Gas and Oil Home Heating Furnaces - Does the supply and return ductwork have to be the same size?
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Darkhorse
05-26-03, 02:20 PM
We have a fifty year old house and we need a new HVAC system. The house is 2300 square feet and a ranch on slab. The current system is a 175,000 BTU input furnace from 1985. The AC is a 5 ton. The new system will be 3.5-4 ton ac and the furnace will probably be a 120,000 btu unit.
Here's the question. They say my current supply ductwork buried in the attic can move at least 2000 cfms of air. The main trunk in the attic above the furnace/ac plenum in the utility room below starts out at 36x8.25. It tapers eventually but travels 65 feet and has 18 branches coming off of it. That's pretty big. They want to size the new return ducting for about 1600 cfm if I go with the 4 ton AC and 1700-1800 if I go with the 120,000 furnace with that. Smaller if I go with the 3.5 ton AC and the 100,000 furnace. The furnace will be variable speed. The return is being replaced due to water in the returns under the slab. If the new return ductwork is smaller in capacity(cfm) than the current supply can it cause problems being unbalanced like that? I've been told it will cause a severe loss in static pressure and air velocity and create heating and cooling problems. I was told it would really hurt latent cooling capacity. The old furnace/ac matched the supply duct capacity but the new units will be smaller and this, along with supply ducting that is too big for the new system will contribute to the problem I was told. Any thoughts? Thanks.
Here's the question. They say my current supply ductwork buried in the attic can move at least 2000 cfms of air. The main trunk in the attic above the furnace/ac plenum in the utility room below starts out at 36x8.25. It tapers eventually but travels 65 feet and has 18 branches coming off of it. That's pretty big. They want to size the new return ducting for about 1600 cfm if I go with the 4 ton AC and 1700-1800 if I go with the 120,000 furnace with that. Smaller if I go with the 3.5 ton AC and the 100,000 furnace. The furnace will be variable speed. The return is being replaced due to water in the returns under the slab. If the new return ductwork is smaller in capacity(cfm) than the current supply can it cause problems being unbalanced like that? I've been told it will cause a severe loss in static pressure and air velocity and create heating and cooling problems. I was told it would really hurt latent cooling capacity. The old furnace/ac matched the supply duct capacity but the new units will be smaller and this, along with supply ducting that is too big for the new system will contribute to the problem I was told. Any thoughts? Thanks.
willywhy
05-26-03, 04:26 PM
Can help with the most of your question.
BUT, FOR SOME REASON IT'S BEEN IMBEDDED IN MY HEAD THAT,RESTRICTING THE RETURN AIR CAUSES THE BLOWER MOTOR TO WORK HARDER AND MAY BURN OUT.
BUT, FOR SOME REASON IT'S BEEN IMBEDDED IN MY HEAD THAT,RESTRICTING THE RETURN AIR CAUSES THE BLOWER MOTOR TO WORK HARDER AND MAY BURN OUT.