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Help Selecting Ducted Heat Pump Size

emma_vt | Posted in Mechanicals on

This forum has been incredibly helpful while we are building our tight, passive house (lower case p) in Zone 6. We’ve settled on an HVAC approach, but my heat pump installer and I are stuck at the corner of Novice Building Science Nerd and Old-School-Fossil-HVAC-Pro-Turned-Heat Pump-Pro. ie, I want to go a little small, he wants to go a little big. 

Manual J (which I did myself, definitely not perfectly, but well enough to be helpful) says we have a total heating load of 34,000, and an average cooling load of 9200, peaking at 10,900. 

The plan is a Mitsubishi ducted heat pump for the main and second floors, and a separate 6000btu wall mount in the mostly finished basement (MSZ-FS06NA with MUZ-FS06NAH). We are between the 12k and 18k SVZ models for the ducted unit (SVZ-KP12NA vs SVZ-KP18NA). 

The cooling capacity of the 12k unit is 5600-12,000 – perfect for our needs. However at 5F it only has a heating capacity of 15k btus and this makes my installer nervous since we do way more heating than cooling in Zone 6. My feeling though is that paired with the 6k basement unit and it’s 10.5kbtus at 5F, it’ll be fine. That’s enough btu’s to stave off disaster if we aren’t home, and if we are home we can fire up the wood stove. 

His preference is to install the 18k SVZ unit, which is a better match heating wise, but its cooling range is 9360-18,000 – which means best case scenario we’re always using it at the lowest end of its capacity. I’m nervous that this will lead to cycling and poor dehumidification. 

I would love some help resolving our friendly debate! My LoopCAD outputs are attached. Thank you!

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Replies

  1. Aaron Hawkins | | #1

    Emma,
    I ran into some similar situations when I was designing my system. I did the initial design work on the system, but because of my inexperience with this equipment, I deferred to the pros.

    I went to Energy Vanguard because of their exposure here, and the information that Allison has presented here. I worked with one of Allison's employees on the design, expressed all of my preferences, and they killed it. My system also uses a CERV2 as an ERV, and shoulder season heating/cooling.

    I would highly recommend using them, as they're super experienced with this type of equipment, duct sizing with them, controls for them, etc.

    In my opinion, it was definitely worth the money to have them do all of the calculations, as well as the duct layouts.

  2. mgensler | | #2

    If you are redoing your ductwork, perhaps look at the Fujitsu slim ducted. The 18k units go as low as 3.1k btu/h. The system part # is 18RLFCD.

  3. Walter Ahlgrim | | #3

    Since this is a ducted heat pump system it will cost you very little to keep the old school guy happy!

    Have him install the size heat pump you think will get the job done let him install the biggest resistance elements that fit you blower and call it a win win.

    From a return on investment point of view it would make financial sense to install a little smaller heat pump with resistance backup. Yes the BTUs from the resistance require 4X the electricity but how many hours a year will you be 5° very few. If the heat pump was sized for say 20° my guess is the heat pump would completely cover the load over 90% of the hours each year. The small HP would cost less have a higher duty cycle and run more efficiently over all.

    Numbers from my HP are 8000 hours heating with 200 hours on resistance. Most of the 200 hours came before the HP was installed and rest is mostly 3 minutes for each of the 2000 defrost cycles.

    Walta

    1. emma_vt | | #4

      Thanks, Walter - that's a really helpful perspective. I was planning to have the ER backup kit installed regardless, but hadn't thought of it in terms of hours of runtime.

  4. Jon R | | #5

    +1 on using a little resistance heat to reduce the gap between HP supplied cooling and heating loads. Even better, a HP with a wider capacity range.

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