Heat Loss Calculation

Why do I need a heat loss calculation?

Your house is always losing heat to the outside. If it is poorly insulated, draughty, or has many odd wings and projections like dormers increasing it's outside area, it will lose more heat. If it minimises these things it will lose less heat. We can calculate how much heat a particular house will lose at a given outside and inside temperature, based on its dimensions and what it's made of. We want to know the worst case, so the Heat Loss Calculation (HLC) is the heat loss for a defined internal temperature (around 21C) and external temperature (normally -2 to -3C, depending on location). The HLC result would change if you do work on the house, for example increasing loft insulation, or building an extension.

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The job of your heating system is to replace this heat loss. It follows that the higher the heat loss of your house, the higher the maximum output of your heating system needs to be. All builders, when they build or significantly update a house, should do a heat loss calculation so they know how big a heat pump or boiler to put in.

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The reason you may not have come across this term before, is that a plumber replacing a boiler will not normally bother with an HLC, they just put in whatever size boiler was there before. And even with a new house, a heating engineer may  not pay much attention to the HLC because a big gas boiler is often the same price as a smaller output boiler, so they just put in a big one because it saves them doing a proper HLC.  Unfortunately this means that British houses have suffered from an epidemic of boiler oversizing.

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And there is a big downside to oversizing gas (or oil) boilers which is this: boilers can only turn down a certain amount below their maximum ouput - eg to maybe 25% or 30% of the maximum. So when your home doesn't need the max amount of heat - which is almost all the time - it can only avoid overheating you by turning on and off throughout the day. This is called cycling and is very inefficient. So the bigger the boiler, the higher the minimum output and the more cycling takes place, the more gas you waste and the more it costs you, compared to if you had the right size boiler. To give you an idea how crazy oversizing can be, our house has a heat loss of 6kW, which means this is the output required from the heating on the coldest day, ie sub-zero. But the boiler had an output of 30kW, ie 5x bigger than needed. It couldn't turn down lower than 7kW. So even on a sub-zero day, it would be cycling, and wasting gas.

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The reason we have a page on this is that HLCs matter even more with a heat pump. Firstly, bigger heat pumps are more expensive and physically larger, so you don't want to get an oversized one. Secondly, cycling is even worse for heat pump efficiency that for boiler efficiency, and the turn down limitation ("modulation") is similar, so if you oversize you will probably use rather more electricity than you need to.

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Equally if course, if you undersize a heat pump or a boiler, it may not be able to keep up with your home's heat loss on a cold day, and you'll be cold.

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For these reasons it is a requirement of MCS that a HLC is carried out prior to any heat pump installation.

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How is a Heat Loss Calculation carried out?

The HLC involves a survey of all your home's walls, floor and roof, room by room, taking account of their dimensions and the material they're made of, to work out a heat loss for each room and for the whole house. This tells you what size heat pump you need, and also how big the radiators or underfloor heating output has to be in each room. The work is normally done by your potential installer, and takes an hour or two. For this reason many installers will charge for the HLC, although they will often give you an initial free quote without it, based on a less detailed guess at your heat loss.

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However, there is a problem with this kind of analysis, which is that it is rather error prone - and once again, likely to overestimate your heat loss. This is because the installer or other assessor looking at your house will often not know what materials it is made of. Even if they do, they might have some rather inaccurate options to choose from in their HLC software, which may be optimised for speed of survey rather than accuracy. And of course there is human error. All this means that it is helpful if you can pay close attention to your HLC, because it matters to you and your pocket and comfort, and you are likely to know more about your house than any assessor can work out in one hour.

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Now you may be asking the obvious: why go through all this vagueness with house materials, when I know how much energy my house uses on a cold day, just by looking at my smart meter! And indeed this is a much better way of doing it, so long as it corresponds to a properly cold day and a properly heated interior. But when HLCs were first developed, there were no smart meters and homeowners were unlikely to have such precisely measured daily heat loss. So we're still stuck with the HLC requirement. However, use your smart meter data for the coldest day to validate the output of the HLC, and don't accept an HLC result which conflicts significantly with your coldest day data. We had one installer who got 13kW for heat loss, which would have been double the coldest day number and given us a disastrously oversized system. See how to get your Coldest Day figure here.

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Can I do my own HLC?

