Guest written by Sune Nightingale Co-founder of Firepower https://www.firepowerheating.co.uk/
Good heat loss calculations are crucial for heating systems – they can reduce installation costs, maintenance needs, and environmental impact while improving comfort. For installers investing in accurate calculations can mean fewer service calls and more referrals and completions.
When people come to us wanting to switch from oil or gas to renewable heating, they often think that the new appliance can just be sized to match their existing boiler. On the face of it this seems logical enough.
However it’s very common for these existing boilers to have been oversized, often by a lot.
Before we’re too quick to blame the heating engineer, bear in mind that they are often forced to work to a price because homeowners often go for the cheapest quote. In this case the main concern of the heating engineer is that they pick a boiler that will definitely be big enough, because if it’s too big then the homeowner is unlikely to notice, but if it’s too small then they will be calling up in midwinter complaining.
Also the building may well have been upgraded in the meantime with, for example, double glazing, extra loft insulation, or cavity wall insulation, all of which will reduce the peak heat load.
Now we definitely don’t want to oversize heating appliances, whether it is an oil or gas boiler, a heat pump, a wood pellet boiler, or something else. The consequences are increasingly more apparent as the cost of the fuel source increases. For the purposes of this post electricity can be thought of as the most expensive “fuel source”, then wood pellets, and oil, and lastly mains gas.
(undersizing is also an issue, less common in my experience, but still to be avoided – I have however limited this post to the topic of undersizing.)
Oversizing impacts negatively on running cost, causes more wear and tear, and can mean more maintenance is needed. All of which ends up costing more, and being more of a hassle and nuisance.
The larger the heating appliance the more it costs to buy, and other associated costs in the system end up rising too, including the time for labour.
The appliance and associated parts also take up more space, and they look bigger (because they are!). And lastly part of the motivation of many people switching to renewable heating is to reduce their environmental impact. A bigger appliance as overall installation uses more materials and energy to make, meaning that the ecological impact will be slightly higher.
So for homeowners an accurate heat loss calculation can:
- Reduce installation cost
- Reduce running cost
- Result in less wear and tear and associated maintenance
- Mean less visual and physical space are used for the installation
- Reduce environmental impact
- Reduce hassle, increase comfort
Systems that don’t work well, or have high running costs are a hassle for homeowner and for the installer, and they serve neither well.
Why exactly is oversizing a problem anyway?
There is a limit to how far below their maximum output that a heating appliances can run. Usually they can turn down to around 25-20% of their maximum output. If the heat demand is less than that then they end up turning off and back on more. Each time a heat pump starts it sucks up a bunch of electricity, or for a pellet boiler the ignition phase tends to be less efficient than when it is fully running and warmed up.
We have to size heating appliances for the worst case scenario so that they will warm a home at the “outdoor design temperature”. This is an extremely rare event.
When applying for the BUS grant for a heat pump or pellet boiler there is a specific heat loss calculation method that must be used to qualify. This method already has the in-built potential for oversizing because it does not account for base temperature (the outside temperature below which a house will need heating – often 16ºC or below), nor thermal mass – which is the effect of the materials a house is made from – an internal brick wall for example or stone floor, acting as a store of heat and therefore to an extent ironing out the times when the outside temperature plummets, especially relevant as these times tend to be brief and at night.
So when applying for the BUS grants it’s even more vital that the heat load calculation is as accurate as possible in every other way.
Here is a graph showing temperatures in Birmingham during the heating season of 2022. The taller the graph the more more often it was that temperature. I’ve marked the outdoor design temperature (-4.3ºC) with a red dot, and then the grey area shows the temperature at which the heat demand in a house would be less than 25% of that peak load. This grey area takes account of the base temperature and so during all these hours it is likely that the heating appliance would need to be switching off and on as it can’t turn down any further. The further we move to the right of the graph the more times it will have to switch off and on per hour. When the outside temperature rises above the base temperature then heating isn’t needed which is why the grey area stops at that point.
How can we be sure a heat loss calculation is as accurate as possible?
- Making sure that our assumptions about the fabric of the building are as good as possible.
- Knowing the software and settings well
- Being able to do each action needed quickly, yet without cutting corners
- Using accurate weather and temperature data.
- Making sure the measurements and details we use are spot on
- Knowing which insulation measures to highlight (the doable/affordable ones), and then being able to redo the calculation based on any upgrades that will happen.
Heating engineers are busy. Doing a heat loss calculation is just one task among the many that they need to carry out, as well as knowing and working to a stack of regulations and best practise. It’s understandable that many want to be getting on with installing, which is after all what they trained to do, especially if they are sole traders or small teams.
For installers accurate heat loss calculations can:
- Reduce service callouts
- Free up time for core tasks
- Get more quotes out of the door
- Increase word of mouth referrals
- Help win more jobs
- Reduce hassle, and support a sustainable livelihood
So for these reasons, especially for smaller teams, it can make sense to use the specialist knowledge and experience of a heat loss calculation expert.
A heating system will last for many years, as will the consequences of initial mistakes. Get it right from the start.
