Diagnosing the cause of central heating fuse blowing

EdSwippet

On return from a three-week holiday, I found the central heating and hot water system in the house dead. The 3A fuse in the separate fuse box (switched) above it had blown. Replacing the fuse and turning on the fuse box switch immediately blew the fuse again. I'm pretty sure the CH and HW were both off the entire time we were away (neighbours did drop in, but I need to confirm this with them).

I removed the controller, standard back-plate and so easy to detach from the wall, and analysed the wiring. There are three wires out from the controller, one that is live when HW is on, one that is live when CH is on, and one that is live when CH is turned off on the controller. It is the last of these that is causing the fuse to blow; confirmed by bridging live to this with a short piece of wire with the controller removed.

For now I have hard-wired live to HW on, and I think that will give us hot water (turn on/off using the fuse box switch! -- certainly the boiler fires up as expected). However, I need to diagnose the cause of the problem.

We have an old gravity-fed system with pumped CH. One thermostat in the hallway, no low-limit override. Mid-1970s installation.

My sense is that it is most likely the CH valve actuator that is at fault. My further guess is that we have a motor on/motor off (MOMO) unit, since this is the only thing I can think of in our system that would demand power when the CH is off. However. the entire CH and HW system is old, so the water pump could well also be implicated -- statistically perhaps more likely as it is a higher current device, but on the other hand not something that is powered when the CH is off, which is the state that blows the fuse. (Does HW only need the pump?)

Anyone in the know here who can confirm, refute, or otherwise refine my logic? The CH valve is embedded under carpet and floorboards and a real pain to access, so I'm aiming to avoid digging around into that until I'm reasonably sure that it is the culprit. (Other possibilities might be things like mice chewing the cables, but no other evidence of this around, and any mice problems we do get are generally in the autumn as the weather turns colder.)

Thanks for any hints.

Mistral001

Have you bypassed the fuse when you said "hard wired"? If so un-bypass it and get a fuse of the correct the amperage. Also check your electricity meter for any large increase in usage while you were away.

nofoollikeold

1. If you have a "Y" plan system, the valve will be a three way one, which does require a live for hot water on, a live for hot water off and a live for central heating on. This is, statistically, more likely than having a MOMO valve.
2. If the motorised valve has one input pipe and two output pipes, it is a 3 way valve and you have a Y plan.
3. If you have a Y plan, the pump will (should) run for both heating and hot water.
4. I agree with you the pump is the more likely culprit, provided 3. above is the case.
5. Photographs of pump and valve(s) would be useful.

EdSwippet

Mistral001 wrote: »

Have you bypassed the fuse when you said "hard wired"? If so un-bypass it and get a fuse of the correct the amperage.

Thanks for the reply. I haven't done that. A correctly rated fuse is still in place. With the controller off the wall, what I have done is run a wire between the two terminals on the controller backplate that the controller would connect to switch on HW. That way should give me HW, albeit manually controlled using the switch on the fuse unit. No CH, of course.

Mistral001 wrote: »

Also check your electricity meter for any large increase in usage while you were away.

Good idea -- I'll take a look.

jk0

I was thinking maybe the pump had seized up over those 3 weeks. Is there anywhere you can fit a spanner to turn it yourself? That might just do the trick.

EdSwippet

nofoollikeold wrote: »

1. If you have a "Y" plan system, the valve will be a three way one, which does require a live for hot water on, a live for hot water off and a live for central heating on. This is, statistically, more likely than having a MOMO valve.
2. If the motorised valve has one input pipe and two output pipes, it is a 3 way valve and you have a Y plan.
3. If you have a Y plan, the pump will (should) run for both heating and hot water.

Hmm, thanks for the steer. I just looked at the wiring diagram for a Y plan system, and that is close what I have. However, the diagram I'm looking at uses the 'HW off' controller output but not the 'CH off' one, and on my system I see the controller's 'CH off' is connected but 'HW off' is unused.

nofoollikeold wrote: »

4. I agree with you the pump is the more likely culprit, provided 3. above is the case.

I thought so too. However, the pump does run fine when I turn on the HW, it's just quieter than when running the CH also, and I didn't notice that it was running last night when testing things. I think that tends to eliminate it as a source of problems, but that doesn't leave much else.

Looking closely at the Y plan wiring diagram, the 'HW off' line runs to two things, the Y valve and the cylinder stat. As already noted though, my system does nothing with 'HW off', and 'CH off' is connected to something, and that something is blowing fuses. Perhaps a dead short to neutral or earth.

Given that the pump seems to be running fine, my money might be on either the valve actuator or the cylinder stat (assuming my weird 'CH off' setup goes to those?). I don't recall the valve actuator ever being replaced, and when it worked it wasn't silent, making enough clunk on start to wake us up on occasion.

The other thing I can't quite work out is what the system will do in the 'indeterminate' state where neither 'CH off' nor 'CH on' is connected to live. Presumably the valve just sits where it currently is, likely in the HW only position, but what about the cylinder stat, assuming that it is involved somehow?

The other mystery is why this should fail under (I think) entirely quiescent conditions. Usually things fail when you turn them on or something moves, not when idle. Mice? Poltergeist? Shrug.

Anyway, thanks again for the pointer to Y plan. Does my description of how 'CH off' is used but 'HW off' unused ring any further (alarm?) bells at all?

nofoollikeold wrote: »

5. Photographs of pump and valve(s) would be useful.

The pump is the exact same model as this one. As mentioned above though, the valve is inconveniently buried under bookcase, carpet and floorboard, so for now I haven't dug it out. That's probably the next step, though it's a fair upheaval.

EdSwippet

jk0 wrote: »

I was thinking maybe the pump had seized up over those 3 weeks. Is there anywhere you can fit a spanner to turn it yourself? That might just do the trick.

Yeah. As mentioned above though, it seems like it's running fine after all. Just so quietly that I didn't pick up on it last evening when testing things.

Either that, or an intermittent pump problem I suppose, and one that is somehow suggesting problems elsewhere. I'm not sure how that would fit in with the fact that connecting live to 'CH off' blows the fuse, though. Presumably the pump is not expected to be in any form of action when both HW and CH are off, yet this is the condition under which the original fuse blew (and still does)?

Heating systems seem to have a fair range of odd failure modes, some where the symptoms don't clearly indicate the problem. Added to which, every heating engineer who has encountered our system over the years has finished the job mumbling about how they never want to see this system again. A strong hint there that we have something that is either wired in an ... um ... unorthodox way, or that is otherwise generally troublesome to work on.

EdSwippet

Update, just in case anyone is still reading.

We removed furniture, carpeting and floorboards to expose the valve and actuator. Although there is no sign of a leak on the ground floor ceiling below it, there's clearly some watery thing going on here, since the base-plate of the actuator is just about nothing but pure rust! It's a two-port Sunvic valve, and probably MOMO based on wiring and the failure mode, although there is absolutely no model number or other identifying information on it

So, a likely looking culprit, just based on its excessively corroded appearance. Even if it's not the actual cause of the problem, it's certainly in need of replacing. Not just the actuator, but the whole valve, otherwise whatever is leaking will just continue. Bigger job than anticipated, but at least it's not winter.