We’ve now been living in our Passive House for about 16 months, although we were still installing and commissioning things early on. We’ve now had a full summer and most of a winter to experience the space and play around with how we run it. Here are a few things we’ve noticed so far specific to summer and winter comfort.

In the summer:

The Passive House software allows a modelled design to have no more than 10% of all indoor hours above 25 C. If you crammed all that time together, it would be over a month of living in a space that’s warmer than that. Our model predicted 2% of hours above 25 (assuming no “free” cooling by opening windows in the evening). Through our first summer, I only saw the interior temperature peak above 25 a handful of times and while not sweltering, I don’t think I would have been ok with a month straight being that warm.

Having said that, we don’t have mechanical cooling, nor do I think we need it. Instead, there are things we have been doing – or will do this summer – to make best use of our dirurnal climate through natural ventilation and to limit the amount of heat coming in.

I played around with our heat recovery ventilator’s boost and bypass modes during the warmest days last summer. Both functions can provide a cooling effect. Boost mode simply increases the airflow. Bypass mode bypasses the heat exchanger, which is useful when the outdoor temperature dips in the evening while it’s still warmer inside. During these times, I often switched the HRV to both boost and bypass to deliver the maximum amount of cooler outdoor air.

Our boost mode airflow changes the entire volume of the house 1.2 times every hour. Given this, I had expected that when the outdoor temperature dipped overnight in our coastal climate, it would have cooled the house quite quickly. This was not our initial experience. A few things I think are moderating this impact:

• Our Zehnder Q600 was still doing some heat exchange even when in “100% bypass” mode. I’m guessing this is a limitation of the heat exchanger design.
• Our HRV has seen a lot of construction dust since we installed it. A dirty filter can reduce airflow by 15-20% and when I recently changed a quite dirty filter, our airflow on normal popped up from 157 CFM on “normal” mode to 184 (and from 232 to 265 on boost mode). Something to pay more attention to this summer.
• I also expect there are other internal gains that may be taking longer to cool on those very warm days. Our downstairs floor slab does get direct solar gain during sunny summer afternoons which likely continues to radiate throughout the evening.

What worked way faster and better was cranking open one east facing and one west facing upstairs window after the temperature dipped in the evening, which created a lovely cross breeze that cooled things down very quickly. On the list for this summer is to get screens installed on those high impact windows so that we can leave them open longer as needed without the wasps and moths also getting in. Sometimes just the sensation of moving air makes all the difference in our perception of comfort. I could see a simple circulating fan being quite useful in houses that don’t have as effective window placement for cross ventilation.

We also spent our first summer with no window coverings and minimal exterior shading. While interior shades are not great for preventing overheating in homes with poorly insulated windows and high solar heat gain, I do expect they will mitigate the late afternoon direct solar gain through our large west facing window. I will be testing this theory this summer, now that we have shades like civilized people. And we remain open to the possibility of adding exterior shading in the future. So lots of “plan b” options – at least in today’s climate – before we would even consider mechanical cooling.

In the winter:

Our main heat source is the Sanden CO2 air-to-water heat pump supplying infloor hydronic heat through our downstairs slab. This works really well, and feels nice on the feet too. We kept the system very simple and did not install any hydronic zones upstairs.

Similar to my comment about summer air distribution, I found that the HRV did not spread the heat from downstairs around the house as much as I’d expected. We do most of our living upstairs and I found as much as a ~2C difference between the upstairs and downstairs temperatures on the coldest days.

We do have electric resistance radiant heat in our upstairs bathroom floor but I’ve been avoiding turning it up too high given the electricity cost. It could make up the difference, but I’d prefer that our heat pump do most of the work given that it’s 2-4 times as efficient as electric resistance. I also noticed – now that we’re getting some early spring sunshine- that direct solar gain through that big west window adds an extra degree upstairs on sunny afternoons. I’ve been idly pondering future options for those cold days without sun, which may include adding a hydronic radiant panel upstairs (fed by the heat pump), or possibly putting a small hydronic coil in the supply air duct.

For now, though, pulling on a sweater is good enough, and I can’t complain about that. My old fleece housecoat, which I would often wear over my clothes in my pre-Passive House days, has hung unused in the closet and is probably due for the donation bin. Thank you, old housecoat, for your years of faithful service, but I just don’t need you anymore.