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Community and Q&A

Solving the Mystery of High Energy Usage in Vacant Duplex Units

tech1234 | Posted in Mechanicals on

Hey guys I got an energy usage question probably mostly relating to a mini split install although I guess the electric water heater could be the culprit… Here’s a little back story, I build affordable rental housing in southern NH in an area with a major housing shortage due to an ever expanding college. The town I focus on allows duplexes on every residential lot without ZBA approval so I focus on buying condemned/fire damaged/etc properties and then build quality duplex housing to the best energy efficient and sustainable level that is financially feasible. I’ve been tweaking some basic designs as I go to ever improve on the ease of build, building longevity, material usage, energy efficiency and renter interface. (I’m a one man show with some subs)

Latest build:
Side by side duplex
Each unit is 1000sqft (500 per floor)
2 story
Stem wall/slab (2″ xps under slab)
2 bedrooms 1 bath upstairs
Open concept kitchen/living room downstairs
Detailed air sealing
2×6 walls, minimal cc flashcoat for air sealing, deansepack cellulose
Triple pane paradigm windows
Fiberglass half light entry doors
R-50 cellulose ceiling
All electric
PV prepped
24k mitsubishi hyperheat 3 head (1st floor 12k wall unit, 2nd floor 6k wall unit in each bedroom)
40 gallon electric water heater 145f set point (mixing valve)
So of course my energy usage was all over the place during the build and finishing, but with the pandemic and other life situations these units sat completely finished with nobody occupying them for one complete month which lined up with the billing cycle of eversource.
Here is how both units sat finished but empty for one month:
12k heads on and set to 64f
All 2nd floor heads shut off
Refrigerators on (no usage)
Water heaters on (no usage)
No other electrical loads
Unit 1- 896 kwh (=$162.70)
Unit 2- 856 kwh (=$156.05)
All of my recently built units are in the same town and the same square footage and the same number bedrooms. (Although they are of varying designs)(single floor living, etc) All of them are full electric and run hyperheats for heating and cooling. My occupied units have lower electrical bills then these partially shut down and empty units… I have a good understanding of the seasonal electrical costs of these units and so I tell new tenants what they can expect for costs. This new build has me stumped though. Admittingly I don’t have a baseline for partially shut down and empty units but I would imagine it would be substantially less than a lived in unit. So I’m assuming if I let people move into these their electrical costs are going to be much greater than the other units I have and I cannot understand what’s causing that. I suspect there’s something going on with the mini split sizing or installation… I’ve had a lot of trouble with subpar work from my HVAC sub and he’s definitely not moving forward with me. (one thing is he has pushed me towards single outdoor/multi heads units and I have found that to be less than ideal) What are your guys’ thoughts on the electrical usage of these partially shutdown units? Am I off base thinking something is not right?
James Walters

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  1. Kevin_in_Denver | | #1

    Read the meter yourself for a few days and when you are there check to see what is on.
    Sometimes the utility will estimate the usage.
    Make sure none of the thermostats are in cooling mode, they could be fighting each other.

    1. tech1234 | | #5

      Kevin, reading the meter myself is a good and simple idea that I skipped right past. Thanks for the reminder. The electric bill for this period says- Reading type: Actual. My guess is the numbers are correct but I will verify.
      The upstairs units are turned off.

  2. user-6623302 | | #2

    Just some thoughts: The building is vacant so you are heating the whole space. No fixtures to take up space, no people giving off body heat. Isn't leaving the upstairs unit turned off like leaving a window open? The outside equipment is running full speed trying to keep up but can not. It is not running efficiently. I think it is different if you have two outside units. Is there some electric resistants heat running?

    1. tech1234 | | #6

      "Isn't leaving the upstairs unit turned off like leaving a window open? The outside equipment is running full speed trying to keep up but can not." - this is not my understanding but I could be wrong. Hopefully an expert will chime in on this.

      There is no resistance heat

      1. brad_rh | | #16

        No, leaving the upstairs unit off is not like leaving a window open.

  3. Expert Member
    BILL WICHERS | | #3

    Each person will contribute about 100 watts worth of heat while they're there, so 1.2 kWh per 12 hour period. No people means no "free" people-powered heat.

    I doubt it's the water heater if there was no usage. Water heaters are reasonably efficient when they're just sitting there.

    I would expect something with the minisplits (was it really cold that month?), maybe a window was left cracked open, or possibly an estimated read like another poster mentioned. If the utility doesn't have good history, their estimated reads will be way off. You can request an actual read and get that corrected.

