First Full Month Heating and Energy Usage

We’ve now been in the house for about two months. But our electric bill runs (roughly) from the 21st to the 21st.  So last week we received our first “full month” electric bill.  Given that it’s the first full month, the information gleaned from the bill is, of course, very preliminary.  But I thought I’d share it anyway.  The important numbers were as follows:

Average Outdoor Temperature During the Period: 53 degrees

Total Electric Usage: 551 KWH

Total Cost for Electricity $90.94

Cost per KWH: 17 cents (all fees and charges included)

The last week of this billing cycle, it got fairly cold.  Nightly lows were as low as 19 degrees.  Daily highs were in the high 20s to mid 30s.  But again, the average temp for the month was a relatively mild 53 degrees

The house is two floors plus a basement.  Square footage is 1,000 per floor.

All lights are LED; 40 and 60 watt equivilants.  Most are 60s.  Only a few are 40s.

The house is completely electric.  Oven is convection.  Cook-top is induction.  Water heater is heat-pump.

We have two Mitsubishi mini-split heat pumps.  Both are 12,000 BTUs.  Each has one cassette.  One cassette is on the first floor.  The other is on the second floor at the top of the stairs.

For all but the last two days of the month, we used only the first floor heat pump.  Some days I didn’t use it at all.  Other days I left it on.  Mostly I was trying to get a feel for how it operated.  I was surprised to find out that the fan on the indoor cassette unit always runs (albeit at a very low rate), even when it is putting out no heat.

The single heat pump, kept the house “comfortable” down to the 19 degree low.  However, when the outside temp got below freezing, it appeared to put out heat all the time.  This kept the first floor at a pretty consistent 67 or 68 degrees at night and 68 to 70 degrees during the day.  The second floor temperature (which is where the bedrooms are) varied; at night it got down as low at 64 degrees, and during the day, with a little help from the sun, it rose to 68 or 69 degrees.

That’s why I put the word comfortable in quotes.  I found this to be quite comfortable.  After all, for us, almost all of the time spent on the second floor is spent under covers.  But my wife was not quite as content.  So for the last couple of days, I turned on the second floor heat pump.  This quickly brought the second floor up to 68 and significantly reduced the working time for the first floor heat pump.  Both heat pumps were set to 65 degrees. I’m not sure why a 65 degree set point on the heat pumps results in a 68 degree environment.  But it does.

A couple of things seem clear to me.  First, at 68 degrees the house is exceptionally comfortable.  Not a hint of a draft, even when the wind is howling. And second, a clear, sunny day makes a measurable difference in the form of a reduced heat load.

Next week, I will install an eGuage energy monitor on the main feed and 12 circuits.  So beginning then, I will have a much better read on the energy consumption for the heat pumps, water heater, and ERV.  I will also install Hobo temperature/humidity data loggers on all three floors and outside. This will give me the ability to monitor and report on the performance of the house over the long term.

Based on this very preliminary information, I have a few initial thoughts.  First, I was a bit disappointed in the first month’s electric bill.  Both the KWH used and the overall cost were higher than I anticipated.  But I’m thinking that this is due more to unrealistic expectations (and an unexpected 17 cent/KWH overall rate) than house performance.

One way to try to put this data in context is to compare this first full-month electric bill to my energy cost from one year ago, when we lived in our prior 2,800 square foot house, built in 1987, which used propane heat. The average temperature last year was 51 degrees; a couple of degrees cooler.  At that house, I followed our energy usage pretty closely.  Electric usage was easy (just read the bill).  But I had to gauge propane usage by my readings at the end of each billing period.  Each night, we set the thermostat back to 60 degrees and at least three days per week we left it at 60 degrees during the day because we were both at work.  We never raised the thermostat past 69 degrees.  But frankly even I thought that was uncomfortable.  It pretty much always felt colder than either of us would like.  At any rate, my total energy cost for that house during the same period last year was $428.43, and my electric usage was 881 KWH.  But remember, we were heating an additional 800 square feet.

It will be interesting to see how things play out in the long run.  Taking a SWAG based on this admittedly limited data, I’m thinking that when all is said and done, we have a good shot at being under 6,000 KWH total energy usage for the entire year.  That would result in a total energy bill of about $1,000.  And if it looks like that prediction is proving to be fairly accurate, I may start looking for a PV system next spring.

Energy updates will follow, once I get the monitoring systems installed.

Final Blower Door Test – .3 ACH @ 50 Pascals

Drew McDowell, the Passive House Rater, came in on Saturday and performed the fourth and final blower door test.  The prior tests were all done manually (I believe they’re called “single source” tests); meaning that the Rater watches fluctuating readings on the meter and uses his judgment and observation to determine an average flow rate. In this fourth test, Drew used that method as a starting point, and came up with a value of about .28 or .29 ACH.  However he then followed up with a computerized test where the PC took dozens of readings in ten-degree increments (i.e. at dozens at 10 Pascals, dozens at 20 Pascals, dozens at 30 Pascals, and so on up to 70 Pascals) and then averaged and charted the results, and used the collective information to refine the computer’s 50 Pascal readings.   Also, unlike the manual test, this computerized test was performed in both directions; first while blowing air out of the house (i.e. depressurization), and then while pulling air into the house (pressurization).

The result came in at .3 ACH @ 50 Pascals.  Copies of the reports are visible below:

Download (PDF, 477KB)

Download (PDF, 259KB)

This was up a bit from the prior test (.24 ACH), which was performed before the installation of insulation.  I’m not sure how that happened, being that it seems to me the insulation and drywall should have made the house tighter, if anything.  But regardless, I’m very happy with the result.

Two observations worth noting.

First, as discussed in a (much) earlier post, we used a trio of products by Prosoco to seal the gap between the windows and sills.  The final product in the trio, R-Guard Air Dam, is essentially a very robust caulk that is used to seal the gap after the installation of a foam backer rod.

The windows were installed about a year ago.  We’ve been in the house now for almost two months, and I’ve been chipping away at the interior work as time allows.  Recently, when I began working on interior trim, I noticed a small gap in the R-Guard around one window.  That prompted me to inspect all of the windows, which led to the discovery on similar gaps at three others.   I suspect that the gaps were due to human error at the time of installation; the thickness of the application in those areas appears to have been very thin and the product appears to have simply shrunk.  Fortunately, the gaps were easy to find and seal because I hadn’t yet trimmed out any of the windows.  But the discovery caused me to think that my slow pace paid off.  Had the interior been finished by a hired trim carpenter, or had it been finished immediately after the drywall was finished (which was last spring), those gaps would have never been discovered.  To be clear, none of the gaps were deal breakers.  But collectively, get enough small gaps and I imagine it’s possible to break the .6 ACH limit.

The second point worth noting is that, during the final blower door test Drew and I went around the house with the thermal camera and smoke pen looking for leaks.  The only weak points we found were the exterior doors (all ThermaTru), which I’ve talked about extensively in a prior post.  Again, obviously the leaks weren’t deal breakers.  But frankly I expected much better given the amount of money those doors cost.  If I had the chance to do it over, I’d be looking at Intus for the doors as well as the windows.