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Community and Q&A

Heat & cool new PGH house w/ one mini split head & ERV?

ChrisCT | Posted in Mechanicals on

We are half way through building a new high performance home in southeastern, CT – 5A. I call it a Pretty Good House +: 2,000sf 2 story, 10″ double stud walls with dense pack cellulose, triple pane Intus vinyl windows, simple shape, open 1st floor, high degree of air sealing and paying attention to the details. I have the manual J from our energy rater and I’m trying to finalize the HVAC design as we are ready for mechanical rough-in.

Manual J shows we need: Heating 11,000 btu 1st floor, 5,300 btu 2nd floor, Cooling 12,500 btu 1st floor, 7,000 btu 2nd floor. I’m an architect and willing to experiment a bit since this is my own house.

My safe design strategy includes a hyper heat head sized for the first floor load and a mini split ducted cassette on the 2nd floor with 2-3 very short runs supplying the bedrooms. A Recouperator ERV is also part of both designs. My 2nd design, somewhat more risky, includes one hyper heat head sized for the whole house and located on the 1st floor. I’m not concerned about heating during the winter due to warm air rising – 2nd floor should be okay. My concern is the south facing 2nd floor bedrooms not receiving AC during the summer. If I exhaust the first floor with ERV pickups close to the floor level to grab the cool air, and then supply the bedrooms on the 2nd floor with the tempered in coming fresh air – will it be enough to cool down the bedrooms a little?

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  1. GBA Editor
    Martin Holladay | | #1

    Q. "If I exhaust the first floor with ERV pickups close to the floor level to grab the cool air, and then supply the bedrooms on the 2nd floor with the tempered in coming fresh air - will it be enough to cool down the bedrooms a little?"

    A. No.

    I suggest you read two articles:

    The first article is Choosing HVAC Equipment for an Energy-Efficient Home: "Some people have suggested that creative ventilation ducting — for example, a system that supplies fresh outdoor air to the living room and hallway, and exhausts stale air from the bedrooms — might equalize temperatures from room to room. [John] Straube rejects that strategy, however. “Ventilation air doesn’t do much to move around heat,” he said. “Ten cfm of 72 degree air to a 65 degree bedroom won’t make any difference to the temperature in the bedroom at all. Open doors work better than HRV ducting.” However, some homeowners don’t mind cool bedrooms. “There are hundreds of thousands of homes that have a single-point heat source — a wood stove,” said Straube. “It comes down to, ‘What are your comfort standards?’ Comfort expectations are varying. If you install this type of system, I wouldn’t say, ‘You will be comfortable.’”"

    The second article is Passivhaus Buildings Don’t Heat Themselves. Note the discussion of the system in the Evans-Rue Passivhaus in Salem, Oregon; read the comments as well as the article itself.

  2. user-659915 | | #2

    Noting that Christopher's concerns are for cooling, not heating of the 2nd floor bedrooms I would say that Martin's comments apply doubly. Considering that the heat gain in the second floor will be greater than on the first and that the temperature gradient is working against you, if you don't add a second head now you will undoubtedly find yourself doing so in the future.

  3. Expert Member
    Dana Dorsett | | #3

    James- the stated Manual-J puts the cooling load on the second floor at only 7000BTU/hr , and 12,500BTU/hr cooling on the first floor. I'm gonna go out on a limb here and say that the stratification won't make the cooling load on the second floor any higher than the first floor. :-)

    That said, a 7000BTU/hr load is a real load, and the cfm provided by an HRV using duct placement schemes won't give sufficient cooling on the second floor for ovecoming the stratification + actual direct load with a first-floor ductless as the only cooling equipment.

    Moving heat with air is only good for about 0.018 BTU per cubic foot per degree F difference, so even with a fairly substantial 10F temperature delta (say 85F upstairs, 75F out of the HRV) at 100cfm (6000 cubic feet per hour) you're only looking at about 1000BTU/hr of cooling supplied by the HRV flow. That may be good enough under low sensible cooling conditions but it won't put much of a dent in the 7000 BTU/hr peak load when it's really hot out- you'll cook.

    Sound like a 1.5ton 2-head multi-split or a pair of 3/4 tonners will do it, but you'll need more than one ductless head to manage this.

  4. wjrobinson | | #4

    Dana, my home is well insulated for a late 80's build. That said it needs very little continuous ac to stay cool. I do see a need for ac up. Better idea is to build living areas above sleeping areas. Then having one head downstairs might work.

    The upside down house is a good idea, yes,no? I think so.

  5. gusfhb | | #5

    My last house, very average late 20th construction I successfully ac'ed with a 5kbtu window shaker in the master and a small pedestal fan blowing it down the hall to the other two bedrooms, dehumidified and within 3 degrees f, downside was noise and frostbite on my right shoulder[window on my side of the bed]
    replaced that with 9k minisplit in the hallway, worked perfectly. Doors are always open.

    Miniplits are so cheap[a wall mounted hyper heat is under 2k] and throw air so well, that I would suggest mounting one upstairs even if you end up barely needing it. If you control the sun loading I think the dehumidifying will keep it comfortable.

