Room stat

[davidstanton]

I have been asked by friends to install a room stat for them. Their system does not have a room stat at present but does have indiviudal rediator stats.

The system is a balanced flue with pumped heating and gravity hot water with a motorised valve.

Is the installation of a room stat, in its simplest for, just a switch on the live feed to the pump?

Thanks for any advice.

 

[croydoncorgi]

No.
The whole point of a room stat is to provide an 'interlock' for the BOILER. When the roomstat is satisfied all the rooms in the house should be at the required temperature - which means the stat MUST be in the right place! - and the boiler will then be OFF so not wasting energy.

If there is gravity hot water, to properly interlock the boiler will also require a relay so that it DOES switch on when water heating and / or CH are needed and OFF when NEITHER is needed.

And you will also need to be careful about control of the pump. If the boiler requires overrun, then the pump must be controlled by the boiler not by the thermostats.

The only thing you don't need to worry about is a bypass - the gravity HW circuit provides all the 'heat sink' needed.

And you thought it was just a roomstat!

 

[davidstanton]

Thanks for the advice.

I am a humble DIYer... I think I will tell my friends to call for Corgiman!

 

[felix]

It's a bit more complex than that. Remember that you'll want the boiler to go off as well but if you wire the boiler and pump in parallel and feed them from your new room stat, how will you get the boiler - but not the pump - to come on when you need hot water.

Come to think of it, how is it all working now. You should have that valve opening and closing as required by the controller and tank stat then, when the valve is fully open, an internal switch should start up the boiler ONLY. Now what happens when your controller demands CH but not HW. How do you get the boiler AND the pump? Perhaps you can't have CH only - but you can because the tank stat can close that valve!

You must start by figuring out exactly how it's all put together. You might find it isn't working properly now. I once inherited a gravity HW, pumped CH system to which a zone valve and tank stat had been added. Unfortunately they'd been added by a moron. When the tank stat closed the valve, the boiler was left bubbling away to itself in an outhouse! The previous owners either didn't know or thought it was normal.

If you can work out what yours is doing you'll have no problem adding the room stat. You have to put it in a room WITHOUT a TRV - and I see croydoncorgi has just beaten me to it. Yes, I had to add a relay to my system too.

As for not needing a bypass. I didn't have one but I did have a boiler with a big heat exchanger and no overheat stat. Even with your present setup you could have the zone valve closed and all rads off. I suggest that you either have a bypass already or else you don't need one.

 

[davidstanton]

Thanks Felix

My original query related to my friends system but your reply describes the system I have which was fitted by the same moron you talk about!!

When I moved in 12 years ago I had a zone valve fitted connected to a hot water cyclinder stat. But, when (if the CH is off and on HW only) the temp is reached and the zone valve closes, the boiler carries on until the boiler stat shuts down the gas. There is no wiring between the zone valve and the boiler. The valve takes its power supply from the emersion circuit, and that's that.

Oh boy! No wonder my boiler bubbles away

 

[felix]

If it really was the same moron you'll have a piece of rotting hardboard floating in your cold storage tank and a bill, thoughtfully left by the previous owners, for "fitting lid to cold tank"!

I'll try to remember how I got round the boiler/valve problem. I know I used a 2PCO octal relay (from the junk box) with its socket fitted into a blanking plate on a surface mount back box. The circuit eludes me at the moment. I'll be back.

 

[ChrisR]

You're close enough to "C Plan" to make it worth a look at how it's wired:
http://content.honeywell.com/uk/homes/Catalogue/Sundial Plans/3.1 C Plan.pdf