Can this Y plan wiring be fixed?

[KnappGate]

I've inherited a Y plan installation that looks like a bit of a mess, but more or less works. The only problem is that the CH won't run unless the time controller has the HW switched on. The time controller has separate setting for CH and HW, the HW will run OK with the CH switched off, but the CH won't run with the HW switched off.

After a few hours of head scratching and working out what is connected to what, I think I know what the problem is: there is no "HW OFF" output from the time controller; the controller has an "HW OFF" terminal, but it is not connected to anything. From my limited understanding of Y plan, this HW OFF signal would normally connect into the cylinder thermostat and provide the "HW OFF" input to the mid position valve when the timer controller had the HW switched off. As it is, the HW OFF is only live when the time controller switches the HW on and the tank thermostat is in the hot position. This exactly matches what actually happens.

Now I want to be able to run CH without the HW on. I assume the right way to do this would be to run an extra wire from the time controller "HW OFF" output to the tank thermostat. It's not practical to do that because I don't have any means to route additional wires to the time controller without digging up plaster, and I'm not willing to do that.

So I'm wondering: Is there any devious way to connect up a Y plan so that it runs CH and HW independently, without a HW OFF output from the time controller?

The reason I'm worrying about this now is that I want to fit a programmable room stat. To work properly, that would need the time controller to switch the CH on permanently and use the room stat to bring in the heating when needed. But as it stands, this won't work unless the HW is also on permanently.

My fallback plan is to disconnect the CH output from the time controller, and supply the room stat directly from the live supply to the time controller. This frees up a wire to use as HW OFF, and I have enough 'spare' wires in place to connect it the rest of the way to the tank stat. This seems quite feasible in practical terms, but feels like a nasty bodge.

How horrified would you be to see a Y plan system with the room stat bypassing the time controller?

Sorry for the long post. If you're read through this far, thanks for your perseverance. I'd be grateful for any suggestions about the best way to sort this mess out.

Edited to add:

In case it's relevant, the time controller is a Danfoss FP715, the existing room stat is a Honeywell one like the one shown here, and the mid position valve is a Honeywell V4073A.

I'm using this diagram as the basis for understanding how Y plan should have been wired up.

 

[matt1e]

yes, use a relay to a simulate a hw off signal for the valves grey
I will do you a sketch if you need one

Matt

 

[KnappGate]

yes, use a relay to a simulate a hw off signal for the valves grey
I will do you a sketch if you need one

Matt

That's an interesting idea. I can understand how it would need to be connected, but I'm finding it a bit tricky to find a suitable relay. There are plenty of very big multi-pole relays which are overkill for this job (and also rather too big to fit in conveniently) but very few small mains powered relays. At the moment the most likely one I can find is from Maplins of all places (hobby electronic suppliers) who claim to sell a small SPDT mains relay for a few quid, but are inconveniently 'out of stock'. If you have any suggestions for UK based sources, they would be gratefully received.

 

[mikely]

My fallback plan is to disconnect the CH output from the time controller, and supply the room stat directly from the live supply to the time controller. This frees up a wire to use as HW OFF, and I have enough 'spare' wires in place to connect it the rest of the way to the tank stat. This seems quite feasible in practical terms, but feels like a nasty bodge.

How horrified would you be to see a Y plan system with the room stat bypassing the time controller?

That's quite normal when using a programmable stat. See below, taken from a Honeywell guide:

Y plan wiring schematic

• mikely
• 3 Feb 2010
• 2

 

[Agile]

Perhaps I am missing somethings?

A programmable stat takes over the functions of a timeclock ( programmer ) AND a room stat. Both these items then become redundant!

Although properly wired it becomes irrelevant but there is no reason NOT to have HW timed to be on when the CH is also on.

Its often forgotten that the HW is thermostatically controlled and once up to temp will be turned off.

The system is most efficient when heating both CH and HW at the same times rather than having the boiler runing just for one purpose!

Tony

 

[KnappGate]

Although properly wired it becomes irrelevant but there is no reason NOT to have HW timed to be on when the CH is also on.

Its often forgotten that the HW is thermostatically controlled and once up to temp will be turned off.

The system is most efficient when heating both CH and HW at the same times rather than having the boiler runing just for one purpose!

Tony

Once the programmable stat is fitted, I would leave the CH switched on at the timer 24 hours a day 365 days a year and let the programmable stat decide when to actually turn the heating on according to my configured temperature profile and the actual room temperature. I don't want to have the HW system switched on 24 hours a day 365 days a year keeping large quantities of water hot for no reason. It's quite noisy and would be intrusive at night, as well as expensive.

 

[KnappGate]

That's quite normal when using a programmable stat. See below, taken from a Honeywell guide:

Y plan wiring schematic

• mikely
• 3 Feb 2010
• 2

Thanks Mikely, that's very reassuring. I think I like this approach better than wiring up extra relays, although the suggestion about the relay was much appreciated and is still being kept in reserve.

 

[KnappGate]

Encouraged by the advice here I've bitten the bullet and rearranged the wiring as suggested. The room stat is now permanently live (subject to the overall CH power switch and fuse) and I have added a "HW OFF" wire between the time controller and the mid position valves "HW OFF" input. I can now run CH and HW separately or together, and the system is working exactly as designed.

Thanks all for the help.