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Will these two heat pumps cycle too much and kill efficiency

cgalgowski | Posted in Mechanicals on

I would like to find out if the following heat pump systems will lose much efficiency from cycling on and off.

The house is a 1 – 1/2 story 2,100 SF farmhouse with 10 rooms.  Peak heat demand at 6 degrees is 33,000 BTU/Hr.  This was determined by two separate Manual J’s, my spreadsheet, and adjusted for the 4 known cords of wood that heat the house.  Blower door ACH50 is 6.5.  The dual system designed by a reputable HVAC firm consists of:

System 1

  1. Equipment used is:

    1. Mitsubishi Hyper heat compressor MXZ-3C30NAHZ2 on south side of house.

    2. MSZ-FH15NA-8 wall head in Great Room.  HVAC Co says this head can send air farther than the FH12 and could get more heat into the kitchen.

    3. MSZ-FH06NA-8 wall head for the Studio.

    4. MSZ-FH09NA-8 for the Upstairs Master Bedroom.

  2. With just my wife and me in the house, this will be installed soon and should suit our needs for the next 10 years as we supplement with wood.  If we leave the house for an extended period in winter, we would drain the water out of the pipes, at least on the north wall.  At 6 degrees outside temp, the heat pump should keep house at 45 degrees. 

 

System 2

  1. Equipment used is:

    1. Mitsubishi Hyper heat compressor MXZ-3C24NAHZ2 on south side of house.

    2. MSZ-FH06 wall heads in the NE and SE bedrooms and Office.

  1. System 2 is planned to be installed about 10 years into the future.

Discussion

  1. System 1 and 2 are designed, so that together they will:

    1. Provide good thermal comport for people with a wide range of needs and desires.

    2. Run the house with no supplemental heat.

    3. The HVAC Company felt to achieve this requires heads in 6 rooms.  Also, limiting the house to two total compressors would help keep maintenance costs down.   Finally, stay with Mitsubishi’s, because parts are available and the there is a 12 year warranty. 

  2. The attached spreadsheet I made indicates the two heat pumps will be supplying much of the heat when the house heat demand for each system is less than 7,200 BTU/HR.  This is the slowest each compressor can run at.  The spreadsheet could be theoretically incorrect wrong.  I am new at this.  Here is how the spreadsheet works:

    1. System 1 (columns A to C) assumes most of the house is available to distribute heat into rooms all at temperature of 68 degrees, except for the upstairs NE and SE bedrooms that will have doors closed.  In reality there will be conduction and infiltration from adjacent rooms.  Also, three heads will not fully distribute heat to achieve an even 68 degrees.

    2. System 2 (columns E to G) assumes System 1 is running concurrently.  The heat pumps share some of the rooms for areas to distribute into.  The Great Room, Kitchen, Utility Room, and Office have no doors that shut.  The bathroom doors are open most of the time.

    3. System 1 and 2 running together (columns I to K) assume room heat demands are commingled and the lowest output of both pumps together is 14,400 BTU/HR.

    4. The following free website, https://www.degreedays.net/#generate, was used to determine temperatures and degree days at various base temperatures.   The temperature data is the 5 year average from 2014 through 2019 from Bradley Field Airport near Hartford, CT.

      1. The interpolation equation to determine the outside temperature when the house system demand is 7,200 BTU/Hr in row 25 is:

= 6 degrees + (System demand – 7,200) / (System Demand x 62 degrees)

      1. The degree day calculations in row 28 performed by the website use a base of 65, even though the house is maintained at 68 degrees.  This is because my house is estimated to achieve an indoor heat of 68 from thermal mass, and has gains from equipment, occupants, and solar without heat pump output.

      2. The degree days in row 31 are calculated by the website using the noted temperature bases.

      3. The percentage of heat load above the base temperature is calculated by the interpolation equation:

= (degree days for 65 base – degree days for 31 base) / degree days for base 65

The key point is this is the percent of heat load supplied when the heat pump is cycling on and off.  But it is not the percent time the heat pump is cycling off and on.  I do not know how to find percent time the heat pump is cycling off and on.  If percent of heat load is not a common metric used to measure how heat pump efficiency drops with cycling, than this spreadsheet might not be that useful. 

So my main questions are, given the data presented so far:

  1. Will these two heat pump systems have a large amount of cycling during the year?

  2. If so, what will be the approximate reductions in HDSPF and COPs from published values in the Mitsubishi catalog and NEEP website.  To convert region IV HSPF to region V, I multiply by a factor of 0.85, a tip I picked up from Dana Dorsett.

 

If these 2 systems are prone to large inefficiencies than other configurations, I would consider are:

 

  1. Using a single compressor with a branch box and perhaps some ducting to get heat and cooling to 6 heads.  My HVAC Company says there are pros and cons for branch boxes and ducted systems along with the geometry of my house.  There will be challenges with running line sets and putting ducts in the attic which is outside the temperature envelope.

  2. Using one single zone hyper heat FH18 compressor in the Great Room with a lot of wood heat supplement to start and then 10 years or so, see what is on the market for multi-zone systems with smaller compressors and lower turn down values.

  3. Adding complexity to this is 10 years from now would be a good time to replace the siding, add a much better air barrier, and maybe 1 inch of foam with R = 12 per inch being developed at the Lawrence Berkeley National Lab (if it works a big game changer).  This could lower my peak demand by 30 %.

Thanks for wading through this.  Any help is much appreciated.

Charles Galgowski

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Replies

  1. Jon_R | | #1

    > I do not know how to find percent time the heat pump is cycling off and on

    And even if you had this number, you wouldn't be able to calculate loss from cycling.

    You are considering installing inverter driven heat pumps that are about 80% ?? over-sized for peak or design day heating load? Around 25% would be ideal for the latter. In either case, your plan will work fine without large inefficiencies.

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