Heat / mechanical design for PNW near-passive home
beth_m
| Posted in Mechanicals on
Hi all –
My husband and I are renovating a 1908 foursquare home in northern Washington State. We are using passive principles though not going for certification. We’re replacing everything in the home except for the framing and wooden floors, and window trim. My husband is mechanical engineer and doing ALL the work himself; I’m attaching a couple of photos of our 120 year old house, currently 15′ off the ground, for your viewing pleasure.
We’ve arrived at mechanical design and heat load calcs and we would love some input from this Brains’ Trust! Apologies in advance for a lot of detail.
The house itself is approximately 3,300 square feet and will be 2 stories plus a full size basement. We are planning R33 exterior walls, R49 attic, R23 ICFs for the basement walls and R20 4″ thick foam insulation on the basement floor (we have the option to increase basement wall and floor insulation if needed as it’s under construction now). We’re using R5 triple pane windows. We are aiming for air leakage of 0.6-1.0 ACH50.
So first question: For wall system design, we are shooting for R33 and thinking of something like the below composition:
5/8 drywall / 2×4 furred to 2×6 w/ R23 Rockwool / Plywood or zip sheathing / WRB / 2.5″ or 3″ Rockwool board / 3/4″ furring strips / Cedar lap siding
Looking for people’s opinion on the interior Rockwool vs. other alternatives (dense pack Cellulose, double stud wall), and on the exterior, plywood with high end WRB vs. Zip system. Any thoughts / experience between these options and impact on true R-value?
We have run heat calculations ourselves using BEopt and Cool Calc. We’ve also received load estimates from two different HVAC companies and one green building consultant. Our calculations come out at ~15,000 BTUhr. The green building consultant gets to ~22,000 BTUhr and the HVAC teams both come back at ~27,000 BTUhr.
Second question: Do the heat calculations make “ballpark” sense? Manual J is a black box to me, but logically and based on other similar examples it feels ok, if we use the 22,000 BTUhr load.
In terms of mechanical design, we’re hoping to use a Sanden for DHW. Knowing that, we’re considering heating options which could include some combination of radiant (we don’t have enough energy from Sanden to heat whole house), mini split, and / or electric wall units, and a Zehnder HRV. We will eventually put in solar panels, but as a next phase.
Radiant: Since we like the idea of using a Sanden for DHW, it feels sensible to use the additional energy (8,000 BTUhr) for radiant in the basement area, whether in floor or wall units. However, there is a significant install cost to designing a separate system for just one floor. The basement only needs ~5,000 BTUhr so we could instead use electric wall units or run a mini split head into the area (see next question). For radiant, we could do the install ourselves so wouldn’t pay installation costs.
Mini-split: the HVAC company of course wants to install an 8 head, 66,000 BTUhr mini split system, and also wants to charge $25K for install. I think this is total overkill. I am hoping we can get away with a 25,000 BTU system, with two heads: one on the first floor (1,300 sq ft: open plan kitchen, living, dining) and one on the second floor (800 sq ft: three bedrooms) using recessed ceiling cassettes. From a heat load perspective, this should be sufficient. What I don’t know is whether one head will be able to reach all corners of the floor it’s serving. Any thoughts? Experiences? Should we go for three heads to service two floors just to be safe? Similarly, you could argue it’s more efficient to only have one system throughout the house and use the mini split in the basement too (no radiant), open to comments here, keeping in mind install is the biggest cost driver!
HRV: And then a Zehnder HRV to help with fresh air, plus move warm / conditioned air around rooms. This seems like a great, no-brainer product, and would also help with some heat distribution. Welcome thoughts.
And, my third question: What do you think the optimal mechanical / heating design could be? Would you ditch radiant and do all mini-split? Or combine mini-split with electric baseboards instead? Or other combination? Would love to hear opinions.
Many thanks for any wisdom you can share!
