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Heat pump size

DADO150 | Posted in General Questions on

Hello, I’m looking to replace my AC  unit with heat pump. I live in N. Central FL & have a 2300sf home, 3 bedroom, R13 walls & r30 ceiling insulation.  The inside unit flows up threw ceiling. The run is about 25ft of 10inch insulted duct. Have about 35 ft lineset under the house to outsude condenser. AC Guy said I’d need a 5ton heat pump. I’ve been looking & found this Mr. Cool unit. Never heard of it so would it be worth while to buy being so new or get something like a Goodman? Also our winters get to 30° for a couple days but summers in mid to upper 90’s. 
Thanks for the help,

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  1. paul_wiedefeld | | #1

    Unfortunately, you’ll have to size it yourself or pay someone who knows what they’re doing. Contractors are often worse than useless at sizing. 5tons sounds 2x too big. What do you currently use for heat?

    Brand doesn’t matter much compared to getting it installed correctly.

    1. DADO150 | | #2

      Ok Thanks. We don't have heat. We use space heaters & a small propane wall mount for those occasional cold days. The 5kw strip in the handler has never been hooked up. With our new addition we felt the heat pump would be more efficient & economical. The current 2 ton AC unit can't keep up with the extra space during the hot daytime temps & runs alot.

  2. paul_wiedefeld | | #3

    A heat pump will definitely be more economical and efficient for heating compared to space heaters or electric strips. It’ll be the same at cooling as an equivalent AC.

    Running constantly and maintaining set point is the goal. Does the 2-ton turn off during the hottest summer afternoons? It could be unbalanced ductwork that’s the problem and not the sizing. Either way, 5 tons is beyond absurd so you’ll either have to hold that contractor’s hand or find a new one.

  3. DC_Contrarian_ | | #4

    You can get some clues from the existing equipment.

    "The current 2 ton AC unit can't keep up with the extra space during the hot daytime temps & runs a lot."

    Running a lot isn't a problem, a properly sized air conditioner should run continuously on the hottest days. It's actually better for them to run continuously, stopping and starting is harder on the compressor.

    Not keeping up is a problem. I assume by this you mean it can't maintain the interior temperature? When this is happening, is it running continuously? If it is, then the current equipment is undersized. You can get some idea of what an appropriate size would be if you keep track of what outside temperature the current unit is able keep up with.

    If the current unit is cycling on and off when it can't keep up then something is wrong with it.

    1. DADO150 | | #5

      Sorry guys I should have said it doesn't maintain temp on hot days. If temp set on 73 & it's 95+out the temp will stay about 74-75 & just run. When temp is 90 it keeps the 73 temp good. The original part of house is 1300sf. The other parts of house is closed off until we get another unit. Really appreciate help.

      1. DC_Contrarian_ | | #6

        Thanks Dale. Could you look up your county in this document:

        And tell us what the 1% cooling temperature is? That will help us figure out if the current system is undersized.

        1. DADO150 | | #7

          It's 92.

          1. DC_Contrarian_ | | #8

            Thanks. It sounds like your existing system is actually sized pretty well. The way a cooling system is supposed to be sized is at the 1% temperature it runs 100% of the time and maintains interior temperature. It sounds like your system can do that, or come close. If it's undersized it's not by much, maybe 10-20%.

            So I'll concur with the others and say five tons is way oversized. You don't want an undersized system, they tend to have short run times which means they don't dehumidify well so you get a house that is cool and clammy.

  4. DADO150 | | #9

    Thanks for info. I do get alerts that the humidity is high & to check for open doors, windows etc. I've been looking at the Mr.Cool systems at Home Depot & Lowe's. I'm not certain at HSPF means on the system other than about $500 differing price. Is this like variable speed? Is that a better choice than simple fan motor? Sorry I'm trying to get an understanding to get best unit for durability, down rod ease of repair & repair cost, & keep electric bill down.
    Thanks for all your help & suggestions

    1. paul_wiedefeld | | #11

      It can certainly be daunting. HSPF is an efficiency rating for heating, like SEER is for cooling. Higher is better, but frankly it doesn’t matter in Florida. Variable speed allows the equipment to run for longer periods, which is more efficient (it’s like driving 55 mph for two hours vs 110mph for one hour). It’s usually much quieter and described as more comfortable. However, variable speed has limits to exactly how low it can run, so can’t overcome massive oversizing. Taking a step back, there might be improvements you can make beyond equipment. If the house is overly humid, a blower door test could help you find and plug leaks.

      1. DADO150 | | #13

        Thanks for explanation. Once I fund an hvac guy/ company hopefully they can get me straight.

  5. walta100 | | #10

    My wild guess is your home is likely to be very leaky in that the cold air from your AC tends to leak out of the house.

    See if you can find someone locally to "blower Door" test and air seal your home this may well be enough of an improvement that the current equipment will be able to keep up.

    It sounds like you have ductwork in your attic and maybe equipment. If that duct work has any leaks, it will have a huge impact on how the system operates. If you find a blower door person, they are very likely to have a “Duct Blaster” to test the duct work and repair if needed.

    From an economic point of view it is never a good idea to replace the existing equipment before it has failed to the point that repair is a poor choice. Economically it will be difficult to justify the extra cost of a heat pump given how little heat your home requires. Lets say the HP is a $800 upgrade and will save you 75% on your heating cost $50 per year. So you save $37.50 per year 800\ 37.5=21.3 year to break even if the equipment lasts that long and if you still own the home both somewhat unlikely. Yes I made up the numbers so do your own math.


    1. DADO150 | | #12

      That's a great point, i understand the made up numbers. Yes there's is insulated flexible duct in the attic. I'll check around for a blower door guy.
      Thanks again.

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