Worcester Boiler keeps cycling (on and off)

[HectorD]

Hi

I have a Worcester Greenstar RI boiler. The boiler is used for both central heating and for heating up water in an unvented cylinder.

When I switch on the central heating, the boiler fires up, all radiators get hot, etc. However, after 30 - 40 minutes, the boilers cuts out and then starts cycling on and off. i.e. boiler fires up for 30 seconds, then cuts out, then after 1 minute fires up for 30 seconds and so on.

When this happens, the temperature set in the room thermostat has not been achieved yet - the boiler is not cutting out because of reaching the temperature

The on and off issue doesn't happen when heating up water in the unvented cylinder - perhaps because heating up its full capacity takes normally less than 30 minutes.

The pump used by the central heating and room thermostat have been recently replaced as they were faulty.

Expansion vessel set to 1.5 bars (when cold).

Any thoughts / comments about what could be causing this issue?

Thanks in advance for your help.

 

[jackthom]

Are the radiators getting really hot and when the boiler starts cycling on and off do all the radiators stay as hot, from top to bottom?

I'm wondering if TRVs are closing on some of the radiators leaving the boiler with a much smaller heating load which it can't cope with.

 

[HectorD]

Good point, one or two of them that are downstairs don't stay as hot as they should. I was assuming that it was because as the boiler cycles, it doesn't get to heat up the system properly.

The rest of the radiators (5 of them, all upstairs) stay fairly hot.

What would be a good way to determine if the TRV are closing?

Also, could the issue be caused by some air in the pipes blocking the water flow?

Thanks.

 

[jackthom]

All you can do is experiment with those TRVs on their highest setting and see what the effect is on the boiler. Those radiators with the TRVs fully open should not be cooling down at all.

 

[D_Hailsham]

Exact boiler model?

How many rads in total?

 

[HectorD]

Not sure about the exact model. I have no indication of it in either the manual or the exterior of the boiler. Any way you know I can find this out?

There are 8 radiators in total:
3 downstairs
3 first floor
2 second floor

All of them, but two, have TRVs.

Last night I increased the boiler temperature by a bit. It used to be set at 58°C, now it is set at 65° - 70° C approximately.

After making this change the boiler didn't cut out any more, and it worked for a long period of time without cycling, the house got warmer too.

Could it be possible that a low temperature set at the boiler was causing the issue? or do you think the issue is still present and increasing the temperature has actually hidden the symptom but a potential issue is still there?

I only move into the house 6 months ago and this is my first winter in there, so I don't really know if this temperature setting is correct for the boiler.

Thanks.

 

[D_Hailsham]

I think you may have cracked it; 58C is a bit low. The water was getting up to temperature, so the boiler stat turned the boiler off - even though the room wasn't up to temperature.