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Heat Load Calculations

  • 23 November 2017

    The first step in designing a heating system for your home is to calculate the required heating load. Here’s how we do it.

    The first thing we do is choose the indoor temperatures we want to heat the house to. As a general rule, we heat living areas to 21°C and bedrooms and utility areas to 18°C.

    We also need to select an outdoor design temperature. This is the minimum temperature that we expect it to get to outdoors. This is dependent on where your home is, and we can get this from weather data.

    Macbook Drawing

    Once we’ve chosen our design temperatures, then we need to calculate the rate at which heat will leak out through the walls, floor and ceiling of each room.

    At the risk of losing readers, I’m going to include the formula for calculating heat loss. It looks like this:

    Rate of Heat Loss =

    Formula for Heat Loss

    A = area of the wall, floor, or ceiling

    ΔT = temperature difference between inside and outside

    R = insulation value of the surface

    The area and temperature difference are fairly straightforward to work out. The insulation value requires a bit more explanation.

    Any insulation product will have an R value provided by the manufacturer. As an example, ordinary timber framed wall with fibreglass batting would have an R value of around 2.

    The R value of the whole wall is determined by the construction method, the material used, and so on. This can be complex to determine but in practice we can make a fairly close assumption.

    Any windows in the wall will have a higher heat loss rate than the walls, so they need to be calculated separately. The type of glass and the frames needs to be taken into account. Aluminium frames lose heat faster than timber frames.

    We need to add up the heat loss rates through each surface for each individual room, so that we can size an appropriate radiator for each room.

    Once we’ve done that we can add all the rooms together to get the total heat loss rate for the house, which tells us what size boiler we need. We can add in a few extra kilowatts to the boiler size if we’re also going to heat our domestic hot water.

    So as you can see, it’s quite an involved process! Using a spreadsheet helps, or you can use specialised software. The tricky part is likely to be getting appropriate R values for the construction. Your architect should be able to help there if you’re building a new home.

    There’s obviously more to know but this is the general overview. Your installer or designer should be able to provide you with their heating load calculation. If they can’t then be very cautious about installing any system they propose. If they get it wrong the first time, it’s hard to fix once the house is built!