Glow-worm 65F short-cycles

[sine]

Hi

We have an 18 year old Glow-worm Fuelsaver 65F. I had a new thermostat (the one in the boiler) fitted a few months ago and since then it’s been short cycling. This is more noticeable now with the heating being used but is still noticeable with HW only.

Probably a stupid question but is it possible to change the on-off range of the thermostat so that the boiler stays lit for longer and off for longer?

 

[sine]

Hi

I've taken a few measurements of how the boiler runs.

When its cold and you have the heating on for a long time after about 40 minutes running from cold the thermostat shuts the boiler off as you’d expect. I’ve taken a few measurements (after it’s been on for 1 hour, so water in system is as evenly heated as its going to get).

Readings with Heating Only
Boiler runs for about 1minute 50secs and is off for just 25secs.
Water output/flow temp at shut-off is 75C and 65C when re-light takes place
Water in/return temp is about 64C all the time.
The temp of the output falls to 70C within 5 seconds of shut-off then more steadily to 65C when re-light takes place.
Boiler thermostat is on 4.25.

The radiators all get hot and the temperature at the top of the centre of the radiators is between 69 and 72C, and they have a temp difference between 8 and 12C.
The by-pass valve is open about half a turn - just enough to give a little flow when the overrun comes on. The pump is on full.