    If you'll be there for a few hours, take a meter read when you get there and another when you leave. Divide that out by how long you were there, and you get the number of kWh consumed while you were there -- and while you knew what was happening. That will give you a baseline to help you determine what is going on.


    1. tech1234 | | #7

      Bill, the lack of people heat is a good point. The temps were cold but my similar units (filled) are all in the same area (one is 300' away) and some of them had bills lower than this unit in the same time period.

      The electric bill for this period says- Reading type: Actual. My guess is the numbers are correct but I will verify.

      1. user-723121 | | #14

        What are the HDD for the electric billing period, we can break down the heating load that way. The duplex units are highly insulated, should come in around 1 to 1.5 Btu's per square foot per heating degree day with straight resistance electric heat.

  4. joshdurston | | #4

    As a possibly irrelevant comparison.
    My single FH09 (1:1) seems to add about around 15kwh per day, definitely less than 20kwh in heating (when I don't have the wood stove going). It gets fairly heavily loaded in moderate temperatures since I keep the other heat sources in the house off and let it handle the load when it's close to freezing.

    Without the heatpump running I'm just over 20kwh per day and with the heatpump the highest I've seen is around 38kwh (haven't broken 40kwh in a day).

    So I'm surprised that your that high unless it's really cold.
    It be interesting to meter the consumption.

    If you have a smart meter you may be able to log in to your utility and check hour by hour and figure out what you base load is to subtract it. The heating load is basically the only variable.

    I did notice my wall mounted head hunted a lot causing the compressor to ramp up and down until I installed a wireless wall controller to remotely sense the temperature. (MHK1). This might be happening to you. You could reduce the hunting a bit by setting the fan to a higher speed and carefully setting the vane angle.

  5. charlie_sullivan | | #8

    First, I love what you are doing--great work!

    Next, I think that given the projects you are doing, I think it would be worthwhile to get an iotawatt or other monitoring system that you can install temporarily on a new build and monitor key electric usage until you need it on the next project. Or leave it in--they are only $200 or so. You can monitor total consumption but also put extra sensors on specific circuits or interest such as the minisplits.

  6. Mark_Nagel | | #9

    A possibility, though perhaps not high in the probability list, is that the heat pump equipment isn't actually operating at the temps that you believe they are (set to). Maybe also adding in a temperature data logger to capture real-time temps (and humidity) could help see what's happening. I have a Lascar (cost is under $100) that I've used in the past for everything ranging from monitoring incubators and brooders to our pump house temps.

    I don't know enough about heat pumps to know whether short-cycling is problematic from an energy perspective (I know that it's mechanically problematic with just about any motors/pumps).

    1. charlie_sullivan | | #12

      Short cycling a minisplit will result in lower efficiency (that is, lower COP). Mechanically, it's not as hard on them as it is on a traditional compressor, because their inverter drives soft start them.

  7. ph_aficionado | | #10

    If your water heater comes with a Wifi module, you can usually check day-by-day consumption in an app.

  8. Expert Member
    AKOS TOTH | | #11

    It is very easy to get incorrect charge in a multi split which kills their efficiency.

    No easy to way to check this without reclaiming and evacuating the system, mostly a last resort if you can't find another cause.

  9. Jon_R | | #13

    If you could find a way to heat with only electrical resistance heat for awhile, you could calculate the buildings performance (in btu/HDD). Once you have that, you can switch back to heat pumps and calculate COPs. Hourly measurements/logging of power usage, indoor temp and outdoor temp are key - wind and sky condition are helpful.

    Solar gain can be a significant difference between buildings of similar design.

    1. tech1234 | | #15

      Jon, good point on heating with resistance heater and calculating loads from that.

  10. brad_rh | | #17

    That is an insane consumption level for an empty, well built house. Something is wrong. +1 for the iotawatt, I have one in my house. You'll need wifi though.

  11. jberger | | #18

    Just to follow-up on Brad's comment about needing WiFi.
    If you go with a Sense meter, you could install via a hotspot on your phone for the initial installation, then use your hot spot service occasionally to let the device phone home until you have internet in the house.

    The meter can only hold the per second data for several hours but it holds the per minute data for weeks or months depending on the volume. Since the units are unoccupied, I'd suspect you could go quite a while without running out of storage on the device.

    It won't get you remote access since you need your phone to provide the hotspot, but it might give you a better picture of time of trends and help you identify the issue. The "device identification" will not work well but just the per minute logs should be useful.

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