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  1. #1
    Join Date
    Jan 2007
    Long Beach, CA
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    Need help with size of new HVAC in SoCal

    I am looking to replace the original floor furnacea with a central heating&cooling system. The home was built in 1953, 1700 sqare ft. located in Southern California (about 5 miles from the coast). I've had 7 estimates, most of them very similar, but one (who performed the heat-load calc.) about 50% higher than the others. The calculated numbers are:
    BTU Heating: 31,540
    Heating CFM: 1,351
    BTU Cooling: 34,618
    Sensible Cooling: 28,553
    Latent Cooling: 6,364
    Cooling CFM: 1,353
    To satisfy these, that contractor bid Trane equipment that was 3 ton cooling and a two stage, 63,000 BTU furnace. That bid is too expensive.

    I am leaning towards the contractor who is bidding Carrier equimpent. However, he recommends 3.5 ton 38HDR air conditioner and the Infinity96 58MVB080-20 (80K BTU, 2-stage, variable speed) furnace.

    I think he is oversizing the equipment. He didn't do a heatload calculation, he's been in the business locally for 24 years and says he knows these homes very well. I want to ask him to bid the 3 ton unit instead and the MVB060-14 (60K BTU).

    Is he oversizing the equipment? Would my proposed equimpent be sufficient based on the heatcalc numbers?

  2. #2
    Join Date
    Aug 2002
    Office and warehouse in both Crystal River & New Port Richey ,FL
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    Depends on the accuracy of the load calculaton,and at what indoor temperature was used in the calc.

    If the carrier guy didn't do a calc.,then I'd be inclined to go with the one you have,If it's accurate.

    You could do your own ,on this site,see hvac-calc above,for a small fee.

  3. #3
    Join Date
    Dec 2006
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    I know you can do the math, but here it is for you:

    Calculated required Heat BTU 31540
    Calculated required Heat Tons 2.63

    Furnace BTU 63000
    Furnace Tons 5.25
    Furnace oversize 99.7%

    Furnace BTU 80000
    Furnace Tons 6.67
    Furnace oversize 153.6%

    If the calculations are correct and they factored in all losses, I doubt you would ever need stage 2 on either furnace. Did the calculator guy use the correct R Value for the exterior walls? Did he know what the window U (R) value truly was? If you guess poorly on both, you could be off by 50% real fast.

    These are just my numerical opinions.

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