Low boiler output

[Bedouin]

Over the last winter our (rather old) boiler has been trouble keeping the house warm. The boiler itself is running continuously but the radiators do no better than get warm.

We've been living in this house for about 6 years, and not noticed this as a problem before - but it is possible that in previous years the TRVs on the rads in the bedrooms may have been turned down lower.

Can anyone think of any likely reason why the boiler should be producing less than 100% output?

The boiler is an old Potterton Netaheat 16-22 - not the most sophisticated unit. I am intending to replace it shortly and will use it as a guide to the correct size of the replacement (which will be bigger) so need to check that the problem is the boiler being underspecified for the house rather than just not putting out its rated output.

Any suggestions gratefully received...

[Andy Hall]

A reasonable possibility is that the heat exchanger is part filled with sludge. clues here would be noises from it, or if it is cycling on and off before the radiators are getting hot.

It is possible that some or all of the TRVs are past their sell-by date and contributing to poor system balance.

Either way, if you are changing the boiler, it will make sense to give the system a very thorough cleaning and also perhaps replace TRVs if they are more than about 7 years old.

It may not be worth spending anything on desludging the boiler, though.

If you want to be rigorous about this, you'd need to do heat loss calculations. However, if the house has been comfortable with 22kW before, going for a 28kW boiler should give a comfortable margin.

[Doctor Evil]

The pump could be running slow. Not on full.

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[Dave Plowman (News)]

Please explain how this could be the fault if the boiler is running continuously, as stated?

[Christian McArdle]

Modern boilers are modulating. In the past, you guessed the heat output and bought a boiler that matched (i.e. 12kW or 14kW or 8kW etc). Now you just buy a 28kW model (which is halfway to heating a palace) and it just turns itself down when less is needed. It means that careful consideration of boiler sizing is no longer required. The only issue is that instantaneous combis need as much power as you can find for reasonable performance when heating water. For heating, any 24kW or 28kW system boiler will heat almost any house. If it doesn't, then you need more insulation, not a bigger boiler.

For smaller houses, the ability to modulate really low is really useful to avoid cycling. Indeed, it is useful on larger houses, too, but not quite so important. Some only go down to 8kW, whilst others go as low as 4kW or better.

Christian.

[Bedouin]

This is certainly a comparatively expensive house to heat - every room has 2 outside walls and the rooms we use most all have 3! Yes in theory more insulation would be a good idea but that is not easy (solid walls and all that); and I also have what is probably the best ventilated under-floor void you've ever seen.

The problem with an over-specified boiler is cost and size so I'd rather get the size right.

Going up to a 28-30kW model will give me modest increase over my current

22kW - but will that be enough?

[Andy Hall]

You'd have to do the sums to know.

There are two major elements.

- Heatloss through surfaces. You can get a reasonable number for that if you are methodical about materials used etc.

- Heatloss through air changes. This assumes that you either use standard numbers suggested by the Buikding Research Establishment or make a reasoned estimate. You can't really know if your dining room has 1.5 or 2.5 air changes an hour.

You can arrive at reasonable numbers for radiator sizing this way. i.e. you can out in enough capacity to be sure of good results. it's hard to do that fo the whole house and sizing a boiler though.

If the house was OK consistently on 22kW (meaning burner on the Netaheat at max) then you whould be comfortable at 28kW and have a lot of margin at 35kW.

A house needing 22kW suggests to me that it's either large or has poor insulation because it's older and you can't put insulation in enough places to make a big difference. If that's the situation, then it's more important to look at heat loss because you will have little to address the variations during a day. The temperature will change quite quickly with the outside temperature. If you have a situation where the insulation is better, there will be less intra-day temperature change and so you don't have as much sensitivity to sudden weather changes.

There's no magic way, unfortunately. If you are concerned, and the house is not well insulated, then I would go for higher power. This will cost more, but not as much as getting it wrong and having to replace the boiler with a larger one......

[Bedouin]

That's certainly true - it has comparatively large surface area - being L shaped, and as a solid walled 1920 construction there is no easy way to improve the insulation further without major work. The low power of the boiler, and the comparatively high thermal mass of the house, means that it is very slow to warm up in cold weather - hence the desire for considerable more power.

That's my feeling too - but the range of boilers in the 35-40kW size is obviously lower than in the more common 28kW area.