Help with HVAC for a green home in Grand Rapids, MI
Hello,
I had a question regarding use of geothermal heating and cooling about for a green home I am getting ready to build in Grand Rapids, Michigan. First, my home is 1633 square feet on the main floor, 1424 on the second floor and 1633 in basement. The walls will be 2 x 6 with blown in fiberglass and 2 inches exterior Rockwool and the goal is to get the air changes per hour low (ideally 1 or under). Windows will be Energy Star U-factor 0.26. Foundation walls will have 2 inch foam interior against concrete and adjacent 2 x 4 framed with fiberglass bats in basement. The ultimate goal is to be net zero capable with 15kW solar panel system.
I was told the home was too large to heat and cool with air source heat pumps and be comfortable so was primarily looking at geothermal. I have been quoted by two HVAC companies, one told me the load calc came in at 50,797 btuh at 7 degrees outdoor temp and recommended a 5 ton heat pump (60,000 btuh). This company said their rationale was there would be a 9000 btu cushion and electric heat coil backups would kick in for any sustained subzero temps (rare, but not unheard of here). They felt 6 ton wasn’t necessary for heating and overkill for cooling. The second company did not give me their load calc but feels strongly I need two 3 ton heat pumps to be comfortable.
Also, I have a friend who is in HVAC and he feels strongly that I should have radiant tubing run in the basement to help with heating in winter and while I was at it I was going to run it in the mudroom, master bath and three season porch.
After reading some of the articles on GBA, I am coming to the conclusion that people are not really fond of geothermal or radiant heat which leads to several questions.
Is there a better alternative to geothermal if I want to have the potential for net zero? If I stick with geothermal is two 3 ton heat pumps overkill (its certainly more expensive)? Should I abort the radiant altogether?
Any insight is much appreciated. Thanks!
Andy
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Replies
Andy,
Grand Rapids, Michigan has an outdoor design temperature (for heating system design) of 5 degrees F or 6 degrees F, so you can use an air-source heat pump from a U.S. manufacturer connected to a conventional forced-air duct system.
All the information is in this article: "Ducted Air-Source Heat Pumps from American Manufacturers."
Don't believe the 50K heat loss number unless it was done by a qualified third party- an engineer, RESNET rater etc., somebody who makes their living and reputation on the accuracy of their numbers rather than installing and maintaining HVAC equipment. Even a code min- house that size would have a heat load between 35-40K @ 0F at 3ACH/50. The only way to stretch it to 50K would be to have excessive air leakage or excessive expanses of window area. YOUR house is likely to come in closer 30K @ 0F, 36K tops and even less at +7F.
That makes heat pump capable of delivering 60K @ 5F about 2x oversized for the house you described. Don't make any decisions on heating without running an independent Manual-J heat load calculation, room by room, zone by zone, floor by floor. This is definitely going to be within the range of a pair of slim-ducted modulating Fujitsu minisplits. (One for the first floor, another for the second floor) or maybe a 3 ton Carrier Infinity Greenspeed air source heat pump w/ heat strips.
I suspect your basement load will come in somw 6-8000BTU/hr, depending on windows and the amount of above grade if the slab isn't insulated. With a couple inches of EPS under the slab it'll be under 5K, and may never have to be actively heated. Don't bother with tubing in the slab without the load numbers first. If the slab only needs to emit 3 BTU per square foot to heat the basement the floor would only be barely above room temperature, which wouldn't add a whole lot to the comfort factor.
I'm aware of a house in VT slightly larger than yours and slightly less insulated, in a location with a 99% outside design temp of -12F that is heated with 4 ductless mini-splits. The notion that your house is "too large" to heat with a heat pump is just silly. It's the size of the heat load, not the house that matters. A 3000' house with a 1500' basement really isn't a problem for cold climate air source heat pumps.
Radiant heat (preferably in limited areas) and geothermal/exchange are great - if you ignore up-front cost. Will usually also provide some efficient domestic water heating.
If you are looking at air to water geo-exchange, then you can add a water tank (thermal mass), resolving any heat pump over-sizing issue other than cost. If very large, it can enable some use of solar PV that doesn't involve net metering. Multi-stage or multiple heat pumps also mitigate over-sizing.
On the other hand, it's perfectly OK and usually cost optimal to under-size somewhat (say 4 tons nominal for an actual 51K design day load) and use electric heat to make up the difference.
When it comes to shutting down when cold (which some air source heat pumps do) or operating within manufacturer specs, be sure to consider record lows - not design temp.
So, this is prescient that I saw this. My house insulation, climate, windows, and foundation insulation are almost exactly the same as yours, just a bit smaller (2200 sq. Ft).
My design heat load at -18C is 18K BTU, but we had a day like that this week, and my air source heat pump didn't have to work that hard to keep up, so, I think it's probably a bit conservative.
I'd say it's unlikely you are at 50k BTU... but you need to get it properly rated by a pro. I had a full on engineering design firm do my house, buildingknowledge.ca.
Great, thanks everyone for the comments! Ill have to get a better load calculation, Ill let you know what I find out!
I found a company to do an independent manual J and one of the questions is about rim joist insulation and it got me thinking. I read some old threads on this site about open versus closed cell and flash and batt, but they didnt pertain to my situation exactly and I am wondering in my situation with the 2 x 6 construction with blown in fiberglass and 2 inches exterior Rockwool what you guys would recommend? I would like to be as environmentally thoughtful and conscious of indoor air quality as I can but also understand the importance of air sealing here. Thanks!
