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Choosing an HVAC System for a Large House

Bend_Hiker | Posted in Mechanicals on

We are building a 5 bedroom 4.5 bath 3,400 sq ft house in the Bend, OR area. If you aren’t familiar with the area, there will be a few days down to 0-5 degrees F, and several in the summer in the 100-105 degree F range.

The home is two stories, with a large 21′ cathedral great room. We’ll be using solar to offset much of the electricity usage, and we are having the house sealed with the Aero Barrier spray down to passive house standards <0.6 ACH and putting a layer of ProClima between the ceiling at unconditioned attic to seal the envelope.

My question is would a ducted min split, ductless mini split, or traditional heat pump with ducts be the best approach?  Maybe some combination?

Our HVAC contractor seems wary of ductless, and I believe prefers a traditional ducted heat pump, though he may not have known we were going for passive house tightness when he recommended that. I don’t have the manual J calculations yet.  Also, what questions should I be asking our contractor, and are there any brands most recommended?

Attached are the blue prints without all the detail. The bottom floor is about 2,200 sq ft and the top floor is 1,300 sq ft.

Many thanks.

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Replies

  1. Paul Wiedefeld | | #1

    Probably some combination in the end, but duct as much as you can. It's more comfortable, efficient, and cheaper to run ductwork instead of oversized ductless minisplits. In terms of ducted minisplits vs. traditional heat pumps, there's little difference and great options abound.

    1. Bend_Hiker | | #2

      Thanks, Paul. That surprises me a bit. So you recommend a traditional heat pump with ducts in the unconditioned space vs. ducted mini split? Or do you mean ducted mini splits?

      1. Paul Wiedefeld | | #3

        Either traditional or ducted minisplit with ducts in conditioned space. A ducted mini-split (especially the high static versions) are extremely similar to a traditional heat pump. The industry has done a poor job of naming these things.

  2. James Howison | | #4

    The "multi-position air handlers" are just a different sized box, compared to the ducted mini-splits. Biggest difference is shape but also whether access has to be from below (ducted mini-split) (and needs greater than usual joist spacing) or can be front/side ("multi-position air handler"). e.g., https://mylinkdrive.com/USA/M_Series/R410A_Systems-1/Multi_Position_Air_Handler-4

    Be wary of the multi-split approach, especially with different sized zones. See https://www.greenbuildingadvisor.com/article/multi-zone-heat-pump-issue

    Also consider that you'll need ducts for ventilation, ERV and/or Dehumidifier. Finally your ability to implement great whole house filtering is much higher with a ducted system. The filters on the ductless mini-splits are not great. https://www.energyvanguard.com/blog/path-low-pressure-drop-across-high-merv-filter/

    We recently swapped out mini-splits for a central ducted system using a side access multi-position hair handler and full duct system. I do think the house feels more cohesive, humidity in particular very quickly equalizes.

  3. Rliebrecht | | #5

    Just calling out the tradeoff of efficiency with central systems. Those pump/handler combos are never near the top of SEER/HSPF ratings, and you need to factor in losses from duct leakage and heat loss along the way. Without excellent system planning, all the old problems of uneven heat/cooling distribution between registers remain (different pressures on different rooms, between floors).

    We have a two single head ductless systems for our 2500 Square foot house (a main floor and walk out basement that's an independent suite.) Unless you want to keep all your room doors closed all the time, you might be able to avoid having a mess of different heads/systems and the associated expense. Still takes careful laod calculation of course.

    1. Paul Wiedefeld | | #9

      A high end traditional heat pump is about as efficient as it gets: mid 20's SEER, HSPF > 13.

      1. Rliebrecht | | #12

        Curious which model?

        1. Paul Wiedefeld | | #15

          https://www.carrier.com/residential/en/us/products/heat-pumps/25vna4/

          This Carrier tests at 24 SEER and 13 HSPF.

          I haven’t looked through many manufacturers, but Fujitsu and Mitsubishi’s ductless and ducted heat pumps seem to have about the same efficiency given the same capacity, with the ducted lines starting at a larger size.

  4. Bend_Hiker | | #6

    Thank you. I am hearing there is little difference in ducted mini splits and traditional heat pump. Is that correct? So what if the ducts for a traditional heat pump cannot be in conditioned space? The crawl space is not conditioned and neither is the attic. I suppose we could build a subfloor over the duct work in the attic, but I don't see how that could work for the ground floor. Also to the point about leakage in the ducts, would the reco be to pressure test them every few years and reseal leaks?

    1. Expert Member
      Peter Engle | | #13

      Getting the ducts into conditioned space is a must-have item, even if it means giving up some interior space. Really.

      You should also seriously consider making the crawl into conditioned space. A conditioned crawl is cheaper to build and performs better in every climate except where flooding is an issue.

