CH boiler fine for HW, won't fire up for CH

[PeterMcC]

I've been brief so as not to waste people's time - sorry if it sounds terse - not intentional.

My central heating system is gravity HW and pumped CH - 7 years old.

Usual bits: boiler, pump, mid-position valve, room thermostat, timer, HW tank thermostat, boiler thermostat.

It fires up for hot water no problem - so I assume that the boiler/boiler thermostat are fine.

It doesn't fire up for CH. I can get a little heat into the pumped radiators by running the HW on full belt. Far from satisfactory but indicating that the pump is OK and that there is no significant blockage in the circuit.

The thermostat seems to be working - negative test: bridged the contacts and it still didn't fire up the boiler. Nice audible click as thermostat moves over the temperature threshold.

I've changed the timer - thought I'd found the problem when I saw the ropey looking contacts on the circuit board but no luck.

When I say it doesn't fire up, it is absolutely inert - not like the fire up/shut down of a faulty thermocouple on the gas valve.

Can the mid-position valve be a candidate?

Any ideas very gratefully received.

[Lurch]

On Wed, 21 Jul 2004 15:25:35 +0100, "PeterMcC" strung together this:

One of the above 2 statements is untrue, it's either got all those components, or it's gravity HW\pumped CH. Either that, or it's a bodged hybrid system!

If you actually have a Y-plan heating system, then yes I would look at the valve. It might be worth you actually checking Honeywells site over and working out what you're doing before going any further.

[Christian McArdle]

I think he's got confused because the DHW itself is gravity fed, even though the primary water through the coil is pumped circulation rather than "gravity" aka convection circulated.

As probably the most unreliable component in the system and capable of just the behaviour reported, I'd tend to agree. Note that usually it is just the motor end that is knackered, rather than the wet part. You can almost always source a replacement motor head, which, assuming it is the problem, is easily fitted with no drain down required.

Christian.

[Set Square]

As others have said, the 3-port valve actuator is the most likely suspect.

In CH-only mode, this has to motor to the CH position - where it operates micro-switches which switch the boiler and pump on. If it fails to motor

*or* if the micr-switches fail to operate properly, you get no heating.

It may be as well to check that the valve itself hasn't seized. Assuming it's the type where the actuator can be removed by taking out a couple of long screws, you should find a D-shaped shaft under the actuator head. You should be able to rotate this in either direction with your finger and thumb or - at any rate - using *light* pressure with a pair of pliers. Assuming this is ok, the problem is almost certainly with the actuator itself.

You say it's 7 years old, so I assume that it *has* worked ok in the past. Seven years is actually quite a long life for a 3-port valve if it's the original! The same symptoms *can* be caused by a wiring fault - but that would have shown up earlier unless something has come adrift recently. For this system to work, your programmer *must* have a live "HW not required" output, and the cylinder stat *must* incorporate a changeover switch with 3 contacts rather than a staight switch with 2 contacts. If you look at the Y-Plan wiring details in

'll see what I mean.

I've just noticed that you've changed the timer. You *have* wired it correctly, haven't you - because that could be the problem if you haven't. (See pervious para).

[PeterMcC]

Lurch wrote in

That was it - thanks.

[PeterMcC]

Christian McArdle wrote in

Thanks - it was indeed the valve.

[PeterMcC]

Set Square wrote in

And, indeed it was.

Timer was changed as part of the diagnosis - and I only swapped out the front plate so the wires weren't touched.

Many thanks for the advice - valve replaced and CH working.