radiators hot when controller on Hot Water only setting


Postby Pembo » Fri Jul 24, 2009 9:11 pm

We recently bought and moved into a house that has a gas back-boiler central heating system fitted, and don't necessarily know why some things are happening as they are. The controller has a switch with 3 settings, CH, CH & HW, and HW. When I set it to HW, there are a number of upstairs radiators that still become hot.
I have opened the cover to the controller and note that there are two independent pairs of connections, one for CH and one for HW, and both have wires connected to them. I presume these are just dry contacts?
There is no room thermostat on the system (and I don't believe the controller has provision for one to be connected - I certainly can't see a connection point other than the two I have already mentioned), but all of the radiators have thermostatic valves fitted. Whilst I know I can set the thermostatic valves to a suitable temperature to shut off the radiators, at least one of the rads has one that is completely seized up.
I know that I can, and will, take action on the seized up valve, but I don't feel that really addresses the true issue, as I know that, even if all of the rads are shut down by their thermostatic valves, if hot water is still circulating around the radiator circuit there will be unneccesary wastage from heat loss whilst this takes place, and I would prefer to really stop the heating part of the system during summer to save on fuel costs.
Is this there anything inherent in the design of some types of system that will cause this to happen? I ask because at a previous house that we were renting that had a seperate wall-mounted boiler, we had an identical problem with the upstairs radiators getting hot, even when set to the hot water only setting.
If this is not so, what else may be causing my problem?
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Postby rosebery » Sat Jul 25, 2009 11:06 am

What's your boiler type: Is it a combi or do you have boiler and HW cylinder?

If its a combi then I'd be suspecting the diverter valve and that prolly needs your boiler service guy to look at it for you.

If you have a more conventional system with boiler, CW tank in roof and HW cylinder but you don't have a room stat or a 3 port valve and heating is only controlled by TRVs on the rad valves then there will always be circulation round the CH circuit and rads will get warm via the lockshield valve on the return.

If you have a conventional system with boiler, CW tank in roof and HW cylinder but you do have a 3 port valve and theres a room stat somewhere that you haven't found yet I'd be looking at the 3 port valve.

If you definateky don't have a room stat or 3 port valve and you want to shut off the CH in the summer then its not just an electrics issue - 'tis a plumbing one as well. Little more information on your current system would be beneficial otherwise it'll be an IF, AND and IF, OR situation in a rather long post.

You may find a little percussive mantenance works on the stuck TRV.


Cheers
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Postby htg engineer » Sat Jul 25, 2009 12:03 pm

Basically it's normal on a gravity HW system, heat will rise, so upstairs rads get warm.

You can turn off all upstairs radiators manually, or get a plumber to alter the boiler pipework to prevent it.


htg
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Postby Pembo » Sun Jul 26, 2009 1:57 pm

Thanks Rosebery - [quote="rosebery"]What's your boiler type: Is it a combi or do you have boiler and HW cylinder?

If its a combi then I'd be suspecting the diverter valve and that prolly needs your boiler service guy to look at it for you.

If you have a more conventional system with boiler, CW tank in roof and HW cylinder but you don't have a room stat or a 3 port valve and heating is only controlled by TRVs on the rad valves then there will always be circulation round the CH circuit and rads will get warm via the lockshield valve on the return.

If you have a conventional system with boiler, CW tank in roof and HW cylinder but you do have a 3 port valve and theres a room stat somewhere that you haven't found yet I'd be looking at the 3 port valve.

If you definateky don't have a room stat or 3 port valve and you want to shut off the CH in the summer then its not just an electrics issue - 'tis a plumbing one as well. Little more information on your current system would be beneficial otherwise it'll be an IF, AND and IF, OR situation in a rather long post.

You may find a little percussive mantenance works on the stuck TRV.


Cheers[/quote]

It's a Baxi Bermuda back-boiler, definitely not a combi.
There is definitely no room stat (though I'm thinking of fitting one for exactly the reason you outline at the beginning of your response - to gain the economy of not having circulation around the CH system when the house is already warm).

I really don't have a clue whether there's a 3-port valve or not - I've not seen one, but I guess that doesn't mean much as my recollection of past systems is that they're often under the floorboards somewhere, and I really don't feel like lifting carpets etc. to find out if I can possibly avoid it.

If I understand you correctly, without a 3-port valve, the water heated by the boiler to produce hot water will still circulate through the primary system (if my terminology's right - the pipework to the rads anyway) hence the effect you describe with warmth getting into the rads via the lockshield valve.

That being the case, what exactly is happening when I switch from CH & HW to HW only? And what do the seperate CH and HW wires from the controller actually control? There are 3 terminals on the controller, each with a wire attached, one CH on, one HW on & one HW off. Would these not normally control a 3-port valve? And if not, then what are they likely to be controlling?

Which brings me to my last point, I mentioned I am thinking of fitting a room stat, which I would normally expect to fit in line with the CH output from the controller. But if I am getting hot rads even when in HW only mode, this is surely going to have very little, if any effect is it? Which leaves me a little puzzled as to where I would connect it.
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Postby htg engineer » Sat Aug 01, 2009 6:43 pm

'That being the case, what exactly is happening when I switch from CH & HW to HW only? And what do the seperate CH and HW wires from the controller actually control?'

When you switch HW only on, there's only power to the boiler, which will come on and off regulated by the thermostat. When you switch CH & HW on there'll be power to the boiler and the pump.


'what exactly is happening when I switch from CH & HW to HW only?'
The pump stops.


Fitting a room stat will not solve the problem of upstairs radiators getting hot/warm, because of what I said in my last post.


htg
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