relay logic for 2-pump wet central heating system

I used a single relay as follows.

The timer/thermostat for the UFH runs the UFH zone motorised valve AND a relay. The valve contacts control the UFH pump, and the relay controls the boiler/main pump.

That way when the UFH is NOT on, but the other heating is, the UFH pump won't run in parallel with the boiler.

If one had a motorised valve with two swiches instead of one, the relay would be unecessary.

Now your diagrams don't show any motorized valves - do you have any? In essence in my system a motorized valve replaces one relay.

Also, your pumps seem somewhat different..I have a UFH circulation pump with a temp reduction bypass valve. This runs continuously as long as UFH is called for, but is not always connected to the boiler..that has its own pump that drives water through the HW, radiator or UFH circuits depending on the zone valves..

And won't work when both CH and HW call for heat at the same time. Don't forget that most stats (particulary tank stats) are change over switches not just "make for heat" and the mains feed doesn't have to go to the common.

Hi

That definitely will not work - the relay would only be energised if one of the thermostats was calling for heat. If both thermostats called at the same time, the relay would not operate.

Also, the neutral path for the relay would be via one of the pump motors which is unlikely to work as expected.

I can't think of a way of achieving your goal with one relay. It seems impossible.

Regards,

Steve

Connect a neon lamp to each pump and tape to an LDR which operates the burner relay. Or use two diodes and a DC relay.

Try this. Requires one changeover relay but changeover relays are only a little more expensive. The relay is shown in the unenergised state, when the heating pump is off. In this state the boiler will fire when the hot water pump is on. If the heating pump comes on the relay operates and fires the boiler.

This circuit uses the Boolean logic identity A or B = A or (Not(A) and B)

L -----------+--------------------------+------------------+ | | | o | o | | | CH - programmer - HW | | o | o | | | | | | | | | | | o | Room o | HW | | thermo | cylinder | o | -stat o | 'stat | | | | | | | +----+----+ +---------->\\

Gawd! ASCCI art is a PITA. Is this more better

L ----------+---------------------------+----------------------+ | | | o | o | | | CH - programmer - HW | o | o | | | | | | | | | | | o | Room o | HW | | thermo | cylinder | o | -stat o | 'stat | | | | | | | +----+------+ +---------->\\

Actually, your first one worked fine. Your second one was way off!

Christian.

Excellent solution! I feel daft for not coming up with it myself :(

My MIL's house (built 1982) has a two pump system but the controls are in '10'/Gravity mode. So the boiler and HW pump is fired from the HW timer and the CH pump is fired from the timer + wall stat.

When the system packs up (boiler is a Netaheat so it may go for years) then I'll sort the whole lot out to a modern standard.

I wonder if there is a relay with two coils either of which will close the contacts?

Not on my screen. They look differant on the preview screen and on the post. Is this because I am using proportional fonts?

Nearly forty years designing digital logic professionally gives me the edge :-)

Yes. You have to use a fixed-width font to draw and view ASCII art, unless you can guarantee everyone who will view it will be using exactly the same font. That's never going to happen, so fixed-width is the only way it works.

Tim

Still, you have a solution now with the SPDT relay.

My [customer's] system is different: no motorised valves, certainly no UFH or mixing valves, just 1 boiler + 2 pumps.

I did think of something like that! Dunno if one can still get LDRs (CdS) or if they'd do for 240V mains.

What I need is AC diodes ;-)

It occurred to me that that's probably how this one's set up. Looks like it used to be a back boiler and somewhere in neolithic times someone plonked a ruddy great floor-standing lump next to the fireplace to replace it. (The flue - which doglegs into the chimney - is a joke, but apparently the consummate professionals that are British Gas keep saying it's OK when they come to do their annual service so the customer won't hear of replacing it.)

I wondered that but it would probably cost vastly more than two ornery relays. :-(

Anyway I see someone has come up with a 1-relay solution so all's well!

D'oh! of course not - can't think what I was thinking of :-(

That would probably be OK (though 'inelegant' as I said) since the pumps are induction motors which take orders of magnitude more current to run than the current which would flow through a relay coil.

I'm aware of that but I can't see how to make use of this to save on relays (if that's what you mean).

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