      1. Bend_Hiker | | #14

        Thank you. I will ask about making the crawl space conditioned.

    2. Patrick OSullivan | | #23

      Bluntly: You don't build a house to passive house levels of air tightness and then put mechanicals in unconditioned space.

      As for ducts, they should be sealed when installed. If done properly, they shouldn't leak later on. (Also, even if they do leak a small amount, it's not a big deal when they're in conditioned space.)

  5. Bend_Hiker | | #7

    Here are also the heat pumps recommend for the traditional ducted system with ducts outside the conditioned space. I was surprised Carrier was recommended since I have read so much about Mitsubishi and Fujitsu here.

    Carrier heat pumps. The 38MGRQ48 is for the lower level (2 Zones) and the 38MBRBQ36 is a single zone for the upstairs.

    The 3 air handlers will be the 40MBAA series and each will have 10kw backup heat strips. The air handlers are in the garage.

    1. Paul Wiedefeld | | #8

      Unfortunately this happens a lot - HVAC is totally neglected until the very end and contractors screw it up.
      1. The heat loss determines the sizing. A new house with that level of air sealing will never need 30kw of resistance heat, that’s >100,000 btu just there! That’s 125 amps! The entire house might come in under 10kw heating.
      2. Installer matters more than brand. Carrier’s great. Their Infinity 24 is one of the best options on the market.
      3. Unfortunately, you’ll often have to tell installers what to do. If you can, get a manual J and that determines what size equipment you get.
      4. The builder needs to get the ducts inside the conditioned space once the heat loss is known. Heat loss, by room, determines the size of ductwork needed. Fitting 100,000 btu of ductwork is much harder than the true lower amount you’ll actually need, especially if it's split between two floors.

      1. Bend_Hiker | | #10

        Thank you. I will reply back once I have the Manual J. Full disclosure, the ducts have already been installed in non conditioned sub floor and attic. I am trying to decide what to do. I think they used flex duct too. I may have them pull them out unless someone has an idea for how to make them be inside the pressure barrier, especially the ones in the crawlspace. The attic seems like I could have them add a subfloor to the attic above the ducts and seal it.

        I am surprised though that the traditional heat pump performs as well energy efficiency wise if properly sized.

        1. Paul Wiedefeld | | #11

          There's very little difference between traditional and mini-split in the ducted realm. A single-head ductless minisplit is the most efficient, but multi-head minisplits are usually the worst option heat pump wise. The ductless heads are usually bad at dehumidification, especially on the smaller end, so the efficiency comparisons aren't really equivalent anyway.

  6. Expert Member
    Akos | | #16

    If you are in Zone 5 there, traditional heat pumps are not the best option. By traditional I mean North American style big square ones with the fan on top. Even the higher efficiency ones such as the Carrier Greenspeed line starts to fall off in capacity in colder winter weather.

    What you want is a cold climate heat pump, sometimes referred to as hyper heat unit. These will generally provide their full nameplate heating capacity down to 5F, this means you don't even need any strip heater backup. I'm in a bit colder climate than you and have no issues using these without any backup.

    The big players are all overseas suppliers but domestic manufacturers have started to offer them as re-badged units.

    You can look at Trane/Mitsubishi or the Carrier/Midea units. Daikin, Fujitsu, LG and Gree also make excellent cold climate units that have a standard furnace style air handler. You can search more here, select "Single Zone Centrally Ducted":

    https://ashp.neep.org/#!/product_list/

    As for your ducts in the attic, the best is to move them into conditioned space. If this is a big problem, the next best thing is to fully encapsulate them in a layer of spray foam. Make sure the register boots as well as the opening where they enter the attic is covered in spray foam. A duct blaster test to make sure they are sealed is also a good idea.

    My guess is a pair of these is in the ballpark to handle your house:

    https://ashp.neep.org/#!/product/34580/7/25000///0
    or
    https://ashp.neep.org/#!/product/33268/7/25000///0

  7. Roger Berry | | #17

    Bend Hiker,

    Based on postings here I looked into the Carrier unit mentioned. I am in a solid zone 6 at altitude so also needing to de-rate capacity. I could not get much useful info from Carrier website, but did find that the owners manual sheet for the 25vna8 unit says it may shut down the heat pump at 10F. The 25vna4 doesn't say anything about low temps.

    One of the links Akos kindly provided suggests at least one Carrier unit can get to 5F reasonably well. Not sure what to make of the manual I found as the model numbers don't match. I will post what else I find out.

    I guess the key is to be sure that you have all the operating parameters available to you before commit to a system. Just wish manufacturers made the information more readily accessible.

  8. Walter Ahlgrim | | #18

    I am in zone 4 my conventional heat pump is set to lock out the resistance heat until it is below 8° outside and when it does kick in the heat pump is still doing most of the work.

