Tag Archives: ERV

HRV Fan Failure: the importance of pre-heating in cold climate zones

We’re in our third winter in the passive house (the second full winter, I guess, since we were only in for part of the first and a lot still needed to be done back then). Several issues have emerged, which are all things that are fixable, but which in retrospect could and should have been prevented. The main one of these is that about a month ago, one of the fans in our HRV failed suddenly, which left us without ventilation in the middle of winter. Up to this point, the Zehnder HRV* has been completely hassle-free – you’d hardly have noticed it was there at all.

This is when it suddenly turns out to be very important that there is a reliable local dealer for your HRV company… and, there isn’t. Pinwheel, who supplied us our system, ceased being an official Zehnder dealer pretty soon after we got our system and despite there being officially named companies that have some affiliation in the province, none of them actually supply or repair Zehnder. Luckily, Zehnder North America have awesome people in their main offices who are prepared to go the extra mile for a customer.

I chatted to a couple of different people, the first, Joe from Operations, about the possible causes of the failure and what we needed. Zehnder agreed to send us a replacement fan immediately with no charge. Excellent. But then what to do about the installation? Well, basically, they got another guy, Gary, the Training and Service Manager, to walk me through it. Yes, I did it! it wasn’t that hard, as it happens, although there were some electrical circuit operations to perform that had to be done exactly right and safely, but I wouldn’t have been able to do without him. I’m pretty confident I could do it again now, if I had to. Gary – you’re awesome!

As to why this happened… well, it’s highly likely that it was something to do with the low incoming air temperature. We noticed when we were installing the system that Zehnder HRVs are not guaranteed below a certain temperature. Apparently Zehnder would never recommend installing one without a preheater, but no-one involved in our original installation – not our passive house advisor, not the dealer and not their contact at Zehnder at the time, ever mentioned this or suggested including a pre-heater. Everyone seems ‘surprised’ now that it somehow didn’t happen. So, a take-away for anyone building a passive house in these cold climate zones: you need a pre-heater for your HRV — don’t forget it or think you can get away without one.

Anyway luckily for us, Zehnder has recently changed the way it does pre-heating for its HRVs anyway. Instead of an internal pre-heater, they now supply an external model that sits in the intake duct. So, we’re installing on of those, and hopefully we’ll have no further problems with fan failures.

I’m still thinking about the implications of all this. A sudden HRV failure makes you very aware of just how much the passive house concept depends completely on this mechanism, and however reliable the mechanism you have, this dependency is a large weakness, a major lack of resilience. On the other hand, we did find that ventilating the house by opening windows for half and hour a day works even in winter when it’s -20ºC outside without cooling down the house too much. We’d been super-paranoid about opening doors for even a few seconds before and, actually, it was fine. We also used a dehumidifier to get rid of excess moisture.

Anyway, a big thank-you to Zehnder North America for their support. They really are a great company – if only a decent Ontario company would take up the challenge of being a proper dealer and servicing agent…

*Zehnder Comfoair 200 UL (Luxe) ERV. You can see more about the system installation here.


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Preliminary winter performance data

We have had temperature and humidity data recorded in the house over the last 4 months as part of a project conducted by Anthony Mach, a passive house designer and Building Science research student at Algonquin College. The preliminary data is now available, courtesy of Anthony. What we have here is essentially the raw temperature and humidity records in two locations: one in the middle of the open-plan downstairs space (1st Floor), and the other upstairs on the landing (2nd Floor). There is a lot of analysis to be done with this data combined with other data on external temperatures, energy use and so on.

