Results 1 to 11 of 11

Thread: Electric Heat kit minimum CFM

  1. #1
    Join Date
    Apr 2020
    Posts
    27
    Post Likes

    Electric Heat kit minimum CFM

    I have a 2 ton Heat Pump paired with 15kW of electric heating. Manual J shows 10kW total required in the winter. The extra 5kW was installed as emergency heat and for when the outdoor temps drop below design.

    My air handler specifies a minimum of 875 cfm for 10kW strips and 1050 cfm for 15kW. Unfortunately, my options with the multi speed fan are 880 cfm or 1385 cfm at 0.5 wc. The high speed definitely makes a lot of noise and Im concerned about it overloading the ductwork or tripping out the fan on high static (it has never done this yet). Its also noisy and doesnt heat evenly at that high speed.

    Would it hurt the system to run the full 15kW at the lower 880 cfm? Or is the 1050 cfm suggested a hard minimum? I do not want to make the system cycle on the thermal limit switches (they are 150F-50)

    For reference, the air handler is a Goodman ASPT29B14.

    Thanks in advance for your expertise.

  2. #2
    Join Date
    Aug 2002
    Location
    Southold, NY
    Posts
    30,708
    Post Likes
    Whats the actual Static Pressure on the unit?

  3. #3
    Join Date
    Jan 2015
    Location
    Bellevue, Washington, United States
    Posts
    1,866
    Post Likes
    Well I'd expect a 2 ton air handler to be running at it's highest speed if it's using a 15 heat strip. Second I would not suggest oversizing your heat strip. You'll spend hundreds more every winter.......

  4. #4
    Join Date
    Jul 2009
    Location
    South Carolina
    Posts
    8,831
    Post Likes
    I'm not a fan of staging heat strips but I think in your case it would be the best solution.
    I would just set the system up to run normally at the 880 or so CFM's with 10kw & then to go to the higher speed only if EM heat is called for along with the other 5kw.
    Gary
    -----------
    http://www.oceanhvac.com
    The best things in life are free but not everyone is willing to pay the price.

  5. #5
    Join Date
    Apr 2020
    Posts
    27
    Post Likes
    Thread Starter
    I was assuming 0.5 static, but have to double check it.

    Part of my problem is my dumb ecobee thermostat keeps calling for the second stage of Aux heat when the heat pump locks out. Ive even got it set to manual staging but for whatever reason it goes for it even when the temperature requirement isnt met yet.

  6. #6
    Join Date
    Jan 2004
    Location
    PA
    Posts
    79,680
    Post Likes
    Quote Originally Posted by reh22 View Post
    I was assuming 0.5” static, but have to double check it.

    Part of my problem is my dumb ecobee thermostat keeps calling for the second stage of Aux heat when the heat pump locks out. I’ve even got it set to manual staging but for whatever reason it goes for it even when the temperature requirement isn’t met yet.
    The static pressure can't be .5" at 880, 1035, and 1385 CFM.

    What temp are you locking out the heat pump, and or aux heat?

    Moving 880 CFM with 15KW of aux heat, will give a temp rise of 54F.
    Contractor locator map

    How-to-apply-for-Professional

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

  7. #7
    Join Date
    Apr 2020
    Posts
    27
    Post Likes
    Thread Starter
    Right now I lock out the heat pump at 20 degrees.

    Is temp rise the driving factor behind these minimum air flows provided in the air handler specification, or is it minimum to prevent the heat kit from cycling on high temperature?

  8. #8
    Join Date
    Jan 2010
    Posts
    20,910
    Post Likes
    Both

  9. #9
    Join Date
    Jul 2009
    Location
    South Carolina
    Posts
    8,831
    Post Likes
    Quote Originally Posted by reh22 View Post
    Right now I lock out the heat pump at 20 degrees.

    Is temp rise the driving factor behind these minimum air flows provided in the air handler specification, or is it minimum to prevent the heat kit from cycling on high temperature?
    There is really no reason to lock a heat pump out at 20 degrees unless you have a dual fuel system. A heat pump is still more efficient than heat strips even down to 0 degrees. The only down side is the efficiency loss during defrost but the Heat Pump is still more efficient at colder temps than strips. Just make sure the defrost is set to 90 minutes. What you might consider is to have the outside thermostat wired to activate the strips along with the heat pump at colder temperatures. That would give you more heat & still get whatever efficiency gain you can from the heat pump.
    Gary
    -----------
    http://www.oceanhvac.com
    The best things in life are free but not everyone is willing to pay the price.

  10. #10
    Join Date
    Apr 2020
    Posts
    27
    Post Likes
    Thread Starter
    Thanks. I actually dont have an outdoor air temp sensor on the unit, we never had it put in since the Ecobee has the lockout function built in. I might go ahead and have one installed if I can use it to activate W1 when temps get cold and leave the lockout to the ecobee when its down below 0.

    As for the full 15 kW of heat, Ill leave that on the high fan speed to prevent the unit from cycling. Right now I have W2 jumpered to speed 5. Its an X-13 type motor. Not sure if the speed taps could possibly back feed the contactor and run the heat strips? If so, Ill switch over to a relay for this function.

  11. #11
    Join Date
    Jan 2010
    Posts
    20,910
    Post Likes
    What's the condenser model number ?

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •