Is our boiler now oversized and causing problems controlling heat output?

Ben84

Well, it's been a while since I lived in this house, but previously the boiler always worked perfectly for many years, giving even heating that was comfortable. It's very old, but it heats up fast and has never failed to heat the house well. Now however, we've installed secondary glazing, loft insulation and cavity wall insulation, and it seems whenever we turn the boiler on the house becomes incredibly hot, then goes cold, hot and cold again and again. Adjusting the thermostat in the hall doesn't help much, I think what's happening is that the boiler is heating the radiators and water in the system which is accumulating heat and continuing to emit it after the house thermostat switches the boiler off, explaining the rush of heat we get after a while. I suspect that it's just over sized now? Whatever the case however, it does seem to have been since we installed all the insulation.

I've also been trying to adjust the boiler thermostat, but it's pretty difficult to control as it seems that despite spinning around across a wide rotation, it really has two settings, inferno and off.

So, any advice gratefully received. Is our boiler behaving like it's over sized, and if so can anything be done to make it behave again?

jayyar66

Having a boiler that is oversize, within reason of course, is not usually a problem. Your problem could be that the thermostat you have in your hall is an old style bi-metallic strip type. These thermostats can have quite a large hysteresis error. For example, if you set it to 20degs and it brings the heating on, the temp in the hall might rise to 22degs before it switches off. It may then fall to 17 or 18degs before it switches the heating back on again. This gives a temp swing of 4 or 5 degs. If you were to replace it with a modern digital 'stat, the hysteresis on these is usually 0.5degs. So if you have 20degs set, it will switch off at 20.5 and on again at 19.5. Therefore maintaining a much more even temp. Of course, it could also be that you need to reposition your 'stat to another part of the house in order for it to sense the temp change more effectively. If this is the case then a digital, wireless, 'stat will sort both these issues out as you could easily reposition it as you felt necessary.
Hope this helps

Mr_Ted

Definately a control problem, i would suspect room and boiler stats from what you say?

Boiler stat should have a variable, not on or off?
Analogue stats should have a + or- 1 degree, but age and wear will affect that!

What is the boiler make & model?

Ben84

jayyar66 wrote: »

Having a boiler that is oversize, within reason of course, is not usually a problem. Your problem could be that the thermostat you have in your hall is an old style bi-metallic strip type. These thermostats can have quite a large hysteresis error. For example, if you set it to 20degs and it brings the heating on, the temp in the hall might rise to 22degs before it switches off. It may then fall to 17 or 18degs before it switches the heating back on again. This gives a temp swing of 4 or 5 degs. If you were to replace it with a modern digital 'stat, the hysteresis on these is usually 0.5degs. So if you have 20degs set, it will switch off at 20.5 and on again at 19.5. Therefore maintaining a much more even temp. Of course, it could also be that you need to reposition your 'stat to another part of the house in order for it to sense the temp change more effectively. If this is the case then a digital, wireless, 'stat will sort both these issues out as you could easily reposition it as you felt necessary.
Hope this helps

Thank you for your advice. I suspect our thermostat is not very accurate, but It has however always worked ok before, so I'm wondering if the problem you're describing can get worse with age? Another possibility, as you've suggested, may be that the hallway isn't such a good place to measure the temperature in the house now that it's insulated. One wall of the hallway is mainly a 1930s window that goes up both floors, so perhaps the new insulation is having little effect in there compared to the rest of the house. So, the thermostat keeps the boiling running too long now when it switches on.

Ben84

Mr_Ted wrote: »

Definately a control problem, i would suspect room and boiler stats from what you say?

Boiler stat should have a variable, not on or off?
Analogue stats should have a + or- 1 degree, but age and wear will affect that!

What is the boiler make & model?

Thank you for the advice. We do have a room thermostat in the hallway as well as a boiler thermostat. The boiler itself is a glow worm majorca 246. The boiler thermostat does suggest a range of temperature, but the reality when I adjust it seems to be either the radiators get incredibly hot, or vaguely warm.

Mr_Ted

Ben84 wrote: »

Thank you for the advice. We do have a room thermostat in the hallway as well as a boiler thermostat. The boiler itself is a glow worm majorca 246. The boiler thermostat does suggest a range of temperature, but the reality when I adjust it seems to be either the radiators get incredibly hot, or vaguely warm.

It sounds like the boiler thermostat is KAPPUT?
If it is failing to shut down completely thermostatically then it could cause a major problem by overheating and possibly boiling the system, thats dangerous!!!
Bad news is that as the boiler is probably over 40 years old the replacement part is no longer available.

Sorry to be the bearer of bad news but it looks like your backboiler and fire need to be replaced:( and I would suggest urgently!!!

Ben84

Mr_Ted wrote: »

It sounds like the boiler thermostat is KAPPUT?
If it is failing to shut down completely thermostatically then it could cause a major problem by overheating and possibly boiling the system, thats dangerous!!!
Bad news is that as the boiler is probably over 40 years old the replacement part is no longer available.

Sorry to be the bearer of bad news but it looks like your backboiler and fire need to be replaced:( and I would suggest urgently!!!

Thanks for the advice. I've got someone coming out to look at it next week. I hope to find out if it's broken, or how to use the controls to stop it overheating the house.

It did however get cleaned and checked back in November, at which point it seemed everything was ok, so I'm hoping it's still fine.