Relay to switch HVAC Unit

Hi, HS. Switching motor loads requires a contactor. Your 10A relay just won't switch off the load safely.

Contactors are built for higher separation speed of the contacts and a farther contact draw distance. They have beefier contacts that have higher temp alloys which can better withstand the heat of contact arcing without melting or pitting (yes, the arc actually melts and splashes the metal of the contact). And many are made with magnetic arc suppression, to help extinguish the arc. All of these are necesary when switching motor loads.

Contactors are available with 12VDC coils. They are more expensive than relays. They also make a lot of noise, can't really help that. By putting the contactor into an enclosure, you can minimize noise. When you go to your electrical supply house, make sure to know the HP rating of the motor you're switching -- contactors are rated for switching by horsepower as well as voltage.

Good luck Chris

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If you have to ask those questions then I recommend you enlist the
services of an HVAC company before you kill yourself or seriously
damage your A/C or burn your house down.

Thanks

With

relay

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12V),

What about one of these?

Are these a bad idea for switching a HVAC unit? If no which one would you suggest?

Thanks

relay

turning

I'd take the approach to tinker in the low voltage controls that should already control the unit start circuit. Adding another low voltage relay in parallel with the one that probably already exist might be an easy solution.

look at that one. you would want the zero crossing type and try to get the

2 pole option with 50 amps and select the input control voltage you want to use.! Dc or AC etc..

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Real Programmers Do things like this.
http://webpages.charter.net/jamie_5

There's no need for additional circuitry for this operation.

So the best thing to do is not add any addition circuitry for this operation.

Yes.

Not the way that you're proposing to do it.

AC works better when it runs longer. That's why it's a bad idea to oversize. The cycle times become too short.

Nope.

You cannot use a relay with the above specs.

Consider this. Your thermostat turns your AC unit on and off. How does it do that? There's no 10A 220V relay in the thermostat, right.

The way that it works is that there's a 24VAC circuit that controls the contactors that are already in place. So instead of trying to insert yet another contactor into the loop, you simply control the 24VAC circuit.

When I replaced my thermostat with a PIC based controller, I used 12V automotive relays to drive the 24VAC low voltage circuits. Since they were in the basement, I didn't worry too much about the click.

Be aware that the you'll need to drive both the AC unit and the fan circuit. While the furnace will cut the fan on automatically, the AC unit won't.

Please stay away from the high voltage, high current circuits. There are low voltage circuits that are a lot easier to manage. This is what John meant be not fooling with anything if you're asking these kinds of questions.

BAJ

This unit is in an apartment building in NYC. Each room's unit is self contained (not central heat/air). I was not able to find any low voltage controls inside of the unit. There is a wiring diagram I could post a link to if anyone thinks that would help.

I thought about wiring into the thermostat, but this would leave the fan running.

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Use the smarthome appliance control rated for airconditioners up to 20 amps. If you have to modify anything, don't do it. As long as it's typical plug into the wall receptacle stuff, you should be ok. But the minute you have to take the covers off, forget it as a bad idea.

Ed

There is

help.

method

12V),

it

So you think that

would be OK? Will this unit be OK and safe, limiting arcing etc as discussed earlier in this post?

The power is "wire nutted" to the BX cable. I could add a small plug pig-tail so that I could plug into the X10 receiver. What do you think?

Thanks

Up until your sentence below I did think it would be ok.

*IF* your unit was cord and plug connected it would be fine. The smarthome unit you posted a link to is rated for airconditioners up to 20 amps, and yours is 8 amps.

Then it does not match what I posted: "As long as it's typical plug into the wall receptacle stuff, you should be ok."

Nope, don't do it. Unsafe, and a possible lease breaker.

Anticipating your next question "what about the other smarthome device?" - you would need to hire an electrician to install that, and I have no idea if that would fall into the possible lease breaker category.

Ed