Bizarre CH/HW wiring - any ideas?

[Mary Hinge]

Been looking at the heating/hot water wiring, no particular reason other than the boiler seems to take up to 5 mins to fire when CH/HW is activated. System working OK other than that.

Looks like a fully-pumped S-Plan with Zone Valves on the 22m returns rather than the 28mm flow, boiler is a Potterton Kingfisher II 30KW, also got tank, room and frost stats. Pump is wired parallel to boiler so not separately controlled or anything. Rather old installation, Zone Valves look ancient and I would guess the boiler must be pushing

20 years old, all open vented.

Wiring is a real mess; doesn't use a standard wiring centre but instead three double pattresses with covers, wires go everywhere. Rather than the standard S-plan wiring for some reason there seems to be some sort of 240v relay (?), also for some reason the white wires for both zone valves are used, as are the CH/HW off terminals in the programmer, and the CH off terminal in the CH stat.

Can't really make head nor tail of the wiring especially as there is a relay to confuse things, any bright sparks (or plumbers ;o)) seen anything like this? Why would you use a relay (assuming that's what it is) in a CH/HW circuit?

TIA,

Andy

[Christian McArdle]

My guess is that the relay is probably a 5 minute delay timer. Probably installed to prevent short cycling. Alternatively, the lot was wired up by a cretin.

Christian.

[Ian Middleton]

I had fiited a similar arrangement in my last house. The relay is to switch on the boiler when the CH is called for by theromstat/timer.

Original setup was gravity HW + pumped CH and thermostatic rad valves and had a timer where you moved notches to select times and a switch to select off, HW or HW&CH. I found the control very poor, house too hot or if trying to do HW and CH from cold house would not heat up till HW was getting hot.

So I fitted a independant timer for CH and HW + a room thermostat. The HW timer contact connected to boiler to call for heat and the relay in parallel to call for heat as well. You then connect the timer CH contacts and room thermostat to relay so when the timer and thermostat call for heat the boiler fires up (you also get HW as well).

Worked very well, put HW on for hour in morning before CH on to allow HW to heat up and got a lot more controllable house temperature.

Replacement boiler had two inputs to call for heat so relay was not needed.

[Alec]

My old oil boiler did that because there was no permanently live feed to run the control sequencer to its start position. I added one and it started in 15 seconds. Rgds Alec

[Ed Sirett]

Most like the zone valves are the type which go around a full circle. IIRC these valves need a connection to make them shut off as wellas a connection to make then open. Hence the relay, although they could have made use of the changeover contacts in the stats.

3 double pattresses seems excessive. I'd reckon getting all the wiring for S-plan in one double patress with a 12-way choc-strip. ( Earth and Neutral 2 terminals each as there are more than 4 wires of each on those).

[Mary Hinge]

Could be either, don't really know much about relays and didn't realise they could act as a delay timer. Is short cycling more likely to be an issue with a 30kw boiler heating a ~125 litre indirect HW cylinder?

[Mary Hinge]

It had crossed my mind that this might be some sort of hangover from a previous gravity-fed system. Surely though when converted to fully pumped S-Plan you would rip all the extra wiring out?

[Mary Hinge]

Sounds like you may have hit the nail on the head, like I said the zone valves appear to be ancient, or at least a plumber mate said so: "Haven't seen those in years" etc. They do appear to be making use of the changeover contacts in the stats and programmer as well. The relay does seem to be associated with the white wire from the zone valves - the orange wires pretty much go straight to the boiler & pump.

BTW Ed, you might have got the wiring into one double patress but you need a second one to house the relay ;o)

Andy

[Dave Plowman (News)]

Since you've got a Kingfisher, it sounds oldish. I have one too - and with a couple of relays. To provide a timed boiler over-run. Home made, I'm afraid, so not much help.

The only real way would be to trace out the circuit. I was a good boy and included mine inside the lid of the box. Not that that would help the average repair man.;-)