Adding A/C

14 years ago

I had a bid to add A/C to my home. The gentleman has since retired and moved and I cannot find the bid with the specs for the coil, etc.........We had gas heat added when we bought the home in Dec. of '08, but opted to not have A/C at the time. I have a friend with HVAC exp. who will help me install it all, but I need to figure out what to purchase. I have "twinned" Payne PG9MAB furnaces, unit size 048100, 100,000 BTUs, single thermostat. If I recall correctly, he was going to install (2) coils and one unit outside. Does that sound feasible? I'm going to guess the house, 2-story, is around 4,500 square feet, although 3 of the smaller rooms have no ductwork.

Also, the furnaces sit on a 4" (what I believe is called) downflow sub-base, built out of plywood. Each has (1) CA return attached to the side and then a second furnace filter between the unit and sub-base for additional return flow (I presume). Don't know if this matters, but I mention it because the plenum sits on top and there isn't a ton of room above before you get to the floor joists.

Is this do-able with (1) condenser unit outside and would I have to get separate A coils? I do recall him saying that he would only need (1) condenser unit.

Thanks

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Comments (5)

dougl99
14 years ago
It sounds like you have (2) 4 ton drive 100k gas furnaces?? If you need more than 5 tons of a/c you could have a problem. I don't think anyone makes a single phase unit over 5 tons. Can you split the duct system? If so, that would be a much better approach. You would be able to zone your home. When you twin furnaces, the furnaces must come on at the same time or you could blow back through the furnace that is off (or the blower wheel will start turning backwards and when power is put to it, it can run backwards).
bold_as_love
Original Author
14 years ago
I'm sure I could split the duct system. Are you talking then: (2) thermostats, (2) evap. coils, and (2) condensers outside? I know that the plenum could be split to equally distribute the ducts. So, I could have one furnace w/ A/C service half of the ducts and the other furnace do the same? They would kick on individually of each other going forward in the summer? I guess I could also install a removable panel, pull it out in the winter, and re-twin them for heat, if I wanted to, huh?

I think the owner of the home (we're now in contract to purchase) twinned them for heat and just assumed we continue on with window units. No way could we have enough window units to cover the house and it gets hot here in the summer. Last summer wasn't very enjoyable in certain rooms!

Thanks
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dougl99
14 years ago
correct. Two completely separate systems.
bold_as_love
Original Author
14 years ago
Thanks Doug. Based on what I posted, how many tons would I need for each, if you can tell me?
dougl99
14 years ago
The info on the furnaces will only tell you what size a/c you can match up. On a single 4 ton drive furnace you would typically be able to install a 3, 3.5, or 4 ton a/c by adjusting the blower speed...............however, a heat load would be required to tell you how much air you need.

Adding A/C

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