OK, I am an experienced roofer who has seen this conversion a million times and I think I can handle it... I think. My plan was to buy ohe of the precharged self contained units that goes on the roof and just do the ductwork and wiring myself (I got my brother to do the 240v and subpanel), but I am stuck with the thermostat. My questions are mainly about the thermosts and ductwork. How much wiring will be needed to wire this unit to work with my current furnace? I currently have the heat only thermostat and would be changing to the heating/cooling thermostat at the time of the conversion, also how should I make the return air ducts? Not what metal etc. since I make them for others to hook up the swamp coolers I feel I can do my own, but where should I route the return for the new AC? Do I need to install all new registers for the return? Or could I just have it return through the ceiling in the furnace closet? Also, will the new AC unit be damaged at all with the furnace blowing out of the registers during the winter? I do assume it will blow hot air back through the ducts and over the exap coils. Thanks a million, Eric

Most newer residential furnaces and thermostats are wired up to operate a conventional Freon based A/C. I have never seen a swamp cooler adapted to a residence. Is the chilled water piped to a coil in the furnace duct work the same as a Freon system? Or is the air diverted to the swamp cooler?

Usually there is a 24 volt AC transformer in the furnace with the secondary winding connected to a red wire running to the thermostat and a blue wire, common, to the gas valve, A/C relay, and fan relay. From the thermostat there will be white wire to the gas valve, yellow to the A/C, and green to the fan. The thermostat is wired to switch the power from the red to the white, yellow, and green as needed with the blue completing the circuit. Most thermostats and furnaces have the contacts labeled R, B or C, W, Y, and G for the corresponding wire colors. It may be wired to have the A/C control wires return to the furnace and its controls and then a second wire goes to the A/C unit. Internal wiring may replace the green wire if the thermostat does not give you the option of fan only or continuous fan. Digital or programmable thermostats may need the blue wire connected to them.

In your case, you will need to run wires from the Y and B terminals to relays that control the water pump and blower for the swamp cooler. The blower in the furnace should come on to distribute the air.

If you pipe cold water to a coil added just down stream from a forced air furnace, you shouldn't need any other changes to the duct work. If instead, the air must go to the swamp cooler, you will need dampers to control it. It might be easier to manually switch them when you change from heating to cooling. Get back with me if so, and maybe we can work it out.

This is a good site for questions were it takes several exchanges of information to work out a solution.

Thanks for the response, it was very informative for the thermosts situation, but I am not addingthe swamp cooler, it currently has the swamp cooler. I want to add an "A" coil to the duct above the furnace and the condenser of my new unit will sit on my back patio. The unit I am adding is a goodman unit with a 4 ton 13 seer condenser and a 5 ton evap coil, I was wondering how can I check if the current furnace moves enough air (on the return) to keep from freezing my coil? Also, if the coil is above the furnace (upflow furnace in a closet) will the heat coming on in the winter cause any harm to the AC system? Or should I try to run it below the furnace (in the return line), so it will not be in direct contact with the hot air blowing on it? Thanks again.

PS my output side of the furnace has 12x20 sheetmetal ducting if it helps.

PS, I got a guy from a local AC place to do my final sweating and charge the system so I am OK for that one.

OK, I misunderstood the question. You are putting a conventional Freon based system in place of a swamp cooler? I think they banned the precharged units at the end of 1994. I paid to have my system soldered and charged in 1995. The technician also adjusted the multispeed blower for the A/C. On my furnace control board, you control the speed of the blower by moving jumpers around. You want a somewhat faster speed for A/C. I think you start with one setting and adjust the speed until the air temperature is right.

The thermostat should be simple, running wires from the Y terminal at the stat to the Y at the furnace and compressor coil. The coil also needs a common as I explained.

Oh, I forgot. The A coil goes downstream of the furnace for down flow or horizontal. That way you don't chill the heat excanger and create condensation there. With an upflow, that could cause condensate to drip down into it. I guess if the pan catches it, it would be OK. Just be very careful to keep the drain clear.

Thank you very much. I have seen many of the newer unit installed pre-charged (Like this one: http://www.alpinehomeair.com/viewproduct.cfm/productID/453056636/p/Goodman_PCK048_1_Self_Contained_Air_Conditioner ) From what I see it is like a bigger version of the ones you put in a window, only it has a return duct as well (that has to be installed). While I am working on roofs and I am watching them install them I have never seen them even pull out a gauge set to chek it, just works from the factory. The one I want also says it is pre-charged for up to 15' of copper line so I don't know. Either way after I install it I want to have a service guy come out to solder it, check the pressure and charge if needed. I got a guy to do it (he works for a local AC company) so I can hopefully get a deal on that as well. Well, thanks a bunch.

PS So basically the thermostat will just suply 24v to the condenser unit when it needs to and turn on the furnace blower fan at the same time? This would be so much simpler if I could find a scematic, but I think I can get it, Thanks again.