Excessive cycling by CH boiler when it should be modulating

[Mrreliableone]

My Glow Worm 18HXI boiler is suffering from way too much cycling - it's either on for about 15 secs and heating up to the set 60 degree temp, whereby it turns off completely, when I think it should modulate. The result is excessive switching of the boiler on and off, and a "slug" of hot water being pumped into the radiators, which because of this don't reach the required temp. The pump is working fine, and all controls (other than the boiler) are working correctly. I just can't get the boiler to modulate on low flame, which would see the radiators hot all the way through, instead of "slugs" of heated water reaching them. The boiler was serviced last week, and the guy said this cycling was normal, but I don't remember it being this way, and I'm fearful of this constant cycling ruining some parts of the system through over use. What I can remember is the boiler running on low flame once the water was up to temp, but it doesn't do this any more, just on then off. Anyone any ideas on how I can fix this please?

 

[piglet11]

Doesn't sound right.
I don't know anything about your model and can only speak for Worcester Bosch.
Mine has a thermostat dial which is set to about 65-degrees C.
The boiler maintains that by gently ramping up and down.
It shouldn't be shutting off unless it has gone past the thermal cut-out in which case it shows a fault until reset or the cause of the overheat (low water content for example) is fixed.
The only time the boiler shuts down otherwise is when either the room thermostat or the cylinder thermostat gets to temperature.
Get the servicing bloke back and ask him how the shutting down is normal.
The last part of your description is more like normal operation when the boiler "throttles back" and is simply maintaining the set temperature.
It is possible that it could cause damage if it is the result of an underlying problem.
Then don't use him again.

 

[Mrreliableone]

Doesn't sound right.
I don't know anything about your model and can only speak for Worcester Bosch.
Mine has a thermostat dial which is set to about 65-degrees C.
The boiler maintains that by gently ramping up and down.
It shouldn't be shutting off unless it has gone past the thermal cut-out in which case it shows a fault until reset or the cause of the overheat (low water content for example) is fixed.
The only time the boiler shuts down otherwise is when either the room thermostat or the cylinder thermostat gets to temperature.
Get the servicing bloke back and ask him how the shutting down is normal.
The last part of your description is more like normal operation when the boiler "throttles back" and is simply maintaining the set temperature.
It is possible that it could cause damage if it is the result of an underlying problem.
Then don't use him again.

Hi, thanks for your reply - I can confirm that no fault code(s) are showing, and would agree that the "throttling back" is what should be happening. I'm also pretty sure this problem was happening before the boiler was serviced - when I pointed out this excessive cycling of the boiler, he said it was normal and was modulating, which to me isn't correct - modulating would be the throttling back you mention to a low flame, not a complete on/off shut down of the boiler. The room thermostat is working as intended, and will close the boiler/pump down when the desired temp is reached. I'm tending to concur with your sentence about thermal cut-out, even though no fault code shows this. What makes me think it could be this is that watching the temp rise when the boiler ignites at low water temp, there is a rapid rise in temp and in fact, it overrruns the set 60 degrees - it hits about 65 in about 10/15 seconds after starting to heat at about 38 degrees. That's when the boiler shuts down completely, and doesn't ignite again until the temp falls to 38 degrees, which takes just a few minutes. So you can see that I have a rapid cycling occurring, not modulation!