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Community and Q&A

Heat Pump Sizing And Modulation

jp381 | Posted in Mechanicals on

Hi I’m Jeremy, and I’m looking for some advice on designing a properly sized heat pump system for my home in Seattle–particularly with respect to how inverter driven heat pumps modulate load. The goal is to convert from a 25 year old gas furnace to free up the utility closet for other uses, transition to more renewable energy, and gain air conditioning. Given the goal of freeing up the utility closet, I’m hoping for the heat pump to be the only source of heat in the house. 

Before getting to the specific questions, here’s some relevant background. The house was built in 1996 is 1700 sq ft and is located in Seattle. It has an entirely open first floor living/dining/kitchen and three bedrooms on the second floor. Two adjacent bedrooms are about 110 sq ft and the master is about 250 sq ft with a large vaulted ceiling. There is also a large landing/bonus area between the master bedroom and the smaller bedrooms also with vaulted ceilings that would make it difficult to run a single ducted heat pump unit that connects all three bedrooms. I calculated a heating load of 21,600 BTU using the calculator at, which I read is the most accurate for sizing heat pumps. 

I had an hvac company out to give an estimate and they recommended a 36000 BTU daikin multizone with a 7k head in each of the two smaller bedrooms, a 9k head in the larger bedroom, and an 18k head on the first floor. They didn’t do a heat load calculation and they were not particularly concerned about oversizing because the inverter modulates the load. However, from reading this forum I’ve learned that the inverter is limited in how much it can modulate the load and an oversized heat pump can short cycle and waste energy.

After doing some research I discovered NEEPs cold climate heat pump list. What a fantastic resource. I entered my location and heating design, and it actually looks like a 36000 BTU daikin Aurora –a different model than the hvac company recommended–may be best for my house because of its high turndown ratio:!/product/26570/28/22000/WA/727930/1. It looks like this model can modulate more than smaller models such that it can actually satisfy a greater percentage of the annual load in the modulating range. But since I’m just a curious homeowner, I wanted to get an expert opinion. Does it make sense that a larger unit might actually be more efficient?

I also noticed that the NEEP website focuses only on the outdoor unit’s ability to modulate. In practice, will the ability to modulate be affected by the indoor head sizing? In other words, since the indoor heads in the proposed design are all oversized and cannot modulate will that cancel out or reduce the benefits of the outdoor unit’s large turn down ratio and result in significant short cycling? If so, an alternative option that I have thought up is to put a 7k ducted unit in the unconditioned attic for the two small bedrooms to share, put a 7k ductless unit in the master, and then a 12k ductless for the first floor. That would at least make the demand each head satisfies more tailored to the room it serves.  Will more right sizing the heads make a difference for the outdoor unit’s ability to modulate and avoid short cycling? 

If I can in fact get the performance the NEEP website suggests from the Daikin Aurora that seems like a pretty good option to me. It will more than satisfy the heating needs even if it’s unexpectedly cold without too much short cycling. But I want to make sure that is in fact realistic before proceeding. Would also love to hear any other ideas for my situation.


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  1. PBP1 | | #1

    An expert member or two may provide detailed answers.

    From an initial look, Seattle (SeaTac) has a heating design temperature of 24 F. In a larger space, over 2100 sq ft, in western MT with a design temperature of 1 F, a 28.6 kBTU/h rated heat pump works just fine (even at -10 F). Thus, 36 kBTU/h is likely quite oversized for 1700 sq ft in Seattle and even 21 kBTU/h may be high (unless low R values and high ACH50). I also have a 9 kBTU/h ducted head serving four rooms of about 600 sq ft total.

    Is it possible to get a unit that slots into the existing duct system?

  2. paul_wiedefeld | | #2

    Use this to calculate heat loss since it's an existing house:

    The idea is that all online calculators require significant guesswork around infiltration, but a fuel usage based calculation is grounded in reality.

    I'm opposed to multi-splits almost always. I think a small ducted system would provide better filtration, distribution, easier maintenance, easier replacement, load matching, etc. Your option to duct some homes and use ductless for others is a good alternative.

  3. user-2890856 | | #3

    Very shortly here , Taco will release an Air to Water heat pump that does heat , cooling and DHW . All controls and control strategy have been built into this unit . Look into it and don't be hasty making your decision as newer better units are coming fast and furious . This one will have good longevity due to the time spent developing and the thoughtful design .
    Taco System M

  4. jp381 | | #4

    Thank you for all the replies. I think I'm going to go in a different direction and get two outdoor units. For the downstairs I'm thinking a single 12k Mitsubishi H2i floor unit. The downstairs has an estimated heating load of 11,750 BTU. And for the upstairs with a heating load of approximately 10k BTU a slim ducted Fujitsu 12K with outlets going to the three bedrooms. I read on here that the Fujitsu 9k and 12k models both modulate down to 3100 BTU. ( Does that mean the choice between 9k and 12k is not particularly consequential? If so, I'm leaning towards erring bigger since the ductwork will be in an unconditioned attic.

    1. andyfrog | | #5

      Supposedly there is up to a 25% penalty when running ducts through unconditioned attics.

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