In an earlier contribution to this discussion,
Alex wrote:

Guys

This could take some time to explain! I have a gas fired central
heating and hot water system that is 9 years old. It comprises an
Ideal 40k Btu boiler, Honeywell 4073 3 way motorised valve, hot water
cylinder and thermostat, room stat, a Horstman Channel XL time control
unit for HW and CH, and a Grundfos 15/60 pump.

The problems started early this year with heating failure. My plumber
changed the pump and three way valve electronic head (with the wrong
item I discovered today). He got the heating running, but if the
heating was on with the hot water (both are pumped via the three way
motorised valve), the boiler would kettle up in about one minute and
click off. So, we compromised and switched the hot water off at the
wall controller and hey presto, the heating would run. We have used
the immersion all summer for hot water. I tried the heating last
weekend, and guess what, with the hot water off, the boiler kettles up
in about one minute and there is no heat. The flow from the boiler is
scalding, and the return stone cold. So, I figure it must be:

1. Pump failure
2. Three way valve failure
3. A blockage

I replaced the three way valve last weekend, and no change. I took the
(seven month old) pump off today and it works perfectly - I ran it
with the bleed screw off and it spins without fault. I took it off its
mounting to make sure it hadn't seized during the long summer layoff.
I changed the Honeywell valve head for the correct one and still no
heating or hot water. I lifted the carpet in the dining room and let
the system drain through for one hour and the water coming out is
CLEAR.

I am absolutely stumped. Basically there is no return flow to the
boiler for whatever reason. I can only imagine that there is a
physical blockage in the return pipe. What do I do, strip pipes and
re-solder new ones in?

There has never been a dreadful sludge build up in my rads - I have
taken them off several times for decoration etc and, OK, the water is
a bit black, but there has never been excessive deposit.

Does anybody have any suggestions as to:

1. What is wrong, and
2. How I fix it?

I am on the verge of calling the plumber out but even he was baffled
earlier in the year.

Thanks

Alex

You say that the CH on its own works ok? So CH return flow is getting back
to the boiler ok.

You say when the HW is on, there is no flow. I assume that the boiler
overheats and trips out, needing to be manually reset. Is this correct?
Under these circumstances, is the pump still running?

If you select *both* HW and CH at the same time, does the boiler still trip?
I think you said that it does, but we need to be 100% sure of this.

IF having both CH and HW on results in the CH working ok but the hot water
not getting hot, there is one possibility which springs to mind. Is there a
gate valve anywhere in the HW circuit - between the 3-port valve and hot
cylinder - to enable the relative flows in the HW and CH circuits to be
balanced? If so, this could accidentally have got closed - maybe by the
plumber who fitted the first 3-port valve replacement. This would stop any
flow round the HW circuit - and any flow at all when only HW is selected.
--
Cheers,
Set Square
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