The curious incident of the boiler in the night.

[Will]

Hello all,

I'm trying to work out what happened the other night, so here's the story..

We have an old Glowworm Fuel Saver Mk2 (no fan) which while old and inefficient is/was still working and having just spent a lot of money on a new kitchen hopefully won't be replaced until late summer at the earliest. We did however replace the two valves, the programmer and the motor.

It has been working perfectly until late Tuesday night. I was sat on the computer when a lot of banging started coming from the airing cupboard. A quick look at the LED's on the Myson valves showed them both to be closed yet the motor was running and I think the boiler was still firing (hence the banging).

I quickly switched the hot water on, the valve opened and the excess pressure dissipated, then CLUNK, nothing.

I went downstairs and checked the boiler, the pilot had gone out because the valve had shut off completely, so it looks like the overheat stat had done it's job.

I left it alone overnight and then tried to restart it the next evening but to no avail. I then went to the plumbers merchants and got a new thermocouple, but this didn't help, so last night I bypassed the over-temp stat and go the boiler up and running again, there was much rejoicing.

So, I'm off to the merchants again at lunchtime to get a new over-temp stat. Testing the old one with my meter shows that the circuit is complete, so it should work, but it doesn't, perhaps there's too much resistance, given that bypassing it allows the valve to latch the pilot open. Either way I'll have a new one soon I hope.

Firstly, thanks to everyone who's ever posted solutions to boiler problems in the past, a Google search has helped me a lot here.

Secondly, why was the boiler & pump on, when the valve was closed?

I've thought of a few possibilities:

1. The valve is knackered. But, it's fairly new and hasn't been a problem for the last few months.
2. The boiler was doing an over-run. But, why was it still firing (if indeed it was)
3. The programmer is knackered. Again, new, not shown any faults before, and doesn't the programmer control the valve which in turn control the boiler?
4. Just one of those things. Probably won't happen again.

Can anyone suggest what happened here, and how I can make sure it doesn't do it again? At the moment there doesn't seem to be a pressure bypass. I am planning on putting a new towel radiator in the bathroom soon, and I'll run it off the primary - would this make for a suitable bypass?

Thanks very much, Will

[The Natural Philosopher]

In which case the switches on the valves are faulty, assuming its wired like mine.

If the valves are off, there should be no current going to the boiler whatsoever.

[Will]

Thanks - I'll keep an eye on it and see if it happens again. Hopefully I'll be able to get away with just changing the powered head.

Perhaps one of the microswitches got stuck? If so, would a squirt of WD40 do any harm?

[Roger Mills]

My guess is that the room stat switched the valve off, the valve closed and switched the boiler off, the boiler needed to keep the pump running to get rid of the residual heat but the water had nowhere to go - hence the overheat trip.

I assume that you have an S-Plan system and that the boiler *does* control the pump in order to provide over-run?

In that case, a by-pass circuit is vital - and I can't understand why you haven't got one. An automatic by-pass valve is best so that it only opens when it needs to.

You didn't say anything about re-sertting the trip. Usually they have a button to press, or else it's necessary to turn the boiler stat right down and then up again. Did you do what was needed? If so, your overheat stat ,may be perfectly ok and may just need re-setting.

[Will]

So there would have been enough heat in the heat exchanger to heat what was stuck in the pipes?

I'm pretty sure that's the case. The boiler has a permant live and a switched live and another cable, which I can only assume is the pump power feed.

I'll get one and fit it! Where would it bypass to, heating or hot water, or either?

I don't think this boiler has anything as advanced as a trip. I think Noah might have installed one on the Ark. I had switched the pot. down to zero and back again, with no luck. The overheat is in series with the thermocouple and is a very simple thingy strapped to the flow(?) with a bit of heat-paste slapped on the pipe. I don't think it has any sort of smarts in it. A new one has been ordered from the merchants, if they can find one.

Thanks for your reply.

-Will

[Will]

I've answered my own question, it just goes back to return. I assumed it would pass through a circuit in order to loose more heat, more quickly. Seems this is not necessary, but I might to it like that anyway.

Cheers! Will

[Roger Mills]

Yes. If there's no flow, there'll be local boiling in the heat exchanger - sending steam into the pipes, which is what you heard.

It needs to be fitted between the flow and return pipes, as far from the boiler as possible - but before either of the valves, so as to bypass *both* circuits.

That's unusual. An over-heat trip is a safety device and normally requires a manual reset. It should only trip under fault conditions (e.g. failed pump or power cut) and when it *does*, the system shouldn't re-start automatically before someone has had the chance to investigate and remedy the fault. If you had a failed pump, you wouldn't really want the boiler to keep firing and boiling, shutting down, cooling down and then doing it all over again!

[Will]

I've just had a look in the airing cupboard and there is just enough room after the pump and before the first valve to insert another T, on to which I'll put a bypass.

Is this the correct thing:

ain't cheap are they!)

Ahhh, OK, then that makes sense as to why the pilot went out. I figure that when the over-temp stat is "activated" it then opens the circuit between the thermocouple and the gas-valve which then fails safe. You then wouldn't be able to re-light the pilot until the over-temp has cooled enough to close the circuit again. You also need to do the hold-the-button-down-for-a-while thing in order for the gas valve to latch - so that would stop it flip-flopping.

Right - I'm pretty certain that you've got to the bottom of it. A new over-temp is hopefully on order, and I'll insert a bypass once I know I'm buying the correct part.

Thanks for your help, I appreciate it.

Cheers, Will

[Ed Sirett]

Clearly there is a problem with the system, (perhaps the wiring is wrong). The boiler should not fire if there is no demand, (perhaps the gas valve stuck on). There should be a bypass in the primary so there is some flow. The overheat did it's stuff, good.

Usually on this type of boiler the overheat cut out resets automatically and the required user intervention will be to relight the pilot.

It might be that the overheat device has failed or it needs to be manually reset.

Either way you now have strayed off the path of what anyone can call good gas fitting, defeating safety controls is the gas fitting equivalent of using a bent nail in the fuse box.

Not only did you bypass the o/h cutout but you have done so on a boiler which at least in principle has an overheating fault as you have yet to identity what caused the fault.

This cause of action gives arguments for the people who would have the whole industry turned into a closed shop with only professionals allowed to buy parts etc.etc.

Rant, whinge, whinge rant, blah blah, no seriously I mean it.

[Roger Mills]

Yes, that'll do fine. There's a cheaper one

if you can wangle an elbowed one in.

Yes, that makes sense. I forgot that you've got a permament pilot light - so if that goes out, it *does* require manual intervention.

[My boiler has automatic spark ignition, and no permanent pilot - and its overheat stat has a button which pops out, and has to be reset before the boiler will fire again]

[Will]

Partly as a thank you, and partly to help Ed's blood pressure, I thought I'd report back and say that fitting a new over-temp stat & thermocouple and installing a pressure bypass valve the boiler is now back in normal operation.

I'm convinced that Roger's diagnosis was correct in that the boiler activated the pump over-run and the extra heat in the heat exchanger along with no path for the hot water/steam resulted in the cut out.

The heating has been working as normal all evening and I'm nice and toasty again.

I did it myself with help from Roger, and saved myself a few quid, and learned a lot about heating systems in the process.

Cheers Roger!

-Will