No hot water - strange low voltage

[SixSevenEight]

Hi,

Last night as a way to see out 2020 I ended up with no hot water in the house...

I've got an S Plan system with Tado extension kit and TRVs etc - which has been working absolutely fine for over a year since install.

I managed to get some hot water by manually opening the HW 2 way valve for now.

I got the multimeter out this morning to check the voltages in the Honeywell wiring centre and I've got some odd results - and I don't know if its the HW Honeywell 2 port valve at fault or the Programmer unit.

At the Programmer connections (left to right) I get:

1. HW Off - No wire
2. HW On - Brown - 80V if the system is off, 102V if CH on, 240V if HW on, 240V if CH+HW on
3. CH On - No wire*
4. Earth
5. Blue - 0V always
6. Brown - 240V always

*There's a separate connector block in the wiring centre I believe that drives this, my CH is working normally.

At the HW valve connections in the wiring centre I get:

Key is - System Off / CH on / HW on / Both on

Brown - 80V / 102V / 240V / 240V
Grey - 240V / 240V / 240V / 240V
Blue - 0V / 0V / 0V / 0V
Orange - 0V / 240V / 0V / 240V
White - 0V / 246V / 0V / 240V

Note above readings taken without manual override enabled.

When HW is called for I don't hear the motor turn.

I also checked the Cylinder Stat, and with HW on I get:

Key is - Stat set high / low
C - 240V / 240V
2 - 240V / 240V
1 - 240V / 103V

My gut feel says it's the 2 port valve, would this be causing the odd low voltages appearing due to leakage between the grey wire and other wires?

It appears to be a newer type valve (with the lump on the top) - therefore based on my research I should be able to switch the valve head over but leave the valve insitu?

Typically this happens on a Bank Holiday....

Any help appreciated.

 

[SixSevenEight]

 

[stem]

You can get all sorts of odd voltages because of the number of interconnected components connected in the system. The most common failure that causes what you describe is the synchronous motor inside the valve.

If you are electrically competent and can do it safely, its quite easy to test the valve by disconnecting the L (brown) and N (blue) wires from the motorised valve black lead, and with a bit of extra flex connect them to a plug with a 3A fuse in. Plug it in and if the valve opens (the manual lever feels loose and floppy when moved) the valve motor is OK, if it doesn't open (there is resistance when the lever is moved) it's faulty.

 

[SixSevenEight]

Thanks - I think that’s already effectively been tested, as when the HW is On the brown is getting 240v and the valve isn’t moving.

Screwfix is open again tomorrow, I’ll try and get a replacement head and wire it up.

 

[endecotp]

I got the multimeter out this morning to check the voltages in the Honeywell wiring centre and I've got some odd results

If you think that everything in a circuit should be on/off, i.e. every voltage should be more or less zero or 240 V AC, then an intermediate voltage probably indicates a wire that is not connected to anything.

If a wire runs parallel to another there is a small parasitic capacitance between them. Because your multimeter has a very high input impedance (which is normally a good thing), you get a voltage.

Testing using something with a lower input impedance, e.g. a test lamp, can reduce the confusion.

 

[oilhead]

A new synchron motor is cheaper and easier to fit than a complete head. If you remove the cover, and restore the power to the valve you can see if the motor is operating.

 

[ianmcd]

Note above readings taken without manual override enabled.

The manual override does nothing whatsoever electrically, it is merely there to fill the system with water without the boiler and pump coming on

 

[stem]

The existing MV head is slightly different to others, in that it has a white wire connected to the normally closed microswitch contact...

...which has been 'parked' in the terminal provided for the white wire from a 'Mid Position' valve.

This white wire is not necessary for an S-Plan and if your replacement head only has orange and grey wires for the switch that will be fine.

Of course if you just replace the synchronous motor as @oilhead suggests, that information is irrelevant.

I've just noticed that the motors are £24 at Screwfix. That seems to have gone up. I think the last one I bought from there was about £10

 

[SixSevenEight]

Going to pick up a new motor tomorrow and see what happens.

Do the wires on the motor fit onto terminals in the head, or will I need to do a bit of cutting and crimping?

 

[stem]

The new motors usually come with a couple of new crimp connectors.

 

[SixSevenEight]

Guessing that isn’t supposed to look like that...

New motor fitted and seems to be back to normal so far - thanks!