CH will only come on if the HW program is on...

26 Mar 2012
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United Kingdom
Hi Guys

Myself and my Dad have had a search around here and found a few relevant posts but I am hoping a heating engineer may be able to confirm our issue.

Brother recently moved into a house with a vented and fully pumped CH and HW system complete with 3 way valve.

On initial inspection it was found that selecting the heating on the control panel also switched on the hot water. This was then found to be due to the fact that he control panel was set to a gravity fed hot water system which this system is definitely not.

Having changed the control point to a pumped system (allowing independent selection of CH & HW) and also the cylinder stat, which was faulty and stuck in "call" mode, the hot water then worked fine, with the 3WV diverting water ONLY to the cylinder when the cylinder stat called for heat.

However, on selecting the CH on at the control panel with the HW not selected, the boiler and pump do not fire up, although we think that the 3WV does open as the motor is heard.

If the HW and the CH are selected on the control panel, with the room stat calling for heat and the cylinder stat NOT calling for heat, the boiler fires, the pump runs and the 3WV diverts to rads only. If the cylinder stat then calls for heat also, the 3WV diverts to both rads and cylinder. Likewise, when the room stat stops calling for heat, the 3WV diverts back to the cylinder only (providing HW is calling for heat).

It seems the CH will work if the HW is selected, whether HW is calling for heat or not, but will not work without the HW selected.

We have identified that this must be a 3WV microswitch issue or a wiring issue. We have mapped out a wiring diagram which I will be looking at tonight but initial inspection looks OK. Would you therefore conclude this to be a 3WV micro switch issue? Is it possible to be a microswitch issue in this case?

Further testing may be required!

Thanks for any help anyone can give!!
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most likely is when installed there was no HW off wire from programmer, probably had a gravity hot water system to start with.
Ah, yes, you are correct in that there is no CH or HW "off" wired in on the back of the wall plate. However, when the control panel was on gravity system, with HW on only and cylinder calling for heat, the 3WV was diverting to both tank and rads, not just tank...?
This might help:

With no power at all, the valve rests in the HW only position.

When the controller and tank stat - working together - call for HW, the valve doesn't have to move. Power from HW-ON at the controller goes through the tank stat to run the boiler and pump.

When the controller also calls for CH and the room stat agrees, CH-ON from the controller goes through the room stat to the valve motor and drives it to the min-position. The HW-ON demand is still running the boiler and pump. Every time the room stat switches - which it does at regular intervals - the valve has to move. You can prolong its lfe by programming CH and HW to run at different times. :idea: :idea: :idea:

When the demand for HW goes off - whether at the controller of the tank stat - that's where the extra wire comes in. HW-OFF goes to the valve. If and only if the valve is already in mid position, HW-OFF drives it all the way over to the CH only position. But what about the boiler? :?: :?: :?:

An extra switch inside the valve routes CH-ON back to HW-ON and from there to the boiler and pump. As long as HW-OFF holds the valve right over, CH-ON and the room stat will control the boiler and pump. The valve does not have to move.

Did that make sense? :) :) :)

However, when the control panel was on gravity system, with HW on only and cylinder calling for heat, the 3WV was diverting to both tank and rads, not just tank...?

Are you sure the valve actually moved? It doesn't take much leakage to make a radiator feel hot. If it did move, the wiring was wrong.
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Thanks for the help guys, much appreciated.

Will compare our wiring diagram with a working diagram tonight and I suspect carrying out further tests. We are for sure missing a HW off from the controller and can't remember off the top of my head where the tank stat off was wired to.

At this point I'm not ruling out a wiring issue (other than lack of HW off), a dodgy 3WV or a combination of both!! But it gives us more to check out!

Thanks again!
Hi guys

As suspected we are missing a HW off from the controller but everything else is correct, fully explaining what is going on.

We were defiantly getting A LOT of water to the rads previously in HW only, but to be fair, the controller was also set to 'linked' mode so I suspect the CH was also calling for heat and we didn't realise.

Thankfully the pump live is currently connected the boiler live beind the controller plate but could be connected to the boiler live in the airing cupboard, giving us a spare wire up to use as HW off to fix the system. Beggs the question as to why whoever installed this didnt do this in the first place.

Hopefully problem solved, we will find out tonight.

Cheers guys.


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