Repairing CH zone valve

[Mike Barnes]

I came home after new year to find the central heating not working. I traced it to the microswitch in the CH zone valve (Honeywell V4043H) failing to close when the valve opens. I've ordered a new powerhead for the zone valve and while waiting for it to arrive I'm wondering if there's anything I should know about fitting it. It looks like just two screws to remove it and replace it. Is there any more to it than that?

In the meantime I've rigged up an override switch to the pump, bypassing the controller and room stat. When I want heat I open the valve manually and flick the switch. I'm very careful not to switch the pump on unless the valve is open, but I wonder what would happen if the pump was switched on with both zone valves (CH and HW, both less than a foot from the pump) closed? Would there be damage?

[Graham.]

Remove the head and latch the lever open. With my conventional boiler, if there is no flow the boiler thermostat would turn it off afrer a short time even if it was on max and cycle.

I dred to think what would happen if the stat failed, I imagine it would discharge boilling water into the header tank and perhaps the relief valve would blow, but as this is inside the boiler casing it would be very messy.

My Honeywell zone valves are the old type with a fixed head and I have replaced the microswitches (salviged from an old microwave oven) and the motors which cost about a fiver each from BES, Oh and the cable. The "wet" valves themselves are 30 years old!

[andy]

Forgive me asking an impertinent question, you are obviously knowledgeable, but is the microswitch really dead. The reason I ask is that I have had a couple of occasions where mine have stopped working because the metal tang that operates the switch had bent ever so slightly away from the switch, enough to stop it making. Bent it back and no more trouble.

A.

[Roger Mills]

Most CH pumps are of the centrifugal rather than positive displacement type - and don't mind being run stalled.

Many boilers require there to be a by-pass circuit, anyway, to allow the boiler to control the pump - keeping it going for a while after the demands are satisfied (and the zone valves closed) in order to carry away the residual heat. Is yours not like that?

[Dave Liquorice]

I've got one like that... Giving it a sharp knock normally gets the switch to operate. It's done it before and I replaced the switch (it's a standard electronics part) but it has now started playing up again. The rack and pinion look a bit worse for wear now so it'll be a new head at some point.

Nope but can be a bit fiddly getting the drive between valve and motor unit lined up so the two units come together properly.

Don't forget that when the valve is motored open it unlatches the manual lever... So if something switches the CH demand off the valve will close.

[Mike Barnes]

Good thought. There are all sorts of possibilities but it's turning out to be an intermittent problem so something is clearly marginal and likely to take a few attempts to pin down. I'd just like it fixed so rather than dismantling and experimenting I decided to order the new part.

[Mike Barnes]

I'm not sure to be honest. I was more worried about damage to the pump or pipes caused by long-term pumping into a small closed space. I guessed that the possibility of such damage is why the pump is controlled by microswitches rather than the valve motor inputs.

[Mike Barnes]

I wondered whether it was meant to be like that... obviously it is. The CH demand had to be off so that I could set the latch in the first place. So I set the room stat to minimum, and left it that way.

[Graham.]

That's why I said remove the head (or disconnect the motor)

[Graham.]

To be honest, I think all the faliures in my valves; motors, microswitches, and cable insulation, over thirty years(!) have been due to heat, because of the proximity of the valves to the boiler.

[Harry Bloomfield]

Repair the old and keep as standby spare.

[Graham.]

My policy has always been repair the old and don't bother with the new, which is why I've ended up with a 30 year old heating system. False economy I know.

[Roger Mills]

No, that's not the reason.

The reason is that this method provides boiler interlock - in other words, turns the boiler off when *both* demands are satisfied but runs it (and the pump) when *either* (or both) demands are present. This is achieved by connecting the microswitches of the two valves (which are independent of the motor part) in parallel - effectively logically OR'ing the two outputs.

[geoff]

In message , Mike Barnes writes

Assuming that the valve isn't so old that you can't remove the actuator head, just make sure that the (probably "D" shaped) slot lines up with the valve shaft. The head is held in place by two bolts and two pins. Check that the valve shaft is free to move and is not tight, which would indicate that one of the rubber seals has gone and will get progressively tighter

Other than that, make sure its not powered up and replace the cable wires like for like

No, not really any hidden catches, which is what you asked

[geoff]

Not until they overheat ...

[Mike Barnes]

Ah, I see, thanks. I was leaving that until it was strictly necessary.