Plumbing question: No hot water

[Andy Johnson]

Hi,

I know very little about plumbing, and am looking for some advice on diagnosing a problem.

We have what I suspect is a fairly average heating system. A Potterton Neataheat boiler and controller box downstairs in the kitchen and a hot water storage tank upstairs in the bathroom. The pump is next to the tank and feeds into the tank and radiator system via a couple of motorised three-way valves.

A few of days ago we stopped getting hot water through the taps. Here's what I've obsrved:

- If we turn on only the heating then the boiler fires up, the pump starts, and the radiators heat-up.

- If we turn on the heating and hot water then the boiler and pump star and the storage tank fills with hot water. The 'lever' on the back of the motor controlling the heating valve moves, so I think we get hot water in this case because the heating-circuit valve is opening on command and the pump is moving hot water through both valves.

- If we turn on only the hot water then the valve motor appears to open the heating circuit valve, but the pump does not activate. Neither does the boiler.

I'm hoping for some advice on what could be causing this. The valves, pump, and boiler appear to be okay. I suspect that the thermostat on the hot water tank is faulty, and the pump and boiler aren't starting because the system thinks that the tank is full of hot. Thats just a guess, though, because I don't know much...

I'd be grateful if anyone can suggest things I should check to solve this. In particular, how to test the tank thermostat. I'm fairly proficient with electrics, and can get access to a test meter if necessary.

Thanks, Andy

[John]

Forget the tank thermostat - if the motor in the valve is energised then it must have a supply to it. the most likely fault is the microswitch in the valve head has failed or the conductor (usually orange and grey wires) from it to the terminals in the junction box are open circuit. (what make of motorised valve do you have?)

[Andy Johnson]

I'm not sure what make the valve is - I can't see any identifying marks on the body of the value, but access is rather cramped. Also, someone (not me) has painted it. The metal seems like plain brass (no zinc finish).

The motor is a synchron motor. About 1 1/2" high. Round on the top and tear-drop shaped on the base with a small toothed wheel emerging slightly off-centre. It looks like this:

It sits under a metal cover above the valve.

Are you saying that when the valve opens fully it trips the microswitch which turns on the pump and boiler? If so, would it make sense to disconnect the microswitch from the junction box and use a continuity tester to see if trips when I turn on the hot water? Or am I still not understanding? :)

Andy

[Set Square]

I'm slightly confused by your description - because you would normally have ONE three-way valve or TWO two-way valves, but not TWO three-way valves. So I will assume they are two-way valves - each having just an inlet and an outlet for the water.

If I am right, you have an S-Plan system. You can see both a schematic diagram and a wiring diagram by looking at S-Plan in

principle of operation is that the motorised valves are controlled by the programmer and thermostats (room stat for the CH valve and cylinder stat for the HW valve). Each valve has an additional set of contacts which close when the valve is fully open. These contacts are totally separate from the motor wires, and are used to turn on the boiler in pump. So as soon as either (or both) valves open, the boiler and pump start up.

From your description, it sounds as if your HW valve is opening (so the cyl stat is ok) but it isn't turning the boiler and pump on. However, if the boiler and pump are turned by the CH valve, the hot water gets hot because the HW valve is open.

So there appears to be a problem with these so-called volt-free contacts in the HW valve. Either they're not closing, or something has become disconnected.

Armed with the wiring diagram from the above reference, you should hopefully be able to isolate the problem.

[Set Square]

You're on the right lines. See my other post. If it's a Honeywell valve (and probably if it isn't!) it's the grey and orange wires you need to be looking at. The internal contacts (micro-switch) should connect these togther when the valve is open. So you could disconnect the remote end of these wires and then use a continuity tester between them. There's probably a junction box in the airing cupboard with all the connections in. [But be careful working on mains electrics - and get help unless you're sure what you're doing!]

[Andy Johnson]

Set Square,

Thanks for replying - you've made a lot of things clearer. And you're right. I have two two-way valves.

I'm not clear what the 'volt-free' contacts are. Is this the microswitch in the motor housing that John mentioned, or is it part of the valve itself? I removed the motor cover, and unscrewed and lifted-out the motor, and I could see a green plastic microswitch underneath with a kind of cam that nudges it as the valve turns. Is this what you mean?

Andy

[Sparks]

Yes,

All the heating controller and thermostats do, is control the opening of these valves, the valves then trip the little switch inside themselves, and turn on the boiler/pump.

So..

You tell the heating controller you want to turn on the hot water, so power is applied to the thermostat strapped to the tank. The stat is not satisfied with the current water temperature, so sends the power along to the valve to open it.

The valve, when it opens, presses a small switch inside it's casing, to turn on the boiler and pump (It is done like this, so if the valve gets stuck in the off position, the boiler does not fire up, and have nowhere to pump the water)

Armed with this info, if you open the Hot water valve, and see what wires are connected to the little switch, then, when the valve is open (and only then!) you could try shorting these two wires together (THEY ARE CHARRING MAINS VOLTAGE BE CAREFUL!!) If this starts the pump and boiler, then you have a faulty switch if not, you probably have a loose connection somewhere! This is what I would do, as I am happy dealing with live wires, however if you are not...

Set the programmer to HW and CH Set the CH room stat high (So it wants the boiler on) Set the HW stat high, so it too wants the boiler on Turn everything off with the main CH fused spur. Make sure everything is now dead before proceeding! Connect the wires going to the possibly faulty microswitch together Turn it all back on (As the programmer is set to CH and HW, and the stat is not happy about the temperature in the house, it all should fire up) Make sure the HW valve has opened set the room stat low or set the programmer to HW only - this should close the CH valve, but hopefully keep the pump and boiler running. If so, then your microswitch in the HW valve is shagged If not, and it goes off, then there is a problem with the wiring somewhere!

Now turn it all off, and put it all back how it was. (Remember the HW Stat!) Whatever the outcome DO NOT LEAVE the two microswitch wires connected, or you may damage something!

Sparks...(Who has never wired up central heating (yet), but thinks he knows how it all works :P)

[Set Square]

Yes - it's electrically separate from the motor supply - and can be used to switch anything that needs to be switched when the valve is fully open. In an S-Plan configuration, it is used (in parallel with the contacts on the other zone valve) to switch the boiler and pump on. When the valve is open, the cam should operate the switch in a way which closes its contacts.

[Andy Johnson]

Thanks to everyone who left comments and help - very much appreciated!

Seems like its probably the microswitch. I'll have a look at it tonight.

Andy

[Christian McArdle]

It is also just as likely to be a loose connection, such as in the terminal box where the zone valve switch contacts are paralleled together.

Christian.

[John]

Nice to see someone with a clear appreciation of the operation and a good idea of how to test it. In other words - exactly so