Air Distribution with Air-Source Heat Pump System
We are designing a new house in Zone 6 using PGH performance targets. We plan to heat and cool the home with 2 x air source heat pumps. Our living room is slab on grade, with a cathedral ceiling (ridge runs N-S) and exterior walls on three sides (W, N and E). The footprint is 24 ft. wide (E-W) by 18 ft. deep. We will add a destratification fan to the ceiling ridge. There is no easy way to duct air to the perimeter walls; I don’t want to run ductwork within the roof trusses, and the architect is understandably hesitant to detail any bulkheads along the interface between the cathedral ceiling and the walls. I can ‘throw’ air from a bulkhead that is 10′ above the floor, and that runs parallel to the north wall about 18ft. away. The peak heating load requires less than 500 cfm of supply air. The question is: How comfortable will it be to sit along the walls in the winter? Notably, my reading/snoozing chair will be in the NE corner, which has 135 ft2 of (triple) picture windows. I would like to avoid resistance heat (not as efficient as an ASHP, and not keen on the aesthetics). Can the better thermal performance of triple glazing achieve acceptable comfort? Or should I at least rough in some electrical power in case we end up wanting additional heat? I am curious to hear about successful solutions to this detail.
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It is hard to get heat to flow down even from 8'. 18' sounds like a stretch unless you use some commercial high throw registers which might need beefier air handler.
I think the better question to ask is your slab on grade a must. Could do something like a shallow frost protected foundation with a crawlspace instead? This means warmer and much softer floors (concrete is hard on feet) and convenient place to house and run services. This would let you put vents directly under windows which will be more comfortable than trying to cover the 18' distance.
Interesting idea. I agree that floor registers would provide the best air distribution, but I am not that comfortable with a crawlspace. It may be simply a Canadian bias (the house is in Nova Scotia). Another suggestion was to use direct buried ductwork, but again, I am not that comfortable with lack of access and possible moisture issues with buried ductwork.
The supply air diffusers would sit vertically on a bulkhead that was 10 ft. above the floor, but 18 ft. horizontally from the north wall. I considered a small terminal booster fan(s) (the heat pump fan does not have a lot of static, and I also did not want to penalize the entire system with higher static), but even then, as you say, warm air tends to float up rather than down.
I'm a bit east of you in the great white North and basements are pretty much universal around me, slab on grade is non existent.
If you want to stick with slab on grade, one option is to put 2x6 (even 2x4 can work with careful design) sleepers over your slab. You can set these to run in the direction needed to run a feed across your living space.
This would let you run ducts through there plus you can lay wood flooring over it. The nice part about running ducts like this is you get budget floor heat in the bays where the ducts run.
Some of the multi position air handlers can be configured into a downflow orientation so you can set it up on top of a plenum in your utility room. All your in-floor ducting can now run this this plenum for a pretty simple setup.
You would only loose a couple of inches of height which can be easily compensated in the outside wall.
A crawlspace is a much more flexible space though for not much extra cost.
Canada is all about crawl spaces? (well, mostly all about basements but they're the essentially same thing)
I'd say consider in slab radiant heating as a potential solution. Or at least roughing it in for future. I'm not the biggest fan of radiant heat as a primary source since it doesn't circulate/filter the air, Thus you still need ductwork for ventilation and AC. I also know Akos isn't the biggest fan of radiant, and you should listen to him way more than me.
This falls back to my opinions that architectural/structural and HVAC should be done by the same person or same team. A house design should encompass how its going to be conditioned, they're hand-in-hand. HVAC shouldn't be an afterthought, its the most important aspect of a house.
Can you post your drawings and the manual J & D, or a BeOpt report so we could have a better idea of the design issue?
Jamie
Jamie and Akos: Thank you for your comments and questions. They touch on several fundamental design issues. In the interests of your time, I will try to provide a brief summary of the steps that led us to the current design, which may help to focus on the topic at hand. That being said, I would be happy to elaborate on any of these points.
1. While basements are common in Canada (I have never lived in a house without one), I was persuaded by some of the comments from the GBA community that they simply become a place to store things that will never be used again (guilty). The land also slopes down to the north, so while the living room is slab on grade, the ground floor of the 2-storey portion of the house behind it that runs E-W, while on the same level as the living room floor, is actually buried into the hill. We could build a basement under the living room wing, but if the only reason for the basement is to run ductwork (and it would be), I'd like to find a less costly solution (hence my initial question). And if I was going to build anything under the ground floor, I would spent a bit more to build a full basement rather than a crawlspace (that was the Canadian bias I was referring to).
2. After much thought, I chose an ASHP heating option for temperature control over in slab heating. The reasons behind this solution include the cost of a water source heat pump and a separate A/C system, and all of the compelling reasons against in slab heating in a tight, well insulated house proffered by a large contingent of the GBA community. (Full disclosure, as a contingency, and as Jamie also suggests, my contractor has convinced me to rough in the piping in case we find the slab cold. In that case I would add just enough heat to the slab to take the cold bite out of it.)
