Compressor size

Discussion in 'General Discussions' started by RonL, Feb 14, 2021.

  1. RonL

    RonL Member

    Concerning compressors sizing compared to how the heat pump is rated. I have read in several places that compressors are rated actually about 1/2 ton smaller than the heat pump rating. When I look up my fhp which is supposed to be 2.5 tons.. but by the name it is an em031.. I would think that would be 31k btu or 2.58 T but the compressor is rated at 28.8 K. So from that I'm assuming the rating of the compressor is only part of the equasion... anyone know how they actually come up with a rating.. especially when heating btu is usually different than cooling. Do they average them some how ? I am still trying my best to get the right size replacement for when my fhp dies. According to sheets I am seeing, 2.5 seems to small but the cost to run a 3 is a lot more.
  2. SShaw

    SShaw Active Member Forum Leader

    There is a method used to size the heat pump to your house's heating and cooling load. It's called a Manual S. See loadcalc DOT net.

    The process compares the heat pump's heat output with your house's heating load, and it compares the heat pump's latent and sensible cooling outputs with your house's sensible and latent cooling loads.

    You get the heat pump's outputs from the manufacturer's data tables, using the expected entering water temperature, water flow, and airflow.

    I don't know how you concluded that a 3T costs a lot more to operate, because it should cost about the same.
  3. RonL

    RonL Member

    I was looking at more amps for the fan and more for the compressor.. about 6a more. But I would guess more btu would mean less running time so it could equal out.. plus less time for the well pump to run. But I will definatly try the loadcalc out. Hopfully i can figure out all of its inputs.
    Thanks for your help !
  4. RonL

    RonL Member

    Tried the loadcalc, and it says I only need 10k btu.. so I must be doing g something way wrong. I'll overplaying with it.
  5. SShaw

    SShaw Active Member Forum Leader

    On the loadcalc page, there is a button on the bottom right "Manual S Based Sizing Calculator." That takes you to a page used to size the equipment. You would need to know the heating and cooling loads for your house to properly size your system though.

    You cannot compute wattage from the amps. Ignore the amps. The compressor, pump, and fan motors are inductive loads, so Watts does not equal Volts X Amps.

    If you want to compare two machines, then note your entering water temperature and look up the performance data (COP/EER) at the expected EWT. I believe you have open loop, so the EWT should be constant and you should know it, or be able to measure it, from your existing system. It would typically be around 50-60 deg for open loop. See below table for an example of what you need to be looking at if you want to compare efficiency. This is also where you get the equipment capacities to use in the Manual S:

  6. gsmith22

    gsmith22 Active Member Forum Leader

    The heat pump is a system made up of components (one being the compressor) that perform a function (moves heat from one place to another). Individual components of the system don't have a tonnage rating. Only the system has the rating.

    In an attempt to put all heat pumps on a level playing field (this is what you are trying to do, right?) AHRI creates a standard test condition and tests all heat pumps at that condition in order to quantify the capacity of a heat pump and resulting energy use. I have attached the AHRI performance tables for a Waterfurnace 7 series (this is what I have so I had it handy). Every manufacturer should have something similar for their heat pump. The "rating" that you are referring to, your heat pump being called a 2.5ton but its name implies a 2.58ton is probably coming from some of the tested conditions in the AHRI rating. WF 7 models have names that match the middle of the road tested conditions so I suspect most manufacturers do something similar.

    Now ignore all of that. Why? Because no one is running their heat pump at AHRI rated conditions. Those conditions include incoming water temp, air temp, fan speed, etc. that don't match what your system will be running at the design day for winter or summer. Your heat pump has to be sized for the design day or it won't be able to heat and cool when you need it most. And that is when you care what the capacity of your unit is. Furthermore, the capacity of your unit changes based on the incoming water temp, air temp, fan speed, etc. Its not constant. You don't have a 2.5ton unit. You have something that probably varies from 1.8tons at the coldest point of winter to 2.8 tons at the warmest point of summer. Your unit will almost certainly have less heating capacity than its AHRI rating and more cooling capacity than its AHRI rating. As SShaw rightly points out above (and I tried to in your other post) find your unit's specification chart, pick an incoming water temperature, chose the water flow, and then read off its capacity and energy usage. Then do the same for the second unit at the exact same conditions. Then you can compare them.

    Attached Files:

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