Here in New Zealand climates, people living in certified Passive House homes need to turn the heater on sometimes. To me as a building scientist and Passive House certifier, this is a blindingly obvious statement. But when a well-informed friend was grizzling about having to run heating in late May in their new Passive House, I had to think about how people’s expectations are being set.
Warning: warm, comfortable indoor temperatures could encourage lip-synching in your underwear.
I am left wondering whether social media posts are creating unrealistic expectations. There are a couple of very enthusiastic Passive House owners in Aotearoa who are very active on various social media channels. They love posting photos of their MVHR control panel etc, showing it’s freezing outside and perfectly comfortable inside. All without any heating, they like pointing out!
Before I go through what might be driving outlier experiences that are not typical, let’s talk some numbers. Because I think it’s important homeowners understand this at design stage. Heating demand and load are key performance measurements that every certified Passive House building must meet. Let’s do some maths, using the actual numbers from my frugal friend’s PHPP file.
Heating demand is predicted to be 14.7 kWh/m2/year. The house is approximately 120 m2.
Annual heating demand therefore: 14.7 x 120 = 1,765 kWh/a.
Realistically, most of that heating will be needed over the course of … let’s say four months. That means 441 kWh per month for May-August. At 25c/kWh, monthly heating charges would amount to ~$110 during winter. Add other electricity consumption and lines charges, and the winter power bill will not be tiny.
My friend settled down thanks to hindsight (the end of May was an unseasonable cold snap), an upgrade to an instantly hot micathermic 2kW heater that hardly spent any time on and, as winter progressed, an understanding of how solar gain effectively warmed the house. The heater turned out to only really be a back-up during extended cloudy spells or cold snaps. They packed the heater away before August was out, by which time they were very happy and grateful to have spent the winter in such comfort. Their good fortune was reinforced whenever they had visitors (first words on entering, every time, “Oh it’s so warm in here!”).
There is no question that occupant behaviour can cause actual energy consumption to deviate from the model. Jason’s favourite story is a woman in the cold snowy corner of the US he comes from, who left her sliding door open all winter so her dog could come and go as he pleased. Actual usage did not match predictions for that high-performance home.
But a lot of other reasons could explain outlier experiences where heating is not required. These are specific to a particular home and its occupants and not generalisable to all Passive House homes.
- Are the occupants comfortable at a lower temperature than most people, or perfectly content to layer up in warm clothes?
- Are there more people in the house than the model anticipated? People produce heat and so do other mammals that might share our living spaces.
- Are the occupants home all day? PHPP assumes the house is unoccupied for a portion of each day.
- Are there other significant heat sources not included in the model? This could include teenagers with a gaming PC, computer servers or even consistent use of an oven.
- Is there a poorly insulated hot water system in the house? PHPP ignores heat losses from the hot water system in winter as a matter of principle. It’s not permissible to install a poorly insulated hot water cylinder and pipes to offset the need for insulation. But in reality we’ve seen a house where heat leaking from the under-insulated domestic hot water system was providing most of the heating needed.
- The climate files used for Passive House design are the best available, but the data is historic. Is this climate zone experiencing especially milder winters? Or is this house located in a microclimate?
- Lastly, did the Passive House designer make multiple very conservative assumptions, thereby creating a lot of margin in the PHPP model? The end result is a building that requires less heat input than predicted.
On the other hand, there can be specific reasons why a home needs more heating than predicted. For instance, I saw this recently with a high-performing home where the couple travelled frequently. Their long absences meant lower than predicted internal heat gains (no humans in residence generating warmth, cooking and running appliances). When they did return, more heating was required to compensate. On another project, the home’s heating requirement crept up over time as trees surrounding the house grew and began to block needed winter sun.
The other thing to keep in mind is psychological. People are pretty quick to adapt to new circumstances and I think comfort inflation kicks in. When you are invested in the benefits and wonders of Passive House living, it’s natural to have high expectations. Your idea of what’s comfortable might change. My friend concedes this was the case for their family. And when I stayed in a Passive House recently that was 19 degrees (while 4 degrees outside), I was disgruntled! I wanted perfect comfort. And so I turned the heater on, and then I was.
My advice then is in three parts.
- Homeowners: understand that even a radically efficient Passive House building needs heat input. We recommend heat pumps! They are the most efficient form of heating that exists; they typically provide 2-3kWh of heat for every 1kWh of electricity used. Plus there’s the not-insignificant benefit of air conditioning, even if only during the worst heat waves.
- Passive House designers: check your client’s expectations are realistic, walk them through what the numbers in the model mean if necessary. Recommend/specify heat pumps.
- Passive House advocates: understand that in a small place like New Zealand, the stories you tell in public forums potentially have a wide impact. Your experience of living in a Passive House is your experience. If it’s an outlier experience ie you do run heating in winter despite your cold climate, recognise this is an anomaly and do not present it as an expected outcome for everyone.