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Feedback on HVAC Design

darko9000 | Posted in General Questions on
Hi everybody,

We live in the Raleigh area, North Carolina, my wife and I are building our retirement home and we wanted to be efficient and tightly built, just the way TUCO from Breaking Bad would say it!

Looking for a sanity check on my HV AC equipment. What I am trying to accomplish is to build the house as close as I can to a passivhaus standard. I am shooting for 0.6 ACH 50 air tightness but would probably be happy with 1.5 ACH 50.
The house is 2 by 6 construction 24 inches on center.
The first floor of the house is approximately 2000 square feet out of which zone 1 is 1336, zone two is 431 and zone three is 227.
The front of the house is facing West.
The walls will be filled with the open cell spray foam, the exterior is zip sheathing which will be completely air sealed, this is followed by two inch thick polyiso insulation, ½” air gap and then exterior brick.
All windows are European tilt and turn, triple pane low E windows, I think they are rated R7 when converted.
The whole house is electric only, (we have 14 kw solar on the shop roof) with an induction cook top, I do not have any exhaust fan in a bathroom or kitchen going outside, however I have just purchased Panasonic Intelli-Balance 200 ERV, 60-200 CFM FV-20VEC1 and I’m planning to run 4 inch hard piping to several areas in the house and picking up the returns from the kitchen and bathrooms.
The roof rafters are made out of 2x 10s, they will be filled with open cell spray foam.
The house is sitting on a sealed crawl space that is lined on the inside with a 2 inch thick XPS Styrofoam, the whole thing will be sealed off with a 15 mil vapor barrier and I will be installing Dehumidifiers.
Later down the road I am planning to install radiant floor heat. A couple of years ago I have installed a radiant floor heat in my shop, and I absolutely love it, so for now I am looking to purchase a heat pump, we would primarily using this for cooling only but heat would be there as a backup.
I have found a place online that will do a manual J calculation for a fee, and these guys came back with 43000 btu for cooling.
My local HVAC contractor wanted to install 5 ton ac with 50 ft of ductwork in crawlspace.
I have read many articles here on green building advisor where tightly built houses can be cooled with 1 ton of AC for over 1000 square feet

So that being said, I’m looking to go with a 3 Zone mini-split as follows:

Outdoor 3 zone condenser 24000 btu

Zone 1:  A 12000 btu 4-way ceiling cassette–which would be main living area, kitchen, Laundry, Half Bath, Foyer and Closet, 1336 SF

Zone 2:  A 9000 btu Slim Duct in the attic which will feed Master Bedroom, Master Bathroom and Master Closet, all piping would be rigid 6 inch, I’m assuming I wouldn’t need to insulate these because I am inside my envelope, 431 SF

Zone 3:  A 7000 btu Slim Duct Guest Bedroom (Bedroom 3), Bath and Closet, same as zone 2.  All duct runs would be 6 inch rigid pipe, 227 SF

Please see attached picture.
Thinking about going with Fujitsu “aou24rlxfzh” mini splits because their slim duct goes down to 7000 btus
Does anybody see anything wrong with this setup? Or even better, have any suggestions to improve on things?
Thank you very much,
Please excuse my spelling, English is not my first language
Darko Erickson

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  1. Expert Member
    Akos | | #1

    I would run your place through one of the on-line load calculators such as loadcalc or coolcalc.

    Based on the description of construction and the fact you have a porch shading most of the house, I doubt your cooling load is above 2 ton.

    The 9k and 7k for the main and guest are probably 3x oversized. Combined that with the fact that it is connected to a multi split and your cooling and humidity removal will suffer. A single 7k for both is probably in the ballpark.

    Once you have the load numbers dialed in, you can look at equipment selection. A 2 zone is probably in the ballpark:!/product/25328

    Your venting plan is mostly fine except for the kitchen. You want a good range hood that is venting outside. It should be wider than your range, large aperture size and good capture volume, quiet ~350CFM blower.

    You also want a largish (~70CFM) ERV stale air pickup in your kitchen kitty corner of your range at least 8' away.