You can do your own HLC for free. It will help if you are reasonably comfortable with numbers, eg using spreadsheets, but more importantly you want to have a reasonable understanding of what your house is made of. This doesn't mean you need the precise specification of every window and brick, which nobody ever has, but you do want to know, or be able to find out, whether for example you have block cavity or solid brick walls. If you have done any significant renovation you are more likely to have the relevant info, or be able to get it from your builder or supplier.

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Software

Don't make up your own spreadsheet. There is one available for free from MCS here. It is comprehensive, but more time consuming than some other options. Specialised software packages are also available, one of the most common is Heat Engineer, which has a small charge for a single use license.

All approaches make use of two main elements in the calculation: dimensions and U-value. U-value is a measure of the insulating ability of a material, measured in W/m2K. The software will give you most of the U-values you will need, by selecting the relevant material, although if you have specific ones, eg suppliled by a manufacturer of replacement windows, then you should put these in instead.

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Heat loss
= U-value x Area of building element x Temperature difference between inside and out.

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The spreadsheet/software will add up the heat loss at defined indoor and outdoor temperatures for all of the many building elements, ie every wall, window, door, roof type. It will also give you calculation sections for the layers of elements, eg inner leaf + cavity fill + outer brick leaf to give you the overall wall/floor/roof U-value. Expect to spend a few hours on this.

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Major building elements

Apart from your own eyes, you may be able to get advice on what your house is made of from: a friendly builder; your house survey when you bought it; your Energy Performance Certificate (EPC), or the supplier, sales material or data sheet for any elements you've replaced. The previous owner may also have this.

 

Walls: the most common UK wall types are:

• Solid brick, normally in Victorian buildings. This type of wall is normally about 220mm thick, which you can check at a window.

• Cavity, in more modern buildings 1930s onwards, normally 270-300mm thick. The inner and outer leaves may be brick or concrete block - often the outer is brick and the inner is concrete block. Cavities will normally be filled in homes from 1980s onwards, though what with may be hard to ascertain.

Floors: What matters with a floor is what is under the carpet, wood etc on the ground floor. Floors are hard to investigate. The most common in more modern buildings is concrete screed. Victorian houses often have floorboards and joists over a void, which can be a draughty arrangement.

Roof: The most common is the pitched, tiled roof, with rockwool loft insulation between joists and, prefereably, over the top to make it up to 300mm. (If you don't have this already get on and do it, it is the best value insulation available! You can buy 200mm rolls of rockwool to lay over the 100mm between joists at right angles.). If you have rooms in the roof you may not be able to see the insulation but should know the thickness from any rooflights. In any modern house or conversion the pitched roof should have rockwool inside it, you will know if not because the loft room will be freezing in winter!

Windows and doors: single or double glazed is obvious, but frames are also important - may be uPVC, timber, steel or aluminium.

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Air changes

Another element you will see in Heat Loss software is called Air Changes per hour, or AC/H. This is a measure of how draughty your house is. Unfortunately it is far more imprecise again than building materials. You may see numbers from 0.3 to 5 air changes per hour, and nobody can look at a house and say what would be right. You can get specialist companies to measure it with air blowers and gauges, but this is not cheap and normally only for new build situations with high air tightness targets. There is significant argument within the building and heating trades about what numbers to use, and unfortunately it makes a big difference to the HLC - 50% or more. If you're not careful an assessor or installer will just plug in a default number like 2 without any justification, and you'll have a completely useless HLC.

Even more confusingly, you may see both maximum and minimum AC/H. A highly air tight PassivHaus (kind of Grand Designs) should have air tightness of <0.6 AC/H for energy efficiency. But a UK home should also be able to purge at at least 4 AC/H, ie that's a minimum when ventilating kitchens and bathrooms with extractors or open windows.

The uncertainty over AC/H can seem to make a mockery of HLC, but armed with your coldest day energy consumption measurement, there is a way around it: do the absolute best you can with the building materials input to the calculation while leaving default or guessed input on AC/H, and then compare with your coldest day figure. Now, adjust the AC/H so that the HLC output matches your coldest day figure! Because the coldest day figure is experimentally obtained, it is real life and likely to be more accurate than the HLC. This approach can also be used to fix other areas of uncertainty too.

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By doing your own HLC you will be a sophisticated customer, able to prevent oversizing, and you may save money by avoiding the £100-300 charge that many installers make (quite reasonably) for doing the HLC for you.