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

 

[carrpaul]

hi there is this a new boiler ? what valves are on the system ( make ) and pump fixed speed or A rated type modulating . might be time to seal the system if your having to shift a blockage , could do with a ton more info on system history age of parts . did you have the sparky and gsr guy looking at this together might help ,

 

[DBatchelor]

hi there is this a new boiler ? what valves are on the system ( make ) and pump fixed speed or A rated type modulating . might be time to seal the system if your having to shift a blockage , could do with a ton more info on system history age of parts . did you have the sparky and gsr guy looking at this together might help ,

Whole system is approx 3 yrs old but it's not been working for a while now. Valves are honeywell 2 ports, pump is a grundfos 3 speed. No, I had the plumber out first with the sparky a week after.

 

[Dotty]

If you've an s plan and it works fine on the heating 2 port but not the hw 2 port which you say is opening fine and sending the signal to the boiler which fails to fire due to lack of movement of water then you're looking at the cylinder.

Look for a balancing valve at the cylinder, make sure that's open at least 1/2 to 3/4 turn. Open it fully as a test firsthand.

Gently crack the nut on your flow to the primary coil and make sure that you've got water coming out. Make sure you don't undo it fully, just a couple of turns to loosen it slightly will do. Then do the nut back up. Make sure that you've plenty of towels down.

If it doesn't go after that you'll need a decent heating engineer in. Certainly not an electrician!

 

[DBatchelor]

If you've an s plan and it works fine on the heating 2 port but not the hw 2 port which you say is opening fine and sending the signal to the boiler which fails to fire due to lack of movement of water then you're looking at the cylinder.

Look for a balancing valve at the cylinder, make sure that's open at least 1/2 to 3/4 turn. Open it fully as a test firsthand.

Gently crack the nut on your flow to the primary coil and make sure that you've got water coming out. Make sure you don't undo it fully, just a couple of turns to loosen it slightly will do. Then do the nut back up. Make sure that you've plenty of towels down.

If it doesn't go after that you'll need a decent heating engineer in. Certainly not an electrician!

Makes sense, they share a flow from the boiler so the obstruction has to be in or around the cylinder.

Not entirely sure what (or where) a balancing valve is, I did (slightly) open the air release on the primary coil which let water out, no air.

 

[DBatchelor]

Would it be worth draining and then refilling the system to see if that dislodges anything? Or do I run the risk of causing more damage/introducing airlocks by doing this?

 

[carrpaul]

put system on to h/water only and unplug flow switch click system on and then link out flow switch and see if boiler fires, could be week pump / blocked pump. scrub all this , just get plumber in go on baxi site and find installer .

 

[DBatchelor]

put system on to h/water only and unplug flow switch click system on and then link out flow switch and see if boiler fires, could be week pump / blocked pump. scrub all this , just get plumber in go on baxi site and find installer .

Thanks for the reply.

I don't think it's the pump tho because that works fine for the Central heating, or am I wrong?

I think you're both right tho, this is a (potentially expensive) job for a heating engineer.

Thanks again for all your help, and I'll update once it's resolved as to what exactly it was, in case anyone else has the same problem.

 

[Pipes Plumbing]

Shouldn't be a horrendously expensive job for a decent engineer to diagnose & resolve for you, certainly not as expensive as parts being changed in error or something going horribly wrong...

Also not as bad as you taking the case off the boiler and breaking the law. Please don't do it, just get a decent heating engineer in. One of the local member may well be able to help you if you pop a post in the 'Looking for a Plumber section'

Good luck getting it resolved!!

 

[DBatchelor]

Shouldn't be a horrendously expensive job for a decent engineer to diagnose & resolve for you, certainly not as expensive as parts being changed in error or something going horribly wrong...

Also not as bad as you taking the case off the boiler and breaking the law. Please don't do it, just get a decent heating engineer in. One of the local member may well be able to help you if you pop a post in the 'Looking for a Plumber section'

Good luck getting it resolved!!

Appreciate the advice, I'll see if I can find someone.

For what it's worth I've just checked and there is flow into and out of the primary coil, so I'm stumped (again!!)

 

[JamesyGoosey1]

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.

The Baxi solo and the promax (same boiler) they have dry fire sensors on them and they are fitted to open vented systems

 

[DBatchelor]

Just to close this one out, I decided to try and change the valve as a last resort. It turns out it was that that was restricting flow, even though the motor and spindle were operating fine - the innards (diaphragm?) must have been buggered!

So the e majority of you were right all along! Moral of the story: just because all appears well, doesn't mean it is!

Thanks again.

 

[Whyme]

Good stuff, glad its sorted and you didnt have to pay us.:grin:

 

[DBatchelor]

Seems I spoke to soon, the error is back. I hear gurgling in the primary coil so it must be air that's trapped somehow because there's only water coming from the air release valve again.

 

[Aquazone]

Don't open the air release when the pump is on. Wait until it stops as it will allow the air to settle

 

[DBatchelor]

Worth flushing with Sentinel X400? Possibly a blockage rather than air in the coil?

 

[DBatchelor]

Quick update for anyone still interested (or coming across this with a similar problem):

I now have enough flow to operate both CH and HW, I assume the air has settled and escaped.

However, now I get a trickling sound from the boiler when either CH or HW valve opens (eventually the trickling stops after ~1min). I assume this means there is air in the boiler (which will hopefully eventually find its way out)?

 

[Whyme]

Yes it usually does pal.