    People get too wound up about rooms getting hot or cold. In my current house we are using our minisplits for heat for the first time this fall. One 9k unit kept the down stairs[1200 sq ft family rm 2 br bath laundry tiny office] within 2 degrees except for the distant office, 3 degrees, through all of october. Nights in high 20s. This is a 1970 raised ranch with no slab insulation, mostly no foundation wall insulation. If this was a modern house I have no doubt we could get away with only running the 'real' heat in the depth of winter. I consider acceptable less than 2 degrees, as defined by the WTWWT spec. As in, what the the wife will tolerate

  6. ChrisCT | | #6

    Martin - thanks for the reply. I can understand John Straube's point that the ERV volume is just too low to make much of a difference. The original idea came from the homes I've read about on GBA with one head. I think Marc Rosenbaum has one Fujistu head at his place and it works for him. I think Carter Scott has some homes with one head on the first floor and there are others that I can't recall - maybe the Up Hill House?
    I may try a middle ground : rough-in the line set and condensate for the 2nd floor cassette (or head) and install it later if we find the two south facing bedrooms are just too hot. The roof overhang was sized to keep the sun off the 2nd floor windows in July and August (and it was gratifying to see it actually work at the end of framing) and I'm not sure this is reflected in the heat load calcs. Perhaps my solar gain numbers come down a bit, reducing the cooling BTU load, making this idea more viable.

    Dana - thanks for the math behind the idea, this is valuable info. A question for you: if I put a single head in the 2nd floor landing, area 5'6 x 7', will the unit potentially short cycle in such a small area, even with all the bedroom doors open? The ducted cassette seems like the best approach for delivery reasons, but I prefer to have the extra efficiency of a wall unit. Keith G. mentions the head in the master and then letting it "leak" out or fan distribute to other rooms - a possibility.....

    Keith - thanks for sharing your experiences in the the world of mini split conditioning. I agree that people can get too wound up about temp differential between rooms. Quick story - our last house was an 1895 vintage, mostly un-insulated, very leaky older home with steam radiator heat. Master bedroom had two exterior un-inulsated walls and NO radiator i.e. no heat source. We would shut our door at night too! If it was 30 outside, our bedroom was probably 45 - and the kicker - we liked it. Guess I have to credit the wife for having a high WTWWT factor.

  7. mackstann | | #7

    Another option is to use ventilation fans (Panasonic bath fans being one option) to move air around.

  8. Expert Member
    Dana Dorsett | | #8

    AJ: My house is a 1920s antique and not so super insulated, and has very little need for continuous sensible-cooling either, but shading factors matter- a lot! If the manual-J was done reasonably well, the 7000 BTU/hr peak is something that WILL be a comfort issue during parts of a CT summer, and HRV flows alone won't treat it, not even a little bit. A fan on the first floor jetting high-cfm conditioned air up the stairwell might though- depends on the layout.

    The mini-split head on the first floor will likely keep the air dry enough to improve second-floor comfort during peak sensible loads, but high-R homes may not get sufficient duty cycle to dehumidify in a high-R house with low sensible loads. The Daikin Quaternity series is a pretty good solution since it has independently settable RH and temp setpoints, and can dehumidify without cooling (or heating) the space.

    If cooling the bedrooms isn't a high priority most of the time (probably isn't), Keith's solution- a $99 window- shaker will get you through the worst of it, for a heluva lot less money than a second mini-split. A 7K peak cooling load isn't much.

    Marc Rosenbaum has stated he would have installed another ductless head in his old place if they were using the other bedroom, and he's probably more temperature-tolerant than most, but you may have him beat! If you managed to make peace with your uninsulated Victorian in an unheated bedroom you're probably going to be just fine. It doesn't hurt to try it out with one head first and adjust as-needed, but an HVAC contractor looking to design a system to meet the load(s) wouldn't take the chance of a call-back, (nor should they.)

    A second head placed at the top of the landing wouldn't short cycle with the adjacent bedroom doors open- convective exchange is powerful if it has a free path. In cooling mode it wouldn't short cycle even with the doors closed, since convective forces would keep it exchanging air with the downstairs. (I know of a guy in Minneapolis who cools is whole house with a Mitsubishi -FE18 at the top of his stairs, letting convection do the air exchange with the first floor.) In heating mode it may even stop, with the doors closed, since the buoyancy of the heated air from first floor would pool at the top of the stairs, keeping it above the setpoint of the upstairs mini-split. In cooling mode a 3/4 ton unit at the top of the stairs would probably keep up with the whole house except for the worst of days- if you were going to pay the extra freight for a Quaternity, making it the upstairs unit is probably the best bet, and you could just turn the downstairs unit off until needed.

    On my daily commute I pass a ~1200-1400' raised-ranch heated and cooled by floor with what appears to be a pair of Mitsubishi FE12s (They might be FE09s). I've never knocked on the guy's door to see how it's working for him, but this is a 1980s vintage not-so-superinsulated house. (He made the mistake of mounting the outdoor units at grade and without protective overhangs, and has to dig them up after every storm.)

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