An HVAC puzzle: find physical room for equipment
The task is to provide the full HVAC suite to a 1949 bungalo in climate zone 2. You’ve already done a good job air-sealing the walls and ceiling.
You can’t put the ducts in the attic. It’s an unconditioned, unvented attic with a vapor diffusion port, and fluffy insulation on the floor. It doesn’t have moisture issues.
You can’t put the ducts in the walls. The building’s 2×4 studs just don’t offer enough room.
You could use ductless minisplits, but how will you balance the three bedrooms? Undercuts and jumper ducts only go so far before all privacy has been eliminated. And what will you do for filtration, dehumidification, ventilation?
What about under the house? There’s an encapsulated crawl, but it’s not one of those nice encapsulated crawls. You’d have a hard time fitting traditional ducts down there. You might be able to get away with something like the Zehnder ComfoTubes, but how will you get them through the 2×4 bottom plate? You could just run them through the floor, but vents on the ground collect dirt at an aggressive pace.
And you’d still have to figure out what to do about the bedroom someone added later, which sits on a slab.
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For what it's worth, the best I can think of is minisplits for temperature control. Throw a dehumidifier in the crawl and cut a couple vents in the floor somewhere out of the way. As for ventilation, I think you are stuck putting it in the attic, and running the pipes for it as far under the fluffy insulation as possible. You could maybe get the ERV into the crawl if you have a closet or something you can run ductwork up to. Either way, tons of penetrations just for ventilation.
Air to water heat pump with fan coils. SpacePak, Chiltrix, Arctic Heat Pumps. Not cheap, but depending on number of rooms may be reasonable. Ventilation with Lunos.
Thanks for the reply. I'm not familiar with....most of those. How does this work? I had baseboard heating growing up but I've never heard of cooling with water.
Basically there is an outdoor unit like an air to air heat pump that is connected to an indoor tank (or tanks if you want to cool and heat domestic hot water at the same time ) either via refrigerant or water lines depending on design. This tank is then plumbed to various fan coil units (similar to indoor mini split heads) via insulated water piping. The fan coil needs heated/chilled water supply and return, power, and a condensate drain. They are also available small enough to properly condition a bedroom without being oversized.
The Arctic heat pump website has some useful information. https://www.arcticheatpumps.com/heat-pump-fan-coils.html. As well as GBA https://www.greenbuildingadvisor.com/article/air-to-water-heat-pump-retrofit
You could also check this out. https://www.probuilder.com/comfort-systems-road-map-plug-and-play-residential-duct-system
Where about are you located?
How large is the house? How open? Is this in CZ2A or CZ2B?
Mechanicals always take up space, this is much easier for a new build but can be done in an existing house. Nobody wants to give up space, in the long run having a good HVAC install is more important than a couple of sqft of floor.
You do have to find space for a mechanical room, if you look at something a slim ducted air handler like a Fujitsu 18RGLXD which can be installed in vertical orientation needs only about 1'x3' space which can easily be in the back of a closet. See post #8 here for size:
https://www.greenbuildingadvisor.com/question/anybody-here-actually-own-a-fujitsu-slim-duct-system
Ducting will still need some space. The best is through interior bulkheads, areas like above built in cabinets or inside and above a bedroom door are great spots as they are hardly noticeable. If needed, with a well sealed attic (this is important part), you can run ducts up there as long as they are buried in insulation.
If there is no spot for ducting, high velocity can be an option. These are small ducts that can be easily fished inside a 2x4 wall. The ducting and equipment is not cheap but can make ducting easier.