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Posted by on December 17, 2006, 9:47 pm
Alright, so I just had a 20KW generator installed to power the whole
house. I have two units, 5 Ton and 3 Ton and I don't want them ever to
run at the same time, because the two would be too much for the 20Kw.
The units are a year old, and I'm using the Honeywell digital
thermostats How can I keep the two from ever running at the same
time? Should I replace the thermostats with two that talk to each
other, or can the Honeywells do that?
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Posted by James on December 17, 2006, 10:36 pm
It would be relatively simple to rig up some logic with a few 24v relays.
Trouble would be finding someone locally to do that for you. Basically you
want each system when it turns on to also power an extra relay. Then put
the normally closed contacts on each relay in series with the thermostat for
the other system. So when one system turns one the relay opens those
contacts and prevents the other system from turning on. A decent service
tech should be able to do it if you can find one willing to.
> Alright, so I just had a 20KW generator installed to power the whole
> house. I have two units, 5 Ton and 3 Ton and I don't want them ever to
> run at the same time, because the two would be too much for the 20Kw.
> The units are a year old, and I'm using the Honeywell digital
> thermostats How can I keep the two from ever running at the same
> time? Should I replace the thermostats with two that talk to each
> other, or can the Honeywells do that?
>
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Posted by ran007 on December 17, 2006, 11:10 pm
James wrote:
> It would be relatively simple to rig up some logic with a few 24v relays.
> Trouble would be finding someone locally to do that for you. Basically you
> want each system when it turns on to also power an extra relay. Then put
> the normally closed contacts on each relay in series with the thermostat for
> the other system. So when one system turns one the relay opens those
> contacts and prevents the other system from turning on. A decent service
> tech should be able to do it if you can find one willing to.
>
> > Alright, so I just had a 20KW generator installed to power the whole
> > house. I have two units, 5 Ton and 3 Ton and I don't want them ever to
> > run at the same time, because the two would be too much for the 20Kw.
> > The units are a year old, and I'm using the Honeywell digital
> > thermostats How can I keep the two from ever running at the same
> > time? Should I replace the thermostats with two that talk to each
> > other, or can the Honeywells do that?
> >
James,
Thank you for your help in the matter. What you're saying makes sense.
I certainly want to at least give it a try. I know anything is always
possible.
I'd like to think Oscar is still being sarcastic and that he's having a
rough day. However after seeing some of his unwitted responses his in
past replies, who can take him seriously?
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Posted by Bubba on December 17, 2006, 11:45 pm
On 17 Dec 2006 18:47:49 -0800, randall.reed@gmail.com wrote:
>Alright, so I just had a 20KW generator installed to power the whole
>house. I have two units, 5 Ton and 3 Ton and I don't want them ever to
>run at the same time, because the two would be too much for the 20Kw.
>The units are a year old, and I'm using the Honeywell digital
>thermostats How can I keep the two from ever running at the same
>time? Should I replace the thermostats with two that talk to each
>other, or can the Honeywells do that?
Simple, just disconnect the Red low voltage wires. The systems will
then never run at the same time......................or ever.
Bubba
"Didnt like that answer? You got exactly what you paid for."
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Posted by thrugoodmarshall@hotmail.com on December 18, 2006, 10:10 am
Since you weren't specific, let's say that it's just the AC function
you're trying to interlock and not a heat pump and it's a 24V AC
system.
Again, you didn't specify what controls you already have (2 thermostats
and some sort of txfer switch, I hope) so what follows is really
academic.
This will get you started.
Please see the "notes" section for some followup you're going to have
to do yourself!
If you want to hack together all these features, you really don't want
to do it with relays--see again the "notes" section.
In this example, the "Y" wire of the first thermostat turns on the
compressor or the 5T unit. Connect the coil of a 24VAC relay from the
"Y" output of that thermostat and the opposite leg of the 24VAC
transformer.
Similarly, for this example, the "Y" wire of the second thermostat is
used to turn on the compressor of the 3T unit. Lift the wire from the
"Y" terminal of that thermostat and land it on the N.C. contact of the
relay. Connect the common terminal (not the coil common--the common
for the N.C. contact you just landed!) to the "Y" terminal of the
second thermostat.
Now, the signal to turn on the 3T compressor has to go through those
N.C. contacts.
This will give the 5T unit priority and both units will never be on
together.
If you want to be able to run both units together when you're not on
emergency power, just put the N.O. aux contacts of the generator
transfer switch in series with the coil of the new relay.
Notes:
Fan:
Typically, the fan output is turned on whenever the thermostat calls
for AC.
Depending on your system, you'll have to deal with this.
One way would be to use a double pole relay and wire the fan signal
through the other set of N.C. contacts just like the compressor signal.
Restart Lockout:
You'll probably want to use a restart-lockout relay to prevent the 3T
unit from starting up again too soon after it's been shut down.
Short-cycle lockout:
Once the 3T starts, you'd probably want it to finish its cycle before
dropping for the 5T. You can install this circuit twice, once with the
5T as primary and once with the 3T as primary. To prevent relay races,
wire N.C. contacts between the thermostat output and the relay coils in
both instances. You get the added benefit of sharing priority equally
if you do it this way.
However, at this point, relay logic becomes silly. You should be doing
all this on paper instead of just going out and buying relays, and at
this point, your design is probably pretty complicated.
Instead, buy a smart relay (a little tiny PLC is what it is) and
program it however you like. Some sources--www.factorymation.com,
www.automationdirect.com.
Have fun, and let us know how it turns out.
randall.reed@gmail.com wrote:
> Alright, so I just had a 20KW generator installed to power the whole
> house. I have two units, 5 Ton and 3 Ton and I don't want them ever to
> run at the same time, because the two would be too much for the 20Kw.
> The units are a year old, and I'm using the Honeywell digital
> thermostats How can I keep the two from ever running at the same
> time? Should I replace the thermostats with two that talk to each
> other, or can the Honeywells do that?
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