Boiler not igniting on CH but will on HW ?

[Justin1]

Hi,

I don't know whether anyone can help me or not with my heating problem.

I have a Potterton 40e Netaheat Profile boiler and it is a gravity system. When my programmer is set to JUST hot water the boiler strikes up nicely and I get hot water. When the programmer is set to JUST central heating the pump runs but the boiler doesn't strike up and all the radiators remain cold.

With both running together only the hot water function is running.

Before Christmas I desludged the system, flushed and flushed again and then left inhibitor in it and since then it was working fine. To try and remedy this problem I have bled the system, drained and refilled under pressure to try and get rid of any airlocks but to no avail.

Could this be the over run thermostat ? Or perhaps the over heat thermostat not allowing the boiler to come on at all ?

Thanks.

 

[felix]

It's not the overheat thermostat because your boiler gives you hot water. The purpose of an over-run thermostat is to keep a pump running after the boiler has gone off so that's not the answer either.

First problem - you can't easily get CH only on a gravity HW system. There has to be a motorized valve in the HW branch to cut off the gravity flow. A common arrangement is for a switch on the valve to start the boiler when the valve is fully closed. This won't happen if the valve is broken. Have you got a valve at all? If not then your system isn't really designed to deliver CH only.

Second problem - if the boiler is on and the pump is running, hot water should be reaching your radiators even if you haven't got a valve in the HW branch. Something is blocking the flow - or maybe the pump isn't actually turning.

 

[Justin1]

I see....

On removing the big plug in the centre of the pump I can see the impeller turning.

I'll remove the pump tonight and see if there is something blocking the inlet/outlet to the pump perhaps.

Cheers

 

[felix]

Good plan. While you've got a view of the impeller you can also check that it's actually connected to the shaft.

 

[felix]

Another idea has just occured to me. Do you have a bypass in your system? This is a pipe allowing water to flow through the pump and boiler only, bypassing all the radiators. There will be a valve in it and this should be almost fully shut. If it's wide open - an easy mistake for anybody who didn't understand its function - your radiators will remain cold while the boiler cycles on and off on its own thermostat. It will spend more time off than on so this might answer your other conundrum.

 

[Justin1]

No I don't have the bypass. Took the pump off last night and was working ok and no sign of a blockage in fact it was remarkably clean.

Had the burners running for a while and now the water within the boiler just isn't getting warm at all. Even drained water from the boiler later and it was cold.

Last thing last night the pilot light wasn't coming in (it isn't on all the time anyway) and the burners weren't striking up. As it's been a few nights now I may have to get someone out as I've exhausted every avenue that I can remedy I think.

 

[felix]

If the burners are running then that water must be getting hot. (First law of thermodynamics - energy cannot simply disappear.) This means that the heat is going somewhere and the only possible place is into your hot water cylinder.

My prime suspect at this point would be a motorized valve between the boiler and the cylinder heating coil. This is the only way to get CH only on a gravity driven HW system. If the valve doesn't close then (a) the boiler heat can escape up the pipe into the cylinder coil and (b) the pump might not start up. Do these symptoms sound familiar? Follow the boiler outlet pipe up to the cylinder coil and somewhere along the way you should find that valve.

The pilot light problem is a completely separate issue.

 

[Justin1]

well, it's all sorted. had a plumber round there for the morning... even confused him. it turned out to be the gas valve, replaced at £110 + labour.

he also identified that the pump was on the wrong way round !! i did take it off the other night but was mindful to put it on back as i'd found it - in fact had i put it back the other way it would have looked odd to me with the motor controller wire coming out the top of the control box on the pump. it seems it's been running like that since the last plumber put it in 6 yrs ago!

bizarre!

Felix thanks very much for all your suggestions and info, i've certainly gained some knowledge from the experience any way!

 

[fishybob1]

i have a potterton profile 40e, the pilot fires up and the main flame kicks in, but after a couple of minutes it cuts out. im guessing its the thermocouple/solanoid? does anyone know if it is a straight forward job to replace said bit?. or if its worth me trying? what are the risks?

 

[twgas]

you really should start your own thread.

But your problem, quite likely, is a rotted flue-RGI needed for this.