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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Replies
The dettson ducts are an odd beast. They're not normal duct work. They are small diameter high pressure things.
Dettson should provide engineering documents to help your installer.
I’m planning to use regular ductwork. If I use their Smart Ducts it will only be in locations where larger ductwork won’t fit, and I’d use multiple parallel runs to make up for the small diameter.
As a heads up, I have tried to spec Dettson before and run into issues with availability, parts support, and a lack of local tech support. Another fellow I was in contact with who has used them described them as faulting on a regular basis and rather cobbled together out of low quality parts. Not much more experience with them than that, and it could be the other fellow just got a bad unit.
If I might make a recommendation, buy a two-stage furnace like the Rheem R96V and just install it to only run on the low stage. Then you have a 26,000 btu/h furnace with a large support network and anyone can service it. No modulation, but if that's close to your heat load, the furnace will run plenty for comfort, especially with a high efficiency building envelope.
You definitely want to size the ducts big enough to handle the full A/C airflow. If you don't, you'll lower the efficiency of your A/C unit and have problems with your coil freezing up and possible compressor floodback. 350 cfm/ton is as low as you want to go, that's a good number for controlling humidity (and smaller ducts!)
On the duct sizing side, without seasonal adjustment or modulating dampers, your system will always be somewhat out of balance as airflows change seasonally. You size for the largest airflow.
"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." No, that's a great idea if you have a zone on that side of the house with high sensible loads. Durodyne makes a nice zoning system that's pretty common around here (Belleville, Ontario).
Energy Vanguard has an excellent series on duct design, it can be found here: https://www.energyvanguard.com/blog/basic-principles-duct-design
Don't forget to include filter, register, damper, and coil pressure drop in your ductsizing.