A Breath of Fresh Air

Have you ever lived in or visited a home and noticed curtains moving on a windy day?  Perhaps you have unplugged something from a receptacle on an outside wall and found the plug to be cold.  These are signs of a leaky house.  In older (and some newer) homes, this is how fresh air is introduced to the home.  So what’s the problem with a bit of fresh air?!  We need to think about this from two distinct but related angles: Indoor Air Quality (IAQ) and Energy Efficiency.

We spend quite a bit of time in our homes – particularly during the heating season.  As occupancy increases, more oxygen is removed, more carbon-dioxide is produced, and exposure to potentially toxic air is at yearly highs because we don’t have windows open.  Our cabinets, furniture, flooring and many other household goods, are constantly off-gassing; further degrading the quality of the air we breathe.  You get the picture…  not pretty.  In a leaky home, fresh air is introduced around doors, windows, lights, receptacles, rim-joists and many other avenues.  Bathroom fans and clothes dryers remove moisture-laden air from our homes, creating a negative-pressure which also draws fresh air in. Air is opportunistic that way and from a health perspective, that’s a good thing.  Even on a still day, the stack-effect will help keep air moving, but it’s not consistent and unless you have expensive monitoring equipment, you have no way of knowing what your IAQ is.

Fresh air is either forced in (wind) or drawn in and although good for our health, it presents a significant problem from an energy efficiency perspective.  That fresh air is either displacing or replacing conditioned air; that is, air you have already paid to heat or cool.  What we need is a means of regulating/controlling fresh air while capturing the energy from outgoing stale air.  Heat-Recovery-Ventilators (HRVs) serve that purpose and have been around for many years.  Most HRVs are installed so they draw air from smelly/humid locations (kitchen, bath, laundry) and deposit the fresh air into the cold air return plenum on the furnace.  In a Passive House we don’t have a furnace or an air handler (typically) so the fresh air is delivered to the bedrooms, family room, office etc. The HRV in a Passive House also runs 24 hours a day, not just when you hit the button before your shower.  This is an important distinction: Consistent, balanced introduction of fresh air is the secret to a much healthier home.

The HRV runs ’round the clock, it harvests heat energy from the out-going stale air, and the air quality is great;  so all is good, yes?  Not all HRV’s are created equal. A typical builder grade HRV is (at best) 50% efficient; only half of the energy in that stale air is recovered – the other half is lost.  If our goal is energy-efficiency, this is unacceptable. Typical North-American made HRVs have a cross-flow heat exchanger, and a small one at that. The fresh and stale air-streams pass through channels that are perpendicular to each other, so there just isn’t enough surface area to recover significant amounts of heat:

Cross-Flow Heat Exchanger in a basic HRV.

Cross-Flow Heat Exchanger in a basic HRV.

Increasing the heat exchanger size will help a bit, but to achieve > 80% efficiency requires a different approach:  Passive House HRVs have a counter-flow heat exchanger and they’re large.  The parallel air-streams and increased surface are make all the difference in the world.  Throw in EC (Electronically Commutated) blower motors and they won’t cost a fortune in electricity to operate:

More Efficient Counter-Flow Heat Exchanger.

More Efficient Counter-Flow Heat Exchanger.

As I write this post, I am aware of only one Canadian manufacturer of HRVs that incorporates a sizable counter-flow heat exchanger.  We were glad to find it, but it would be nice to have options, and going with a European model is not only costly, but not Canadian eh!  I’ll spare you the details of modelling an HRV in the PHPP software. Just know that it entails gathering all the performance specs, and that’s not a simple task considering North-American testing methods (www.hvi.org) are different from those in Europe.

There is one more rather unique feature of many Passive Houses that dramatically increases the efficiency of an HRV.  That’s all I’m saying for now, you’ll have to wait until my next post to find out what it is…

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Welcome…

As the title would suggest, this blog is a means of documenting the design, construction and challenges associated with Kingston’s (in Ontario, Canada) first Passive House.

What is “Passive House” ?

It is the world’s most comprehensive and ambitious energy-modelling standard.  Although relatively new to Canada, the Passive House Standard (PassivHaus in Germany) dates back over 20 years, has become very popular in Europe and is gaining traction all over the world.  Passive House is non-prescriptive, meaning it does not require you to use specific materials or building systems.  Stick-frame, ICF, SIPS, and even straw-bale can be used.  At the heart of this standard is the ‘Passive House Planning Package’ (PHPP) which is a very thorough energy modelling software tool. PHPP gathers every detail of the house (orientation, areas, volume, windows sizes / placement, all wall/ceiling/floor assemblies) and computes energy consumption, taking local climate data into account.   Although there are many requirements for meeting the standard, the most widely referred to are:

  • Air-tightness – 0.6 ACH50 (air changes per hour at 50 Pascals pressure – positive and negative) is the maximum allowable.  Passive houses frequently go well below this number, but for comparison’s sake, an R2000 home allows 1.5 ACH50 and modern code-built homes are normally in the 4-8 range.
  • Ventilation – A home this tight requires active, controlled ventilation to remove indoor air polutants and bring fresh air in, while recovering over 80% of the heat (or cooled) energy from the outgoing stale air. The HRV or ERV runs continuously and Passive Houses are very healthy to live in for this reason.
  • Annual Space Heating and Cooling demand – A passive house is designed to use no more than 15 kWh/(m2a)… that’s 15 kilowatt-hours per square meter of floor area, per year.  Essentially this gets heating down to a level where the house can be heated without a conventional furnace or boiler.  The house still requires a heat source but it’s almost negligible in that it can be delivered to the rooms by post-heating the fresh air from the HRV/ERV.
  • Windows – They must have total (frame, spacer/glazing and installation)  U-values of 0.8 W/m2K which translates to R-values of 12.5 or better.  Common double-pane windows are roughly R4 so this marks a dramatic improvement in window performance – justified by the fact that windows are the single largest source of heat-loss in a typical home. Passive House windows are triple-pane with one or more low-E coatings and must also have a high Solar Heat Gain coefficient (SHG of 50% or more solar transmittance) to take full advantage of solar gains in the heating season.

More details on the standard will pop up as the build progresses.  For now it’s all a numbers game… taking an optimized (for Passive House) floor plan and adjusting glazing, wall, roof and floor assemblies in PHPP in order to meet the standard.  These numbers will of course be verified prior to certification.

Stay tuned!