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Community and Q&A

Calling all fellow Nerds: Heating and Cooling a low-load Multi Family?

RICHARD EVANS | Posted in Mechanicals on

Im in the early stages of planning a ultra low- load Multi Family Project in Zone 6A.  Planning on a two story building with 8 units total, each between 550 and 600 interior, conditioned square feet (4900 square feet total).  I did a rough heat loss calc and assumed a load of about 2,800 btus/hr at 99% Design Temp for each unit.  I have not done a cooling load calc.  

Heating these would be easy and cheap if I just used electric baseboard heat.  But how would I cool the units in the hot but brief summers of western NH?  Units will have Tilt and Turn windows and I would prefer not to have window ACs hanging out. 

I thought about ductless mini splits but this seems expensive and I will have to come up with space to keep 8 condensing units outside.  Also, even the smallest mini split would likely be over-sized. Multi-splits seem like a nightmare.

Would a ducted mini split work?  I thought about having a Ducted 9k Fujitsu on each floor.  But how would I regulate the thermostat across 4 units? What about the make up air?  Can you run ducts through fire/party walls?

Finally, I thought about a Magic Box option as I will need ventilation as well (21 CFMs per unit).  I’ve played with a CERV machine a couple of times and was really impressed.  The Minotair V12 looks really neat too. This seems like the most expensive option though and would require two holes in the air barrier for each unit. 

So, in summary, What would you do???

1. Option 1:  Resistance Heat. But Air Conditioning?? 
2. Option 2: (2)Centralized Ducted heat pumps and (2) centralized HRVs. 
3. Option 3: Magic box in each unit. 
4. Option 4: Better ideas?

Hope to hit the PHIUS standard with this but low loads are creating some challenges! 

Thank you all-

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  1. Expert Member
    Akos | | #1

    There is probably no cheap easy answers here. You definitely don't want a ducted unit split between apartments, this creates issues with smells and sound.

    Probably the simplest is a split in each apartment, I've installed 9k units in similar sized apartments without issues. They are over sized but they modulate low enough to still work quite well. It is not that hard to hide 8 outdoor units, roof, garden feature, balcony or strategic jog in the building. With a cold climate unit, you have the heat also taken care of, small savings.

    The only other option I can think of is a building chiller with hydro coils in each apartment. I can't see being able to justify the cost of that, plus you would end up paying part of the tenants utilities.

    For the ventilation, a spot ERV (Lunos or Whisperquiet ERV) in each unit might not be too expensive. You can do a shared ERV with fire dampers, but you are getting close to the cost of dedicated units.

    1. Expert Member
      RICHARD EVANS | | #3

      Thanks Akos, this is exactly what I needed to know! (But didn't want to hear, haha)

  2. Expert Member
    BILL WICHERS | | #2

    Mini splits seem like a good fit here. Resistance heat is probably about the most expensive option out there, especially in areas with high electric rates like the northeast!

    You’ll get more BTUs per watt of input electrical energy with minisplits, and you’ll get cooling ability when you need it too.


    1. Expert Member
      RICHARD EVANS | | #4

      Thank you Bill, it's a price difference of at least 30k up front but it sounds like this is the only feasible way to achieve some cooling. This is the feedback I needed.

      I estimated that even with resistance heat and $.20/kwh it would be about $200 per year for heat per unit. But definitely no cooling.

      1. Expert Member
        Akos | | #5


        Since your loads are low, you can save a bit by not going with a vapor injection cold climate unit. For example LG's non hyper heat unit (LSU090HSV5) comes with a pan heater and can modulate down to 500BTU for heat but still provide 5kBTU at 0F.

        There are also some considerably cheaper brands with similar capability, I use an EMI heat pump (rebranded Gree) for a studio.

        1. Expert Member
          RICHARD EVANS | | #8

          Akos, thank you again! The smallest units will have a heat load of around 1000 btus/hour at 47 degrees. The LG product you referenced is the only mini split that I could find with a minimum btu of around 1,000 at 47 degrees. This is very helpful.

          I noticed on the NEEP spreadsheet that the COP for the minimum BTU output was really low for the LG unit. (COP of only 0.51 @ 5 degrees for minimum output). Does that seem right to you?

          1. Expert Member
            Dana Dorsett | | #12

            >"I noticed on the NEEP spreadsheet that the COP for the minimum BTU output was really low for the LG unit. "

            Different models of LG have very different min-mod efficiencies depending on which compressor technology is used. Some are pretty crummy, others are GREAT. In US climate zone 6 or colder the additional efficiency at low temp (and at minimum modulation) of the "hyper heating" compressors can be " worth it" despite the higher up front cost over the lifecycle of the unit.