    I think you should consider having the upstairs on a separate system this will give you much better control between the two floors.

    I think a vented crawlspace is a poor choice for comfort and will cost more to build and operate than a conditioned crawlspace. The way I see it the walls of the crawlspace have a much smaller surface area than the floor of the living space.

    My list of must haves for a modern heat pump.
    1 Variable speed compressor.
    2 Variable speed indoor blower.
    3 Two electronic expansion valves.
    4 Communicating thermostat made by the equipment’s manufacture.

    If the HVAC contractor wants to over size (in our opinion) the resistance heat, I see little reason to fight with him over that issue. The operating cost will be exactly the same. If it is 300% oversized it will use 3X the power but only run 1/3 as long = the same number of kWh. Yes, you will buy more wire and the equipment will cost slightly more.

    Walta

  9. Bend_Hiker | | #19

    I wanted to post a small update. The HVAC company has provided some new units and proposed single head systems rather than multizone, but has not provided the Manual J claiming that is proprietary information.

    Is that common - that Manual J is considered proprietary and not available for the customer?

    As FYI, here are the new systems recommended. As mentioned above, home will be sealed to passive house standards and we'll be using a lot of solar to offset energy use. We are in zone 6A (97707 zip code).

    2 Carrier 38MARBQ24AA3 variable speed, inverter drive heat pumps with drain pan heaters serving 1st level.
    1 Carrier 38MBRBQ36AA3 variable speed inverter drive heat pump with drain pan heater serving 2nd level.
    2 Carrier 40MBAAQ24XA3 variable speed fan coils with 10kw backup heat strips serving 1st level.
    1 Carrier 40MBAAQ36XA3 variable speed fan coil with 10kw backup heat strips serving 2nd level.

    1. Patrick OSullivan | | #24

      It often makes sense for Manual J, S, D, etc. to be done by an independent third party that doesn't have a vested interest in what and how much equipment you install.

      Then, HVAC companies should bid to that designed spec. Unless you have evidence and experience to the contrary, the average HVAC company will not spec the proper gear for an above average house.

      HVAC companies that deal with commercial jobs often tend to work better in this model. Commercial jobs are more likely to have the HVAC spec'd by an engineer for contractors to bid against. They're used to this dynamic. Contractors who only do residential are more used to spec'ing and installing whatever they decide by gut feeling.

  10. Jamie B | | #20

    "Is that common - that Manual J is considered proprietary and not available for the customer?"

    NO!

    That's a garbage move. If you haven't paid them anything yet, I'd consider finding another contractor.

    1. Patrick OSullivan | | #25

      Yep. Like a nervous kid on a math exam, if they're not prepared to show their work, they're likely cheating.

  11. Bend_Hiker | | #21

    Okay, interesting. I may have to do that then. I'll contact my GC.

    1. Expert Member
      Akos | | #22

      Not quite. The contractor is not obliged to give you any information unless you paid for their service to do it or is part of the contract. Man J is something you need to submit to the city (or at least you have to here) so it is not something that is proprietary.

      I'm sure if you discuss compensation for the man J, they will be happy to provide it. The better option is getting it done by an HVAC designer whose job is to size equipment properly.

      Based on the initial proposal, I don't think anybody did anything more than a BTU/sqft rule of thumb.

      Installing almost 90K BTU of heat output at 5F into a well sealed 3400sqft house is silly. That puts you about 25BTU/sqft which is above an un-insulated double brick house here in the north edge of Zone 5.

      Adding in 100k BTU of resistance backup is even sillier. Never mind that it is way oversized, but you'll have to pay to increase your electrical service to supply something that will probably never be used.

      My suggestion is to get somebody competent to run a proper man J/S on your place and select the equipment and backup from there.

      If you have multiple one-to-one units, I would skip resistance backup completely. You are not in cold enough climate to need it and if one of the units does fail, the other one will keep the house mostly comfortable until it can be repaired.

  12. Allan C | | #26

    Just curious, was an HVAC design and heat loss calculation signed by a qualified HVAC engineer not required for the building permit? There are required in my jurisdiction as part of the building permit submission and you can't get get a permit without it.

  13. Walter Ahlgrim | | #27

    I true manual J calculation requires several hours of work measuring each room and every window determining the R value for each type of wall window and ceiling. That is hundreds of dollars in value it does not seem right to demand a copy for free if they really did do one. My gut says they are unlikely invest that amount of work in a bid but some do.

    7 ton of equipment for 3500 sqf, sounds like 200-300% of what should be required. I hate rules of thumb but less than 1000 sqf per ton with good insulation and air sealing does not pass the smell test.

    You may want to consider reading this article and hiring Energy Vanguard to do the HVAC design.

    https://www.energyvanguard.com/blog/air-conditioner-sizing-rules-of-thumb-must-die/
    Walta

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