Any ideas?

[TeaPea]

Hello, I have a Worcester Heatslave boiler 12/18. It will run for around 4 minutes, then switch off, for around 5 minutes, and repeat. Adjusting the room temp has no effect, and the house never warms up as I suppose its never running for long enough. Any ideas? Thanks,

 

[steadyon]

1. Circulation problem, so boiler fires, flow heats up, return doesn't, difference in temperature too great, boiler shuts down. Flow cools and off we go again.
2. First things to check (bit general as I'm not familiar with your boiler):
2a. Pump. Is it working?, Is it failing?
2b. Motorised valve(s). Working?

 

[TeaPea]

Hello steadyon,
Thanks for your reply.
The pump is working, and isolating the motorized valve makes no difference, and niether does adjusting the room thermostat. Warm water is returning to the boiler via the radiators, but not enough to warm the house. Do you know if there is an internal thermostat on the boiler?
Many thanks.

 

[green grant]

you have a thermistor is out of range. If you have a multi meter and have access to the resistance range (in operation) values then you can test it to confirm OR...buy 2 (CH and DHW) and replace them.