure of 3 bar. For a low cost, the model has a 50 mm layer of polyurethane thermal insulation and the ability to connect a heating element.

Hajdu AQ PT 500 C

One of the best buffer tank models for its price, equipped with one built-in heat exchanger. Volume – 500 l, permissible pressure – 3 bar. An excellent option for a house with an area of ​​150-300 m2 with a large power reserve of a solid fuel boiler. The line includes models of different sizes.

Starting with a volume of 500 l, models (optional) are equipped with a layer of polyurethane thermal insulation + an artificial leather casing. Installation of heating elements is possible. The model is known for extremely positive reviews from owners, reliability and durability. Country of origin: Hungary.

S-TANK AT PRESTIGE 300

Another inexpensive buffer tank with a capacity of 300 liters. The design is a storage tank without additional heat exchangers with a maximum permissible operating pressure of 6 bar. The internal walls, as in previous cases, are made of carbon steel. The main difference is a significant, environmentally friendly layer of thermal insulation made of polyester material using NOFIRE technology, i.e. high class thermal and fire resistance. Country of origin: Belarus

ACV (ATsV) LCA 750 1 CO TP

A productive, expensive buffer tank with a volume of 750 liters with an additional tubular heat exchanger for domestic hot water, designed for boilers with a large power reserve.

The internal walls are covered with protective enamel and there is a high-quality 100 mm layer of thermal insulation. A magnesium anode is installed inside the tank, which prevents the accumulation of a layer of solid salts (the kit includes 3 spare anodes). It is possible to install heating elements and additional instrumentation. Country of origin: Belgium.

Calculation

The power accumulated by a heat accumulator (TA) is calculated based on the volume of the container, more precisely the mass of the liquid in it, the specific heat capacity of the liquid used to fill it, and the temperature difference, the maximum to which the liquid can be heated, and the minimum target at which it can still be carried out. heat intake from the heat accumulator to the heating circuit.

Formula:

• Q = m*C*(T2-T1);
• m – mass, kg;
• C – specific heat capacity W/kg*K;
• (T2-T1) – temperature delta, final and initial.

If the water in the boiler and, accordingly, in the heating element is heated to 90ºС, and the lower threshold is taken equal to 50ºС, then the delta is equal to 40ºС. If we take TA water as filling, then one ton of water, when cooled to 40ºC, releases approximately 46 kW*hours of heat.

The stored energy must be sufficient for the intended use of the heat accumulator.

To select the required volume of the heat accumulator, it is necessary to determine:

• The time during which the accumulated energy in the heat exchanger should be enough to cover the heat loss of the house;
• The time during which the coolant in the heat exchanger must heat up;
• Power of the main heat source.

For periodic operation of the boiler during the day

If it is needed to switch the boiler operation only to night or day mode, when heat is supplied for a limited time, then the TA power should be enough to cover the heat loss of the house for the remaining time. At the same time, the boiler’s power should be enough to heat the heating unit in a timely manner and, again, to heat the house.

Let's assume that a solid fuel boiler is used with firewood only during the day for 10 hours, the estimated heat loss of the house for the coldest period of the year is 5 kW. 120 kW*hours are required per day for full heating.

The battery is used for 14 hours, which means that it needs to accumulate 5 kW * 14 hours = 70 kW * hours of heat. If you take water as a coolant, you will need 1.75 tons or a heat exchange volume of 1.75 m3. It is important that the boiler must produce all the necessary heat within just 10 hours, that is, its power must be more than 120/10 = 12 kW.

If the heat accumulator is used as a backup option in case of boiler failure, then the stored energy should be enough for at least a day or two to cover all heat losses in the house. If we take the same 100 m2 house as an example, then heating it will require 240 kW*hours in two days, and a heat accumulator filled with water must have a volume of at least 5.3 m3.

But in this case, the TA does not necessarily have to heat up in a short period of time. A boiler power reserve of one and a half is enough to accumulate the required amount of heat in a week or two.

The calculation is approximate, without taking into account the reduction in the thermal power of radiators depending on the temperature of the coolant and air in the room.

Prices: summary table

Wiring and connection diagrams

As an additional heat source, for example, instead of an electric boiler, a tubular electric heater (TEN) can be used. Most modern models already provide for its installation via flange or coupling mounting. By installing a heating element in the corresponding pipe, you can partially replace an electric boiler or once again do without lighting a solid fuel boiler.

It is important to understand that these are simplified, not complete connection diagrams. To ensure control, accounting and security of the system, a security group is installed at the boiler supply. In addition, it is important to take care of the operation of the CO in the event of a power outage, because The energy generated by the thermocouple of non-volatile boilers is not enough to power the circulation pump. Lack of coolant circulation and heat accumulation in the boiler heat exchanger will most likely lead to a rupture of the circuit and emergency emptying of the system; it is possible that the boiler will burn out.

Therefore, for safety reasons, it is necessary to ensure that the system operates at least until the filling is completely burned out. For this, a generator is used, the power of which is selected depending on the characteristics of the boiler and the duration of combustion of 1 load of fuel.

Installation of a heat accumulator

Regardless of what type of storage tank you install, you will need the following in any case:

• Ball Valves;
• Circulation pumps;
• Polypropylene pipes of various diameters;
• Check valves;
• Thermal sensors;
• Pressure relief valve;
• Wiring;
• Electronic pump control unit;
• Thermal accumulator;
• Expansion tank;
• Tools (various wrenches, pipe soldering iron, FUM tape, and so on).

Important advice! It is recommended to install the heat accumulator closer to the boiler. This will save you some money, and heat loss will also be significantly reduced.

All water or antifreeze is poured out of the CO (it is usually poured into a special container, since buying a new one would be too expensive).

1. A container for thermal expansion of water is connected to the upper open part of the storage circuit, and a safety valve must be connected to it in case the pressure in the system is exceeded.
2. Taps are mounted on the tank nozzles. In general, you don’t have to do this, but if you want to carry out repair work without draining the coolant, then you will have to install ball shut-off valves.
3. A pump is connected to the lower outlet pipe of the heat accumulator.
4. The pressure terminal of the heating system is connected to the upper terminal of the heat accumulator.
5. In the same area, a temperature sensor and an electronic control unit for the thermal storage system are installed. If you do not know how to configure the unit’s operation, you should seek help from a specialist.
6. The main part of the CO through which the coolant circulates is connected to the adjacent terminal, located at the top of the thermal tank.
7. A second pump is connected to the return line of the battery to circulate water. This device is needed to move the main heating circuit.
8. Then automation is built in to regulate the normal functioning of the second circulation pump.
9. If your heat accumulator has a second storage circuit inside the tank, then it is also connected in parallel to the CO.
10. If additional equipment and temperature sensors do not have standard power cables, then work is carried out to connect the power supply.
11. Lastly, the system is connected to ground, after which the advanced heating system is diagnosed and started for the first time.

Note! All security systems and additional equipment are best installed through quick-release fittings, which are usually called “American” fittings.