Heater Efficiency Calculation: Formula & Equation

Heater Efficiency

A heater’s efficiency varies based on its type. Although electric heaters have an efficiency of about 100%, the efficiency of gas heaters covers a wide range, which is less than 100%. Using general definitions and assumptions, in this article, we will explain how the efficiency of various types of heaters can be calculated.

Heater Efficiency
An example of a gas room heater (Reference: gasheaterservicesydney.com.au)

Definition of Heater Efficiency

Imagine that you need to heat your house. You can obtain heat by converting electrical energy directly into heat as is the case for an electric heater. Almost all electrical energy is converted to heat. Energy input into the house to achieve a given level of heat is equal to the amount of this heat. Thus, the electric heater is 100% efficient because energy not directly transformed into heat will eventually be transformed into heat.

The problem here is that that’s not really a useful way to think about efficiency since any type of energy will likely become heat energy pretty quickly in your home. According to this viewpoint, computers, televisions, and refrigerators are 100 percent efficient at heating your home, since although they perform tasks other than generating heat, the energy they use to perform those tasks is converted into heat very quickly.

Heat pumps, on the other hand, heat your house by moving heat from the outside and transferring it to the inside. Temperatures inside and outside determine how much energy it needs to complete this process. Based on the temperatures inside (Ti) and outside (To), we can calculate the amount of energy required to operate an ideal heat pump (a Carnot engine) as follows:

 

E=\left(1-\frac{T_0}{T_i}\right)\times dH

 

It is the Joules of work required to transfer dH Joules of heat energy from the outside to the inside; if the outside temperature were higher, the number would be negative, meaning the heat pump would get energy from the outside.

In comparison to the idealized heat pump, the electric heater for given inside and outside temperatures has the following efficiency:

 

\eta =\left(1-\frac{T_o}{T_i}\right)

 

Electric heaters work at zero percent efficiency when the indoor and outdoor temperatures are the same. Suppose that the temperature outside is 0 °C and the temperature inside is 25 °C; in this case, the efficiency of the electric heater is about 8.4%:

 

\eta =\left(1-\frac{273}{298}\right)=0.084

 

Note that the temperatures in the above equation are expressed in Kelvin units.

In other words, since electrical resistance heating uses all incoming electric energy to produce heat completely, it can be considered 100% efficient. Despite this, coal, gas, or oil generators are responsible for supplying the majority of electricity. They convert approximately 30% of the fuel’s energy into electricity. Aside from the generation and transmission losses, the cost of electric heat is often higher than that of heat from combustion instruments, such as natural gas, propane, and oil furnaces.

As a result, the cost of natural gas can be less than that of electricity per energy unit, making natural gas a more cost-effective heating source compared to electricity, although this may have nothing to do with the real efficiency of the devices converting energy to heat.

Heat pumps can be very effective in certain environments. However, waste oil furnaces (particularly if you have access to plenty of used motors) are effective in terms of energy conversion and cost-effectiveness, even though they are not theoretically as efficient as heat pumps.

Heating with heat pumps is among the most efficient options out there. Even though all electric heaters are 100 percent efficient, these heaters can actually use heat from the outside to heat your home.

Most climates would benefit from heat pumps over electric resistance heating if electricity is the only option due to their ability to use 50% less electricity. It is an exception to this rule in climates with dry conditions and hot temperatures or mixed temperatures (hot and cold). Since there are few heating days in these dry climates, the high cost of heating has no economic significance.

An electric resistance heater may also be an option if an existing heating system cannot be extended to supply heat for a home addition.

 

Electric Room Heaters 

Because all the electricity they consume is turned into heat, electric heaters are considered 100% efficient; however, they are not cheap to operate. The operating cost of a heater can be calculated by looking at its power rating, expressed in kilowatts (kW). In addition to producing more heat, higher power ratings also have higher costs.

