Adding wireless thermostat to CH system

[Steve]

I have a CH system with gravity-fed HW. The controller has two output terminals, one (nominally the HW connection) connects to the boiler and the other (nominally the CH connection) to the pump. The controller arranges that when there is demand for CH it switches both outputs on. I assume this is all pretty standard.

I've got a wireless room thermostat/controller (Towerstat RF) which I want to use to control the CH. It only has one output (it has connections for live, neutral, and output). It has two temperature settings, for day and for night. So I'm thinking I will have to use it alongside the existing controller so that I still have a timer for the HW.

I can't work out how to wire everything together so that the new one controls the CH and the old one the HW. If I wire the new one to the pump and boiler, and the old one just to to the boiler for HW, then the pump will come on with the HW as they'll effectively be wired together. I thought that I could set the old one to be on permanantly for CH, and take the output of that to the new controller. But then the boiler will be on all the time. Basically I can't see a way of getting thermostat controlled cental heating 24 hrs a day without the HW being on all the time. I guess this wouldn't be so much of a problem if there was a thermostat on the HW tank, but there isn't.

Can I achieve what I want with the kit I've got?

Steve

[Roger Mills (aka Set Square)]

No - but your system is very wasteful of energy, and worthy of an upgrade, anyway!

The simplest upgrade for a gravity HW/pumped CH system is to convert it to C-Plan - for which you will need a motorised valve in the HW circuit, and a cylinder stat. This gives you independent control over HW and CH and only runs the boiler when one or other (or both) are calling for heat, and only runs the pump when CH is calling for heat.

Have a look at C-Plan in

you do this, you will have to remove the link in the existing programmer which turns the HW on when CH is selected. If your room stat is a programmable one - providing different temperatures at different times of day - you can use *that* to time the CH. In that case you would use the timer in the old programmer just for HW, and set CH to 'constant'.

Hope this makes sense!

[Steve]

Yes, it is, and that's exactly what I was hoping to do with the system as it is, until I realised that would mean the HW being on all the time. I was hoping for just one improvement for now, with the upgrades you've suggested some later. But if I can't do it that way, then so be it. Thanks, Steve

[Steve]

A thought occurred to me - could I not achieve what I want using a double-pole relay to switch the CH (one pole for pump and one for boiler)? This would avoid the problem, I think, of the pump coming on when the HW only was on?

By the way, how do you fit a thermostat to a HW tank when it has pre-applied insulation rather than a jacket? Do you have to scrape off some of the insulation?

Steve

[Roger Mills (aka Set Square)]

Yes, I was thinking along the same lines after reading your previous reply. I don't think that the relay needs to be double pole - single will do. If you wire the coil in parallel with the pump, you can switch the boiler on whenever the pump is running - but can still have *just* the boiler when HW-only is selected. that would give you a *degree* of independence - but the HW would still get (potentially too) hot whenever the CH was on - whether you wanted it or not, and there would still not be a boiler interlock - so the boiler would continue to waste energy keeping itself hot unnecessarily.

Just that. You cut out a chunk of insulation - 50mm x 30mm or whatever - so that the stat makes contact with the copper cylinder.

[Steve]

Oh yes, very clever. I think I'll go for this simpler approach for now to at least give me a thermostat on the CH, and go for a stat on the tank and a valve when the weather's warmer and I can be without the CH while I do it.

Many thanks for all your help.

Steve