Radiators heating up when water only is selected

Hi All
My heating system seems to cause the radiators to heat up when only the DHW setting is on. My system consists of a condensing boiler (Greenstar 29HE Conventional), fully pumped with two isolating valves, frost protection thermostat and auto-bypass valve.
I do not think the frost stat has anything to do with the problem since this overides all timer settings to switch the boiler on and the problem occurs only when DHW is on.
I suspected the auto-bypass valve but setting this at its maximum setting has not cured the problem.
I have checked the CH isolating valve to see if its opening but that does not seem to be the case yet nearly all the radiators are heating up, some quite considerably.
Any suggestions before we melt in the heat.
Ta1 Tricky Dicky
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In an earlier contribution to this discussion,

I presume that by Isolating Valves you mean motorised zone valves?
Is this a new system, or has it worked correctly before?
Assuming that it has been plumbed correctly:
a) the auto by-pass simply provides a short cut between flow and return, to allow water to flow when the pump is running but both zone valves are shut. Whether this is open or closed should not have any effect on the radiators getting hot
b) the *only* path to the radiators is via the CH zone valve - so the water must be flowing through this. Even if it doesn't appear to be *opening*, it pretty certainly isn't fully *closing* when it is supposed to. Are the pipes hot in the vicinity of this valve?
I suppose one other possible explanation - and it's a bit of a long shot - is that some sort of one-pipe gravity circulation is occurring through the return pipe. If this is the case, only one pipe of each radiator will be hot - and the other will be cold.
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On Tue, 20 Jul 2004 20:43:03 GMT, "Tricky Dicky"

This could be a phenomenon known as reversed circulation.
It can happen if the returns from the radiators are not brought to a common point before meeting the return from the hot water cylinder coil.
It can easily be diagnosed because the pipes to the affected radiators will show a reversal as to which gets hot - it will be the opposite one to that which heats up when the CH is run.
The cure is to replumb the returns from the radiators together before the cylinder return joins on the way back to the boiler.
.andy
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Hi Andy
I think you are right with the reversed circulation. The system is a mixture of old with a new boiler which we installed last summer, it was Sept. we got it commisioned and never had DHW without CH till this spring. With the original boiler the return from the cylinder was the first return in the system. The new boiler is sited in the garage and it now makes the cylinder the last return in the system.
Are there any other solutions to the problem since it would involve ripping up floors and laying another several metres of pipe to do as suggested.
Would re-balancing the radiators restrict flow? I have noticed that those fitted with thermostatic valves do not warm up.
Richard
wrote:

DHW
this
occurs
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On Wed, 21 Jul 2004 11:11:12 GMT, "Tricky Dicky"

That shouldn't matter if there are then no heating returns on the boiler side. The problem arises if some heating returns are upstream of the cylinder return and some downstream.

Rebalancing is not going to help.
What you could do is to fit one or more check valves at appropriate places.
BES have these as swing check valves but there are others which only fit vertically in a pipe run.
You will need to sketch out the system to work out the places.
Another solution would be to use motorised zone valves at appropriate places and have them paralleled up to the main diverter valve or if they could be related to natural heating areas of the house, local thermostats. This would give you some added benefit of zoned heating control.
.andy
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