Boiler cutting out - no hot water but heating OK

[impreza280]

I have a Baxi Barcelona condenser boiler. It was working fine, apart from leaking flow and return isolator valves after he changed the EV and PRV. My plumber changed these (not a full drain down but wait for a vacuum to be created - it's an unvented megaflo system). Then after a couple of days it has started to fire up for about a minute then cuts out, but no warning lights. I wondered if there was an airlock and I regenerated the air gap in the megaflo which cured the issue, but now it's doing it again. Any ideas?

 

[Gasguru]

The air gap in the cylinder has nothing to do with the boiler operation.
You mean it randomly fires up or only fires up for a minute when there's a demand?
How come that boiler hasn't been skipped...it was junk the day it was launched..over 20 years ago.

 

[impreza280]

When there's demand i.e. the hot water isn't hot, then it fires up but then only stays fired up for less than a minute. It does it quite often. Difficult to tell if there's a problem with the heating as it's underfloor, very well insulated house and the boiler pump does enough to push it round the house without the UFH pump coming on, unless it's really cold. It's 20 years old and has been trouble free until this.

 

[Gasguru]

Is this actually a system boiler with the pump inside?
Feel the flow and return pipes..is the flow getting very hot?

 

[impreza280]

duplicate

 

[impreza280]

Yes, it has a pump inside it. Flow and return pipes below boiler are very hot - I'm wondering if that's due to the heating. Also wondering if there's a stuck valve on the underfloor heating and hot water is being returned to the boiler therefore it thinks there's no need to run. I will isolate the heating, turn the immersion off and see what happens.

 

[Gasguru]

After only a few minutes both pipes should still be cool with the flow perhaps 10 to 20 degrees above the return.