Editor’s note: David and Kayo Murakami Wood are building what they hope will be Ontario’s first certified Passive House on Wolfe Island, the largest of the Thousand Islands on the St. Lawrence River. They are documenting their work at their blog, Wolfe Island Passive House. For a list of earlier posts in this series, see the sidebar below.
Along with the thick insulation and sealed building shell, the heat-recovery ventilation (HRV) system is one of the most important features of any Passive House.
An HRV system is really very simple. In essence, it is a ventilation system with a heat exchanger, so that in winter, when cold air is drawn into the house, it is warmed with the heat from the stale, outgoing air (and vice-versa in summer). However, it is also more than this. The HRV ensures a uniform temperature throughout the house, taking the air heated by the sun through the large south-facing windows and distributing it to other rooms.
Our system is also a little bit more than this. With the addition of a very small thermostat-controlled heating element (drawing about the same amount of power as a toaster), it also becomes a low-pressure forced-air heating system. This will be the only artificial heat in the house, and we don’t think we’ll need it that much even in Ontario winters.
Choose a system carefully
It’s worth noting that not all HRVs are created equal. Most manufacturers make claims about the efficiency of the systems they make, but few are actually independently tested and verified, and this includes the leading companies in North America. This doesn’t mean they are bad, but you can do so much better.
Our HRV (actually, an ERV, but more on that later) is the ComfoAir 200 from Swiss company Zehnder, pretty much the best manufacturer of HRVs in the world. Their tested efficiency is the highest; their systems are the quietest; and their ribbed plastic Comfo-Tube ducting is much better than the old square section metal ducting you might be used to. Our system was supplied, with minimal fuss, by the excellent Hans, from Zehnder’s Ontario dealer, Pinwheel Building Supplies.
You need to plan for the installation properly from the beginning, though. We didn’t, but we got lucky. The unit itself takes up a fair bit of room. However, they can be mounted on walls or ceilings. Since we have massively over-engineered cross-laminated timber (CLT) ceilings, we did the latter, mounting them up and out of the way in our machine room. There just happened to be enough room.
There are a lot of ducts, too — serving fresh air registers and exhaust grilles in different parts of the house — and the ducting goes up through what would have been our chimney openings. Again, this was luck rather than judgment. We also didn’t get the design for the HRV system ducting done before we ordered the timber components, so rather than having pre-drilled fittings, the builders are having to drill through the thick ceilings to make the vent holes.
Adding the heating element as cold weather arrives
We finally had the little Thermolec 2-kilowatt heating element installed in the HRV intake in December, months after the system was installed, but just in time for a cold snap.
This element is a bit like a toaster and heats the air coming into the HRV manifold that distributes heat around the house. It is controlled via a thermostat, which is downstairs and not too far away from the kitchen. We are aware that it’s likely to be a bit warmer there than elsewhere, so we keep it at 20ºC (68ºF) knowing this is likely to mean 18ºC (64.4ºF) on average around the rest of the house.
All the other heat comes from passive solar, i.e. the south-side windows, but it’s been quite dull recently, so that has not been such a big factor. In addition, we’ve been having further problems with the Motura sliding door. We already had to have it taken off and some new sealing rubber put on, but it’s still not tight.
This was not apparent in the recent blower-door tests, we think, because when you pressurize the air inside the house, the sliding door would have been pushed outwards, against the seals. However, at other times, this is not the case, and the higher we have the HRV settings the looser it gets.
Compared to most houses, it’s not at all bad, but there is still an appreciable draft around the edges of the sliding door. We have contacted the supplier and await some advice. We might just push some of the left-over rubber gasket from the CLT construction into the edges and tape them up, just for the winter.
A footnote on our ventilation system
There was an unexpected development with what we had thought was our HRV. Malcolm Isaacs of the Canadian Passive House Institute had stopped by to carry out the second, and we hoped final, blower-door test and make some final tweaks to the ventilation system to make sure it was correctly balanced.
While he was there, we discovered one curious thing: our “HRV” actually appears to have an energy recovery ventilation (ERV) core. This means that actually, it not only uses a heat exchanger to transfer heat from the outgoing exhaust air to the incoming fresh air, thereby keeping the air in the house both warm, but also balances the humidity. This is interesting, not least because we only paid for an HRV, so far as we know.
What we now think is that Pinwheel, the retailer from whom we bought the HRV, already knew when we bought it that they were going to stop being the Ontario dealer for Zehnder (they officially stopped at the end of 2016), and therefore were selling off whatever units they had in stock. They probably didn’t have any ComfoAir 200 units with an HRV core left, so they just gave us one with an ERV core instead. That’s just fine by us!
The only potential issue is that it probably isn’t the latest model — all the latest Zehnder units apparently now come with a pre-heater for low temperature operation. Without this, ERVs are not, I don’t think, guaranteed to run below about -10ºC (14ºF). But that’s quite common here in winter, indeed it has been -14ºC (6.8ºF) for the last few nights and much lower at times. But we’ve not had any problem with the unit at all.
Still not running at peak efficiency
We are recording temperature levels for a project at Algonquin College, but that data won’t be available until it’s downloaded from the recorders. In the meantime, we are just keeping an eye on performance using simple thermometers, particularly when it’s cold outside. And last night, it was very cold outside.
When we got up, the external thermometer was reading -15ºC (5ºF), so it could have been even colder during the night, and the winds have been fierce here, so the effective temperature would have been around -25ºC (-13ºF). When we woke up, the temperature inside was 14ºC (57.2ºF) upstairs and 16ºC (60.8ºF) downstairs. With some sunlight in the morning and a few hours of an additional 1-kW heater, this was soon up to 19ºC (66.2ºF) downstairs and 18ºC (64.4ºF) upstairs, and it has pretty much stayed that way since, as the winds blow around the house.
This is pretty good. But we shouldn’t need the additional heater, and until we resolve the problem with the Motura door, one way or another, we will not be able to get the more consistently warm temperature levels we would like. It’s certainly not cold — it’s very comfortable — but the house is not yet operating at its optimum.