How to measure the efficiency of combustion equipment and can the efficiency be increased somehow?

One of the most important parameters that indicates the quality of any device is its efficiency. If we talk about the efficiency of the combustion equipment we use for heating homes, it is obvious that with increasing efficiency, fuel consumption will decrease, and thus also fuel costs. Similarly, we can state that with decreasing efficiency, we have to add more fuel to the combustion device in order to get the same heat.

The average user has a minimal idea of ​​how efficiently his combustion equipment works, i.e. stove, fireplace, fireplace insert, stove or boiler. Did you know that if you heat in an open fireplace, approx. 90% of the heat is not used for heating, but simply put, "goes up the chimney"? Yes, an open fireplace is only about 10% efficient. Do you want to know how efficiently you heat your home? The aim of this article is to offer a simple procedure that will enable an approximate determination of the efficiency of your combustion equipment in domestic conditions.

The efficiency of the combustion device expresses the degree of efficiency of using the energy contained in the fuel for heating purposes. Efficiency can be determined either by direct or indirect methods.

The direct method of determining efficiency is based on the consideration that the efficiency of a combustion device for heating expresses the ratio between the energy used (that is, the amount of energy - heat - that we obtained by burning the fuel and used for heating) and the supplied energy (that is, the amount of energy in the fuel - calorific value and fuel consumption). Direct determination is an accurate method, which, however, is difficult to use for ordinary users due to the problematic nature of "home" determination of the performance value.

The indirect method of determining efficiency is based on the following consideration: an ideal machine or device works with an efficiency of 100%, but in reality no real device is Perpetum mobile and its efficiency is therefore always less than 100%. The actual efficiency is lower by the various losses, so we can say that the efficiency is equal to 100% minus the sum of the individual losses in %. If we determine the main losses, we can relatively reliably determine the efficiency even in domestic conditions.

The most important factors in terms of effectiveness:

  • insulation of the house or heated room
  • perfectly tight fireplace
  • type of wood and % moisture content of the wood being burned
  • the size of the radiating surface
  • method of controlling the stove

In a simplified way, we can talk about the following losses for small combustion devices:

  • Loss caused by the leakage of combustibles in solid residues (in the ash) - black ash still contains combustibles that could burn if we throw it in the trash, it will no longer warm us. Sintering and incomplete combustion sometimes occur when burning coke, and this loss can reach more than 10%. If we put fuel on the grate that contains a large proportion of fine particles (unsorted coal), this part can fall into the ash tray and if it does not burn in it, the loss of combustibles in the ash will increase significantly. For hot water boilers, this loss usually ranges from 2 to 4%. When burning wood in a fireplace stove, we can count on a value of around 0.5%.
  • Loss due to leakage of combustibles in the flue gas. The goal of every user of combustion equipment is the perfect combustion of the combustibles contained in the fuel (carbon, hydrogen). If combustion is perfect, carbon (C) burns to carbon dioxide (CO2) and hydrogen (H) to water (H2O). If the combustion is incomplete, the carbon will only burn into poisonous carbon monoxide (CO) or it will not burn at all (soot). Various hydrocarbons (CXHY) are also a typical product of incomplete combustion. CO and CXHY are flammable gases and if they did not burn, they could not release the energy contained in them (calorific value [J/m3]). Simply put, we don't get warm from the fuel we don't burn. For fireplace stoves with a CO concentration of up to 0.1% (very good stoves), this loss will be approximately 1%, but in the case of worse combustion at a CO concentration of around one percent (by volume), this loss can reach values ​​of up to 6%.
  • Heat loss from solid residues. It occurs when we put hot ash out of the ashtray, which gradually releases heat to the surroundings as it cools. In devices with a one-time delivery of fuel to the boiler and stove, this does not normally occur, so we can consider a zero value.
  • Loss caused by the sharing of heat to the surroundings through the walls of the boiler. The primary goal of a hot water boiler is to transfer the heat of the flue gas to the heating water and not to heat the boiler room. It depends on the thermal insulation of the boiler walls, the size and temperature of the boiler surface. For conventional hot water boilers, this loss is no more than 2%. The greater this loss, the warmer it is in the boiler room. It is not a loss for fireplaces, fireplace and tile stoves, fireplace inserts and kitchen stoves, as these are devices whose purpose is to heat the air in the room where they are installed. These devices are about device performance and that's what we want and what the device is designed for.
  • Loss caused by heat leakage in flue gas, the so-called chimney loss (loss due to sensible heat of flue gas). Chimney loss represents that part of the heat that so-called "flies out the chimney". The heat of the flue gas could have heated the heating water for the radiator or the air in the heated space, but for some reason this did not happen. For well-functioning combustion devices, this loss is absolutely dominant (the largest). Therefore, in the next part of this article, we will only deal with the chimney loss and neglect the other losses.

