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Community and Q&A

Home energy monitoring programs

Gerald Blycker | Posted in General Questions on

I am building a net zero energy home to Passive House standards in the Seattle area. In addition to having it be as efficient as possible, my goals were to use renewable materials (no foam), eliminate thermal bridging. have a protected air-barrier and to keep the OSB warm. I’m using dense packed cellulose in the R-50 walls and R-85 roof (cathedral ceiling with parallel chord trusses). The wall assembly is a double stud system with an inner 2×6 load bearing wall and an outer 2×4 wall. The OSB is on the outside of the INNER 2×6 wall and acts as the shear wall and air barrier, with all seams sealed with Prosoco joint and seam filler. Densglass is used as the exterior sheathing on the outside of the 2×4 wall, then Tyvek with a 3/4 inch rain screen and the cedar siding. Placing the OSB air barrier sandwiched in between the walls keeps it warm by having 2/3 of the wall insulation outside of it and protects the air barrier from being compromised, even during any future possible remodeling. I must say that it did look odd during framing to have a 2×4 wall on the outside of the OSB. Many builders stopped by to ask what was going on. I will be using a ductless minisplit air source heat pump with one head (heating load of 6,300 Btu/hr= 1,860 W) and using the Zehnder HRV system to circulate the hot air throughout the house.

I teach physics and environmental science and would love to monitor the performance of the house, both for my own interest and to collect data to help the building science community continue to improve on best practices. Does anyone know of a home energy monitoring program that is looking for new energy efficient homes to include in its study? I would like to monitor both energy consumption and performance (temperature of various rooms).

If I can’t join a program or research project, can anyone recommend monitoring equipment I could use to monitor the house myself? I would like software that can also log production data for my PV system.

Thanks for any suggestions.

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  1. GBA Editor
    Martin Holladay | | #1

    I hope that some GBA readers can help you with your main questions. In the meantime, I have a comment.

    You wrote, "I will be using a ductless minisplit air source heat pump with one head (heating load of 6,300 Btu/hr= 1,860 W) and using the Zehnder HRV system to circulate the hot air throughout the house."

    The HRV will not circulate hot air throughout your house. The HRV will circulate 100% outdoor air, which won't be hot (except in July, of course). During the winter, the temperature of the air distributed by the HRV will always be cooler than the indoor air temperature.

  2. Expert Member
    Dana Dorsett | | #2

    I don't know of any program, but there are multiple variations on electrical circuit dataloggers that an track power against time on a circuit by circuit basis. This company's lineup are pretty flexible & popular:

    I'm curious how the decision to use DensGlass & Tyvek in lieu of plywood / OSB / plank / fiberboard sheathing & #15 felt, came about, given the ".... renewable materials..." design goal? There are many fiberboard sheathing products that would be more vapor permeable than the 4-perm GP DensGlass, drying quickly into the rainscreen gap, and other asphalted versions that would be variable permeance. (GP Stedi-R is north of 15 perms, #15 felt is variable permeance, north of 5 perms when wet.)

    Even a 3/4 ton mini-split would be ~2x oversized for your 6300 BTU/hr load at any King County location's 99th percentile temperature bin (mid to high 20s F.) It may be under sized for your cooling load though, if (as with most PassiveHouse designs) you went big on the south facing glass. Your heating/cooling balance point is in the 40s Fahrenheit, and then only on dark days or at night. It will cycle on/off rather than modulate, and the standby power use may cut significantly into it's as-used efficiency. If you go that route, kill power to the circuit when you don't actually need to run it, since it will rarely be needed during the heating season. In Seattle you can usually use nighttime ventilation cooling strategies, as long as you have a means of killing the window gains.

    Mitsubishi's 3/4 ton dedicated single mini-ducted SEZ/SUZ KA09NA is less oversized than most single head wall coil types, and would have you covered (8300 BTU/hr @ 25F outdoors/ 70F indoors , according to the table on p.26:

    That means it would at least be modulating with load at least part of the time.

    By contrast the FH09NA delivers 12,500 BTU/hr @ 17F- 2x overkill, and only has about a 3:1 turn-down ratio, which means it would modulate only rarely, spending most of it's time idling in standby , cycling on/off.

    The Fujitsu AOU-9RLS2 is similarly ridiculously oversized for your miniscule load, and would cycle more than modulate, spending all if it's time in standby:

    Rather than using the HRV for heat distribution, right-sizing a mini-split ducted system and distributing the heat with short duct runs allows you to close doors and still get VERY good room-to-room temperature balance. At 0.018 BTU per cubic foot per degree-F, an HRV chews a lot of power, and requires big room-to-room temperature differences to move any heat at all, which is not very efficient.. The net efficiency would likely be higher with a mini-duct cassette pproach, using the HRV for ventilation only, not heat distribution. That is despite a dramatically lower rated HSPF as-tested for the mini-duct unit (HSPF=10) compared to high efficiency 3/4 ton wall coil type units (HSPF 12-13).

