Radiators warm but not hot (boiler short cycling)

[cambsdiy19]

My radiators are getting warm but not hot enough to actually heat the room properly. I have a SE12 FF boiler with hot water tank, the hot water seems ok (possibly because it has the tank to accumulate it and keep it warm). I checked the boiler and noticed that it fires up for about 10 seconds, then shuts down for about the same time and then refires. The pump is working and can be heard. According to the manual this scenario suggests an air lock so I’ve bled all the radiators although there was no air in them. My next guess is possibly a frozen condensate pipe? We’ve had consistent subzero temperatures for over a week now. It’s not that easy to access but I will try and pour some warm water over it.

Would anything else cause this behaviour? There is at least water running to the radiators although it’s not very hot, and seemingly no issues with hot water to the taps or shower. I've tried the more obvious things like checking the boiler temperature and checking each radiator setting.

 

[stem]

This is a difficult question as there could be a lot of reasons, so you might have to engage the services of a pro to make an inspection of your system. Based on your symptoms, here are some suggestions that might help:

I got asked by a neighbour about a similar problem. Turned out that they had been trying to improve things themselves without understanding what they were doing and had opened up a gate valve on a manual by-pass that should only have been cracked open a tiny fraction. The result was the majority of the water from the boiler was now flowing around the by-pass pipe with only a small amount going around the radiators. And, as the water wasn't loosing much heat the boiler was short cycling.

The failure of an automatic by-pass (if fitted) can cause the same symptoms.

It could be a problem with a partial blockage in a pipe. Especially if the system has microbore pipes. Although this doesn't usually effect all of the radiators.

Is the circulation pump on the correct setting for your system?

Is the boiler thermostat working properly. check that the flow pipe from the boiler is hot, which it should be, or is it only slightly warmer than the radiators?

There could be a fault with a motorised valve on the hot water cylinder in that it's not shutting off properly and much of the hot water is circulating through the hot water cylinder continually instead of going to the radiators.

Has anything been changed recently?

It won't be a frozen or blocked condensate pipe, if it was the boiler wouldn't work at all.

 

[gas112]

ideal classic se12ff ?
That is not a condensating boiler

 

[cambsdiy19]

This is a difficult question as there could be a lot of reasons, so you might have to engage the services of a pro to make an inspection of your system. Based on your symptoms, here are some suggestions that might help:

I got asked by a neighbour about a similar problem. Turned out that they had been trying to improve things themselves without understanding what they were doing and had opened up a gate valve on a manual by-pass that should only have been cracked open a tiny fraction. The result was the majority of the water from the boiler was now flowing around the by-pass pipe with only a small amount going around the radiators. And, as the water wasn't loosing much heat the boiler was short cycling.

The failure of an automatic by-pass (if fitted) can cause the same symptoms.

It could be a problem with a partial blockage in a pipe. Especially if the system has microbore pipes. Although this doesn't usually effect all of the radiators.

Is the circulation pump on the correct setting for your system?

Is the boiler thermostat working properly. check that the flow pipe from the boiler is hot, which it should be, or is it only slightly warmer than the radiators?

There could be a fault with a motorised valve on the hot water cylinder in that it's not shutting off properly and much of the hot water is circulating through the hot water cylinder continually instead of going to the radiators.

Has anything been changed recently?

It won't be a frozen or blocked condensate pipe, if it was the boiler wouldn't work at all.

Thanks for your detailed response. The flow pipe is only slightly warmer than the radiators and the return pipe is only slightly cooler than the outlet. Nothing has been changed recently, a plumber installed a new shower pump although it draws directly from the hot water tank so don't think it would have affected the boiler operation.