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Replacing an HVAC Vent in an ICF Build

prometheanfire | Posted in General Questions on

I am in climate zone 5 (chicagoland). ACH50 hasn’t been run but is expected to be low (less that 1.5, probably lower).  House will have an ERV for ventilation and is an walkout basement.

I’ve run a manual-j via cool-calc and get ~20k btu heating and 32k cooling (I imagine the extra cooling is because of south facing windows), with an SHR of 0.863 (I think this is used to get the desired sizing?).

I originally planned to use two fujitsu ARU18RGLX / AOU18RGLX units for their turn down ratio, one for the basement, and one for the main level.  They would be using a doing ducts to each room.  I’ve included a table for some statistics.

HVAC estimated 32k cooling 20k heating BTUs total (1 unit per floor)
Brand Name Notes Heating at 5°F (99% spec) Heating at -13°F Cooling at 95°F (99% spec) Cooling at 105°F Minimum Heating / Cooling
Fujitsu AUU18RLF better turn town ratio, better pan heater 20.7k unkn (16kish) 16.8k 12.3k 3100 / 3100
Mitsubish PEAD-KA18NAHZ slightly higher capacity, lower turn down ratio 21.6k 16.4k 16.8k 8800 / 9320

My question is if I need to do ducts to each room or can use a single wall/ceiling unit per floor or go with some ducting.  I’ve heard that well sealed ICF has low temperate variance across a house (at distance from the HVAC exhaust) but would love to hear from someone with more experience.

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  1. user-2310254 | | #1

    When this questions pops up, the experts always ask about bedrooms. When occupied, will the doors be open or closed? If closed, it's likely the rooms will be more comfortable with their own supplies and returns.

  2. kyle_r | | #2

    In a new build I would go with a ducted system. Going ductless can make sense when retrofitting because of the difficulty of adding ductwork to an existing house. But a properly designed ducted system will get you comfort in each room.

    How will the Erv be ducted?

  3. prometheanfire | | #3

    Ya, closed doors and new build has me leaning toward ducted anyway.

    ERV is planned to be ducted from bathrooms and to bedrooms, mainly.

  4. user-2310254 | | #4

    Just make sure the air handler is sufficient. In a previous house where we were retrofitting a ducted mini, my HVAC designer had to go with a bigger unit to ensure the right level of static pressure.

  5. prometheanfire | | #5

    Ya, the one I'm leaning toward (fujitsu) can do 0.8" for static pressure, it's a thing I'm paying attention to.

    1. kyle_r | | #6

      I’m not sure where I’m Zone 5 you are, but keep in mind the Fujitsu 1:1 ducted units don’t have pan heaters.

      1. prometheanfire | | #7

        That they don't include a base pan heater kinda discounts a bit (they only seem to do it with multi-zone (which don't go high turn down ratios) or wall units).

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