No hot water but CH OK

Quick update ! I'm now hoping it's not the motorised valve as I can't find one, I guess the HW is a gravity based system. Since I've been home (and the heatings been on) the top of the HW cylinder now feels warm but the bottom is still cold - so would this point towards a blockage somewhere. Any suggestions of how I track it down and get rid of it.

Thanks again

Jim

Yes - the best thing to do is have someone turn the room stat off/on while you watch the valve - does it move?

Then do the same with the stat on the cylinder, with the heating on and off.

If it does not move at all, then you need to test the wiring to the valve, to make sure you are actually getting power to it. If so, then you need to test the stat on the cylinder to make sure it is switching properly.

It's probably the valve, but could easily be the stat or the wiring!

Sparks...

How many large diameter water pipes come out of the boiler (two or four)?

Does the cylinder have a thermostat fitted?

If the HW system *does* use gravity (convection) circulation, you could have a blockage or - more likely - and air lock. Make sure there is some water in the fill & expansion tank in the attic, and that its ball valve hasn't stuck shut.

Don't suppose you got a manual thermostatic valve (Cyltrol etc.) on the return pipe from the coil in HW cylinder? I had one of those many years ago and - just occasionally - the gravity circulation would take it upon itself to go the wrong way - with the result that the Cyltrol sensed the temperature of the incoming water rather than the return - and promptly shut. Result: no hot water. Once the system had cooled down, it was ok again next time it started up. It only happened about 3 times in 8 years.

OK, there are 4 pipes from the boiler. 2 go downwards, one of which has the pump and 2 go up towards the airing cupboard.

I can't see a thermostat on the cylinder

I've turned the backup immersion on so at least we had hot water this morning !

Cheers

Jim

OK, that does indeed sound like a gravity fed system, so no three port valve to worry about.

Do you have any automatic valves close to the cylinder where the pipes to and from the boiler connect? (may look like a kind of thermostatic ra valve)

If you do then that would be the most likely susspect. If not, then a boiler fault or a pipework blockage sounds more probable. Since the heating is ok it looks less like the boiler.

John thanks for the input. Had a chance to do a bit more fault finding;

I've switched the CH off at the programmer and let the HW tank cool down. When I switch the HW on the boiler fires up but only runs for 5 mins. At the HW tank the top pipe feed is very hot but the bottom is cold so I guess that the blockage/problem is within the tank somewhere (suppose I should have done this test first but still learning about the system works !)

Is this going to be a case of the simplest thing to do is switch out the hole tank or is there a clever trick to clearing blockages.

Thanks

Jim

But what happens after 15minutes, 30 minutes, one hour etc.? There must be

If you have a tankful of cold water, you'd expect the return flow to be pretty cold to start with - because all the heat in the incoming water is being transferred to the tank. You'd also expect the boiler to cycle on and off on its own thermostat - because you can't transfer heat to the HW cylinder as fast as the boiler can produce it.

If the return pipe is *still* cold after a couple of hours, you've probably got a problem. Otherwise you may not have.

One quick check is to make sure there is water in the header tank. Gravity HW is often the first thing to go if the primary circuit startsto dry up.

Thanks to all that replied but bizarrely the water seems to be working OK now, at least when I got home tonight the water ran hot !

Hopefully just a temporary blockage/ air lock that's cleared itself but I'll monitor it closely for the next few days!

Cheers

Jim

Was it air or hydrogen gas? When was it last flushed? Do you have an inhibitor in the system? Do you have to do regular bleeding of any radiators, and does the water come out clear or black? Have you gone up to look in the header tank to check that the water is not rusty/gungy?