Heat Loss Calculator

Use our calculator to estimate the total heat loss of your home through walls, floors, ceilings, windows to determine heating capacity requirements.

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How much BTU do I need for my house?

Heat loss is the transfer of heat from inside to outside through the building envelope. Knowing what heat loss is, how to reduce it, and what it means when choosing heat output will help answer one of the most common questions we are asked: Do I have enough BTUs to heat my room? This figure is usually specified in kilowatts (kW) or British thermal units (BTU).

The rate of heat transfer in walls, floors and roofs is measured by the heat transfer coefficient, which in turn shows how well parts of the building transfer heat. A low heat transfer value means a well insulated building, while a high value indicates a poor thermal performance of the building. Accordingly, to know the heat loss of a building as a whole, you need to know the heat transfer value for the walls, floor or roof.

Our heat loss calculator contains all necessary data and allows users to estimate heat losses through enclosing constructions without any special knowledge. The only thing you have to do is to specify the minimum temperature of the coldest week of the year, indicate the area of the room, as well as enter the layer thickness and other features, depending on the fields to be selected. Note that you can select two layers (external, internal) for each part of the building. To get the result press the "Calculate" button.


Theoretical basis for calculating heat losses

The main formula that estimates the heat loss through parts of the building is as follows:

Q = S × ((tin - tout) / R)

  • S – area, m2;
  • tin – internal temperature, °С;
  • tout – outdoor temperature, °С;
  • R – thermal resistance, (m2 × °С)/W.

To calculate the thermal resistance of the wall the following formula is used:

Rsum = Rm + Rin + Rout

  • Rm – main material's thermal resistance, W/(m2 × °С);
  • Rin – thermal resistance inner wall layer, W/(m2 × °С);
  • Rout – thermal resistance outer wall layer, W/(m2 × °С).

Deeper still, it is the thermal resistance values for each layer are equal:

Rm = L / λ 
Rin = 1 / αin
Rout = 1 / αout

  • L – material thickness, m;
  • λ – material conductivity, W/(m × °С)
  • αin – heat transfer coefficient inner surface, W/(m2 × °С);
  • αout – heat transfer coefficient outer surface, W/(m2 × °С).

All parameters are selected according to the regulations of their country (they may vary slightly up or down).

Heat losses for infiltration are calculated by the formula:

Qi = 0.28 × Gi × c × (tin - tout) × k

  • Gi – airflow rate, m3/h;
  • c – heat capacity of air, 1.006 kJ/(kg × °С)
  • tin – internal temperature, °С;
  • tout – outdoor temperature, °С;
  • k – coefficient for the effect of counter heat flow in the constructions (0.8 by default).

The exhaust air flow Gi, which is not compensated by the supply air, is determined as follows:

Gi = 3 × S

  • 3 – air exchange rate for apartments, m3/h;
  • S – area, m2.
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