Radiators don't stay as hot as when first on

[Kevin David Greig]

Hey hoping someone can help me out with a question I have with my central heating. When I put the heating on the rads gets nice and hot but once on for a an hour or so they cool down a bit still hot but not as hot and takes the house a while longer to heat up. Also the kitchen radiator I replaced as cold at the bottom and warm at the top but still get the same issue, this one is also last to heat up by about half an hour. Thanks

 

[SimonG]

Second issue sounds like balancing, first issue, do you have thermostatic radiator valves? What they set on?

 

[Kevin David Greig]

Second issue sounds like balancing, first issue, do you have thermostatic radiator valves? What they set on?

Thanks for the reply, I have tried tuning them all off except the bypass one and still the kitchen one does not get as hot as it should like the rest of them. The thermostats are set the max (5) boiler is a 34cdi Wochester 6 years old. Tried setting boiler to max they get scalding hot then after an hour or so they get cooler again but not soo much so where it is causing too muchbpf an issue just wondering what it could be

 

[Best]

Have you a Zone stat and where is it positioned?

 

[Kevin David Greig]

Have you a Zone stat and where is it positioned?

Zone stat is in the living room on the wall opposite the radiator.

 

[Best]

Zone stat is in the living room on the wall opposite the radiator.

Good that the wall stat is not near a radiator.
The motorised valve that wall stat works maybe should be tried with actuator off and valve on to see if rads improve.
Just my thinking of how I would first do checks

 

[fezster]

Has the boiler been range rated to match your heating requirement? How big is your house / how many rads?

If the boiler is oversized, you may find it short cycles if it's unable to dissipate all of the heat it is outputting which will cause it to turn on for a short period, shut down and then wait a period of time. This would cause the radiators to then cool down before the boiler fires again and the cycle repeats. Can you observe what the boiler is doing when the rads cool?

 

[Kevin David Greig]

Has the boiler been range rated to match your heating requirement? How big is your house / how many rads?

If the boiler is oversized, you may find it short cycles if it's unable to dissipate all of the heat it is outputting which will cause it to turn on for a short period, shut down and then wait a period of time. This would cause the radiators to then cool down before the boiler fires again and the cycle repeats. Can you observe what the boiler is doing when the rads cool?

Hey there, will have a look at observing the boiler, we only have 7 rads 3 of which are small 2 x 300's and one 200 in one of the bedrooms. 3 bedroom house.

 

[johnduffell]

34 kw boiler sounds very oversized for a small system like that. the minimum burner output might be 8kw on a boiler like that. Those rads might only have 5-6kw output total, so the rads will shoot up to temperature before the burner cuts out. Once cut out the burner will wait 20 minutes or so before firing again.
As above not much you can do about it other than fitting bigger radiators to get a faster warm up, or a boiler with a lower minimum output.

 

[Kevin David Greig]

34 kw boiler sounds very oversized for a small system like that. the minimum burner output might be 8kw on a boiler like that. Those rads might only have 5-6kw output total, so the rads will shoot up to temperature before the burner cuts out. Once cut out the burner will wait 20 minutes or so before firing again.
As above not much you can do about it other than fitting bigger radiators to get a faster warm up, or a boiler with a lower minimum output.

Thanks for reply, Worcester have advised that the boiler can be range rated, can any heating engineer do this?

 

[johnduffell]

Thanks for reply, Worcester have advised that the boiler can be range rated, can any heating engineer do this?

Range rating only changes the maximum output, it wouldn't affect the minimum. The problem occurs when your radiators aren't dissipating the heat from the boiler when it's running on minimum. There's nothing you can do about that short of a new boiler, as the minimum is a function of the design of the burner and gas train. Some boilers are only 1:4 modulation and some are as high as 1:10 due to better design. That improves the real world efficiency but would be a higher end boiler.

 

[Kevin David Greig]

Range rating only changes the maximum output, it wouldn't affect the minimum. The problem occurs when your radiators aren't dissipating the heat from the boiler when it's running on minimum. There's nothing you can do about that short of a new boiler, as the minimum is a function of the design of the burner and gas train. Some boilers are only 1:4 modulation and some are as high as 1:10 due to better design. That improves the real world efficiency but would be a higher end boiler.

Thanks again, do you think this is why the kitchen rad never gets fully hot? It has been replaced.

 

[johnduffell]

no that would be the balancing issue mentioned. Since you tried turning the others off it could just be too much resistance in that radiator or the bypass is open too far. Check that the valves both ends of the kitchen rad are open fully (and then back 1/4 of a turn from the stop). If it's on a long length of narrow pipe it could just be too much resistance.

 

[Kevin David Greig]

no that would be the balancing issue mentioned. Since you tried turning the others off it could just be too much resistance in that radiator or the bypass is open too far. Check that the valves both ends of the kitchen rad are open fully (and then back 1/4 of a turn from the stop). If it's on a long length of narrow pipe it could just be too much resistance.

I thought the bypass should be open fully, yes the pipes are the thin microbore. So a combination of issues here then. Will have to run the boiler to death then go from there, can I get the pipework updated for this one rad?

 

[johnduffell]

no you should design the system have reasonable resistance at all points to allow the design temperature drop across all radiators.
You can overcome higher resistance by closing the bypass further and turning the pump up.
You must ensure that on the chosen pump and bypass setting, the specified minimum water quantity can flow through the boiler. If there is enough flow elsewhere in the system, the bypass should close. If the bypass is still open when the furthest radiator is calling for heat, you would get the problem that most of the water goes through the bypass.

You can update the pipework as you wish, there's no need to replace a boiler when changing pipework. But first it sounds like you need to set up your bypass/pump to get adequate pressure difference.

 

[Kevin David Greig]

Ok so had the boiler range rated to 15kw lowest setting is 7.5 and what a difference! All the radiators are holding a steady heat! Thanks for all your help

 

[johnduffell]

nice work didn't expect that so glad you had a success!

 

[Kevin David Greig]

nice work didn't expect that so glad you had a success!

Was not entirely sure myself glad it could all be done from the front panel, they said when it was serviced they set it to max once they do that it resets the setting apparently? BG done the service for me when we moved in.

 

[fezster]

Has the boiler been range rated to match your heating requirement? How big is your house / how many rads?

If the boiler is oversized, you may find it short cycles if it's unable to dissipate all of the heat it is outputting which will cause it to turn on for a short period, shut down and then wait a period of time. This would cause the radiators to then cool down before the boiler fires again and the cycle repeats. Can you observe what the boiler is doing when the rads cool?

You're welcome!

 

[dan_the_plumber]

What control have you got? Are you actually achieving the room temp you are asking for?

 

[Kevin David Greig]

What control have you got? Are you actually achieving the room temp you are asking for?

Hi yes I am now much quicker now the boiler has been range rated