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Community and Q&A

HVAC – Continuous Operation or More Zoning

user-6504396 | Posted in Mechanicals on

Should I run a ducted heat pump system with ECM motors continuously or go with a more zoned approach with the units coming on and off as needed?

We are designing the HVAC and ventilation design for our new house in the Pacific Northwest, Zone 4 marine. The house is 2,200 sq. ft. with 2/3 of the square feet on the first floor and a smaller second floor. The majority of the windows face NW toward the view, 2×6 walls with Roxul batts, 10″ SIP panels for the lower level floor, vaulted gable and hip scissor truss roofs, high solar gain windows. The passive house heat load calculations put our peak load at 21,600 btu/hr with a design temp of 22F. Typically we have 3 months a year where we turn off the HVAC in our current house (same area) and ventilate naturally.

We have decided to go with a Mitsubishi or like heat pump. We will have electric radiant floors in the 2 bathrooms and a propane free standing stove in the kitchen. I am thinking of using the Ultimate Air 200DX ERV, eliminating the bathroom exhaust fans, and using the boost mode on the ERV. I am looking at the Mitsubishi multi-position ducted indoor unit that has ECM motors, 3 air speeds and very quiet indoor and outdoor units. We are trying to get all of the ducting in the conditioned space.

We want high comfort in the house without a lot of cold and hot areas which is why we moved away from mini-spits especially with our low heat load. The question we are working through now is should we have a single unit and run it full time or zone the upstairs and downstairs and have the system only come on when needed. Minimizing ducting for the HVAC and the ERV would help us.

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  1. GBA Editor
    Martin Holladay | | #1

    This is an easy one. You can't easily heat and cool a two-story house with one zone. You need to zone the floors separately (or install at least two ductless minisplits -- one upstairs, and one downstairs).

  2. user-6504396 | | #2


    Thanks. That helps.
    How do you feel about continuous operation of the units with ECM motors? I've been told it improves comfort and we have moisture challenges here, so keeping the air moving is sometimes a good thing.


  3. Expert Member
    Dana Dorsett | | #3

    Can you share the model name of the Mitsubishi ducted unit you're looking at? Most of the larger air handler ducted heat pumps have a fairly limited turn-down ratio compared to single zone ductless or even multi-splits.

    Ductless head (and mini-duct cassettes) on multi-splits still modulate and the better ones all have ECM drive motors on both in the compressor unit and the heads/cassettes, but the minimum modulated output of the compressor sometimes limits just how much the heads can modulate. Sizing the heads/cassettes appropriately for the zone load goes a long way toward keeping both comfort & efficiency pretty high.

    A 1.5-2 ton 2- 3 zone multi-split with a combination of wall coils & mini-duct cassettes might be better than trying to get the upstairs and downstairs temperatures to balance with a single zone ducted system.

    Most 1.5 ton single zone mini-splits (mini-duct cassette or ductless) have sufficient capacity at your 22F outside design temp to cover the whole house load. It's possible that separate mini-splits, a 1-ton or 1.25 tonner downstairs, and a half ton or 3/4 ton upstairs might come in cheaper up front, and operate at higher efficiency & comfort.

    BTW: "...we moved away from mini-spits especially with our low heat load. "

    A heat load 21,600BTU/hr @ +22F isn't a very low heat load at all for a 2200' house. A well designed and reasonably tight code-minimum house can hit that mark. My sub-code 2400' antique 1.5 story (+ 1500' of insulated but not directly conditioned basement) comes in at about 28,000 BTU/hr @ 22F.

  4. user-6504396 | | #4


    Thank you for the response. I really value your perspective in this forum.

    You are right on the heat load. It will be essentially be a code built house. We are shooting for a 1.0 ACH50, but we have a lot of window area. The other energy upgrades just didn't pay out so our plan is to execute everything very well but not add things like exterior insulation, etc. I used the phrase "low heat load" mainly out of habit talking to HVAC contractors who were trying to sell me 60k btu/hr systems with 6k multi-split units for all of the individual rooms. When I tell them I only need 20k btu/hr, they tell me how extraordinarily energy efficient my house is.

