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  1. #1
    Join Date
    Jul 2004
    Location
    Kent, WA
    Posts
    129
    I'm looking at options for a heat pump replacement of an oil furnace. Manual J says I need about 57K BTU at my design temp of 28F. If I go with a 4 ton HP and airhandler, I'm wondering if the airhandler is OK with the combination of resistance + heat pump heat. The most resistance heat you can buy for most 4T air handlers is 20KW and I was going to go with 15KW. The heat pump may only make around 30K BTU on a cold day. Full up resistance heat adds about 50K BTU. So I could have 80K BTU being produced when its real cold out.

    Am I guaranteed that I won't trip out on high limit when the full 15 KW is being used along with the heat from the heat pump? How do you figure the max BTU limit of an air handler? Do you need to find its max allowed temp rise and then use the installed air flow rate to see if you can push enough BTU's to keep from overhating? If so, what is a typical temp rise limit for most air handlers, as no specific equipment has been selected yet?

  2. #2
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,387

    Thumbs up Heater Selection

    Originally posted by suemarkp
    Manual J says I need about 57K BTU at my design temp of 28F. .. I was going to go with 15KW.

    Am I guaranteed that I won't trip out on high limit when the full 15 KW is being used along with the heat from the heat pump?

    What is a typical temp rise limit for most air handlers, as no specific equipment has been selected yet?
    NOTE: Mfg would not be selling 20 kW heater
    if it was not perfectly safe.

    Delta T ( Temperature Rise)
    80,000 BTUh / 1.08 * 1600 CFM =~ 45'

    Room Temp 70'F
    Delivered temperature at diffuser would be roughly 115'
    at blower full-speed

    Flexible duct is rated at 250' F
    http://www.thermaflex.net/content.php?page=mkc

    I would use the 15 kW heater assuming you have
    < 100 total hours < 30'F
    and you don't mind in the lower 60's
    when outside temperature may dip close to 20'F.
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  3. #3
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    Air flow will be fine,If ESP is low enough,and A/H is on the correct speed for 15KW.

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