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Resources for properly sizing heat pump(s) when installed as a secondary HVAC system

n2bragg
n2bragg Member Posts: 1
in A-C, Heat Pumps & Refrigeration

About me: DIY homeowner who is crazy enough to try anything with the help of the internet

My house: 1925 Bungalow in Chicago with forced air and central air. Frame construction, mostly plaster/lath, original windows with storms.

HVAC Situation / challenge:

  1. Upstairs does not have cold air returns so the central air does little to cool in the summer. (This was the fault of the previous owner, I'm not the guilty party for that one)
  2. Basement is too cold in the winter
  3. Generally want more individual control for each room

Possible solution: Keep the existing HVAC system as it is working pretty well. Add in a multi zone minisplit for the upstairs (2 bedrooms, 1 bath) and the basement. I assume I would often run both systems at the same time.

Specific Question: What resources are available to help me determine the appropriate size of minisplit for this application - specifically taking into account the existing forced air / central air system?

I have seen a number of resources (most notably eComfort) that assist sizing a system - going into a great level of detail, how many feet of wall are facing which direction, number of windows etc - but they all seem to be based on the assumption that the minisplit will be the only HVAC system installed. I understand having an appropriate sized system is important - even having an oversized system has drawbacks related to system longevity. So, I'm interested in looking before I leap - how should I be calculating my needs taking into account the existing system?

Comments

  • pecmsg
    pecmsg Member Posts: 5,978

    You size Heat Pumps for cooling. If like in your situation the heating load is larger multiple Systems need to be used.

  • DCContrarian
    DCContrarian Member Posts: 1,117
    edited April 26

    I recommend this piece if you really want to do a detailed job:

    https://www.greenbuildingadvisor.com/question/how-to-model-and-predict-electricity-usage-for-a-heat-pump

  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 10,383
    edited April 26

    For the second Floor, design the appliance for the necessary cooling needed. Most of the miniSplit heat pumps you are going to have to choose from are using variable speed compressors with ECM motors. That means that there is a computer program in the control that measures the needed cooling based on several temperature sensors in the system and will ramp up the motor speed to make the compressor do more work (more cooling capacity) and ramp down to a slower speed when the needs is much less (lower cooling capacity). So your air conditioner will never actually be over sized as long as you select the right one within reason. Don't use a 5 ton compressor if the maximum you will ever need for that space is only 2 tons. The motor will never get slow enough for the times you need it, and will short cycle which is the reason you don't want to oversize anything. Short cycling is not efficient and will cause premature equipment failures.

    In the basement situation you will rarely use the cooling system, if ever. That is because the central AC for the house will end up down there by gravity, duct leakage and the fact that there is very little heat gain when the basement walls are below the grade. So that one you may need to add more cooling that you actually need in order to get the heat required in the winter. So I might propose you size the basement system as if it was a regular room and not a below grade room.

    Selecting the indoor unit depends on your personal preference. if you don't mind the wall hung indoor units, then that is usually the easiest way to go.

    With 2 bedrooms up there and a bath, you may select to use 2 small wall hung indoor units and let the bathroom just use the overflow, if any.   There are also folks that elect to install a short duct air handler (that has fan not powerful enough for a long ducted system like the central heat & air you have)  I have used those system in the bathroom ceiling and then draw the return air from the hall and have 3 short ducts to each of the 3 rooms.  One of the rooms will need to have the thermostat so you need to choose wisely (not the bathroom).  This is accomplished by lowering the bathroom ceiling about 6 or 7 inches.  

    The basement can also use the wall mount or any of the other indoor options.  There are floor consoles or a ceiling cassette or just use a standard wall hung unit.  Once you select the indoor units, then you can decide on what the outdoor unit(s) will be.   In your situation I might use a single larger unit that can accept several indoor units. Some of them can connect to as many as 8 indoor units.  In your home I can see a maximum of 4 but I might select 2.  One console for the basement and one short duct for the second floor. 


    Hope this is helpfull

    Edward Young Retired

    After you make that expensive repair and you still have the same problem, What will you check next?

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