            The LAN/LAU120HYV1 or LAN/LAU090HYV1 units are both in the latter category, and I'd have no issues whatsoever specifying them for a place with a 2800BTU/hr design heat load, despite the shoulder season cycling:



          2. Expert Member
            Akos | | #14

            You have to be careful with the NEEP database. Lot of the manufacturers play it pretty loose with the power consumption of the drain pan heater. This is partly why some units seem to have much higher COP at low modulation than others.

            I believe the LSU090HSV5 includes the pan heater in the power calc, whereas the ones Dana linked do not.

            At low modulation, a 90W heater is comparable to the power consumption of the compressor. It also looks like they have terrible control for it as it is always on in cold weather.

            About hiding the outdoor units. In Asia, a lot of times the outdoor units mounted inside underground parking structures. If your place has this, you can probably take around 3000BTU/1000sqft without overheating/cooling the basement.

          3. Expert Member
            RICHARD EVANS | | #17

            Dana, Akos- you guys are awesome! Those look like the perfect fit after all. Massive Thanks!

          4. Expert Member
            Dana Dorsett | | #18

            >"You have to be careful with the NEEP database. Lot of the manufacturers play it pretty loose with the power consumption of the drain pan heater. This is partly why some units seem to have much higher COP at low modulation than others.

            I believe the LSU090HSV5 includes the pan heater in the power calc, whereas the ones Dana linked do not."


            It's the same manufacturer, (LG) filling out the same forms required by NEEP. While there are sometimes data-entry typos, I have no reason to believe that the folks at LG somehow didn't understand the COP reporting does NOT include pan heater power in one case, and did understand it in the other.

            It's a pretty straightforward form. On page 3 (of 4) it states:

            "Do not include the power required for defrost
            cycling or drain pan heater operation in the table."


  3. Jon_R | | #6

    At least take a look at a hydronic heat pump (like the Chiltrix CX34) with fan coils.

    Don't expect any latent removal from over-sized mini-splits. Nor adequate distribution into closed door bedrooms.

    1. Expert Member
      RICHARD EVANS | | #9

      Thank Jon! I know nothing about the Chilltrix. I will research it.

      I worry too about the AC shutting off before it has a chance to dehumidify.

  4. johns3km | | #7

    You'll need dedicated heat in each bedroom per code, so you'll have to throw some electric baseboard in their regardless if you go with a split in the main area.

    1. Expert Member
      RICHARD EVANS | | #10

      Thanks John! I am more accustomed to the single and two family code book. Lots to learn here. This makes the resistance heat only option more and more appealing.

  5. walta100 | | #11

    Get real 2 parents and 4 kids in 600 sqf you can’t build jail cells that small.

    And you are going to use 36 sqf for a stair case?

    Tilt turn windows that sounds like the low cost option. NOT

    Remember PHIUS standard has no dollar signs in there equation because it makes no economic sense.

    You will need a very sharp pencil to make this type of project fly.


    1. Expert Member
      Akos | | #13

      Housing size is very region specific. I'm in the land of high density, a 600sqft is large by modern standards. Small doesn't have to be crummy, with good design and attention to details it can be actually very nice.

      I like to think of it as a challenge, if you sharpen your pencil, eliminate lot of the wasted space of modern construction, it can definitely work. Might be hard to believe, that even with a family of 4 (family of 6 seems to be rarer nowadays), it does not feel cramped at all.

      Overseas, family apartments tend to be even smaller than this.

    2. Expert Member
      RICHARD EVANS | | #16


      I don't anticipate a family of 6 living there. As it turns out, there are couples with no children and even unmarried people that need housing. :-)

      I agree with you on keeping a sharp pencil. I am still early in the planning stages and hope that it will all work out. Recent PHIUS certified projects in the area were met with great press. I see it as free marketing. I also happen to believe in the PHIUS mission. Most of my savings will go into this though so I will plan diligently and conservatively to stay on budget.

      1. Expert Member
        Akos | | #19


        Couple of pictures from a 550sqft 2 bed apartment I did. One thing that makes a big difference is increasing the ceiling height in the living space, if you can always go for higher.

        Kitchen is one place you can save a lot of space with compact appliances. 24" fridge/stove, 18" dishwasher and 18" deep counter tops. Finding smaller sinks is a challenge, normally I buy a large dual bowl sink in cut it in half.

        Try to eliminate circulation space where possible, these are just wasted space. Interior partition walls are 2x3 where there weight on it, 2x4 on flat elsewhere.

        Small bathrooms are your friend. 5x6 for one with a tub, 4x6 with shower only. Sounds small but they work.

        You can never make the living space too bright. I'm not a fan of pot lights, but they do work in this application.

  6. Expert Member
    Akos | | #15

    One last more I just remembered. In condo land here it is common to have water source heat pump units in each apartment.

    These can be quite small, 6000BTU unit:

    The building has a water loop that each unit connects to, this could from a chiller or a geo loop.

    I've only looked at the replacement cost of these, which tend to be quite high. Might be different for a new build.

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