Hot rads but no hot water

[Paulie]

I have an old Thorn M type boiler which has worked fine until a few days ago when I replaced a leaking rad valve and had to partially drain the system. The sytem tops up via a header tank and once I bled the rads the heating worked fine. Now I have no hot water. If I put the controls on to HW only, nothing happens. Usually, HW is always there when the CH is on and there's no control position for CH only, so if the boiler is lit, you get HW.

Any ideas?

Regards,

Paul.

 

[weareleeds]

Is there a bleed cap on the circs @ the cylinder, is header tank in loft full, has ballcock stuck in up position, is it fully pumped.

 

[Paulie]

Thanks for your interest.

If you mean the HW cylinder, I didn't see a bleed cap. There are two tanks in the loft , the smaller of the two I assume is for topping up the CH, the other for the upstairs taps incl. HW via the boiler. I'll pop up there and check the tanks.

I had a look at the cylinder and didn't see a stat. How is the boiler switched as regards HW?

It's a fully pumped system.

I know most people would call a plumber but my experience is that they tell you to get a new boiler because they don't feel it's worth their while fixing a minor fault. In my last house, I had one tell me that when it turned out to be a faulty fan which I replaced for £25.

Cheers,

Paul.

 

[Blasphemous]

I know most people would call a plumber but my experience is that they tell you to get a new boiler because they don't feel it's worth their while fixing a minor fault. In my last house, I had one tell me that when it turned out to be a faulty fan which I replaced for £25.

Yeah but don't tar everyone with the same brush Paul And after all, you say it was only one Plumber who told you that. As you might guess, there are one or two Plumbers and/or experienced people on here giving advice totally free of charge and in their own time, weareleeds being one of them

Check the Header tank first to see if it is full/working.
Is the pump running?
The problem could be totally unrelated to the rad valve you replaced.
Let us know how things go.

 

[ELZ4742]

isnt that a gravity system if the HW comes on with the CH, not fully pumped?

 

[Paulie]

Blasps,

You're right.

However, in three separate instances when local plumbers were asked about a problem on three different systems, only one of which was my own, the answer has been the same: new boiler. In each case it was fixed by replacing relatively inexpensive parts, namely a fan, a PCB and diaphragm.

Anyway,

At the moment he pump runs but when the CH is on. I assumed it was fully pumped because I can hear the pump running when the boiler is on. How could I tell a gravity fed system? Don't they have larger pipes so the water circulates via convection?

I'm not at home so I can't check the tanks but there was only cold coming from the hot tap and I could hear the usual noises as the tank fills after say, running a bath.

Someone told me to check the cylinder stat and the motorised valve.

I don't know where these are. I couldn't see a stat on the tank nor a valve near the boiler but I assume they exist.

In my old place, the stat was on the HW cylinder and the valve adjacent to it.

It could be coincidence that the HW problem happened after I partially drained the rads, but I'm not convinced.

Thanks again,

Paul.

 

[Paulie]

I had a look here: http://www.diydoctor.org.uk/projects/boilers.htm

As I have two tanks, mine will be like the one in the top diagram. Now I can see how the HW side works.

Looking at it, I suppose my problem could be a stat or the motorised valve.

 

[Paulie]

I checked the tank this morning and it fills up when I run the hot tap. I then back-filled the system by opening both taps on a mixer and covering the outlet ubtil water ran from the overflow. The boiler and pump operate when I turn the control to HW but then stop after a minute or so and the pipe into the HW tank doesn't get hot.

 

[Paulie]

Realised I'd need to back fill the HW heating coil. Done that but still no HW. The boiler fires when HW only selected but goes out after a short time. There's a faint sizzling from the HE area after the gas switches off so I suspect that it's overheating due to the pump not running unless on CH/HW.

 

[Paulie]

I wired up the immersion but I needn't have bothered as the HW is now working, albeit only when the CH is on. Seems hotter than before, too. The pump doesn't run (earlier I thought it did) when HW is on, the boiler cutting out after a short while. I'll check to see if power is switched to the pump when HW is selected. If it is, it may be the windings that have gone, This would make sense if the pump ran at a lower speed when HW only is on. If not, the boiler controls are next on the list.

 

[Paulie]

OK. I spoke to a helpful chap at a firm that does fans and PCBs in Watford who've been very helpful and good value when I had a prob with the glow-worm fan thermistor problem.

With his help I was able to understand my system and realise that it was gravity fed on HW and pumped when on CH/HW. He said I had a blockage in the coil circuit that the pump overcame. He said to try sludge remover and it seem to have done the trick. The gravity side works. Not brilliant but it's only been circulating for a few hours.

If I ruffled a few feathers by appearing to tar every plumber with the same brush then I apologise. The help and advice is appreciated.

Need the HW working for the shower I'm about to fit. When, of course, the tiler's finished.