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Project: Adding 1000 sq ft of basement onto a 2-year-old 3300 sq ft house. House has existing gas forced-air furnace and 4-ton AC unit outside.
In order to heat/cool the basement, my HVAC contractor is giving me a choice (whatta guy!)
1) Install a new gas forced-air furnace in the crawlspace and a new AC compressor/coil
or
2) Turn the exisiting furnace/AC system into a multi-zone system. Top floor is one zone, main floor is one zone, basement is one zone.
Prices are not too far off since labor in solution #2 is much higher than solution #1. But, equipment costs are lower in #2.
I like the idea of 3 tstats and 3 zones. Any warnings/caveats I should know about?
Replies
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Three zones.
J
*Go with the additional unit for the following reason. If you only want to cool the smaller area, your four ton unit will cool it, but will not remove the humidity. Air conditioning, without getting into re-heat units is a fuction of both temperature and humidity. To get rid of sufficient humidity with the larger unit you will have to cool down the smaller space to an unacceptable temperature. I have a 4000 ft sq. office buidling - and I have four separate airconditoners. Works fine, utility bills are low (about 351 per month in worst case and this includes a fair amount of mechanicals such as a large air compressor, a 2 hp vacuum pump running almost continualsly, and a few other gagets. For what it is worth. From a DYIer.Dennis
*You need to find out if your 4 ton unit is over sized. (most A/C units are.) If your unit was running less than 75% of the time on the hottest days last year then it has more than enough over-capacity to handle 1000 more square feet of basement. Adding the basement to the conditioned space will allow the unit to run longer and give better dehumidification than you have now. A 3 ton and a 1 ton unit would be better, but I assume that you want to keep the origional unit.If the 4 ton unit is not over sized then follow Dennis's advice.
*Good point, I didn't think of the correct sizing of the first unit. I have been using 18,000 per ft sq. in the Midwest. Dennis
*I have seen a 5 to 1 variation between cooling load per square foot in actual houses. Overhangs, trees, insulation, roof color etc. make so much diff. in the heat gain in the summer. Without math and simulations, runtime will supply the answers. The properly sized unit should be running almost all the time on the hottest days.
*Thanks for your feedback Dennis and Ron. One of my frustrations (being a computer engineer) is that the HVAC contractors around here don't like to do heat-loss calculations. So, it's not clear if we have the thermal/airflow capacity or not for the additional basement space. I'm hoping to get the final bids from the contractor next week. Then we'll see how he calculated the amount of heat/cool needed and how much I have left in the existing system.