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HRV system and mini-split systems

James Stufano | Posted in General Questions on

Hi GBA – my main question has to do with new construction and what type of HRV system does one put in a 2000 sf barn ranch single floor home if they are using mini-splits?  but I also have other questions imbedded in my post . In my case we have 12 foot ceilings with floor and attic trusses. The attic will have closed cell foam under roof deck so attic will be open. It’s 84 feet long x 26 wide so attic looks like a bowling alley down the middle that has an Advantech sub-floor. 1 st floor has a 1 inch thick Advantech sub-floor.

I am torn on what HVAC system to install. Plumber said best bang for the buck is forced hot air with boiler, ducts in walk-out basement that is not insulated. Insulation will be in the floor trusses. When I mentioned air-handlers the plumber frowned as I am thinking who installs a boiler with a single fan with ducts to everywhere anymore?  Plumber said with 12 foot ceilings we are going to want the ducts in the floor and I am not so sure that is true as he claims you will not get warm with heating from above. I would think with today’s tight homes this is not true.
I also hate forced hot air. 

I was thinking Mitsubishi Mini splits that would provide air and heat, but I am wondering about maintenance of say 5 or 6 ceiling cassette units. I think a couple times a year you have to clean each one.  I know Mitsubishi makes two other types of ducted units – one is an actual air handler and the other is other is smaller. I’ve seen Matt Ressinger use the smaller units to do a master bedroom, closet and bathroom, so I would need about 4 of these with each one ducted. I think pluses also are 1) it’s easier to just change the air-handler filters rather than open and cleaning each ceiling cassette unit 2) Installing two outdoor mini-split outside units on each end of the 84 long house would simply be two air conditioning condensors if I had to go with forced hot air with two air handlers. 

I am getting away from my question which is what type of HRV system does one put in a home or  if they are using mini-splits? 

Does one have to add duct work in addition to the mini-split ductwork if I use the ducted version?  This new build has MB on one end, two bedrooms on the other end, and a open concept in the middle of kitchen and living room as one. 

I figure if I needed more heat, I could add radiators to give the radiant feel and use the Mitsubishi units for air and backup heat.  We also will have a wood burning stove in the center area for real cold days – I’m in a Zone 5.  

They make a Mitshubishi unit that goes down to -4 and one that goes down to -14. Can I ask what units others in Zone 5 what Mitshubishi mini-splits they used and how did they perform with a -4 vs a -14 models. 

Thanks

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Replies

  1. Expert Member
    Akos | | #1

    First item. ALWAYS insulate the basement walls not bellow the first floor. This is less insulation, more energy efficient and keeps the basement much more comfortable.

    Before even thinking about your HVAC also have somebody qualified (not your HVAC tech) do a proper heat/cooling load calc for your building.

    It is good to figure out if any of the rooms have very large variation of heat/cooling through the day and weather you need zoning.

    Two separate heating systems an expensive proposition. Cold climate mini splits (look for hyper heat units) can cover your heat load even load with capacity to spare in Zone 5 even on the coldest of days.

    You want to stay away from multi split systems with many heads. Most new construction builds the rooms have such small load that even the smallest unit is around 4 to 6 times the heat load there. Oversizing by that much is bad for comfort and is a recipe for low efficiency.

    In colder climate with tall ceiling you want registers near or pointing at the floor. You need good mixing to keep the colder air from stratifying at the floor level. You can do this with a wall/floor mount mini split as well provided the space is open and you don't have huge windows.

    Generally the best setup is a wall mount unit on the main space and a ducted unit for the bedrooms each on it OWN outdoor unit. Again, you don't want a multi split. With your layout, you might need to go with two ducted units. With the long footprint this would reduce the amount of ducting through the house and still allow for a bit of zoning.

    For new construction HRV/ERV should be ducted separately. You can save a bit of money buy sharing the return ducts with your HVAC. Fresh air supplies should run to all your bedrooms and living space. There will be some very long duct length, but the flow is low enough that you can do it with modest size ducts.

    A bit of electric floor heat in smaller areas is not expensive to put in and operate. A bit around the entrance, bathroom and kitchen work area makes a bit difference in comfort.

  2. James Stufano | | #2

    Thanks Akos.

    In your first sentence you said "First item. ALWAYS insulate the basement walls not bellow the first floor. This is less insulation, more energy efficient and keeps the basement much more comfortable."

    Was wondering if you were saying I should or should not insulate the basement walls which are below the 1st floor when you said NOT below the 1st floor

  3. Expert Member
    Akos | | #3

    You insulate the basement walls, not the basement ceiling.

    The reason you insulate the basement walls is that you want the basement as warm as possible and close to the same temperature as the rest of the house. Most of the heat loss is through the perimeter walls, thus the insulation there. There is actually very little loss in most climates through the basement floor, insulation there is usually done more for comfort than energy savings.

    A warmer basement means a basement that is lower RH, thus much less chance of mold. Cold basements always end up musty.

  4. James Stufano | | #4

    Thanks - My building inspector as well as my architect both said the basement ceiling has to be insulated
    on this new construction project since under the basement floor was not insulated. House is in Connecticut Zone 5. I like your thinking as I had asked that same question several months ago and was told the ceiling floor needed to be insulated since new codes stipuate the roof is R49 and the basement floor has to be (I think it's ) R38.
    So the R38 has to go in the basement ceiling I'm told

    1. Expert Member
      Akos | | #5

      I think there is confusion about floor insulation. The R38 is for floors exposed to outdoors (overhangs, house on post and beam foundation). The R38 is not needed for houses with unvented crawl spaces or full basements.

      For example for houses with basements, our code calls for R12 batts plus R5/R10 rigid insulation on the basement walls. There is no requirement for insulating the basement ceiling. I'm in Zone 5 area as well.

  5. Brad | | #6

    Insulating the basement floor or the basement ceiling to R38 is nuts, and I don't think the code says that.

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