Mid Position Valve query plus.

[Johno]

Hi What do the three selectable settings W M and H represent on a central heating Mid Position Valve? Also, how would a central heating system with thermostats on the radiators (Except for the radiators in the bathroom and toilet, these have the traditional twist open/close type valve) know when to the turn the boiler off? There is no thermostat anywhere else in the house (As far as I can see), like in the hall or landing!

What would cause the very brief sound similar to ball bearings rattling around an empty radiator mean when switching on the central heating. I hope it goes without saying that the radiator is not actually empty of water. The sound only lasts about two seconds. The radiators are being bled of air regularly. Many thanks Johno

[Set Square]

W = Hot water H = Central Heating M = Mid position (both on together)

Also, how would a central heating system

It wouldn't! (unless there's a flow switch somewhere). Is there any evidence that the boiler *does* turn off when all demands are satisfied - or does it keep itself hot by cycling on its own stat?

Could be a small amount of air moving to a different position - or maybe its the TRV rattling? How often do you have to bleed the rads? Is there any corrosion inhibitor in the system? [When you bleed, check the 'air' with a lighted taper. If it burns, it ain't air but hydrogen - signifying corrosion due to no/insufficient inhibitor].

[Johno]

Thanks for the reply/advice. Just to clarify these are queries concern two different central heating systems, both belonging to friends of the family.

Concerning my query about the thermostat, what is a flow switch and how will I be able to tell if the central heating system has one? All I know is that the water from the hot tap gets extremely hot, scalding hot in fact. We had put this down to the small thermostat on the emersion tank. Which has a dial which can be set between 45 and about

80 degrees (If I recall). It was set to 50 degrees and moves freely between 50 and 80 degrees but when I tried to move it down to 45 I had to use a little bit of force and there was a noticable click. And with it set to 45 the boiler did not turn on to heat the water. I moved it back to 50 and the boiler started. So may have a problem with this thermostat also. I will suggest your method of testing for corrosion in the system. Johno

[Set Square]

I very much doubt whether you have got one. As far as I am aware, they're only used in systems which have a by-pass circuit - to provide a flow path when all the TRVs are closed. The switch would be in this circuit. I've never actually seen one - but it would almost certainly be a pipe fitting, with an IN and OUT and with a couple of wires going to it. They're not used very often because they require some complex logic to get things going again once they have shut the system down. Are you SURE that there isn't a room stat somewhere? Does the wiring all come back to a wiring centre like the one shown under Y-Plan in

so, terminals 4 and 5 would have to be bridged if there's no room stat.

Sounds like the tank stat is shot. If it's working properly it should control the hot water to 55-60 degC. See what happens if you replace it. Note that the new one needs to have change-over contacts and needs to be wired exactly as per the Y-Plan diagram in the Honeywell reference (above).