    With the single ceiling unit in your living space, you might also want some electric resistance heat under the tiles in the mudroom and kitchen for a bit of extra warmth.

  2. darko9000 | | #2

    Thanks for your reply Akos,
    Unfortunately, 7000 btu concealed duct is the smallest I could find and i really wanted to keep master bedroom separate from guest bedroom

    1. kyle_r | | #3

      Hi Darko,

      I’m just curious as to why you want the Master Bedroom zoned separately?

      1. darko9000 | | #4

        Thanks for your help Kyle,
        I thought that separating master bedroom, bathroom and a closet from the guest bedroom bathroom and a closet would be a good feature. If the in-laws are coming over or anybody else is coming over, I thought that, that section have it's on thermostat would be a good feature so people are not complaining that it's either too hot or too cold for them.
        We might not be running a AC in the guest bedroom section a lot and I figured the 7000 BTU for 237 square feet was an overkill but 7000 BTU for a master bedroom side which is roughly 432 sf would be somewhat OK.

        I'm not quite sure how to approach this, I feel that I would be OK having both bedrooms and bathrooms on a single zone with 7000 BTU cooling, I'm just afraid that I might regret this later down the road.

        I also thought that ERV would be OK as far as fresh air exchanged in the house. Wasn't considering installing hood for the kitchen because we cancelled our gas and we're going all electric so we using an induction cook top, and I would be running all the returns from the kitchen and bathrooms to the ERV

        1. this_page_left_blank | | #7

          You need a vent hood for the cooking itself. Just frying a single egg will send the particle count in the air through the roof and keep it there for hours.

          1. kyle_r | | #9

            Trevor, if I remember correctly from a post a while ago you had a recirculating hood with an erv vent in the kitchen and was happy with the performance. Curious if you changed your mind and think that a range hood should be vented outside?

          2. darko9000 | | #10

            "Trevor, if I remember correctly from a post a while ago you had a recirculating hood with an erv vent in the kitchen and was happy with the performance. "

            This is what i was planning on doing.

          3. this_page_left_blank | | #14

            I did ay one time believe that the recirculating hood with nearby ERV/HRV exhaust would be adequate, based on the advice of others. I don't remember ever saying I was happy with the performance. I was pretty much blissfully ignorant of what the performance was until I actually set up a particle counter and measured it, which is when I found out it doesn't really work as advertised. I still actually have this setup, but it's near the top of my list of things to renovate once the weather turns nice. I meant to do it last summer, but couldn't come to a decision on the details.

          4. kyle_r | | #15

            Thanks Trevor, it’s helpful to hear from someone who has tried it and measured the effectiveness.

          5. darko9000 | | #16

            Hey Trevor, thank you very much for your feedback, this is a huge help.
            Well to start with I was thinking about doing the kitchen exhaust system with a motorized damper, something similar to what these guys did.


            But now reading your feedback that the ERV was not adequate for doing this I'm going to run all my piping and leave it in the ceiling so I can do this later once we move in.


  3. paul_wiedefeld | | #5

    What did the heat loss come in at? A well insulated house in a southern climate with an insulated floor is generally not worth adding in-floor heating to - you might only run 75 degree floors on design day.

    1. darko9000 | | #6

      I know it is probably not worth it to have it installed by a professional company, but I like the comfort of radiant heat, it is going to cost me $1500 and my time so it is worth it to me to have this for the next 30 years

      1. this_page_left_blank | | #8

        If the house is well sealed and insulated, the warmth coming from the floor will be imperceptible. This is especially true in a climate that doesn't get very cold. Unless you're in the habit of leaving the windows open in the winter, you might never experience that warm toes feeling you're looking for.

        1. darko9000 | | #11

          You might be right, i do not know for sure but im wiling to experiment. im doing it myself so it is only material cost.

          1. paul_wiedefeld | | #12

            What’s the heat source for the floor?

  4. darko9000 | | #13

    The heat source is an instant hot water heater. I did the same thing in my shop 6 years ago

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