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Replies
A passive house would redistribute heat within the shell faster than the heat escapes out. So although you need to calculate heat losses and gains per room or space, you don't necessarily have to have heating and AC per each room like with a leaky, poorly insulated home.
Since you already have designed a passive house, congratulations for good design work, and I'm not sure you need every room or space to be heated and cooled. It pains me when I hear people suggest expensive HVAC solutions, when more insulation and air-sealing (including better quality windows and doors) could bring the heating or cooling load down more economically, and provide resilience in case of outages. You've already designed a very effective shell so don't need expensive, complicated HVAC.
A kitchen will generate a lot of heat, so you might pay special attention to cooling there, and not needing as much heat for any open areas connected with the kitchen. Assume you are paying attention to solar heat gains (east and west windows in summer, south windows in winter). That might help explain some differences among the wide variation in calculations? I'd rely on your own, as others may add "fudge factors" to make certain your system is powerful enough to handle the most extreme (rare) weather. HVAC companies typically oversize so much that it reduces comfort, with less precise control of temperatures and less dehumidification when cooling. Challenge them, don't go with their numbers.
Most people use basements for storage, a workshop or play space, or otherwise don't need perfect thermal comfort there. Basements have significant thermal mass in the concrete floor and walls, and temperatures would tend to be quite stable. A portable electric heater for occasional use, or a single built-in electrical heating unit may suffice when needed. The same may hold true for at least some other rooms, given your passive quality shell.
It seems a mini split is a reasonably economical solution for a passive house. Typically it is recommended to have one head per floor. (Maybe not bother with the basement?) Heat rises, cool air falls, so one head on the main floor would be unlikely to cool the upstairs, nor would the head upstairs be likely to heat the main floor effectively. But unless you are demanding about temps in bedrooms overnight with closed doors, you may not need more than two heads for your home. Or maybe a ducted mini split head upstairs for bedrooms. Avoid oversizing, you need half of what the HVAC company has suggested, and you have the calculations to set them straight. I'm not the expert on mini splits so others will likely have more helpful suggestions.
The Zehnder system will help provide fresh air to all rooms including bedrooms, and some help with humidity control, but won't do as much for temperature redistribution. Air has little thermal mass, so is not a very effective way to move heat, unless the ducts have air MUCH hotter or cooler than the rooms needing to be heated or cooled, and they are moving a LOT of air, unlike HRVs/ERVs.
You can always use a portable electric heater for those rare situations of sustained, very cold weather. Or an installed heating unit for a room that needs a bit of a boost, but it would be uneconomic, less efficient and unnecessarily expensive to install a mini split head in each bedroom etc.
Also congratulations on doing so much of the work yourselves! I built a house solo and it was one of the great experiences of my life. Plus you'll save yourselves some money!
Robert - thanks so much for the detailed response! Very comforting to hear we're on the right track.
Re: a ducted mini split on the second level - I like this option for distribution but am concerned about ducting in my attic (for condensation / efficiency reasons). Should this be a concern? Or if we cover with enough insulation should it be fine?
Thanks again, Robert!
Its not a good idea to put ducts in the unconditioned space in the attic (i.e., above the attic insulation, or without the attic insulation above it). If the ducts are next to the ceiling drywall, and are covered with insulation, they are almost within the conditioned space (good). The ceiling drywall R-value and air film at the ceiling don't amount to much R-value (about 1.5) vs. your attic insulation (R-49). You could cover the ducts with rigid foam if the attic insulation is loose fill; then its easier to keep the insulation at R-49 or even higher.
Very important that the ducts are well air-sealed also! No leaks to lose conditioned air and allow unconditioned air into the ducts. Much more important in the attic than somewhere else inside the building envelope.
You also may be able to find some places on the second floor (e.g., along the ceiling in closets or hallways, or next to walls at the ceiling where you can cover them with drywall).
Thanks, Robert! This makes a lot of sense to me and seems like a great and logical solve.