There is no good reason to use electric heaters as the primary source of heat since they are expensive. Consider your heating needs and the type of heater before purchasing, or you could end up paying more for a heater that you won’t feel comfortable with. As halogen heaters have a low power rating (but also produce less heat), they are often the cheapest radiant heaters, while oil-filled radiators are usually the cheapest convector heaters because they have a thermostat to control the temperature.

The following figure shows different types of electric heaters:

Heater Efficiency
Different types of electric heaters: 1) Bar fire, 2) Convector heater, 3) Oil-filled radiator, 4) Radiant fan heater, 5) Radiant halogen heater (Reference: cse.org.uk)

Gas Room Heaters 

Gas heaters work differently from electric heaters in their efficiency, as not all of the gas in the tank results in heat output (some is lost as moisture, exhaust gases, or light). However, main gas is approximately three times cheaper than electricity, which means that the running costs are typically comparable.

It’s possible to operate gas heaters using mains gas or LPG (liquid petroleum gas), also called bottled gas. It is important to note that prior to purchasing a gas heater, you should check to see if there are any safety regulations, as some heaters require flues or chimneys for the discharge of combustion gases and moisture.

Portable gas heaters do not need flues; however, you still need to keep the room well-ventilated. Carbon monoxide risks can arise if not ventilated appropriately, while water vapor produced by gas heaters can cause condensation, which can result in damp and mold problems.

A gas heater is rated based on its heat output as well as its efficiency, which should be considered together when purchasing. The heat output tells us how much heat will be provided to a room. If two heaters produce the same amount of heat, the heater with the higher efficiency will cost less to operate than the heater with the lower efficiency. In other words, it will use less gas to maintain the same level of heat.

Obviously, the costs will vary, but it’s important to compare them. Heaters are at full power without thermostatic control for one hour, so the comparison is performed based on the results of this process.

Heater Efficiency
Different types of gas heaters: 6) Traditional gas fire, 7) Gas convector heater, 8) Open gas fire, 9) Bottled gas heater (Reference: cse.org.uk)

Radiant gas fires have an efficiency of about 60%, with a typical output of about 5kW. The source of heat of this gas heater is radiation.

A gas convector heater is about 60% efficient. The typical output of each is 3kW. Convection is the predominant heating mechanism.

Gas open fireplaces offer an average efficiency of 28% and 2.5 kW output power. In addition, the heat source is radiant.

Bottled-gas heaters are more efficient. Typically, this gas heater type has an efficiency of around 92 percent. Additionally, they typically produce 4kW of power. Furthermore, it is a radiant heating source.

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2 thoughts on “Heater Efficiency Calculation: Formula & Equation”

  1. Alan Mainwaring

    Ever since my early days of learning about thermodynamics I have always been puzzled by for example heat pumps. For example, a Coefficient of performance of 3 is quite typical. That is for every 1kWhr of energy consumed you get 3KWhr of heat. Now at first sight this sounds like you are getting something for nothing.
    A common reason given is that you are moving heat, not generating. I think I have a good back in Physics but I feel this statement explains nothing. I call it explaining away. I feel a more insightful reason is to do with a puzzling concept called LATENT HEAT of vaporization. I have always been fascinated using the water analogy that liquid water at atmospheric pressure will absorb huge amounts of heat to change its state from liquid into gas and yet the temp stays constant. Heat is going in yet the temp does rise. I note that going past a heat pump on a cold day, the outside unit is much colder than the outside air. Thus heat is moving into the outside evaporator, but the evaporator does not rise in temp. Why? Its because of this latent heat of vaporization of the refrigerant, for example it might be liquid ammonia at high pressure being converted to gaseous ammonia its boiling point might be for example -30 deg C, so that the outside air at 0 deg C contains enormous amounts of heat that gets eventually moved into the insulated room. Whoever came up the heat pump it is an extremely clever invention. As I said heat is weird and many people confuse it with temperature and just using the explanation that you are just moving heat really is a very superficial explanation

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