Duct sizing for tiny furnace
My build is going to use a 12-30kBtu modulating gas furnace (model c30-M-V) and 24kBtu heat pump/air conditioner from Dettson.
The low and high heat airflow ratings are 300 and 420 CFM, respectively. For 2 tons of AC it will flow about 800 CFM at max load. I’m in Canada (Ottawa) on the colder edge of CZ6A, so surely heating dominated but we still need cooling ~3 months/year and have a decent latent load in the summer.
How should I go about sizing ducts for this unit? I’m tempted to size for 420 CFM (high heat) as a balanced approach, but I don’t want to choke the system (or worse – have high noise) when using AC or have grossly oversized ducts for when it’s modulating near low heat (which will be the majority of the time it’s heating, probably).
I thought about having an additional “zone” at the front (South) of the house, only used for increased airflow during AC season. Is this a dumb idea? Ideally I could use a powered damper to turn this zone on and off depending on which mode the furnace is in. I’m referring to the Energy Docs duct sizing tool for guidance.
I haven’t contacted Dettson yet to ask their advice. I thought I’d post the question here and see what some knowledgeable high-efficiency builders had to say first.
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