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Thread: Blower Size

  1. #1
    Join Date
    Apr 2011
    Posts
    14

    Blower Size

    My gas furnace and central a/c were installed by previous owners 15+ years ago in the course of putting an addition on the then-40-year old existing house. Both pieces of equipment are grossly over-sized. I have had Manual J run and it would indicate max heat load of slightly more than 80,000 BTUs an hour and max cooling load of just over 3 tons. The old equipment is an American Standard single-stage 5-ton condenser and an American Standard single-stage, 140,000 BTU input, 80% AFUE furnace with a 5-ton blower. I have been told that the maximum flow that the supply ducting can handle given its size is 1500 CFM. As you can imagine, we have had lots of problems over the years with short-cycling, coil freezing in summer, very uneven heat in winter, etc. I have wanted to replace both but wife was reluctant to spend to do it while system "still worked." Tech came in recently to start up furnace and found a cracked heat exchanger (no surprise). Rather than invest in significant repair under the circumstances, I decided to take the opportunity to re-do the system properly. We have a local contractor (who seems very competent and comes highly recommended both by other homeowners and tradesmen I respect). He proposes to install the following:

    1. Bryant high efficiency multi stage gas fired furnace model 997 MA 080 V capacity 78,000 – 31,000 BTU’s input. AFUE of 97%. Multi Stage Gas Valve. Variable Speed Motor
    2. Bryant model CNPVP 3721 cooling coil capacity 3-tons.
    3. Bryant Two-Stage Puron Condensing Unit model 187 BNA 036, capacity 3 -tons. Combination coil and condensing unit has a Seasonal Energy Efficiency Rating of 16.5.

    My question: Is the 3-ton blower in your view the correct choice (that is, should it match the tonnage of the condenser), or would anything be gained by going with a larger blower, say 3.5-tons, which the supply ducting should be able to handle?

  2. #2
    Join Date
    Jun 2007
    Location
    Greenwood Indiana (Indianapolis)
    Posts
    420
    in order to get a bigger blower, you would need to purchase a larger furnace. It sounds like the contractor has chosen the appropriate sized equipment. If you have a 3 ton a/c then you need 3 tons of air flow (1050 cfm's- 1200 cfms) you will like the variable speed blower for several reasons, but mainly that it is a smart motor and will deliver you the correct air flow, even if the filter starts to get a little clogged, at all times.
    As iron sharpens iron so one man sharpens another Proverbs 27:17 NIV84

  3. #3
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,599
    Moving more than 1200 CFM on a 3 ton A/C can cause higher than desired humidity unless in a desert type climate.

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