Central heating wiring question

[Dan S. MacAbre]

Okay, I'll check the zone valves later. I'm curious to see how the CH one is controlled with no room stat.

[The Natural Philosopher]

Absolutely. I have run like that. Its not ideal BUT with TRV's as they shut down one by one,. assuming you have a bypass so some flow occurs, what you end up with is a very hot short loop that isnt going to the rads at all, and the boiler thermostat itself will cycle on and off to keep that loop at the boiler stat temp - say 60C or whatever.

I will say as general advice, that if you are reasonably knowledgeable about matters electrical. faced with a bodged rats nest, often the simplest thing to do is disconnect everything and document where wires go, and rewire the whole system according to one of the 'standard plans'

Adding an overall stat is not hard with todays RF ones. Neither is fitting TRV's to a drained down CH system.

You may need to add a motorised valves but that's not rocket science, and a documented control box where all the wires come together is a nice touch.

[The Natural Philosopher]

I think that is actually mandatory with building control these days.

[Dan S. MacAbre]

Yes, the manual for the boiler has several wiring diagrams, two of which use two zone valves. All of the diagrams start with the power coming from the timer. This thing actually has power for the boiler/timer coming from a round-pin socket/plug. All the other sockets in the house, and even the other one that sends power up into the void to the pump are square pin. It's an 80's house, so the guy probably just had a spare round-pin socket, and used that. Or something.

[spuorgelgoog]

Or has it fallen off the HW cylinder so is measuring air temp not cylinder temp?

Owain

[Dan S. MacAbre]

Fortunately, the boiler, pump, and valves are physically close, electrically linked through a big hole in the boiler cupboard ceiling. So working out how it is wired shouldn't take too much time. Each radiator does have a TRV, and I'll just have to hope that there is, indeed, a bypass somewhere.

Interesting to know that the room stat isn't strictly necessary.

[Dan S. MacAbre]

Another thing to check :-)

[Davey]

The system I had to work out earlier had apparently been attended to by a professional, but it had a dead room 'stat, and had lots of other weird wiring mistakes. Although it had normal separate DHW and CH systems, if one came on, they both came on. I worked out what went where, then figured out what wires to move to where to make it work. It was the thread "Heating System Puzzle", if you wanted to look it up.

[Martin Brown]

It could be that the tank stat has failed and never says OK any more... my parents did that once with scalding water from the taps.

It is an odd failure mode for midsummer. More often you get the opposite that the CH is satisfied so it fails to heat the hot water.

Could be other timers but then you would have more than one system timer but there should be thermostats somewhere. My parents is like that with thermostats and zones being switched at night to only heat the bedroom in use (and with a lower set point).

The usual cause of sudden changes resulting in either mad hot hot water or insanely hot radiators is that a valve has stopped opening or closing when commanded to do so by the thermostat (or the thermostat is dead).

Find the thermostats and then you might be able to test it by altering the settings.

[tabbypurr]

It is possible if you have all TRVs, but the result is varying temp and wasted gas. It's a right bodge.

NT

[tabbypurr]

Nothing wrong with round pin plugs, they're occasionally used to prevent other things getting plugged in somewhere.

NT

[Brian-Gaff]

If its only just started to occur, I'd think incorrect wiring an unlikely cause, more like a fused together contact in the clock itself. I've seen these actually brake off and wedge it all up solid, but this was years ago, how old is this boiler. If you see Fred Flinstone as the service engineer, its probably time for a new one! Brian

[Graham.]

Precisely what I must have been doing yesterday, 39 years ago, according to this!

I drew it as a schematic rather than a terminal to terminal representation, because I understand diagrams better when drawn like this.

Randall time switch and 2X Honeywell V4043 valves, apart from the room stat it's probably exactly what the OP has got, and I hope it helps.

[Ash Burton]

Nothing wrong with round pin plugs per se, except that as the plug has no fuse protection in it, the circuit fuse or mcb will most likely be rated in excess of that required for the boiler/timer unit and possibly the flex connection to it.

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[Roger Mills]

It *is* necessary to meet current regs - but wouldn't have been when this system was installed.

Current regs require "boiler interlock" - which means that the boiler gets turned off when there's no demand from any of the zones. If you just have TRVs and no room stat, the boiler wastes energy by keeping itself hot when it doesn't need to.

[Davey]

My one as described for my project started out like that, noting cables and their colours, and then changed to a more formal wires-and-terminals version as the wiring became clear(er).

One point worth making for the OP: Make sure you know where every cable goes to at both ends. You can find devices of which you were totally unaware! (believe me).

[Johnny B Good]

The boiler stat is left to do the temperature control. When TRVs are fitted on all bar one radiator (or a heated towel rail) to provide a permanent return loop, it works tolerably well although gas consumption will be trivially higher than one with a room stat to shut off the boiler completely when the room selected to 'pay the piper' has reached its target temperature.

Upgrading a 30 to 50 year old CH & HW system using the classic indestructible cast iron boiler with a room stat or two won't provide much improvement in gas economy (by the time the stat(s) call for more heat, the boiler and pipework will have cooled down (not stone cold, admittedly but nevertheless enough to cause a lag in raising the radiator temps) requiring the burner to run for an extra minute or three for the system to come back up to temperature which will partially at least (if not significantly) nullifying the efficiency gain of using room stat(s).

If you have a system using the new fangled lightweight destructible condensing boilers, a room stat or three makes a lot more sense. On an older system with a cast iron boiler and TRVs, the economies of adding one or more room stats becomes questionable. This is one of those rare cases where the saying, "If it ain't broke, don't fix it!" does actually have some basis in fact. :-)

[bert]

The boiler gets its power through the valves which get their power from the timer (and normally through a room thermostat). Sounds like one of the valve heads is U/S. I don't quite see the point of zone valves without individual timers or thermostats. Are you sure they are not hot water/central heating controllers?

[Davey]

And I was taken to task a while ago for saying the same thing...

Oh well, that's life.

[Dan S. MacAbre]

Well, I've had the floorboards up, and made a few drawings, and I think I've got my head round it now. There is no room thermostat - the timer output for the CH goes straight to activate the motorised valve, so it's basically acting as a relay. I can understand this if the timer output is not capable of driving the pump and the boiler; however, it's a mechanical timer, so I don't think it's /that/ delicate :-)

I see from other posts that not having a room thermostat is valid, if you rely on the boiler to respond appropriately, however, mum and dad are likely to turn it high in Winter, which seems to be a bad idea in this configuration. It would be easy to add a room stat into this wiring, I think.

I was a bit confused because mains sockets and plugs have been used to make some of the connections. So what I thought was the main power feed to the whole thing (because it is right next to the boiler) is, in fact, just the connector coming down with the power from the motorised valves.

Anyway, it's obvious now that a knackered CH motorised valve will keep it all going regardless of what the timer etc. does, since they have an uninterrupted feed from the mains. So this is what I need to check. I suppose having it open all the time the heating is on doesn't help. The actual valve underneath it feels stiff, so it may just need freeing up.

But the whole thing is a bit ropey. The heart of it is a terminal block dangling in the void space. I need to at least put it in a box, or something.