Water, but no heating. Possible 3PV?

8 Jan 2016
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United Kingdom

I've have a question that might help me clarify what the fudge is going on with my system, in particular a Honeywell 3PV:

When I switch DHW off and turn the stat up for CH the 3PV motors open but the boiler isn't firing up. I've checked for 240v on terminal 7 from the orange wire and I only get around 40v. On terminal 6, I have about 70v.

It's my understanding that when the 3PV motors open, I should get 240v from the orange on terminal 7 to keep the boiler fired up.

Am I right in thinking the valve head has gone, or is there something sinister going on (before I go out and buy a replacement)?

Thanks in advance,

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If the valve moves to the correct position, there is 240V to the white wire (CH on), then there should be 240V coming out of the orange wire (Boiler & pump on). If not the valve head is faulty. See this helpful diagram of how the switching works inside the valve head.
Ahhhhhh. That makes more sense. Very helpful diagram by the way Freddo.

When I took the cover off and watched it motor over, everytime it would only make contact with S1. The only time when it made S2 is when I motored it by hand but then it would bounce back as the lever rested in the gate.

Could it be more cost effective (Read gash) to be able to realign the gear teeth so when it does motor over, it moves far enough to make S2? (I only say this because when I had the head off and motored it by hand, the teeth kept slipping)

Cheers again,

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HW off, CH on;

Grey=around 70v
Orange=around 40v

I'm really tempted to try and make S2 with something non conductive and see if the boiler fires up
You need 240V on the Grey when HW is off. Your fault is elsewhere and your valve is fine. I expect you are not getting HW Off signal from programmer as it is faulty or there is a loose wire somewhere.
Well.......that's just pi$$ed on my chips.

I was certain it was the valve head as S2 isn't making when it motors over
It won't make S2 because without power to the grey wire the valve will sit in the mid position and not go to CH only. See here and click the link about mid positon valve operation and it shows what each wire does.
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Just had it set to CH only, valve motored and made S1, I made S2 with a non metallic pokey thing, a relay inside the valve head clicked (A LOT) and the boiler fired up until I let go of S2.

I've checked and checked again for loose wires and I'm certain the programmer (Honeywell ST9400C) is ok as the relay clicks every time on/off is selected and the voltage disappears from the relevant terminal
Yes but voltage needs to appear at the hot water off terminal for the valve to work properly. HW off is terminal 1 on the programmers base and connects to the valves grey wire along with HW satisfied from the cylinder stat.

To prove it switch the CH and HW on at the programmer, turn the clyinder stat down so it clicks off, and make sure the room stat is turned up. The valve should move to the CH only position and the boiler and pump should run, as S2 will now have closed. If it does, the fault is either the programmer not sending power to the HW off terminal when HW is switched off at the programmer or there is a break or loose connection somewhere between termianl 1 and the grey wire on the valve.
You're a star Freddo, but I'll to give it a shot in the morning as the nipper is in bed and the airing cupboard is in her bedroom.

From memory of fiddling around with it earlier, I DID try that and the valve didn't move. It would only move with HW off. As soon as HW is switched on, the valve motors back.

Thanks again, standby for more stupid comments tomorrow.



I've just re-read this thread and realised how novice I sound.

Now having more than a hunch of how the system is supposed to operate, it is clear that the programmer isn't playing ball so I'm going to check each terminal properly to confirm before I splash out my hard earned money on a new programmer.

Thanks again for your patience and help,

I have no problem spending time helping someone learn who is interested. Central heating wiring isn't difficult when you know how each of the parts function, and follow the wiring diagram carefully. Yet I have come across a number of heating systems that have not operated properly for years, as they have been originally wired wrong.

If you are not totally sure the programmer is faulty or can't easily/safely test for power at terminal one as it's behind the programmer, you can easily check the HW off wire is continuous and that everything else is good by:
  1. Mains power off, and programmer off of base.
  2. Make note of wire going to terminal 1 of programmer (mark it with tape if it isn't obvious by colour.)
  3. Disconnect the wire that was in terminal 1 and connect it into terminal 4 with the other wire in terminal 4.
  4. Programmer back on base and power on.
  5. Set CH only on and check room thermostat is turned up, and the valve should move fully over to CH, and the boiler and pump should run. If it does you definitely need a new programmer.
  6. Mains and programmer off again and move wiring back to how it was.
If it doesn't fully open the valve and switch the boiler and pump on, then there is a break somewhere in the HW off wire. The cable between the programmer and wiring centre might be joined somewhere and have a poor connection for example, or the wire may have been damaged by drilling or nailing through the cable!

If you do need a new programmer, I recommend one of these.
That sounds like my kind of fault finding........

I'll try it later when the kids aren't wrapped around my ankles. I was thinking of splitting the unit apart and sticking the unit back in place by the pins so I can get my probe down the back of it while it's still on.

On a side note, I checked voltages on the white, grey and orange wires in the 3 configurations and I think this will confirm your suspicions:

HW only, White=0v, Grey=240v, Orange=0v

HW&CH, White=0v, Grey=240v, Orange=0v

CH only, White=240v, Grey=73v, Orange=42v

Am I right in thinking that the reason there is only 73v on the grey during CH only is due to (apart from terminal 4 on the programmer not outputting the required 240v) the rectifying diode/resistor configuration.

Re: the programmer, that is exactly the one I have now, and I'm bidding on one on fleabay as we speak.

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