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helpDIY · Dec 23, 2009, 05:53 PM

I have a forced air furnace and the only return duct is located in the living room where there is a 18k BTU AC. In the summer time, can I connect the AC to the furnace thermostate so that it control the AC on/off and at the same time turn the furnace blower on to cerculate the cool air to the entire house. Sound crazy. But I would like to try it. Thank you for your help

EPMiller · Dec 23, 2009, 06:24 PM

That will be a real rig job, possibly not even doable. You would be better off just turning the fan switch ON, letting the air handler circulate constantly and let the internal tstat control the AC unit. You could easily get into a situation where the AC will run continuously because it is not large enough to satisfy the tstat for the whole house. Don't go there.

KISS · Dec 23, 2009, 07:46 PM

Technicly, it is possible.

The easiest method depends on a lot of things.

You would have to run a low voltage wire from the furnace to the AC unit area and I'd recommend that be a low voltage dry contact.

A transformer, relay and a box and your all set. Certain FAN CENTERS have what you need, but it won't look pretty.

EPMiller · Dec 25, 2009, 10:21 AM

There are units that you may not be able to get to a switch point that will not cause unwanted operation. Adding a long loop of tstat wire would not work well in series with the thermistor on some AC units that I am familiar with, and it could be tough at the circuit board level.

Just $0.02.

KISS · Dec 25, 2009, 04:54 PM

18 AWG wire would not be an issue. It just depends what the AC does with an opne thermister. Better yet, who knows if it has one. It would also depend if it detects an OPEN and stops or if it's an NTC or PTC thermister.

If it's NTC, you can always parallel a resistance. I guess you could do it if it's PTC too, but the NTC version means when disconnected it reverts to normal.

NTC means (Negative Temperature Coefficient). Resistance decreases with increasing temperature.

What would be helpful is a wiring diagram if there is one on the unit.
Or if you can get a service manual.

If your still interested, post a pic of the inside usig Go advanced/manage attachments

EPMiller · Dec 25, 2009, 08:02 PM

KISS, I agree with you that it if the unit used a mechanical thermostat it should be fairly easy. I was inferring that, in the absence of a good switching point, the resistance value of a long run of wire added to the thermistor could throw off calibration, although that could possibly be fairly easy to compensate for. I also wonder if EMI from that loop could also possibly play havoc with the thermistor buffer amp. The 2nd I can't speak to authoritatively, but I have some experience with strange results when I tried to rig things like this.

I guess if it is a thermistor controlled unit, you could measure the resistance of the thermistor at say 70 deg and then use some local relay switching to just switch 2 values of resistance into the thermistor circuit. One that calls the AC and one that indicates it is satisfied.

Still sounds like a fiddly rig job, although now that I've thought it through, I have a good application for the idea next cooling season. We have some Fujitsu mini-splits that NEED an external tstat and even the importer claims that the factory part mentioned in the manual doesn't exist. :confused: :rolleyes:

KISS · Dec 25, 2009, 08:24 PM

That's really all you would have to do. Switch in an off (60F) and switch in an on (85F) resistance. Add a disable switch and you should be done or pop the relay out of the socket if you use one.

If I did use the furnaces transformer, I'd put a 1 A fuse or so in series with the (Y) lead at the furnace.

I did a toxic gas alarm and I would not drive lights in other panels from a power supply in another room. Just didn't make sense. I used only contact closures in each panel.

So the local power supply drove the indicator. Yea, it costs about $20 more for the weird relay. 24 V AC/DC with indicator and took up less than 1/4" of space on a DIN rail.

KISS · Dec 25, 2009, 08:52 PM

Her eis a typical 10K thermister curve. Wire resistance is the least of the problems. But I would not like to run the thermister wire 50-100' (2-way). Keep the relay local.

KISS · Dec 25, 2009, 11:38 PM

Here is a real simple AC unit. http://www.airconditionparts.com/fil...ice_Manual.pdf

Note a line voltage thermostat. Too simple.

A contactor in parallel with the tstat fed from a 1 A fused source. Put a connector on the unit and with the connector removed it reverts back to a normal AC unit.

Fuse at heater unit prevents problems.

hvac1000 · Dec 26, 2009, 04:49 AM

Sounds like a bunch of trouble to me . This little toy might work well and with few mods.

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