3. The house design has progressed in a collaborative and iterative manner with the design team (the architect, structural engineer and the HVAC designer (me)) accommodating the various pulls and pushes inherent in any design process. If anything, the HVAC designer has not quite kept up to the architect.
Returning to the original question of HVAC options, the raised floor option is a clever solution.
However, it introduces costs and construction details, while not insurmountable, I would like to avoid if we could.
Responding to Jamie's request, I have attached a floor plan of the 'north wing'. (North is up.) The living room has the cathedral ceiling. There is an enclosed mezzanine above the kitchen and dining room (south of the dotted line) that creates a 9 ft. ceiling in that half. I am able to supply heated air along the northern edge of the mezzanine for the living room, directed towards the north exterior wall, some 18 ft. away. (Heated air for the kitchen and dining room is supplied via grilles located in the ceiling under the mezzanine floor.)
I can also run ductwork along the east and west walls where the cathedral roof structure meets the walls, thereby delivering air downward along the exterior walls from a height of 9 ft. Alternatively, I could run a supply duct above the collar ties and then down the face of the north wall and into an E-W bulkhead. Neither of these options, perhaps understandably, excite the architect. Exposed ductwork has also been suggested, which I am not averse to.
The notion that simply introducing the air into the space (as opposed to right under or over the load concentration, in this case, the windows) comes from a perhaps unfounded reliance on the performance of well insulated walls and triple glazed windows, as well as the prevalence of using ductless splits to condition open areas. Placing the grilles in the mezzanine wall would seem to replicate these local head end units, although I acknowledge that the grilles could not fully mimic the automatic deflector in some units that direct heated air downward.
Responding to Jamie's other request, attached are the basis of the load calculations. It is a custom spreadsheet that I developed for the house; I have not used a Manual J calculation (nor a Manual D analysis, at least yet). I recognize that this load calculation approach may be controversial, and that it risks distracting from the original question, but I offer it to assist in the diagnostics of the design question.
Thanks for reading this far, and your helpful suggestions to date. Any additional suggestions based on the foregoing information would also be welcome.
Quick correction. The floor plan insists on rotating 90 deg. CCW. North is to the left (i.e., the living room is on the north half of the floor area).
Some of this depends on what your finished floor will be. If you are going with concrete in zone 6, floor heat is about the only way to get barefoot comfort. Even there, you might run into overheating if you keep the floor hot enough to be felt, for example on a bit bellow freezing day here, my floor temperature is only 1.5C above the room.
If it is wood flooring, you can get decent comfort with a strip of resistance floor heat by the windows and entrance with the supply vent 18' away. The resistance heat in this case is only there to take the edge off and should not supply most of your house heat.
I think the simplest solution would be to figure out how to get ducts there. Bulkheads are not as bad as they may seem when designing. Once built they tend to be not noticed at all. Not the best picture but you can see the supply running above the windows on the right side of the picture.
Akos: We plan polished concrete floors in most of the ground floor, and I suspect you are correct about barefoot comfort, and the delicate balance between enough warmth and too much heat. And electric strip heaters would be a good option for the few colder nights we have. In the end, I think I am leaning towards bulkheads or exposed ductwork. Thanks again for your feedback; it is helpful.
Take a look at the table here:
http://www.healthyheating.com/Thermal_Comfort_Working_Copy/Definitions/floor_temps.htm
Your polished concrete floor would have to be at 27C for barefoot comfort.
At that puts out 11F*2BTU/F/sqft*600sqft=13200BTU.
Once you get your heat load sorted, I'm pretty sure that will be a good portion of your design heat loss for the whole place. This means outside of the coldest of days, the floor will be either too cold or the house too hot.
I think better use of the concrete polishing funds is get some wood flooring on there. I'm a big fan of engineered click for this, just make sure it is rated for heated floors if adding it in.
Don't do polished concrete for the heat. It's a lousy floor, hard and unforgiving.
If your home will be modern or contemporary, exposed ductwork can look sweet. It will set the style tone for the whole house.
A slab has a LOT of embodied energy. FHB featured a slab-less basement. You could probably do slab-less, but then you wouldn't have polished concrete.
.02
Thanks all. After mulling over the comments, I suspect we will proceed with overhead ductwork (exposed or in a bulkhead) to deliver the heat where it is needed. I am also reconsidering polished concrete (which we have flipped and flopped on already). While I like the look, I also acknowledge that it is a cold and hard surface, as has been pointed out.
I'm in Maine. Our living/dining/ kitchen room is much bigger than yours. About a third of the ceiling is 8', the rest goes from around 10-14'. Slab on grade, with 4" of reclaimed XPS underneath. Passive House level airtightness, windows and insulation. A single air source heat pump heats the entire space just fine. I can sit a few feet away from our south facing 11'x11' window array and not notice any chill, even though the heat pump is around 20' away.
My point is that comfort is all about the building envelope. Make it robust enough and a single heat pump will keep you as warm as you want.
As for polished concrete, we like it. I wear socks in winter. It's hard, but as a practical matter, I don't know that it's any harder on your body than any firm wood floor.