DIY Doctor UK DIY Forums

I'd like a new programmer for my CH. But before I go and get one I need to clarify somehting.

As far as I can tell, the system is what I think is called a Y plan. There's a boiler and a pump that pushes hot water round both the CH and the heating coil in the hot tank. There's a 3 port valve that I believe is capable of controlling the HW and CH circulation independently.

But the programmer can't control the CH and HW independently. If it calls for CH, the HW comes on as well.

Was that a bit of a false economy by the installer? Would it make more sense to have a prgrammer that does have completely separate CH and HW controls?

sounds like a faulty actuator to me.

Yes there are some old programmers out there that physically stop you from putting the central heating on without the hot water. I presume you have one? The tricky thing is these have internal wiring differences to the normal "Y" plan. The system you describe providing you have stat on the hot water cylinder does indeed seem like a "Y" plan.

Maybe it was all plumbed up correctly, but the sparky who fitted the timer just bought the wrong one, and fudged it? Possibly it's a DIY job and whoever wired it just made it all up.

Before you buy a programmer, assuming that's what you intend, you'll need to examine the wiring first to see what needs to be done to convert it.

Yes there are some old programmers out there that physically stop you from putting the central heating on without the hot water. I presume you have one? [/quote]

Yep. It's an old Sunvic that the previous owner put in--he told me he was a heating engineer, but perhaps not much of an electrician :(


[quote]The tricky thing is these have internal wiring differences to the normal "Y" plan. The system you describe providing you have stat on the hot water cylinder does indeed seem like a "Y" plan. [/quote]

There's a thermostat on the cylinder, yes. And on the CH. The cylinder stat behaves in a way I don't quite understand though. I've identified the HW 'ON' from the programmer and traced it to a core in the flex that goes off to the thermostat. Of the two other cores in that flex, I can see one turning ON (going live) when the tank is too cool, but the other doesn't seem to switch over. It stays live no matter what the temperature of the tank is. I thought the thermostat was a changeover switch.

[quote] Maybe it was all plumbed up correctly, but the sparky who fitted the timer just bought the wrong one, and fudged it? Possibly it's a DIY job and whoever wired it just made it all up. [/quote]

It's a bit kludgy, certainly; the interconnections are through a 7 section choc-block. But it seems to work OK. I just thought that it might be more elegant and possibly cost a bit less to have completely independent controls.


[quote] Before you buy a programmer, assuming that's what you intend, you'll need to examine the wiring first to see what needs to be done to convert it.[/quote]

Well, I've traced all the connections though the choc block and I've found a couple of Y plan wiring diagrams--I just need to compare one with the other and see if they match up.

If the programmer has been replaced since the system was installed, it may be that all the wires you need are there so you may be alright. The problem though is going to be in identifying the correct wires. Not easy! Don't attempt any alteration unless you feel confident because picking up on someone else's wiring will be a challenge. Working out the wiring from the snarl in the junction box will be fun especially if other fingers have been meddling as well.

Don't be surprised at funny wiring. The wiring plans in my opinion are totally over engineered for what they need to do. Just follow the diagrams, and it should work. Ho ho.