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:

<pre><tt>

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 -----------+--------------------------+-----------------------+

 

[The Natural Philosopher]

[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:

<pre><tt>

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 -----------+--------------------------+-----------------------+

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..

 

[Dave Liquorice]

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.

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.

 

[stevelup]

However I think I've seen somewhere a single relay solution (but can't
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.

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

 

[Homer J Simpson]

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.

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.

 

[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 |
| | |
| | |
+----+----+ +---------->\ <--+
| | | \ A
| ----- | o
| | | | |
pump (X) | RLA | pump (X) --------
| ----- | | BOILER |
| | | --------
| | | |
N ------+---------+---------------------+-------------+

Dave

 

[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 |
| | |
| | |
+----+------+ +---------->\ <--+
| | | \ A
| ------ | o
| | | | |
pump (X) | RLA | pump (X) --------------
| ----- | | BOILER |
| | | --------------
| | | |
N -----+---------+--------------------+--------------+

 

[Christian McArdle]

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

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

Christian.

 

[stevelup]

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)

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

 

[Ed Sirett]

[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:

<pre><tt>

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 -----------+--------------------------+-----------------------+

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?



--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html
Choosing a Boiler FAQ http://www.makewrite.demon.co.uk/BoilerChoice.html
Gas Fitting Standards Docs here: http://www.makewrite.demon.co.uk/GasFittingStandards

 

[dcbwhaley]

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

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

 

[dcbwhaley]

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

Nearly forty years designing digital logic professionally gives me the
edge

 

[Tim Auton]

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

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

 

[Homer J Simpson]

What I need is AC diodes ;-)

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

 

[John Stumbles]

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..

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

 

[John Stumbles]

Connect a neon lamp to each pump and tape to an LDR which operates the
burner relay.

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

Or use two diodes and a DC relay.

What I need is AC diodes ;-)

 

[John Stumbles]

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.

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

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]

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.

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

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

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]

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.

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).

 

[John Stumbles]

Try this.

---8<---

Great - thanks!