Tag Archives: natural ventilation

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.


Passive Cooling

As our place is a passive house, it is pretty much sealed tight in the colder months and works via a mechanically very simple Energy Recovery Ventilation (ERV) system from Swiss manufacturer, Zehnder. As regular readers will know, we added a 2kW heating element (about the size of a toaster) which turns the system into a very gentle (un) forced air heating system. This is all we have and need by way of heating.

So what do we do now it’s the summer and it gets hot? We unequivocally do not have Air Conditioning (AC). Okay, we are in Southern Ontario and this is not a tropical climate. But we started from the principle that we wanted for this to be a Net Zero house not just a Passive House. So, thinking about cooling was also designed into the place.

In some ways the answer is laughably simple: in the summer, we open windows. However, as you’d expect, exactly what’s going on is not quite as simple as that.

  1. Reducing passive heating. First of all, we have very large windows on the south side of the property in order to passively heat the place in the colder seasons, but we don’t want those window to heat up the house in summer. Of course, in summer the sun is higher in the sky which means that it can be relatively easily prevented from hitting windows, even big ones, directly by having enough shading. Together with our architect, we worked out the overhang needed on our roof and the width of the porch that would be needed to provide summer shading but allow the sun in at other times. Considering all the trouble the builders went to getting our rafters in place and all the insulation to fit in perfectly around them, it’s particularly satisfying that this works exactly as designed!
  2. Thermal mass. Look at cave homes. Why do they stay cool? Because of the thermal mass of the earth / stone around them. You can still design houses for thermal mass, even if you don’t want to live in an actual cave: rammed earth, for example is an amazing building material for (very) hot climates, and one we considered early on (see here). But there are plenty of good modern versions based on this principle. Our house is a combination of solid wood (CLT) and wood fibre insulation, which isn’t in the same league as rammed earth or stone for thermal mass, but is good enough to make a difference in our not-quite-so-hot summer temperatures.
  3. Air flow. There are two basic ways to take advantage of physics to create natural through-flow of air in a house to provide significant cooling. Our house is designed for both.
    • To cool a particular room or floor, cross (or wind) ventilation is the easiest answer – this means taking advantage of pressure differences at one side of a building by opening a window (or two) open at each end of the space you want to cool. You can encourage this process with ceiling fans and/or ceiling fans (and the latter may be a good idea if you live in any area where you don’t feel comfortable leaving downstairs windows open). Our cross ventilation is designed to make sure the air exits through the toilets and the pantry, which has the added benefit of removing air directly from places that need it most.
    • For whole house passive cooling, you need to think about stack ventilation (and/or the related Bernoulli’s Principle). There is a really good explanation of both here, but simply put it means taking advantage of the pressure differences and wind speeds at different heights, so cool air enters the house at lower levels and hotter airs exits through the highest points. Our attic has two windows at either gable end these are open all the time in the summer, day and night.

Cross ventilation (image from Sustainability Workshop)

Stack ventilation (image from Sustainability Workshop)

The particular windows that open themselves are European tilt-turn opening windows. They can be fully closed, fully inward-opened, or tilt-opened, which means that they are still locked but have enough of a gap to allow air flow. Because our internal doors are designed with the air flow for the HRV in mind, they also have enough of a space underneath for summer air flow. The one small problem we had to deal with was the lack of insect screens – European windows generally don’t have them, because clearly they never had to deal with the number of flying creatures we get on the edge of Lake Ontario. If we had had them manufactured as part of our original window order, it would have been ridiculously expensive. So we just got a local manufacturer to fabricate some to order. They were slightly bemused by our request for screens that had to be friction-fitting – i.e. not for pre-made slots or clipped in etc. because we don’t have anything like that – but eventually they worked out that the best solution would be to simply make the screens very slightly smaller all around and put heavy-duty stick-on weather-stripping all the way around. It works fine. We couldn’t get a colour to match our dark red frames – they only do brown or white – but the brown doesn’t stand out, and at less than $900 for all the screens we aren’t complaining

Tilt-turn windows (image from http://glowindows.com)

So, yes, we open the windows.

And does it work? The short answer is ‘yes.’ As we discussed in a previous post, both our passive house advisor and the researcher who analysed the winter performance of the place argued that we would need mechanical cooling in summer. We aren’t going to say that they were wrong yet, because it hasn’t been the hottest summer ever so far. But we haven’t felt that we needed even the most basic fan so far and from casual readings of the temperatures, they are not getting anywhere near the 26ºC inside that is apparently the threshold for needing mechanical cooling. In fact, when it’s in the high 20s outside, it is still in the low 20s inside the house.

Our conclusion is that for Ontario, energy-draining AC is completely unnecessary if your house is designed sensibly. And that may well be true for much hotter climates too. There are good traditional examples of thermal mass design and natural ventilation from all over the world, but he changes we need can also be combined with technical innovation. Most architects, engineers and builders already know how to build houses that work without excessive energy use, but the current housing (and wider) market provides incentives in the opposite direction, while we know that we have to act fast to prevent the worst that will come from global warming. My view is that we need significant government intervention in housing policy. The governments of some hotter countries are taking this more seriously – look at this Australian government site, which is packed with good advice on passive cooling: http://www.yourhome.gov.au/passive-design/passive-cooling

However, it may be that we just have to stop building ridiculously in (and even abandon) certain large cities in completely inappropriate places. In North America, Houston, Las Vegas, and especially Phoenix are totally unsustainable and will only get worse. Phoenix is, I think, the fastest growing city in the USA, particularly popular for retirees, and yet it’s likely to be so hot (and lacking in water) as to be virtually uninhabitable in a hundred years or so: http://www.salon.com/2013/03/14/tk_5_partner_5/ (just one of many articles on the subject, and there are academic studies on which this is based). In those places, designing for passive cooling is going to be a matter of life and death not just comfort, in a post-oil world. It’s a daunting prospect.