The Heat Geek Range Rating Method

March 9, 2020

Range-rating heating appliances can help reduce boiler cycling, keep the appliance cooler, and more efficient and generally prolong its life. 

If you have an advanced weather compensated controller (article difference between advanced and standard to follow) your boiler is automatically range rated on a continual basis provided it is set up correctly. However, with load compensation, TPI or on-off controls the boiler does not know the outside temperature and so has to assume the worst-case scenario of –3 outside. 

Installers are increasingly aware of how oversized boilers can reduce efficiency, but by just how much they are oversized we think would surprise many. Reducing the maximum output can help address this but how do you range rate it?

The usual method is to take an estimate of the load and just reduce it down to that which could be nowhere near the actual load. Here are 2 methods that will range rate to your property's actual heat loss! not theoretical. It will also help with the understanding of how heat is transferred on a theoretical level, and squeeze out those last fractions of a % of efficiency as well as potentially improve comfort. Method 2 will do all this more accurately and also identify your real-world 'radiator design temperature'.

Before starting it's important to understand what radiator 'design temperature' means. What we mean is the temperature your radiators are designed to operate at under maximum load conditions, i.e. at -3 outside temperature. It typically refers to the mean temperature of the radiator, however for the purpose of this method, we are referring to the required flow temperature. This is affected by how much insulation a property has AND the size of the radiators.

Traditionally radiators were designed with an 80°c flow temperature in mind, however as insulation has improved these same radiators can now run a lot cooler, radiators are normally well oversized due to contingency also meaning they can run even cooler.

For method 1 your design temperature will have to be assumed for your property, we would suggest a flow temp of 70°c for less well-insulated houses or smaller radiators, and 45-50°c for better-insulated houses or houses with underfloor heating and or bigger rads.

Method 1

  • First, turn the heating off
  • Turn all TRVs to max
  • Turn the boiler flow temperature and target room temp up to their max
  • If it's not –3 outside, open some windows
  • Range rate the boiler to the minimum it will allow and allow to fire.

Now watch the flow temperature, you'll eventually see it will level off, hopefully much less than the maximum flow temperature of the boiler. 

  •  If the system does reach the radiator design temperature then you have a finished range rating. Unfortunately, your property's heat loss is above the boiler's minimum output. You no longer have a modulating boiler and compensation controls will do nothing. However, it's still best rated down as this will elongate the cycling effect your system will be suffering from whether you have previously realised it or not. But it's essentially going to a non-modulating boiler much like the boilers from 20+ years ago.
  • If your system hasn’t reached max flow temp then great. You can now incrementally up the power output until the flow temperature reaches the 'design temperature' of your radiators, which is typically a flow temperature up to 70°c with an average of 60°c. Be sure to take your time only increasing it a bit at a time and giving plenty of time for the flow temperature to plateau. How long you leave it to plateau will depend on your property size, a flat may take 10 mins every time you adjust it, and a larger house could take an hour! importantly make sure the room temperature doesn't rise above your usual indoor comfort temperature (say 20°c) by opening windows as necessary as this will give an inaccurate heat output. 

To speed things up, when you first turn on the system on at its minimum output, measure the difference from the room temperature to your current flow temp. If the flow temerature plateaus around half way between the target room temp (say 20) and design flow temp of 70, in this case the plateau would be around 45oc, then youll be looking at doubling the power output. Increase it by 40 % to come just below. If its ¾ of the way, then up it by around 20% etc. 

  • Once your flow temperature has plateaued at your design flow temperature you have your maximum range rating.

This is the maximum output of your system and near the maximum requirement of your property, there is little to no point in increasing your heating output beyond this point. However, we would now advise adding between 5 and 20% for intermittent heating. Failure to do this will result in the system taking too long to heat up. The less additional intermittent power you put in the more you will elongate your run times and keep the boiler cooler (read as improving 'efficiency'). The more additional power you allow the better the responsiveness of your system. 

It's worth mentioning that this additional intermittent is only required when coming off set back when it's –3 outside. If the temperature is higher than this, for example when it's 12oc outside, even with no intermittent heating power added your boiler is still double the size it needs to be. So minimizing this additional power can be helpful and may only show up as small issues in winter. 

You can also minimise your intermittent heating power requirement if you have a small setback differential. I.e. if your nighttime temperature is within 3oc of your daytime temperature. Also if your heating is regular and on for longer periods. If your heating is erratic and/or only on for short bursts you should have a higher %. 

We would hazard a guess that most combination boilers in flats will be left at the minimum range rating setting and even many boilers with poor modulation in houses, which is a real shame. It's also noteworthy that flats will probably only require a max flow temp of about 50°c, which will mean your input is even lower. Most systems in flats will easily reach a 50°c temperature so again can be left at the minimum output and non-modulating. If you want to understand the issues of this lack of modulation please read our modulation article and our low temperatures systems article. 

If you really want to go full Heat Geek and refine things further, here's a method that will give you your real-world actual heat loss AND design rad temperature.

Method 2

Note; requires a cooler day if possible, as your system may need to run too hot to complete the task.

  • Set the internal temperature to the maximum, and make sure windows are closed.
  • Start range rating as above from minimum with TRVs set to max.
  • Keep lifting the output until the house temperature just starts to reach 24°c above your outdoor temperature.

The 24°c we are targetting is to emulate the hyperthetical differential between an external temperature of -3°c and the internal temperature of 21°c. Perhaps your area dictates an outside temp of -2°c and your comfort temperature is 20°c, if so adjust to a 22°c differential.

  • Once your house reaches your target differential of around 24°c you have found your max heating output required.
  • Your radiators will be running slightly hotter than in design conditions. To work out your radiator 'design radiator temperatures' from this, take the difference between the design's external temperature (usually –2/-3) and take it off your actual outdoor temperature. Then deduct this from your current flow temperature.  
  • Add intermittent heating %


So if it was currently 5°c outside temperature, you would set your internal temp to at least 30°c. When you achieve 29°c indoors note your flow temperature once it's plateaued and maintain 29°c. If your flow temperature was 72oc, then we would deduct your differential between the design outdoor temp (-3°c) and your actual current outdoor temp (5°c), which is 8°c, from this flow temperature. This gives 64°c and means we would require 64°c flow temperature at –3°c. 

You can then also set your max flow temp at 64°c or perhaps a bit higher to allow some responsiveness or improve your basic WC curve.

If you are working on a heat-only boiler, or system boiler where it cannot tell the difference between heating demand and hot water, please note that doing this may make cylinder reheat times suffer. 

We don’t expect many will be going around doing this on every job by any means, but doing it at least once or twice, perhaps in your own house will help to learn actual real-life heating loads of properties in comparison to theoretical or rule of thumb. And in the case of method 2 also help engineers understand just how cool they can run their systems to maximise comfort and efficiency. It will hopefully help engineers question specking oversized boilers, helps you understand the importance of good modulation, and gives a good insight into the theory of how heating works 

How low did you manage to get yours? Were you surprised? Let us know in the comments below! 

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    Heating Sensei

    March 9, 2020
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