No hot water but central heating fine

[DBatchelor]

Hi,

Could someone please help with the following; my boiler fires perfectly for central heating but I'm getting dry fire error for hot water. System is (I believe) an S plan with a Potterton promax SL 12. I have had a plumber tell me it was an electrical problem but have since had an electrician check all connections and everything is fine - motorised valve operates, pump runs, signal gets to boiler but it still doesn't fire.

Looking through the fault finding section of the boiler instructions for dry fire everything seems ok and it says next step is remove/replace flow switch, but surely this is working for central heating to work?

Any help gratefully received.

 

[Dotty]

Please don't go messing around with the boiler.

Make sure the valve to your hot water cylinder is actually opening. It might be ok electrically but the valve itself might be stiff.

Also have you drained the system down?

 

[DBatchelor]

Please don't go messing around with the boiler.

Make sure the valve to your hot water cylinder is actually opening. It might be ok electrically but the valve itself might be stiff.

Also have you drained the system down?

Thanks for the response.

I have removed the motorhead from the valve and operated it manually, no problem there.

I will try and drain it tomorrow, I assume you mean turn on the lowest hot tap and allow air in through one of the valves on the tank?

cheers

 

[Dotty]

Don't drain anything. I meant have you drained the heating system for any reason?

If you're on an s plan as you say and the heating side is working ok then you've either a blocked primary coil in the cylinder or you've an air lock.

 

[Whyme]

What do you mean by dry fire, that usually only applies to combis or system boilers that can sense via the pump that there's no resistance.A normal open vented boiler wont detect anything like that.Do you mean it overheats and shuts off on hw.

 

[DBatchelor]

Don't drain anything. I meant have you drained the heating system for any reason?

If you're on an s plan as you say and the heating side is working ok then you've either a blocked primary coil in the cylinder or you've an air lock.

Definitely an S plan. Yes the system has been drained previously, but this was after the problem began. All hot water taps have full flow (using immersion currently) and as the hot water/central heating share a flow from the boiler it must be a blockage in the coil?

 

[DBatchelor]

What do you mean by dry fire, that usually only applies to combis or system boilers that can sense via the pump that there's no resistance.A normal open vented boiler wont detect anything like that.Do you mean it overheats and shuts off on hw.

Thanks for the response.

I'm only going on what the manual describes it as (i.e. The flashing light sequence indicates dry fire fault).

 

[Whyme]

Definitely an S plan. Yes the system has been drained previously, but this was after the problem began. All hot water taps have full flow (using immersion currently) and as the hot water/central heating share a flow from the boiler it must be a blockage in the coil?

or a dodgy valve head.

 

[Dotty]

^^^^^^agreed.

Try moving the actuator lever to the manual position and let us know what happens.

 

[DBatchelor]

or a dodgy valve head.

The (Honeywell 2 port) valve is working fine; opens and I can hear flow, microswitch engaged, pump runs and signal goes to boiler which clicks as if turning on but then I get this dry fire error