Results 1 to 7 of 7

Thread: cfm per btu

  1. #1
    Join Date
    Jan 2007
    Location
    NE OHIO
    Posts
    778

    cfm per btu

    how much cfm do you need for 70000btu furnace, new install with 2 10inch round returns same supply

  2. #2
    Join Date
    Sep 2002
    Location
    I don't know
    Posts
    2,903
    General public's attitude towards our energy predicament: "I reject the reality of finite resource depletion and substitute it with my own; energy is infinite, we just need an alternative storage medium to run the cars on. The economy can grow indefinitely - we just need to "green" everything! Technology is energy! Peak what?"

  3. #3
    Join Date
    Jan 2007
    Location
    NE OHIO
    Posts
    778
    HUH

  4. #4
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,875
    Depends on its efficiency and the allowable temp rise.

    For a 55F temp rise. At 80% eff, 940CFM, at 90% eff, 1060 CFM, and for 95% eff, 1120 CFM. Whcih your 2 10" aren't good for any of the above CFMs.
    Contractor locator map

    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

  5. #5
    Join Date
    Jan 2007
    Location
    NE OHIO
    Posts
    778
    its a 95 didn't think it was enough, our wonderful installers Thanks beenthere

  6. #6
    Join Date
    Mar 2011
    Location
    Northeast Missouri
    Posts
    61
    The formula I have used for a condensing furnace is to divide the input BTU by 10,000 and then multiply that number by 150(cfm for a condensing furnace) So roughly 1050 cfm, your return is a little short. I would watch that heat rise cause I bet your on the high end of it or even over. And as a side note I have been down that road with our installers too, just remember keep it professional.

  7. #7
    Join Date
    Jul 2008
    Location
    Rochester NY
    Posts
    4,721
    Here's a great example of expectations and reasonableness being totally disconnected. Homeowners buy replacement furnace (maybe not in this case, but I'll use this as a common example) and expect the hvac company to know everything about issues with the house, the duct work, etc, all based upon the "free estimate".

    Your free estimate got you little to no engineering. By getting 6 free estimates you put a lot of cost on 6 hvac companies, which really means YOU pay for that in your price.

    We understand the consumer thinks this is how to get a furnace as "cost effectively" as possible, but once you signed that free estimate, unless your bid specification included a lot of diagnostics and design, you got the furnace quoted installed as quickly as possible.

    Did you have a specification?
    Which makes more sense to you?
    CONSERVATION - turning your thermostat back and being uncomfortable. Maybe saving 5-10%
    ENERGY EFFICIENCY - leaving your thermostat where everyone is comfortable. Saving 30-70%

    DO THE NUMBERS! Step on a HOMESCALE.
    What is comfort? Well, it AIN'T just TEMPERATURE!

    Energy Obese? An audit is the next step - go to BPI.org, or RESNET, and find an auditor near you.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
Comfortech Show Promo Image

Related Forums

Plumbing Talks | Contractor Magazine
Forums | Electrical Construction & Maintenance (EC&M) Magazine
Comfortech365 Virtual Event