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Red Squirrel 07-08-2010 06:10 PM

Making portable AC more efficient
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I have a 12K BTU unit, which I believe is equivalent to 1 Ton. It cools my office or bedroom in about 15-30 minutes, however lot of that air gets sucked in through the condenser side vent, then blown outside. This is very inefficient.

I was wondering if it would make it more efficient by somehow adding a duct to the condenser side intake and ducting that outside, so I have an intake duct and exhaust duct, both going outside, so I'm not sucking air out of the house.

Then I got thinking, why not hook this up to the furnace so I can distribute the cool air in the house... but do you think 1 Ton would be enough for a 1100 square foot split level house?

I attached a picture. Do you think this would work, or would the furnace blower be working against the AC's blower? Could I simply just hook it up to the return or would that conensate the heat exchanger... the air coming out of the AC should technically be rather dry given it already went through the evap coil which took most of the humidity.

For the ducts, I'd use insulated ducts, and apply them to the portable unit with aluminum tape, or vp tape, whatever works better. IF this works I'd set it up in a permanent matter so I can easily bypass it in the winter.

yuri 07-08-2010 06:20 PM

Anytime you dump cold air into the return of the furnace it will cool the heat exchanger which could then sweat and corrode. Illegal now except for electric furnaces. If it corrodes the limit control and it later fails when you need it you could die. The portable unit will never overcome the larger pressure of the furnace fan and will overload and burnout. If you monkey with the flows of air for the portable or any fan it can interfere with the flow the manufacturer designed it for and will overload it and burn it out and could start a fire.

Red Squirrel 07-08-2010 06:25 PM

Sounds bad, but I don't understand the heat exchanger condensate part though. How is it different then cold air from a central air system entering back into the furnace return? By the time the house temp drops to the point the thermostat makes the system stop, I'm sure lot of that cold air has went back through the furnace several times. Hooking up the portable to the return sounds like the easiest and less worry of dealing with pressure in the plenum, and I could also test it out from upstairs before I rig anything permanent. I'm not doubting what you said, but just curious as to why it would be an issue.

Currently I have my unit in my office, and the return of the furnace is just outside that room, and I tend to turn on the furnace blower to circulate air, is this bad?

yuri 07-08-2010 06:31 PM

This is all theory. Moisture condenses on the coldest surface. If the heat exchanger or limit control hits the dewpoint then it will sweat. Not sure if that will ever happen but it can. Southern Ontario has high humidity levels. Running your furnace fan all the time can cause it to burn out sooner if it has poor quality sleeve bearings. Will use $1/day in electricity also.

Red Squirrel 07-08-2010 06:43 PM

Oh ok, but is it more likely to happen with a setup as I suggested (not the picture, but hooking it up straight to the return) then with central AC? From my understanding both the central AC and a portable unit will remove moisture as air passes through the evap coil so when the air hits the plenum it should be rather dry, at least I would think. Guess it's best not to take chances when messing with this stuff though.

yuri 07-08-2010 07:16 PM

"Guess it's best not to take chances when messing with this stuff though."

The air from the house will be humid and condense on the exchanger, not the air from the portable. What your mix will be who knows:whistling2:

beenthere 07-09-2010 05:34 AM

Cold air blowing over the furnace heat exchanger will make it sweat inside the heat exchanger. The heat exchanger and flue passages will have air from either the basement/mech room, or from outside if direct vent. The moisture in this air will have a high enough dew point that it condenses in the heat exchanger and flue passages. And rots the heat exchanger out.

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