Heating Pump conundrum

[Maxster]

Calling all heating engineers, I have a conundrum for you. When we first turn our room thermostat up, all the radiators in the house come on and are very hot. Also the pipe that links the hot water tank to the heating pump is also hot so the water is obviously being pumped round the radiators ok. However after an hour or so, the radiators start to cool down, even though the temperature on the thermostat hasn't been reached. The water in the taps is still hot so the boiler is heating the water ok. Any ideas why the radiators cool down? The only way to get them hot again, is to turn the heating off altogether for a couple of hours until the radiators are cold and then switch it on again.

[AlBags]

Are you sure that there is not a TRV bypass fitted anywhere?
The flow through the hot cylinder .. is it damped right down on a gate valve near the cylinder?

[Maxster]

Thanks Albags, unfortunately, that's a bit technical for me. The system is 15 years old and last year was the first time that this happened, and after a while it sort of fixed itself. But this year I think we need to get someone to come out and see to it but I wanted to get some sort of idea what could be wrong, before I call someone out.

[AlBags]

What boiler is it?
Is it a pressurised system with a filling loop from mains cold you charge up on a gauge?

[Maxster]

The boiler is an Ideal Classic and is in the kitchen. We have a large hot water tank in the airing cupboard upstairs, but we don't have a cold water tank, just comes from the mains. Coming off the water tank are 2 Grundfos selectric pumps, one on each side, One says Heating Pump the other Boiler Pump. I could understand it better if it didn't work at all, but it's almost as though once the initial tank of hot water has been sent through the radiators, if never gets the chance to heat up again while the heating is on. But then again, there is really hot water in the taps. It's a mystery!

[terence7556]

what temperature is the cylinder stat set at?

[Maxster]

There is no temperature gauge on the cylinder, but the temp on the boiler is at maximum, 6.

[The Builder]

I'm no a heating engineer Max ........... so I'm a bit confused over the layout of you system. I'm guessing it's a conventional boiler indirectly heating an unvented cylinder of hot water. No roof tank for the HW, but surely there should be a top-up (header) tank for the heating .
Also, I think I can see what Al and Terence are thinking .......... there must be some kind of diverter valve and cylinder stat arrangement to tell the boiler whether it should heat the water, or heat the rads (put very crudely).
If so, it sounds like the diverter (motorised valve?) is staying on HW selection, and refusing to cross over to heating.
Someone will have to take a look Max ............ either that or tap the diverter with a hammer (don't quote me on that though) ..........

[Maxster]

I've had a look in the cupboard where the cylinder is and have found the label from it. It's an IMI Flowmax Mains Pressure Hot Water System, Ref: FM4651 combination D/Pump 140 litres. Don't know if this helps. It's copper and is coated in foamed polyurethane. There are about half a dozen pipes coming out of it so I wouldn't know which one to 'tap' - thanks anyway, I'll get someone in to look at it.

[Maxster]

To The Builder - hypothetically, if I were to give the diverter a little tap with a hammer, very gently of course - where exactly would I find it.? There is a little plastic box attached to the side of the tank that has wires coming out of it, one to the heating pump and one to the boiler pump - is that it?

[The Builder]

ahhhh .............. that sounds more like the cylinder 'stat to me Max ............ maybe ..... perhaps we should wait for a heating engineer to come along. There are some excellent ones on Answerbank..............

[bednobs]

that sounds like a mr bednobs' solution - hit it with a hammer!

[AlBags]

That is a HYBRID system .. using an Ideal Classic with possibly two pumps being used. There is obviously a control problem or there may be heating priority because of it.
Don't you know who fitted it? .. Get a Heating Engineer to work it out.
We can't do much more here.

[Maxster]

And the culprit was ...... a seized boiler pump.....thank you to John the Heating Engineer the house is now toasty!