Active Cross-Ventilation vs. Ducted Minisplits
In Vancouver BC (zone 4), continuing on the journey to electrify the whole house, starting from hydronic baseboards (high-temp fin-tube) upstairs and down, driven by a natural gas combi-boiler. In the end state, I’d like to get both heating and cooling, though cooling only needs to be in some of the rooms.
Overall heat load is 32 kBTU/hr for the 2600 sq ft of conditioned space (2 storey, slab-on-grade, stick built). I’ve attached house layouts for the 2 floors, with notations in each room giving the model cooling/heating loads for a 20F design temp.
I’m now starting to lean back away from ATW and looking at mini-split heat pump systems. The problem is that a number of the rooms, particularly the bedrooms, have heating loads which are below the minimum capacity of the ductless mini-split air handlers which I’ve found so far, so even if I were so inclined to follow the advice of the one outfit I contacted (who asked me what I meant by a “Manual J”) to put a ductless air handler in each room, it seems like I’d not exactly get good performance — the air handler would be constantly short cycling. Some of the rooms (eg, D-Living, @ 4 kBTU/hr) could be handled with their own air handler, but many of the rooms, especially the upstairs bedrooms, would be overwhelmed.
So question for the experts out there — what is your advice?
1. Use ducted mini-splits to group some of the rooms together (eg, U-Bed1, U-Bed2, U-Bath), and ductless mini-splits for others.
2. Use somewhat bigger (and fewer) ductless mini-splits as point sources, and use through-wall ventilation plus added return vents to circulate the air and even out the temperatures. eg, 1 mini-split for U-Living, with fans to circulate to the 2 bedrooms upstairs. To get 3 exchanges per hour, we’d need 60-100 cfm, depending on the room. The house itself is an older (1988) house, so even with the additional work I’m putting in on air sealing, I doubt I’ll get it much below 4-5 ach50 from where I am now without laying out a lot more money than I have budget for.
3. Same as (2), but rely on passive (unpowered) ventilation.
4. Go central air handling, which could work with either mini-split or the ATW systems, and just run ducting all over.
I’m not against ducting, but especially upstairs there isn’t any good place to put the ducting other than through the unconditioned attic space, buried under insulation.
Thank you in advance for your help.
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