A few things to note when looking at these charts:

  1. We were only half-moved in for most of November – we started using the kitchen sometime in the second week.
  2. When we moved in (around the 21st November), the HRV (which actually turned out to have an ERV core – for more on the differences, see here) had still not been properly balanced and we were still only using the system on its lowest setting.
  3. After the HRV had been balanced properly on the 4th December, we started using it on the middle setting, with boosts after baths and during cooking.
  4. We only had the 2kW Thermolec heating element, that works with the HRV, installed on December 14th. Up until that point we had only been using a single 1kW space heater. If it was cloudy in the morning after this point we used both, but if it was sunny we didn’t need the later.
  5. However, that installation coincided with a serious cold spell where external temperatures dropped to -25ºC or lower.
  6. We were away from the 20th to the 30th of December, and had the HRV just ticking over, which means that the house would have had almost no internal heating. You can see the drop, but what’s remarkable is that the place still never got below 13ºC.
  7. Once everything was back to normal and functioning properly, from early January, the temperatures in the house were generally between 17ºC (average night-time low) and 19ºC (average day time high) upstairs, with the extremes being 15.5ºC and 21ºC; and 18ºC (average night-time low) and 20.5ºC (average day time high) upstairs, with extremes of 16.5ºC and 22ºC. The difference is probably explained by a combination of the use of the extra heating downstairs, the passive solar effect from the larger windows, and generally that there is more activity downstairs for more of the time.
  8. The humidity has generally been where you’d want it, between 40 and 50%, gradually drying out as winter goes on. Our HRV having an ERV core helps in stopping the place getting too dry.

“The most airtight building in Ontario”

On Saturday, our advisor and supplier, Malcolm Isaacs from the Canadian Passive House Institute, visited us again to carry out the second (and hopefully, final) blower door test on airtightness, and to do some final tweaks to the Heat Recovery Ventilation (HRV) system to make sure it is fully balanced.

On the latter issue, we discovered one curious thing: our ‘HRV’ actually appears to have an Energy Recovery Ventilation (ERV) core. This means it actually not only uses a heat exchanger to transfer heat from the outgoing care to the incoming air, thereby keeping the air in the house both warm and fresh, but also balances the humidity (for more on the differences, see this Ecohome.net article). This is interesting not least because we only paid for an HRV, so far as we know…

However, the big news is something much bigger. When Malcolm last visited to do our preliminary blower door test at the end of September, various things weren’t ready, in particular, we had a malfunctioning and only partly sealed Motura sliding door, and the equipment wasn’t able to accurately record the figures we needed so we could only get a rough result. Even then we got a pretty good indication that the house was already well inside the Passive House standard. In the meantime, the door had been properly sealed, and Malcolm had obtained the component that would allow for the test to be done more accurately.

As the house was being pressurized, we went around with a little camera that measures temperature differences, to check for air leaks. We also discovered that one of the HRV ducts that goes through the outside wall was not actually as sealed as it should have been, despite the gaps between it and the wall having been filled with sprayfoam. So we had to seal up the edges with some more of that expensive but very effective Siga tape.

The process of doing a blower door test is outlined in the blog entry about Malcolm’s previous visit. You need only recall that you pressurize the house to 50 Pascals, and then record the amount of air that has to be added to the house to maintain that pressure. From these numbers, with the house volume, you can calculate the Air Changes per Hour, or ach50. The highest Canadian Standard, R-2000, is 2. The Passive House standard is 0.6. Last time we’d measured just above 0.4.

Malcolm did his measurements and then went away to do the calculations back home. From here I’ll just quote directly from his e-mail to us today:

“I’ve just finished recalculating the blower door test results from my visit yesterday. We did not do too badly, with an overall n50 airtightness of 0.185 ACH @ 50 Pa for both overpressure and underpressure. To the best of my knowledge this house is the most airtight building in Ontario […] This result shows the high quality and performance of all components used in the house, as well as the significant extra efforts of each of us who worked on the airsealing. The impact on overall energy demand in PHPP [the software used for calculating Passive House performance] is very significant: it drops overall specific energy use from 14.6 kWh/m2y (assuming 0.6 ACH)  to 12.5 kWh/m2y.”

For those of you who don’t speak Passive House, Malcolm provided a summary:

“Speaking as an engineer, we have a special technical term for this result: KICKASS.”

Quite. 🙂