  3. Expert Member
    Dana Dorsett | | #3

    Martin- I haven't looked at the specs for the Zehnder, but several HRVs have a recirculation mode option, which does move modest amounts of heat from room to room. That's at truly piss-poor heating efficiency when the delta-Ts are low, as will surely be the case here.

  4. David Meiland | | #4

    You might want to talk to the folks at WSU Extension's Energy Program in Olympia, and see if they know of any research going on that might apply.

  5. Gerald Blycker | | #5


    My understanding is that the HRV is exhausting the stale warm air from bathrooms and the kitchen and is replacing it with an equal volume of fresh air from outdoors. The warm air is transferring more than 90% of its heat to the incoming cold air in the heat exchanger. So, yes, the air from the supply vents will be cooler than the air it is replacing, but not that much cooler. I would certainly prefer to circulate only warm air throughout the house without losing any heat, but the Second Law of Thermodynamics won't allow that either. The purpose of the air tight assembly, R-50 walls and R-85 roof is to keep the heat inside the house as long as possible. The HRV system was designed for .34 air exchanges per hour. One way of looking at that means every 3 hours all the air in the house is replaced and I lost 10% of my heat. I'm not sure if that level of ACH is overkill and maybe the air would still feel fresh at .1 ACH. My thinking is, since I need the HRV to provide ventilation and it will be moving warm air around the house, why put in extra ducting if it's not needed? The problem is that I won't know for sure if it's needed until everything is closed up and I've lived in it for a winter. I've heard that it is common for Passive Houses to use one ductless minisplit head and have the HRV circulate the warm air to the rest of the house. Is this not the case?
    I've talked to some HVAC installers who recommend at least two heads, but even the smallest 3/4 ton heads would be more than 3x my heating load. That's assuming, of course, that the PHPP (Passive House Planning Package) software is accurate. It seems like if I go with a ducted system to directly supply heat to living spaces, the system will be grossly oversized. Another option is to wire the bedrooms for possible wall heaters down the road in case they are needed, but I just love the "free" heat you get from a heat pump.

  6. GBA Editor
    Martin Holladay | | #6

    While most of your analysis is correct, my point is the following: if you are worried that a single-point heating system (the ductless minisplit with one head) will cause room-to-room temperature variations, you should know that your HRV won't help solve the problem.

    The more you run your HRV, the more you will be introducing outside air at a somewhat lower temperature than your indoor air. That won't make cold rooms feel warmer. It will make cold rooms feel colder.

  7. Gerald Blycker | | #7


    Thanks for your detailed information. I love the idea of not relying on the HRV to distribute heat throughout the house. Unfortunately, the house wasn't designed for traditional ductwork. I have some leftover 3" diameter Comfotube (Zehnder's ducting) left over from my HRV installation, which can easily be routed through my walls. Can these be used with a mini ducted heat pump system? How many rooms could a slim duct system supply air to? Are return lines needed for a ducted mini split system?

    Another option would be to put in one 3/4 ton ductless minisplit head in the living room with an open floorplan (see attachments). I would then use a Panasonic inline fan to pull warm air from the living room and send it to the bedrooms on the second floor. Our living room is adjacent to an open stairwell, which is also open directly up to the attic loft area with cathedral ceilings. I am concerned about hot air collecting up by the peak, so I could have a separate inline fan bringing air from the peak down to the study on the first floor. Would it be easy to put thermostats on these inline fans so that they turn on automatically as needed? Of course, adding ducted runs with inline fans will reduce the overall efficiency of heating system, but they might not need to be used very often.

    How reliable are PHPP's heating loads (assuming the home is actually constructed according to the input parameters)? Can I assume that my actual heating load will be within a 10% margin of error? Some HVAC installers have suggested putting in two 3/4 ton ductless minisplits (one on each floor). When I mentioned that it would be overkill for my 1/2 ton heating load, they said that because of the variable inverter compressor, a 1 1/2 ton compressor would use the same amount of energy as a smaller 3/4 ton compressor if they were both producing 6,000 Btu/hr. That seems to contradict what you wrote regarding a larger compressor spending more time in standby mode. I don't want the heating system to be any larger than necessary. It seems like 3/4 ton should provide enough heat, it's just a matter of evenly distributing the heat.

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