Also, I just added a third question in my original post as new question #1 (hope I'm not taking advantage of this group too much - you've been so helpful already) - given your knowledge, would also love your input on insulation design.
Thank you!
Beth,
You'll find it's hard to right-size the system to the small loads and still accommodate your layout with proper distribution and zones. Low-load houses are difficult and the options are thin so I don't fault your HVAC bidders too much. Trust your own numbers but more importantly be decisive about how much temperature imbalance you can tolerate between closed doors in the same zone.
To throw a dart at the board, on a typical four square layout I would lean toward a ducted minisplit on the second floor feeding each room, a single ductless on the main if the plan is at least 'semi-open', then either the DHW radiant or just baseboards in the basement. Maybe a small baseboard or two at the far reaches of the main level away from the ductless.
Minisplits will start at about 9K btu/hr heating, so two is plenty for your home. Nothing wrong with using multiple heat sources for the distribution you want though. The HRV will mix things a little bit but closed doors will still lead to some temp imbalance amplified on sunny afternoons and cold nights.
Sounds like a great project.
Best of luck,
j
Jason - thank you! I agree with your dart throw!
See question to Robert - same applies here. What are your thoughts on ducting in an attic? I know in the south this is common, but in the PNW I've heard it's a bad idea.
Thanks again! Beth
Internal gains are not what they once were due to LED lighting. A house full of incandescent lights derived a considerable amount of heat from them, not so with LED. Cooler air tends to settle in the lowest level even with superinsulation, I would have controlled heat and cooling at all levels. You are spending a lot of time , effort and money on this project, make it comfortable.
Great point, Doug! Thank you for the help!
Passive heat flow depends mostly on air movement and heat load. Even open interior doorways are often a problem - you can calculate the BTU with this (with appropriate substitutions) and make sure it exceeds the room load. On the other hand, if you want some passive room more conditioned that the actively conditioned main room, it can't be done. Fan coils can address this.
doorway CFM = 60 * 1/3 * (7 * 2.5 * .6)* ((32.2 * 7 * (2.78))/266)^(1/2)
7' high door
2.5' wide (ie, 17.5 sq ft)
2.78C (5F) temp difference (1.67C/3F would be better)
BTU = CFM x Delta-T x 1.08
Do account for kitchen exhaust fan load (it can be huge). Think about the Sanden freezing if the power fails.
Very helpful, thanks, Jon! And good point on the Sanden.
Since you have a basement, the simplest would be a single mid static ducted unit there feeding the whole house.
The loads in your rooms are low enough that you won't need very large ducting and you can probably run most of it inside a couple of bulkheads or a small service cavity.
This would let you have cooling and heating to all your rooms even with the doors closed. I live in a highly insulated house and can tell you the bedroom get uncomfortable overnight without cooling.
Make sure to run a largish return up to the ceiling of the 2nd floor, this is the most important item for good cooling comfort.
If you are careful with your design and pressure drops, you can share part of the ducting with your HRV/ERV, so you won't be adding that much more ducting, just slightly larger ones.
As for the Senden, it doesn't make sense to have two types of heat source in the house. A couple of supply registers off the ducted unit is all that you would need for the basement. A standard HPWH is much cheaper and would use only slightly more electricity.
Thanks, Akos!
Given our house is old (2x4 studs and no original ducting) it will be a challenge to duct the whole house, hence why I am leaning towards a mini split system.
And I tend to agree on the Sanden. It's perhaps more of a dream than reality.
Thanks for responding! Beth
You can often create chases in corners with a bump out and hide vertical duct runs in there. Horizontal duct runs can be hidden with a valence type of arrangement. If you’re careful, this can sometimes be done in a way that fits in with the architecture of the house and isn’t really noticeable unless you know exactly what to look for.
Sometimes you can run horizontal ducts between floor joists too. It’s like a big version of wire fishing. It helps to have an eye for it which is something that usually takes some experience.
Bill