Closed cell is pretty standard for use on rim joists. Lots of people use kits like those from foam it green and the Dow frost packs. What you want to avoid is keeping the wood wet. Your assembly sounds like it won’t be a problem, but make sure you have a capillary break between the rim joist and the top of the foundation wall.
Bill
Since you are possibly having a tight building envelope, low heating and cooling loads, plus installing a 15kWH PV system, I would highly advise you to consider going all electric with an air-to-air heat pump, and hopefully installing ES appliances and lighting, electric fireplaces and an induction cooktop, . That's what I spec on all my houses, which are ZERHs. Just a thought!
Ha, I am currently trying to convince my wife that electrical fireplace and induction cooktop is the way to go.
My builder usually does 2 inch closed cell or 5 inches of open cell in rim joists. I was leaning open cell because my understanding is its "greener" and still an effective air barrier. Is that flawed thinking?
For about $50-60 you can get a single-burner portable induction cooktop, I have one made by duxtop. Have your wife play around with it, it’s an inexpensive way to test out induction cooking. Note that not all pots are compatible with induction cooking.
Bill
I also read prior comments suggesting that 1 inch closed cell and then R15 rockwool batts may work. Thanks for any advice!
For US climate zone 5 framed walls that works for meeting code-min, but not the best performance or bang/buck if you're framing with 2x6 and leaving 1" of cavity depth empty. It's both cheaper and higher performance to put 1" of foil faced polyiso exterior to the sheathing of a 2x4/R15 wall. Even a 2x6/R23 rock wool wall would be slightly higher performance than the inch of closed cell + R15 in a 2x6 cavity.
The 1" continuous polyiso + 2x4/R15 wall slightly better than code-min, but substantially lower performance than the "...2 x 6 with blown in fiberglass and 2 inches exterior Rockwool..." initially described.
For basements building a 2x4 wall an inch from the foundation (as a depth guide for the foam) and going with an inch of closed cell + batts in the cavities works.
Electric fireplaces look great and there are no chases to build and no penetrations on the envelope and roof to deal with (aka leaks).
Induction cooktops are more accurate, faster and requires smaller exhaust fan and no make-up air equipment.
OC foam around the rim joist is best when you install rigid foam (CC foam) on the outside of the wall. Since you have Rockwool, either would work. There is not that much area to cover to be concern about GWP.
Thanks Armando.
Dana, I think I may have confused you, or I am confused. I am still doing the 2 x 6 wall with blown in fiberglass and 2 inch exterior rockwool, I just wanted to know what my options were for insulation at the rim joists.
Andy
That makes a bit more sense. Your response #10 didn't specify where the 1" foam + R15 rock wool was going- my initial instinct was that it must be referring to the basement walls, but since it wasn't specific I moved beyond that assumption.
Yep, my bad. I would still appreciate any additional thoughts on the rim joist insulation :)
Andy,
Q. "My builder usually does 2 inch closed cell or 5 inches of open cell in rim joists. I was leaning open cell because my understanding is its "greener" and still an effective air barrier. Is that flawed thinking?"
A. In a cold climate, closed-cell spray foam would be a better choice for the interior of a rim joist. You're right that either type of spray foam is an air barrier. The reason that closed-cell spray foam is better is because it is a vapor barrier, unlike open-cell spray foam. The closed-cell spray foam prevents outward diffusion of water vapor, and therefore keeps the rim joist dryer.
For reference, I built a 3070 sq. ft. house in Macomb, MI in 2014, 1500 on the second floor, 1570 on the first floor, over a 1570 basement. We used a 5 ton, variable compressor, variable speed blower ground-sourced heat pump, with a horizontal ground loop. The forced air system is split into separate zones for the first and second floors. The basement is "unconditioned." We added a 9.4 kw solar array in 2016. The array is divided in two, 20 panels for electricity, and 13 panels for the HVAC and hot water. This was primarily done to put the heating system onto DTE's time of use meter - be careful with such a decision, as it can go the wrong way. The electrical side of the system let us be very close to net zero over the year, while we use still buy about half our our energy for heat. We also used an induction stove and all electric appliances. I did not plumb the house for natural gas, although it was available at the street.
Walls are R-38, double stud construction, and the blower door tests resulted in an ACH around 2.
In the winter, the house is very comfortable. We keep the thermostat at 73 degrees - I can't confirm that this is the actual temperature with a separate thermometer. In two and half winters, I have only seen the system dip into the electric reserve heating coils once. The system varies between between running at low power and high power. In the summer, the system keeps the house very cold, and always seems to be running in low power. We only run the AC on the second floor, and let the cool air spill down from the second floor, which is more than adequate to cool the first floor.
The system is certainly oversized for cooling. Its probably oversized for heating. If it were not a variable capacity system, this would be more clear.
You may not have a to-water heat pump, but conceptually, your time of use rates (avoid Summer use from 11am to 7pm) are a good fit for cold water storage (a large buffer tank). But perhaps your solar panels cover most of this.
The system the utility uses for net-metering and time-of-use divides the year into 4 segments - summer and winter, peak and off-peak. For example, energy credits created in "summer-peak" cannot be applied to "winter-off peak." This results in some of my credits being trapped in a segment where they cannot be used - I create way more kwh in the summer peak period than I can use. Offsetting this problem is that the time-of-use rates are advantageous enough that even though I cannot take advantage of all of the kwh credits I create, I am still coming out ahead of the game. If I had a greater number of panels dedicated to the HVAC / time-of-use system, I would be better off with just the standard net metering arrangements.
More info - I use the de-superheater from the heat pump in the summer to pre-condition hot water in 55 gallon tank (electric tank, power turned off) before transferring to an 85 gallon tank (electric) to raise to the final temperature.