Central heating wiring question

[Dan S. MacAbre]

My mum and Dad's boiler wants to stay on all the time. I thought the best thing to do, besides just switching it off for now, would be to try and work out how it's all wired together, and then work out possible causes. I've only so far had time for a quick look, but something has struck me as odd. They have two motorised zone valves, but there is no room thermostat anywhere. I'd always thought that each zone valve responded to a thermostat. Is there any valid configuration that has two zone valves, but no room thermostat?

[Muddymike]

Is there a thermostat on the hot water tank? If so is it set higher than the boiler thermostat? If it is that is what is causing the problem.

Mike

[tabbypurr]

No room stat would explain why it's always on :) You need one, and would expect 2 with 2 zones.

NT

[Dan S. MacAbre]

I'll check. This has apparently only just started happening, and neither of them has admitted to changing the tank stat. I don't think they could even find it. There is a timer, which has, AFAIK, worked until now. A mechanical clock type, with a red/blue wheel for manual control in the top right corner. Even when that is switched off, the boiler still lights, but I thought the boiler got its power from the timer.

[Dan S. MacAbre]

I'd have thought so, but if there is one, it's not on any wall anywhere. I'll have to see how it's connected - I've a suspicion that it's powered open all the time.

They say that it's only just started doing it, but maybe they've only just noticed it. Funny thing is, even when you unplug the power to the timer and boiler, the pump still runs. It has its own power supply, and didn't stop till I pulled out another plug in the boiler cupboard. I think the whole thing is probably completely wrong.

[tabbypurr]

Turning the primary temp down could cause it

Wired wrong then.

NT

[tabbypurr]

Sounds like the system is wired up utterly wrong.

A separate overrun to the boiler could explain that.

I suspect so.

NT

[Dan S. MacAbre]

I think it probably is. It's that sort of house - everything you look at has been done wrong. Previous owner had a reputation for bodging everything, unfortunately.

[Davey]

Is there a junction box somewhere near the boiler or 'stat? That might help provide information by identifying the wires connected inside it. Otherwise, you will indeed have to do what I had to do earlier this year, start from scratch with a pad and pencil, and make your own schematic. Keep careful note of changes of colours where wires from different components are connected.

The sudden change in operation could possibly indicate that one of the zone valves has failed Open, thereby telling the boiler to run all the time. Also, what temp. is the boiler thermostat set at?

Look for as much information as possible on components, ie Manufacturer, model, etc so that you can look them up online.

But two zone valves and no room 'stat is a real puzzle. What controls them? I am intrigued.

[Dan S. MacAbre]

I think it's in the void, along with the pump. I'll have a look at it this evening.

I did that with my own CH maybe ten years ago. Quite instructive.

ATM, the thermostat is switched to off. Still comes on, though.

I'll find out later.

[Davey]

That does not sound right at all. You definitely need to start sketching, including the boiler internal wiring! There may be no limit to the bodges done to the system.

[Dan S. MacAbre]

Yes, this seems the oddest symptom to me. I've no doubt that it is wired incorrectly, but as long as the boiler stat was working, no-one noticed. And, it seems, the timer did actually switch it on and off as required, since mum says that putting the timer switch in the permanently off position no longer does anything. Now, perhaps, something is broken in the boiler. Maybe some frost protection device or a PCB, if it even has one? It's an old Glow-Worm thing, but to me that means there should be fewer things to go wrong :-)

[tabbypurr]

Well at least the worm is still glowing.

NT

[Dan S. MacAbre]

:-) Too much, right now.

[The Natural Philosopher]

Reminds me of years and years ago. Bloke said 'my car battery keeps going flat ever since I fitted these air horns'

Well there was supposed to be a relay, that the horn push would activate, that would then drive the air horns. Somehow the relay was permanently on across the battery and the contacts were in series with the horn push and the horn itself.

So it worked...BUT....

...a prime example of swap the wires till it works...

[Graham.]

It sounds as if they have a Randall time clock controlling both of them, probably got a cylinder stat in the DHW circuit. I wonder if they have TRVs on the rads?

No room stat? Well gas was cheap and installers were stingy in those days.

The Randell does allow the DHW to run independently of the CH, but not vice-versa, even though a typical fully pumped "S" plan does allow it given the chance.

Dan, One valve should open when "hot water only" is selected on the time clock and both should open when HW+CH is selected

When each valve is open (the motor remains energized and stalled by design) a cam operates a micro switch and powers the gas-valve and pump. The two micro switches are in parallel. My guess is one of these micro switches is stuck, or a valve is stuck open

[Dan S. MacAbre]

Good grief!

[Dan S. MacAbre]

I searched that, and they do indeed have one of those. Big wheel in the middle, little red and blue one in the corner.

So it's possible there is a workable, if less than optimal, configuration that can work without the room stat?

Okay, I think I can easily check that, since there are two of them to compare against each other.

[Roger Mills]

Depends on the system. If it's an S-Plan system, each zone valve should be driven by a thermostat (one room stat and one tank stat) and the boiler is then switched on by contacts on the zone valves which close when the valves are fully open. It's quite a common fault for a valve to stick open and turn on the heating even when there's no demand from either of the stats.

[Roger Mills]

That certainly sounds wrong! the whole system should be powered from a single switched fused spur so that it can be isolated in one operation.