As of January 2022, we’ve lived a full year in our new house, the Birches @ WTF. With a year’s worth of data, I’ve taken the opportunity to analyze our overall energy use.
The house uses propane for radiant heating and domestic hot water and electricity for air conditioning and heat recovery ventilation. The radiant heating and domestic hot water both use some electricity when they run but, for now, I’ve included that in the electricity base load. We also run the air distribution system all year round as it helps to even the temperature differences out in the house; this too will be reflected in the electricity base load. In the future, I may get an electrical monitoring system (something like a Sense system) to do a more precise allocation of electricity.
To start, here’s our total monthly energy usage for 2021:
I used a factor of 7.0859 kWh/litre to convert the propane usage to kWh. It is clear that the house uses more energy for heating than cooling.
Computing Energy Base Loads
My next step was to determine the non-heating and non-cooling base loads for propane and electricity respectively.
For the propane base load, we did not run the radiant heat system in June, July, August and September. Therefore, our propane use over that period was entirely for domestic hot water. I averaged our usage over these four months and got 18.4 kWh/d. It is in line with what I would expect for our household size of 5 people.
Calculating the base load for electricity proved more challenging as we had some big one-off electricity users in the spring of 2021 (specifically, electric heaters in our workshop and greenhouse). After removing these months, I was able to use parts of May, June and October as well as all of November and December data to compute the electric base load. The final average was 32.6 kWh/d.
Using this information, I was able to refine the first chart into the following:
(right-click and select “Open Image in New Tab” to see a full-size version)
You can clearly see the Other (E) electricity usage in January through May. It surprised me that those workshop and greenhouse heaters used in total 2.5 times more energy than the house used for air conditioning in 2021.
Annual Heating and Cooling Energy Demands
With these splits done, I could now sum up our annual heating demand (14,020 kWh) and cooling demand (1,411 kWh) for 2021.
So is that performance any good? The gold standard for low energy building design is the international PassivHouse standard. Although we did not explicitly design the house to PassivHouse standards, we did implement many of the PassivHouse ideas into our design:
- Triple-glazing on all windows and doors;
- Heat-recovery ventilation designed into the air circulation system;
- Explicit solar designed to let winter heat in while blocking summer heat;
- Concrete floors and subfloors to retain solar heat, especially in winter;
- Very tight construction which yielded a Air Changes per Hour under 1.0;
- No fireplace or gas stove.
The PassivHouse standard takes into account the livable surface area of the building. For our house, the livable surface area is 347 m2, which yields the following:
|PHI Low Energy Building Standard
|The Birches @ WTF in 2021
From these numbers, we can see that the performance of the house is very good, especially if you consider the house sits exposed in the middle of a field with only a small treed windbreak to the northwest of the house and exposed in all other directions.
Here’s another way to look at the house’s energy performance: our 40.4 KWh/m2/a is well below the expected performance of a modern house:
More Detailed Modeling for Heating and Cooling Energy Use
An useful website for energy performance monitoring is DegreeDays.net. From their website:
“Buildings require more heating in colder weather, and more air-conditioning in hotter weather. DegreeDays.net provides the data to quantify this and help monitor, manage, and reduce energy consumption in millions of buildings around the world.”
Using their free service, you simply identify for a local reliable weather station and request the degree-day calculations for the time period of interest. The closest reliable weather station to White Tree Farm is the London Ontario International Airport (CYXU).
Other than the weather station selection, the other key parameter in the degree-day calculation is the base temperature. In layman’s terms, the base temperature is the ambient temperature at which the building requires heating or cooling. For example, a building that can store passive solar energy will be able to go to lower ambient temperatures before heating is needed. For example, as I write this on a cold but sunny winter’s afternoon, it is -10 deg C outside. And yet, it is 25 deg C throughout the house despite the radiant heating last running 10 hours ago.
Heating and cooling have different base temperatures and they are not the temperatures you run on your thermostats.
Determining what base temperature you should use is really only something you can do after you have a year’s worth of data. DegreeDays.net provides a regression tool to allow you to find the best Heating and Cooling base temperatures for further analysis. For the Birches @ WTF, the regression tool suggested that the heating base temperature (HDD) should be 11.0 deg C and the cooling base temperature 22.0 deg C. Our actual experience with the house aligns with these numbers. The house retains heat well and we are comfortable in spring and fall with no heating. On the other hand, because the house retains heat so well, the ambient temperature does not have to go much above 22 deg C and we feel we need to run the air conditioning.
Predicting Energy Use from HDD and CDD
As well as finding the best HDD and CDD base temperatures, the regression tool generates a fitted model relating the daily HDD and CDD to the expected energy usage.
- Heating: Daily Demand (kWh) = 7.423*HDD_11
- Cooling: Daily Demand (kWh) = 7.456*CDD_22
These equations allow a prediction of heating and cooling demand if the daily HDD_11 and/or CDD_22 is known.
I am pretty pleased with the performance of the house. It is a comfortable living space and the energy use requirements are very much in line with a high performance, energy efficient house. There are a number of changes we could make over time to further optimize the house’s performance.