The size of the chimney loss is most influenced by two parameters:

  • flue gas temperature
  • the amount of flue gas, without knowledge of which we are unable to determine the efficiency of the combustion device

  1. An example of placing a thermometer behind a fireplace stove - if we are talking about the temperature of the flue gas, it is the temperature of the flue gas behind the combustion device (between the device and the chimney). Without information on the flue gas temperature, it is not possible to determine the efficiency of the device. Here, complications can arise with finding a suitable and accessible place for the flue. As a guide, however, we can get by with measuring the surface temperature of the flue with the help of touch thermometers.
  2. Determining the amount of flue gases is a significantly more complex task compared to determining their temperature. Measurement is not easy, but on the other hand, it is clear that the amount of flue gas is closely related to how much combustion air is supplied to the combustion device (in most cases sucked in), i.e. with the excess of combustion air (the ratio of the amount of actual air to the amount of air theoretically needed ) we work. The amount of flue gas is approximately the same as the amount of combustion air. Each combustion device has an area with an optimal excess of combustion air, when it achieves the best parameters. The amount of combustion air is affected not only by the setting of all control elements for the supply of combustion air (dampers, etc.) and chimney parameters, but it is also important to know how tight our combustion equipment is. If we close all control elements of the supply of primary, secondary or of tertiary combustion air (I will perform a leak test) and the flame continues to burn and does not go out visibly (can only be tested on a combustion device with a glass door), it is obvious that the air is drawn in outside of these regulating elements and there is no point in adjusting them, because they do not regulate anything. These leaks need to be found and removed. A simple method of finding leaks is to approach the source of smoke (e.g. a lit cigarette) to the combustion device in which the fuel is burning and look for the place where the smoke is sucked into the stove or boiler. In most cases, the biggest source of leaks is in the door, and to eliminate it, it is enough to replace the sealing cords on the side door and replace the adhesive seals on the ashtrays and glasses. When we have the air under control - the change in the setting of the control elements will be visibly reflected in the change in the size and color of the flame. We can then set and influence the intensity and quality with the regulatory elements. The goal of sealing leaks is to limit the supply of false air, which negatively affects the operation of the equipment. However, the goal is not to completely prevent the supply of air to the device, but to limit the supply of false air to inappropriate places in the combustion chamber of the device. The control elements will allow a sufficient amount of air to be brought to the right place in the combustion device. We only close the control elements during the leak test, never after adding fuel.
  3. Furthermore, we have other possibilities of increasing efficiency if we reduce the temperature of the flue gases that come out of the stove, reduce the amount of heat going out through the chimney and thus increase efficiency again. We reduce the temperature of the flue gas by the amount of fuel burned (we have to add less), by adjusting the control elements of the combustion air supply and the intensity of cooling the stove. With classic fireplace stoves, cooling the casing is problematic, with hot water stoves, we can increase the flow of heating water and lower the temperature of the return water. For warm-air fireplace inserts, we increase the cooling intensity by opening all flaps for the supply of heated air into the room or by turning on the heated air fan. If we reduce the flue gas temperature to 250 °C, we reach an efficiency of around 70%. At 200°C the efficiency would be around 77%.
  4. We should keep the flue gas temperature in the range of 150 to 250 °C. For hot water boilers, it should be approximately from 150 to 200 °C. If the temperature is too high, a lot of heat will "fly out" through the chimney and we have a large chimney loss. But be careful, we cannot reduce the temperature of the flue gas indefinitely, because if the flue gas is too cold (below 150 °C, it depends on the composition of the flue gas - water and SO3 content), condensation of tar substances and water vapor may occur, and at low flue gas temperature, reduction of chimney draft. Condensation reduces the life of the chimney and combustion equipment. Again, this is a compromise where we remove as much heat as possible from the flue gas for heating purposes, but only to the extent that our boiler or stove does not corrode and the chimney "survives" (corrosion, wetting of masonry).

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