Central heating system with no thermostat (again^2)

[Robin]

I've now visited elderly relative armed with the helpful advice about her central heating system given in December. May I check what I think I found?

The b/g in brief was that British Gas had told her she had to leave one radiator on full as she had no room stat. What I found was:

a. a Glow-worm Economy 40B boiler (in a vented system); b. the installation instructions for the boiler require a bypass, with a lockshield valve, between the flow and return; and c. what seems to be a such a bypass fitted in the airing cupboard - several meters from the boiler. (I only say "seems" as one end is behind the HW tank where I'd need an endoscope to prove it).

Does that all confirm BG's advice being (no doubt well-intentioned) unicorn droppings?

She's agreed to let me fit a room stat in the spring. (She has a sensible view of my ability to get things right first time.) There's an RWB252 programmer which she really, really does not want to change. That allegedly has a standard backplate. So can I simply drop a wireless thermostat as recommended by Adam between the "CH On" output and the pump? (Plus the manual by-pass switch Adam also helpfully recommended and which I know from experience with her security lights she'll find a great comfort.)

[ARW]

Remind me. was this a gravity fed system with no motorised valves? If so then the stat does go between the programmer CH output and the pump. The HW output is the boiler call.

[Roger Mills]

If it's a gravity HW system - where the boiler can look after itself without the pump running, I wouldn't expect a by-pass to be needed (but ICBW)

If it's a fully pumped S-Plan, Y-Plan or W-Plan system (or even a half-pumped C-Plan system) the stat needs to go in the place shown in the appropriate wiring schematic (to which Adam will doubtless provide a link).

[Robin]

Sorry I didn't spell it out better. It's a vented system but I *think* fully pumped: there's a BGMVSP-23 "mid-position actuator".

I am (now) forced to admit that even after looking at the Wiki diagrams I wasn't *sure* where it fell in the alphabet :(

[Roger Mills]

If it's got a 3-port mid-position valve, it's pretty certainly a Y-Plan system.

The stat needs to be inserted between the programmer's 'Heating' demand and the valve's white wire. If the system is wired using a conventional

10-way wiring centre, the stat needs to be connected across terminals 4 and 5 - having first removed any strap between those two terminals.

[Big Les Wade]

Robin posted

What's the function of this bypass? (I seem to have one too.) Can't be to protect the pump against being activated when both HW and CH valves are shut, because the live supply to the pump and the call-for-heat wire to the boiler are both switched off whenever that happens, aren't they?

[Roger Mills]

Depends on how the pump is wired. If it's wired dirctly in parallel with the boiler's switched live, then you're right. However, if the boiler requires pump over-run, the pump will be connected to dedicated terminals on the boiler and will be controlled by it - and will continue to run for a while after the boiler stops. This is when the by-pass circuit is needed.