relay logic for 2-pump wet central heating system

[John Stumbles]

[I tried this query on the uk.d-i-y newsgroup without getting a solution. I'm x-posting it there for info to that group as I hope it may be useful to others, with followups to s.e.d]

I need to fix up proper controls on a wet (what I think leftpondians call 'hydronic') Central Heating system that has separate pumps driving the space heating (CH) and water heating (DHW) systems. Each part of the system has a time switch and thermostat determining when the pump for that system's hydronic circuit needs to be driven. When the CH or (Boolean-OR) DHW systems are running the boiler must also run. The boiler has a 'Switched Live' terminal which, when energised, causes it to run.

I know I can do it with 2 relays with their contacts wire-ORed to drive the boiler Switched-Live:

L -----------+--------------------------+-------------+--------------+ | | | | o | o | o | o | | CH - programmer - HW | | A | B o | o | o | o | | | | | | | | | | | +-----------+--+ o | Room o | HW | | thermo | cylinder | o | -stat o | 'stat | | | ---------- | | | SL | +----+----+ +----+----+ | | | | | | | boiler | | ----- | ----- | | pump (>) | RLA | pump (>) | RLB | | N | | ----- | ----- ---------- | | | | | N ------+---------+----------------+---------+---------------------+

However I think I've seen somewhere a single relay solution (but can't remember where!). The only way I can think of doing it would be to wire the relay coil between the live terminals of the two pumps which I suppose would work but seems inelegant.

240V~ L -----------+--------------------------+-----------------------+ | | | o | o | o | | CH - programmer - HW | | A o | o | o | | | | | | | | | | o | Room o | HW | | thermo | cylinder | o | -stat o | 'stat | | | ---------- | ----- | | SL | +---------| RLA |----------+ | | | ----- | | boiler | | | | | (>) pump pump (>) | N | | | ---------- | | | N -----------+--------------------------+-----------------------+

[stevelup]

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

[dcbwhaley]

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 | | | | | | | +----+----+ +---------->\

[dcbwhaley]

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 | | | | | | | +----+------+ +---------->\

[Christian McArdle]

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

Christian.

[stevelup]

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

[dcbwhaley]

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

[dcbwhaley]

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

[Tim Auton]

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

[John Stumbles]

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!

[John Stumbles]

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.

[John Stumbles]

---8

[Homer J Simpson]

Yes. In Outlook Express you select View : Text Size : Fixed Font to view these.

[Christian McArdle]

I just set my proportional font to be Courier New.

Christian.

[stevelup]

You've been desigining digital logic circuits nine years more than I've been on this planet so how can I compete :)

[jasen]

when you're reading on google groups and hit an ascii schematic select "show original" (under "show options") and you'll get a fixed pitch font rendering of the message (including the diagram).

Bye. Jasen