    I was thinking of using the MVZ-A24AA4, MVZ-A24AA7 (difference between AA4 and AA7?) or PVA-A24AA7 indoor unit if I went with a single zone ducted for 1st and 2nd floor. If I was going to have two zones one for upstairs and one for down, I was thinking of using a smaller multi-position unit for the first floor and the SEZ-KD09NA (or NA4.TH or NA4R1.TH ??) or for the P series PVA-A12AA7 horizontal ducted unit for the 2nd floor. I have been to Mitsubishi's MyLinkDrive website. I have trouble figuring out all of the options and compatibilities. I was considering the P series for their anti-corrosion unit as it will be 50 ft. from a salt water bay.

    I looked at using multi-splits and wanted to be able to heat and cool the 4 bedrooms/offices. One thought I had early on was to use 3 of the SEZ-KD09NA units (I don't like the look of the wall units) and an MXZ-3C24NA outdoor unit. One indoor unit for the upstairs; one for the downstairs office, laundry and guest bedroom; and one for the kitchen/dining and great room. But one contractor I talked to said the ducting runs would be too long downstairs for the low static pressure those units put out. I'm attaching the upstairs and downstairs drawings here. I think the units could go above the lowered ceiling of the upstairs storage room for the upstairs unit. The ceiling above the space outside of the bathroom downstairs for that unit. And, maybe the ceiling in the master bath closet for the great room/kitchen unit (ducting to come down the wall inside the closet). The ceilings in the kitchen/dining, guest bedroom and the entire upstairs are vaulted and will likely have conditioned attic space.

    If I could do the above, similar to what you suggest, I think the zoning would work out well and I wouldn't have to redesign my floor joists to accommodate all the ducting. I really like this/your idea better than a single ducted unit, but was told it would not work without using wall units and leaving he bedroom doors open because of the restriction on the ducting length for the horizontal ducted units. What do you think?

  5. Expert Member
    Dana Dorsett | | #5

    The MVZ series has only 3 blower speeds, and are not fully modulating. IIRC the best you get out of them is a ~2.5:1 turn-down ratio (minimum output is ~40% of maximum output.)

    The SUZ/SEZ Mitsubishi mini-duct have much wimpier air handlers than the Fujitsu xxRLFCD competition, which takes a more careful duct design to manage. The minimum modulation on the SEZ-KA09 is also pretty high (4,800 BTU/hr @ +47F.) They run ~10-20% lower HSPF efficiency than the Fujitsus to boot.

    If looking at a multi-split solution with all mini-duct cassettes, a 2 ton 3 zone Fujitsu AOU24RLXFZ or might be a better fit. The compressor has about a 4:1 turn down ratio, and the RLFCD cassettes can each throttle back to 3000 BTU/hr (any size), which would be less on/off cycling than a 2 ton MVZ or SEZ-KA solution.

    If you can't stand the look of wall blobs and are looking for an all Mitsubishi solution, their SLZ-KA series ceiling cassettes might still work for you. The SLZ- KA09 has about a 4.5:1 turn down ratio, and can drop back to 3100 BTU/hr @ +47F (comparable to the Fujitsu heads/cassettes). The KA12 has an even bigger turn down ratio, and the same 3.1K minimum.

  6. user-6504396 | | #6


    Thanks. I'll start checking out the Fujitsu units to see if I can make those work with an all mini-duct cassette design. The ceiling cassette will possibly work, but only in the great room as the rest of the ceilings in the large rooms are pitched.

    Does Fujitsu have a document site similar to Mitsubishi's where I can look at the different choices and the technical information for each? I couldn't find a link to one anywhere on the GBA website.

  7. Expert Member
  8. user-6504396 | | #8


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