Potterton boiler - CH but no HW

7 Jun 2007
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United Kingdom
I have an older looking conventional Potterton gas boiler in the house I've just moved into (not sure of model but can find out if needed) with a hot water cylinder and two motorised Honeywell valves. Tank thermostat set to 70F and boiler thermostat on 3 (out of 5).

CH works fine. Radiators are hot within 15 mins.

HW - either nothing (no noises at all). Or, the boiler fires for less than a minute and then trips out. If the CH is on at the same time this keeps the boiler firing and the HW then works (but only luke-warm) but the house is like a sauna at this time of year!

The motorised valves are set on AUTO, but allow you to move the switch to MAN/ON when the boiler fires for the HW and you can hear them whirring - but moving them into the MAN/ON position (as sometime suggested on this forum) doesn't keep boiler firing.

The pipe at the top of the cylinder never gets more than luke-warm.

Any ideas???
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It sounds like a failed motor valve which is moving but not operating the microswitch.

To get hot water you could try manually setting the HW valve to the "on" position and then turn on the CH and you should find the HW warming up. Only propblem is that your heating will be getting hot as well but you might be able to turn the rads off.

On the other hand if it does warm it up from cold without the heating on then the thermostat is faulty or out of calibration. In that case you should turn it up from 55°C and see if you can hear a click and if it brings on the HW warmup.

Thanks for those replies.

Breesey - roughly how much is a new stat for the cylinder and is it a job that a novice could do?

Agile - do you mean turn off all the rads individually? At least then I would have HW while I get it sorted and not fry in the process!
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janderton said:
Thanks for those replies.

Breesey - roughly how much is a new stat for the cylinder and is it a job that a novice could do?

Agile - do you mean turn off all the rads individually? At least then I would have HW while I get it sorted and not fry in the process!

Stat is about £10 - £15 and yes you could do it. Disconnect - reconnect!

Yes, Agile means turn all the rads off and open HW 2 port manually, this will send hot water through the cylinder coil.

Seems like the valve is working and calling for heat only to find that the stat decides its hot enough already.

Changing the valve won't break the bank either.
I hope you don't mind me muscling in on this topic. I had a flame safety valve replaced on an old (but very reliable Ideal Standard boiler). The pilot light had gone out and this is something that happens periodically. Everything else was working fine. When the plumber asked me to switch on the hot water on the programmer (and he was still touching something in the boiler) the circuit tripped. When I reset the fuse, the hot water would not fire the boiler but the central heating did. He was not sure whether the programmer was faulty (it has been fine for 20 years!) or the motorised valve, despite my giving him the wiring diagrams on the paperwork and he had checked the wiring. He is now bringing back a new front of the programmer (tonight) but I am concerned that whatever tripped the power may happen again because it is not clear what caused it. I feel guilty that I am questioning his judgement but this will be getting expensive. The question is - is there any way that replacing the safety device could have short circuited something else??
(I am a bit fed up with having cold showers for a week!!)
janderton said:
Tank thermostat set to 70F

70F is way too low and is probably only luke warm!

Try setting to about 130F as it should be 60C in modern money ;)
To gas4you
Thanks for the reply. I sort of thought that but didn't like to say!! He has just been again and fitted the new programmer front but it hasn't solved the problem. He has gone away to get a part and will now look at replacing the motor on the valve. I'm relatively happy now because he has jammed the valve open and I can get hot water until it is sorted. If it isn't that, could it be the cylinder stat? I was only worried that it isn't clear what caused it originally and electrical safety bothers me!! By the way he pointed out that I had a 13amp fuse, so that didn't help. It's been there 20 years but didn't like whatever challenged it!
Thanks again
You need to try cyl stat at 130F before you decide it is faulty. Whether to let him touch electrics is another matter :LOL:
The cylinder is heating up fine with the valve jammed open so I assume the stat is OK - it's set at about 65C. Hope when the motor is replaced all will be back to normal!! It's made me realise how we take hot water for granted.
Sorry, I'm back again (last posted reply sent too soon!). The plumber (having repaired the thermocouple, managed to short circuit something else so I had no DHW but CH demand would fire boiler) did something to the zone valve so that presumably it was by-passed and no cold showers for a while. Now, a couple of weeks later, he has returned and either replaced or repaired the zone valve (not sure which). The DHW and CH appear to be working but the water is far too hot, despite the boiler thermostat now on its lowest setting and the cylinder stat also at its lowest. While the zone valve was jammed open (or whatever he did) the water was very hot so I reduced the thermostat positions, but I assumed that when the zone valve was repaired/replaced, the temperature would be as it always has been - satisfactory. I now wonder whether the cylinder stat has also been damaged and whether I could replace it myself, or is this not permitted? Otherwise, is there still something wrong with the zone valve still? I know I should get him back, but it's getting rather expensive and I don't feel very confident that he knows what is going on - even though he is corgi registered. Any advice would be so welcome! Thanks Jonquil
If you're comfortable with electrics you can replace the cyl stat yourself, if you're sure that is at fault :rolleyes:

Check whether the new hw zone valve hasn't be set to manually open.

Can you here it opening and shutting as the cyl stat is turned up and down?

You don't need a corgi to repair these problems, just somebody who is competent. Your chap is obviously not :eek:
Thanks Dave
I've just come home and tried what you suggested.
When I turn up the cylinder stat I can't hear anything happening in the zone valve. The valve set at auto but when I move the switch to manual I can't hear anything at all and there is no resistance (and I know I used to be able to hear a whirring noise and feel some resistance).
I don't mind replacing the cylinder stat (although a mere female, I have rewired irons and vacuums, and I can use common sense and switch off the mains and make sure the wiring is neatly done with no stray bits of wire!!).
I now wonder whether the zone valve is set open still. I think I understand - that the cylinder stat may not be working so there is nothing to make the zone valve close or that the zone valve may not be working so the cylinder call to close the valve is not being received. Also, it could either be the zone valve itself not working or the wiring wrong. Have I got all this right?

I'm tempted to replace the cylinder stat myself and if that makes no difference, to call the man back and make sure he sorts it out.

I appreciate that plumbing is never straightforward (though I find it interesting!) but it's a bit frustrating to have him keep going away and leaving me to find it not working. (Every other plumber I have ever had has been great, but you are all so busy!!)

Best wishes
If theres no resistance on the valve then it is constantly open for some reason, hence the really hot water.

Try turning off the hw on the programmer and see if the valves closes (check resistance on lever) or failing that turn off power to heating at switched fused spur and try valve.

If it stays open without any power then sounds like valve knackered. if it closes when no power then look at possible faulty cylinder stat. You really need to check electrics with a multimeter to be sure.

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