Gas Boiler Problem

It may be the room thermostat is faulty or not correctly wired to the boiler control circuits. A couple of years ago the wire of our room thermostat was accidentally cut by a builder. The radiators became hot and at times would vibrate. Turning down the boiler thermostat helped a little. Only when the room thermostat was correctly wired into the HW/CH circuit terminal box were things back to normal. We have a condenser(not combination) boiler. The hot water has a cistern with a thermostat fitted. This is also wire linked into the circuit box. Motorised valves direct the water from the boiler to which ever circuit requires heating according to the HW/CH thermostats.

The room thermostat is a Honeywell with neon light indicator for on/off state.

What thermostat has been fitted. What boiler is it ?. If you turn off the hot water and then turn down the room thermostat, does the boiler still fire and cycle (it shouldn't - just taking the room stat out of the equation)

James

I don't think it's an issue with the room stat as it happens with either the CH or the HW calling for heat. When the room temp is high enough the the boiler will switch off. The problem is the boiler itself is getting way too hot before the boiler stat switches in.

Thanks

Dave

I'm sure the HW/CH thermostats would switch off the boiler way before the boiler stat is activated. 

If you say the room stat switches off the boiler then maybe it's the HW stat that's the problem. Is your boiler a condenser (with HW cistern) or combination type or even older ( with HW cistern and balanced flue).

Hi James,

The boiler is a Potterton Neatheat Profile 50E.

The STAT that was fitted according to the label on the bag:

 part number 907729 which is a T'stat control kit k36 GC No 248147.

The paperwork inside the bag quotes a Potterton part No 404504 K36-L1014

My boiler manual quotes a Control Thermostat CL6 PO155 part no 404500 & GC part no 382889

With the HW turned off if you turn the room stat down the boiler turns off and only turns back on when the room stat drops to required setting.

Looking at the replacement stat part no it isn't the same as the original but I assumed it was the equivalent as perhaps you can't get the original now but it would function between 55 - 82 degrees. 

The replacements paperwork does quote all the various Potterton  Models from 30E up to 100E including mine.

Dave

Hi Graham,

If you discount the HW as this has been switched off on the programmer and only set for CH.

If I turn the room stat up high so it is always calling for heat the boiler switches off...eventually with very hot rads and very hot boiler. 5 mins later it will switch back on and run for 15 mins.

I'm guessing the boiler and rad temperatures must be about 80 degrees after those 15 mins...they are very very hot.

It is operating as if I've switched the boiler stat up to MAX but it's set to the absolute min. 

Thanks 

Dave

Dave,

             Leave the HW switched off, leaving CH on. Set room stat to a reasonable temp. i.e. 21deg. cent. Get a thermometer . Position it near the room stat. Check if the room thermostat is switching off the boiler at 21deg. cent. If not it could be the room stat.

Next turn off the CH with HW on. If the boiler keeps running at the frequency you describe  even though the water is too hot to touch, > 70 deg, cent. it could be the HW stat. (presuming you have one attached to your hot water cistern and assuming you have a HW cistern). I think in some systems when you have room and HW stats fitted the boiler stat may be disabled as a controller. In any case the boiler stat is a safety devise to ensure it doesn't overheat.

p.s. do you have motorised valves fitted and are they functioning correctly.

Ok. - I had a similar problem with our Potterton Suprima. This had two problems,  the burner locking out and needing the reset button pressing and a couple of times leaking water from a valve above boIler. The boiler itself would run for long periods, the temperature control on the boiler not making much difference at all.

After checking out all the central heating wiring (correcting a wiring error since the house was built stopping the water coming on without the central heating being enabled) i could find no issue but was lucky to walk past the boiler one day and caught it trying to start without the flue fan running and locking itself out. Pressing the bolier casing near the control panel gave a few clicks and the boiler started up and ran. After checking all the connections to the boiler control PCB, I took the PCB out and checked it out and found the problem. Lots of dry joints and a couple of cracked joints. One on the boiler stat potentiometer which stopped the flow temperature being set correctly, one on the fan which occasionally stopped the fan working locking out the bolier and the final one powering the pump. This would stop running, the bolier would continue heating the water until the overheat stat would kill the burner. Not good. I remade all the bad joints and a couple more which I didn't like the look of and re fitted the PCB. The bolier has worked perfectly now for the last year with no problems at all. All I'd say is mechanically the Pottertons are good but suffer from electrical problems due to shoddy manufacturing of the control electronics. Dry joints on these cause a number of issues that most heating engineers miss or can't be bothered to diagnose and recommend a new bolier. I'd suggest, if you can't do it yourself then I'd suggest finding an engineer who knows that type of bolier and will know the strange faults they can exhibit. 

James

Thanks Guys,

Graham,

Have tried your suggestion leaving the HW off and only running the CH. The room stat is in the hall, it's a digital one which gives you the currect temp of the room, with it set fairly high the boiler runs continually for a considerable time, rads get extremely hot as well as boiler and eventually boiler will turn off for a short period, so the stat does work but not at the temp it should.

In fact I have just monitored the boiler and it has run continously of 40 mins before turning off, I could here the water in the boiler, not exactly boiling but very close.

The room stat was still set high and calling for heat.

I do have 2 motorised valves which both work ok.

James,

" After checking all the connections to the boiler control PCB, I took the PCB out and checked it out and found the problem. Lots of dry joints and a couple of cracked joints. One on the boiler stat potentiometer which stopped the flow temperature being set correctly"

An interesting point...I actually changed the PCB 6 months ago after it failed to start. Some time before that it was attempting to fire up but failed, it would this for between 6 to 10 attempts and then would run ok. I changed 3 electrolytic capacitor and it cleared the fault and worked for about 1 week then totaly failed to start. hence the replaced PCB. I did check the new PCB for DJ's and there none.

I had the boiler serviced about 1 month later and it ran OK and on minimum setting would run for 90 secs and then be idle for 3 mins until the Boiler stat was changed.

I will wait a few days till this cold spell clears then I'm going to check that the stat is sitting correctly on the flow pipe....Also if thermal paste was used on the stat...the manual states it should be used but not sure if Fitter used it or if this could be causing an issue. I will also re check the PCB,

I assume when the boiler stat temp is reached the expansion of the alchohol results in an open cct instructing the PCB to switch the boiler off.

thans once again

Dave

Dave - the bolier stat on yours should be a thermistor connected to the PCB. The overheat stat is a small round bimetallic type switch. Apart from the heating wiring fault, our boiler had been fine until I had it serviced and the engineer had disturbed the PCB when disconnecting one of the large multiway headers so if you've not done so already just recheck all is ok. 

Cheers

James