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?
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.
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
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?
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
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
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
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
Now, the signal to turn on the 3T compressor has to go through those
This will give the 5T unit priority and both units will never be on
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.
Typically, the fan output is turned on whenever the thermostat calls
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.
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.
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,
Have fun, and let us know how it turns out.
On 17 Dec 2006 18:47:49 -0800, email@example.com wrote:
You just need a couple of 24v relays that have a normally closed set
of contacts. Wire each of them in parallel with the active wire
(usually y) and ground of each thermostat and then tie the active
wires thru the NC contacts of the 'OTHER' relay.
The first call from either thermostat will shut off the Y line to the
On 17 Dec 2006 18:47:49 -0800, firstname.lastname@example.org wrote:
I assume you dont care if they both run when on normal power and its
that if you switch to emergency power when you dont want both units
If that is the case I would have a relay powered off the transfer
switch or generator which would de energize one or the other a/cs.
The relay, when energized, would break the control wire through a set
of N.C. contacts to the condensing unit (your choice) compressor 24v
Consult the installer of the generator for a specific wiring location
to power up the relay when the generator starts. Done.
you're forcing the customer to choose 1 of 2 when he could have both.
the two 24v relays are the best choice. the ho can purchase them off
the web & install them himself. once in place, operation is automatic.
customer has the option of using either ac, but never both at the same
now put a sticky on each t-stat that says:
for ac, turn off the other thermostat
Actually I could see working the third relay into the picture. Use a
115v relay with two nc contacts. Wire it to the generator so it is
pulled when the generator is running. The nc contacts can be used to
bypass the other two relays so that when the generator is not running
both systems can run at the same time. If you have a problem with
hi/low voltages in proximity then add a 24v transformer at the
generator and use a 24v relay for the third one.
Sorry but thats an idiotic idea imo. Now your forcing him to always
use only one a/c at a time during non emergency conditions. The two
relay idea is ok, but only if they are in play when the generator is
Probably could use a bank of relays and really get crazy with redundant
switching. Every time the stat calls.... click clack click clack
relays pulling in and dropping out. Make sure to mount them on the
duct work. :)
Only takes three to include the capability to activate the single
system limit while the generator is on. One relay powered by the
generator to enable the system, then one relay for each system to lock
out the other system while running.
I agree it's starting to reach the point where a control circuit with
solid state switches would be the preferred solution. But it takes
less background knowledge to rig up a small logic system with three
relays than it does to lay out the circuit for it using solid state
stuff. For a one off requirement the relays are still the way to go.
You could argue it's a potentially common need and design a control
board in a box that can do it. I'd go ahead and make it smart enough
to handle three or four systems since most of the whole house generator
customers tend to have bigger houses with multiple hvac systems. Then
market it to the generator companies.
Power's Mechanical wrote:
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