Honeywell boiler wiring

[Davey]

I have just downloaded a Honeywell Wiring Guide to help me understand the system in this house, but I have one question that I will ask here: What is the function of the thermostat mounted on the side of the hot water tank, and the motorised valve in pipework near to it? The system is an oil-fired Hot Water and pumped Heating arrangement, gravity feed, with loft-mounted expansion tank. The problem that prompted this is that the house hot water is getting hotter and hotter, even with the boiler thermostat at its lowest position, so I started to investigate what I have, and found the devices mentioned above, with no idea what they are for. The boiler will fire if asked by its own 'stat, even with the local external timer set to 'Off'. I reduced the tank-mounted thermostat setpoint a tad, and I think I have more control at the boiler, but something's wrong somewhere. Any useful help and advice welcome.

[Dave Plowman (News)]

The tank stat and motorised valve allows the hot water temperature to be set at a lower figure than the boiler temperature. Basically, the thermostat turns off the flow from boiler to tank when satisfied.

The likelihood is the valve has stuck or failed. You can sometimes replace just the electrical part (if that has failed) which saves having to drain things down. If it does come off easily, you could check the actual valve moves by hand.

[Graham.]

Typically there will be a second valve in the radiator pipe work controlled by a room thermostat. Also typically, the timer and thermostats do not control the boiler and pump directly, they only open the valves. When the one or both valves are fully open a cam in the valve operates a microswitch. It is these microswitches that fire the boiler and turn on the pump.

Are you sure the DHW is not also pumped?

[Davey]

The boiler is as I have seen in diagrams, there is one heated core, with connections on one side which go to the tank and hot water, and the other side goes to a pump and the radiators. There is only this one pump, the DHW is gravity only. The CH is controlled, successfully, by a room thermostat, no problems there.

I checked the tank 'stat on the bench and it appears to make and break properly. But when it was back in place, and everything connected, operating it had no effect visible at the valve, which seems to indicate a bad valve/operator, yes/no? I can see a gearset which looks as though it is a small cog on the motor shaft, engaging with a large cog on the valve stem, or vice-versa. Nothing moved. It is a Honeywell valve, mentioned frequently in the Wiring Diagram, so I should debug it further, and find out what part of it is bad, I believe. Would this valve failure result in the DHW getting hotter than it should, then, letting the boiler 'stat be the only control? Thanks for assistance.

[Roger Mills]

Chances are that you have a C-Plan system. [Look at the Honeywell C-Plan schematics and see whether they match your system].

When wired correctly and working correctly (either of which may not be true!) it provides independent control of HW and CH systems as well as providing boiler interlock. That is to say, the HW valve will close once the cylinder stat is satisfied, and the pump will stop when the room stat is satisfied. The boiler will shut down when *both* demands are satisfied.

If this is not happening, there's something wrong either with the wiring or with the operation of the valve. The wiring needs to be 100% correct

- making use of the change-over function of the valve's auxiliary switch

- for things to work properly.

[Davey]

Thanks. The C-Plan looks right. go back to the website. It does sound as though the Zone Valve has failed, and is telling the boiler, correctly, that it is still open. But it shouldn't be, is the problem. I will check again, that, with the timer switch on (I can't be sure if it was on or off yesterday), that the valve responds to the tank 'stat. If it does, then it looks as though the microswitch is faulty. Thanks again for all the help, I think I now have enough information to dive back in.

[Davey]

Correction: delete "> go back to the website". It stayed behind after a deletion during editing.

[Graham.]

What is puzzling me is why the water is getting hot at all if the valve isn't energised. There is a lever at one end that allows it to be opened manually and there is also provision to latch it open. When moving the lever you should feel considerable resistance from the return spring and gear train. If there is no resistance the valve is either stuck open, or it *is* energised. Turn off the power and see if it closes.

[Man at B&Q]

I had that.

When the tank is cold, and the programer says ON, the thermostat contacts are closed and energise the valve motor which opens to allow circulation in the primary circuit. The contacts on the motor close and fire up the boiler.

What can happen is that switch in the valve fails stuck closed so the boiler is always on, pumping through the bypass loop (or trying to circulate by gravity in the OPs case) regardles of what the programmer or stat are doing.

MBQ

[Davey]

Answering both messages:

I think the valve is stuck open, making the switch contacts all the time. I found the manual lever yesterday, although I could not find any description of what it did. It moved with zero resistance, with everything powered down. It has only a few degrees of available movement, though, limited by the length of the slot in the casing. How does this translate to 90 degrees of motion at the valve? Is it actually a switch? This will be my first place to resume debug when I get back to the problem. Thanks again.

--=20 Davey.

[Dave Plowman (News)]

Remove the electrical part so you can get at the actual valve and make sure that isn't stuck. Some WD40 and moving it end to end may get things working again - for a while.

[John Rumm]

Have a look at our excellent pages on this:

you can get all the diagrams in a couple of downloadable docs as well:

the system in this house, but I have one question that I will ask here:

First identify the type of "plan" being used, then we can work out the exact function.

[John Rumm]

Its usually a mechanical linkage that rotates a part of the gearing mechanism in the base of the electrical part of the valve. The wet part of the valve may only need a few degrees of rotation anyway. (some simply have a ball on a stick that covers a outlet port - to open it you juts need pull the ball away from the round hole it is plugging).

[Davey]

Thanks, but I think I have already found out what Plan is in use, and what is probably the problem. Read on.

[John Rumm]

Yup, saw that - see my comments elsewhere. (the docs above are still good though!)

[Davey]

Good, that's the way it should be.

I had never considered Heating-only mode, but I can see where it would be useful.

Ah! So the term 'Zone Valve' does not refer to a Heating Zone, but to the Hot Water Tank Zone. Ok, I think, but not intuitive coming to it, ahem, 'cold'.

Not the case here.

That is the case here.

Ok, now I know how it's all supposed to work. Thanks.

[Davey]

>>>>

>>>>

Thanks. See other reply, to Roger Mills. I now have enough information to tackle this with a test meter at the junction box, as a starting point. I will see if the valve is being given the correct instructions, and if it is, then I go deep into the airing cupboard and start opening things in there.

[Roger Mills]

Based on the symptoms which you report, my first port of call would be the mechanical part of the valve. I'd whip the actuator off, and expect to find the shaft seized solid rather free to rotate.

[Roger Mills]

The term 'zone valve' is a generic term for a valve which controls one or more zones. Your valve could equally easily be used in a fully pumped S-Plan system in which case there would two or more of them - one for HW and one for each heating zone. A 3-port mid-position valve is also a zone valve which controls HW-only, CH-only or both together, depending on the position of its paddle.

A C-Plan system is rather a special case, since gravity HW systems are relatively unusual these days - and a single zone valve used in this way enables you to have independent control of HW and CH plus boiler interlock - none of which would otherwise be possible with a gravity HW system.

[Davey]

Maybe. I reported elsewhere that I was able to move the manual actuator with no resistance at all. Does that support that theory? It may be now disconnected from the valve stem, assuming it is a mechanical linkage, not a switch. Thanks.