In article , says...
In an earlier contribution to this discussion,
R Taylor wrote:

Miles wrote:
OK I'm no expert but I will try to describe this problem as best I
can:

The central heating system in our house is a fully-pumped,
open-vented Y-plan system. Now it should be possible to have either
the central heating or the hot water on separately, shouldn't it?
The problem we are experiencing is that the radiators start to warm
up when only the hot water is on.


pluber brother in law : "there's a lot of heat to dissipate, it's got
to go somewhere"

hth

RT

From your original post, it appears that you've got a Y-plan system, and
that you've already replaced the 3-port valve - so we have to assume that
that isn't faulty.

I suppose that it *could* be wired wrongly - so that the valve always thinks
there is a CH demand even when there isn't.

It seems more likely to me that the whole system is *piped* wrongly. Hot
water should come from the boiler outlet, through the pump to the inlet of
the 3-port valve. One outlet from the 3-port valve should go to the inlet of
the coil in the HW cylinder. The other outlet should go to the inlet (flow)
side of all the radiators. The outlets from all the radiators should be
connected together, and taken back to the boiler inlet. The outlet from the
HW coil should also be taken back to the boiler inlet. If the HW and CH
returns are connected together, this must *only* be done after the point
where the *last* radiator return connects in. If there are radiator returns
*either side* of the HW return, you have a possible return path that goes
through some of the radiators - even with the CH port on the 3-port valve
closed. I suspect that this is what is happening.

Under these circumstances, the radiators won't get as hot as they would be
with the CH on, but they will get distinctly *warm*. Does this fit the
facts?

Thanks, that's very helpful, and yes, the radiators do not get as hot as
when the CH is on. Also, it is only mostly the upstairs radiators that
are getting warm, some of the downstairs ones remain cold.

If this *is* the problem, you need to correct the pipework rather than
faffing about with additioanl valves.

So it sounds like I do need to investigate where the pipes are running
to confirm this problem. I think I will check the wiring first, as the
other poster said, just in case, because it's a lot easier than pulling
up floorboards.

I don't know if this is a bodged former DIY job or a design fault when
the house was first built ~10 years ago, because like I said, the
previous owners (who were in 7 years I think) said it has always been
like this.