Gas and Oil Home Heating Furnaces - hvac sizing
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11-05-01, 02:55 PM
I need to replace old heating/ac system. My home is 2000 sq. ft., brick and wood siding, 2 x 4 studs with fiberglass insulation. 178 sq. ft. window space. 8 ft. ceilings. Duct work under house. I live in central Arkansas, mild winters, hot and humid summers. This will be electric a.c., gas heat. What tonnage unit should I use and what size return air vent should it have. Thanks
resercon
11-06-01, 10:03 AM
The easiest way to determine what size unit you need is to go to the old heating system and read the specification plate. For your home it should read 100,000 Btu's input and 80,000 Btu's output. If the house is the same when this heating system was put in, meaning to say no additions have been put on, then you would require the same amount of output that's on the plate. With high energy efficient heating system the input number will lower but the output would be the same. In other words, if the furnace is 90% efficient, the input will be 90,000 Btu's and the output will be 81,000 Btu's. If you really want to know the load calculations find a book known as ASHREA and look in manual "J".
Jay11J
11-06-01, 11:12 AM
Resercon,
You can't always go by what your old furance is rated for. Lot of times in the "Good Old Days" they over sized your furnace to keep you very warm!
Raymond,
I'd suggest you bring in a good Rep comapany to size your new equipement for your home. Like I said earlier, Don't go by what you have now.. Not knowing how old of a system you got. One home I've done had a 150,000 btu furnace and come to figure everything out, it only needed 75,000 btu system, and the homeowner was SOO Pleased with it! :-) She said she's no longer hot, then cold when it had a big swing in temp.and she said her gas bill was cut almost half!
You can't always go by what your old furance is rated for. Lot of times in the "Good Old Days" they over sized your furnace to keep you very warm!
Raymond,
I'd suggest you bring in a good Rep comapany to size your new equipement for your home. Like I said earlier, Don't go by what you have now.. Not knowing how old of a system you got. One home I've done had a 150,000 btu furnace and come to figure everything out, it only needed 75,000 btu system, and the homeowner was SOO Pleased with it! :-) She said she's no longer hot, then cold when it had a big swing in temp.and she said her gas bill was cut almost half!
Mike Swearingen
11-06-01, 01:31 PM
And MY lesson-learned story...(You CAN get too large of a system).
I remodeled my 1977 home in 1998, including all new Andersen windows and doors, which drastically improved the energy efficiency of my home.
I hired one of the "best" HVAC contractors in the area (based on their reputation for excellent service), and paid them $6,500 for a new system (with no competitive bids) for
their best Carrier hi-efficiency heat-pump, programmable thermostats, zone-controlled, etc., etc., etc.
Long story short...they installed a 1998 4-ton to replace the old 1977 4-ton without doing ANY energy audit of the remodeled home. I did not know any better. I thought that they did.
It never worked properly from Day One as far as dehumidifying our home. We had mildew all over walls, furniture, carpets, etc., and some of our wooden furniture finish was ruined.
Round and round...Carrier rep finally came out, inspected everything, and told me that they should have installed a 2.5-ton system.
All they have done, however, is to slow the fan down to minimum speed, and adjusted the system down to 3.5-tons (which I didn't even know that they could do.)
It works O.K. now, as far as the dehumidifying, but I have never seen the promised 30% energy cost reduction with the system that they sold me.
I'm living with it, but I'm not happy (and I could refer a LOT of business their way, but I don't.)
I wish that I'd had J11j's advice back in '98.
Mike
I remodeled my 1977 home in 1998, including all new Andersen windows and doors, which drastically improved the energy efficiency of my home.
I hired one of the "best" HVAC contractors in the area (based on their reputation for excellent service), and paid them $6,500 for a new system (with no competitive bids) for
their best Carrier hi-efficiency heat-pump, programmable thermostats, zone-controlled, etc., etc., etc.
Long story short...they installed a 1998 4-ton to replace the old 1977 4-ton without doing ANY energy audit of the remodeled home. I did not know any better. I thought that they did.
It never worked properly from Day One as far as dehumidifying our home. We had mildew all over walls, furniture, carpets, etc., and some of our wooden furniture finish was ruined.
Round and round...Carrier rep finally came out, inspected everything, and told me that they should have installed a 2.5-ton system.
All they have done, however, is to slow the fan down to minimum speed, and adjusted the system down to 3.5-tons (which I didn't even know that they could do.)
It works O.K. now, as far as the dehumidifying, but I have never seen the promised 30% energy cost reduction with the system that they sold me.
I'm living with it, but I'm not happy (and I could refer a LOT of business their way, but I don't.)
I wish that I'd had J11j's advice back in '98.
Mike
resercon
11-06-01, 01:35 PM
Raymond I'm sorry about the E-mail, I didn't know it was you. All the answers to my questions are in your post. Heating in ASHREA is based on two premises. The first is that the system must be large enough to heat your home on the coldest day of the year for your area you live. The second is the system must be able to heat your home with in 6 minutes. All this comes down to a default equation that can be applied to all areas in the United States. Let's say the equation is 1600 sq.ft. = 80,000 Btu's output. To apply this to your home you divide 1600 into 2000 = 1.25 and multiply that by 80,000 = 100,000 Btu's output. Then the varibles are multiplies to this number to determine the proper size unit for your home. Since your area has mild winters, let's say this varible is .8 the actual value would come from your State's Bureau of Vital Statistics. Therefore if I multiply 100,000 by .8 = 80,000 Btu's output. Other varibles are attic and wall insulation, type and size of windows, types floor, etc. All these are given values that are far more consistant than the weather. Without the actual value for weather patterns in your area, all I would be doing is making an educated guess on the proper size for heating in your home. ASHREA uses tables which does the calculations for you. If you want to know about your return system click on the little red house below this message and read topic " Combination "