Boiler cuts out, no hot water but central heating is fine.

[WoodleySaint]

Hi all,

I'm getting desperate.....

I have this problem where my gravity fed Potterton Profile 80e boiler keeps cutting out and the water in the hot water cylinder is not getting hot, however all the radiators in the house, which are pumped, are really hot and the house is toasty even on a cold day. We have lived here for 12 years and not had a problem with it prior to this.

The feed to the hot water cylinder from the boiler is very hot to touch but the return is cold, which I guess is what you would expect if the cylinder water was getting hot but it's not!

I spoke to a boiler engineer and he said that it was probably the cylinder which is clogged up with scale, so I replaced it but still have the same problem. (That was a waste of £500 and a days work!)

To ensure that the 28mm return pipe to the boiler was not blocked I turned on the header tank feed which connects just by the cylinder, (this was when the HW cylinder was removed and having blocked off the outlet,) and water flowed straight out of the drain valve by the boiler, so no blockage there.

For some reason the water is not flowing around the cylinder loop. There is decent rise from the boiler to the cylinder and there are no potential places for air to be locked. Hot water is not venting into the header tank either.

Any suggestions would be very gratefully receieved, I really don't know what to try next other than have a new boiler fitted (which I can't afford) and I'm not sure that that would even fix the issue.

Cheers

 

[Charnwood]

Motorised valve problem?
Cylinder stat problem?

Is the feed pipe hot right up to the cylinder?

 

[Murf520970]

arent modern cylinders incompatable with gravity fed systems ?

 

[ianmcd]

what size are the connections to the coil on the cylinder ? if they are 22 mm it is designed for fully pumped and wont circulate on gravity, for gravity the coil would have to be 28mm, why not just make it an S plan ? if you could swap a cylinder yourself you can alter some pipe work and fit a motorised valve its not hard to do

 

[WoodleySaint]

Thanks for the replies folks. Here are the answers to your responses:

Charnwood:
Is the feed pipe hot right up to the cylinder?
Yes, the feed pipe is hot right up to the cylinder
There is no motorised valve, the CH has a pump, the HW uses graivity.
There is no cylinder stat. I'm not sure how the boiler know how to stop and start, I guess there is some temp sensors in the boiler???

Murf520970:
I bought a direct replacement for the cylinder, it's an indirect cylinder.

ianmcd:
the pipes from and to the boiler are all 28mm as are the connections on the cylinder.
I'm not sure what an S plan is??
The pipework around the boiler is quite tight, I'm not sure whether I could fit a motorised valve. Still flummoxed as to why it stopped working.

 

[Murf520970]

the boiler will always heat the hot water whilst on for heating , as you say its gravity fed & cant in its present set up be controlled without converting it , the only thing that prevents circulation are air locks really . I was called out recently to look at a boiler & the complaint was no hot water , it became apparent very quickly that the heat exchanger of the boiler was choked up which stopped the boiler working properly which inturn prevented it from heating the water enough to get it to move up to & through the cylinder , i would check that your expansion tanks got water in it & has filled the system up & then consider having a pro look at it if that dosent reveal anything

 

[WoodleySaint]

Thanks Murf. One of the first things I looked at was the header tank, that's full and the ballcock works well. I had tried draining and refilling the system before putting the new cylinder in.

What did the boiler you looked at get choked up with? I've been assuming that the boiler os OK as it heats the CH well, perhaps the pump is pushing the water around the boiler but when it relies on gravity it's constricted somewhere??

Thanks again for the advice, my sanity is still in the balance though!

 

[Murf520970]

theres 2 tanks , the larger one fills the fresh water inside the water tank as you probably know & the smaller one fills the boiler side up , if the smaller tanks dry it would cause problems , however if alm the rads are heating up its probably ok , of the pipe leading to the centre of your tanks gettin hot & the return pipes cold then i can guess its an air lock or the tanks incorrect . The boiler i looked at was choked with soot & the flame was so poor it barely gave any heat out hence poor heat transfer etc , are there any non return valves in the pipework that could be blocked

 

[WoodleySaint]

Just to confirm that both header tanks are working and are full. (I had to drain the HW tanks as the isolating valve leaked when I replaced the cylinder.)

Yep, the pipe which connect to the middle of the cylinder is really hot, the return is not.

I really cant see where there can be an airlock, the pipes rise from the boiler to the cylinder and the header tank feed to the system is on the return side to the boiler and I tested that there is no blockage when the cylinder was out.

There are no non return valves in the copper pipework, is there one within the boiler itself?

 

[Murf520970]

yes agree unless the tanks incompatable i cant see alot else to cause any problems, try cracking the tank return union slightly & draining a bit of water & see if any hot water comes through

 

[WoodleySaint]

Thanks Murf. I replaced the cylinder with the same make as the old one and got the same symptoms, so I don't think the cylinder. (This has worked well for the 12 years we've lived here too.) It all points towards the boiler....???

 

[WoodleySaint]

Forgot to include... I'll try your suggestion of cracking the return union tomorrow.

 

[Murf520970]

ive just read your first post properly this time of course it points to your boiler if its cutting out etc , get someone to look at it asap