need advice re cental heating/hot water

[mike13]

Will try to keep it brief,central heating /hot water available when it has not been on for a while, however once it has warmed up then cooled slightly it will not fire the burner up.Boiler cycles but burner will not fire until hours later.Scottish gas have been out four times,changed pressure switch,fan etc, but problem still persists.Baxi combi boiler approx 9 years old.Any heating engineers have an idea

Thanks

[metalsteve]

Have the same symptoms on my Baxi solo 3, mine is only getting the water/rads luke warm. Had a really good engineer out last night who diagnosed it as faulty gas valve. basically the boiler is going through the sequence and applying power to the valve solenoid but its not always opening. seems to work once from cold and then not fire again for an hour or more.

mine should be repaired by monday so i'll let you know.

[mike13]

Thanks for that,having to spend days confined to the house,getting frustrating,thank goodness for the maintenance contract

[Arthur Jackson]

It could be a number of things. Boilers tend to operate in the same sequence. Fan, proved with a pressure switch, run igniter, open gas valve, check for flame. The problem occurs when a PCB is asked to do the checking and thermistors get involved. Baxi-Potterton do have a bad name for PCB faults.

[mike13]

Arthur,two times when the engineer has uncrewed the front cover,the burner has then fired up making them think along the lines of faulty pressure switch,the last visit the engineer had a circuit board with him,but on seeing the burner fire up,thought it would be a waste to fit a circuit board.However he left yesterday and the fault has occured again this am.He is due back on Tuesday on a separate premise,will see then what the next step is.Thanks

[Arthur Jackson]

Hmmm... I'd take a very close look at the venturi where the pressure switch tubes are connected. Flue obstructed? It's not rocket science to put a multimeter on that pressure swicth, though.

[caziques]

It's probably nothing to do with it but......

I used to rebuild overdrive units, and a not unusual problem was the overdrive working when cold, and then not working when warm.

The solution was always a replacement solenoid, something was causing the it not to work when uo to operating temperature. Even after stripping down and checking everything, some still wouldn't work when hot.

A new solenoid cured the problem.

Does the gas supply use a solenoid on a conventional boiler? I only know about heat pumps (and yes they do have a solenoid, when it fails the heat pump is permanently on cooling).

[andySC]

If boiler fires & stays lit when the front cover is opened then it's certainly a flue fault. OP you mention that's your boiler is a Baxi Combi...is it an 80/105 E? Does it show any flashing LED's when it faults?

I'd bet on a faulty air pressure switch or blocked venturi. Be worth checking the impulse tubes ( silicone tubes that connect venturi to APS) to see if they are intact. These can perish & split.

Also see that the fan is getting 240v. This is powered via the PCB & As has been mentioned previously Baxi PCB's are glitchy.

[mike13]

Update

This problem appears to have been cured by a new pcb,a couple of the replies appeared to point towards this.

Thanks