HVAC Choices for “Zero Energy” Homes

November 13, 2017

Everyone has asked the question "So what kind of HVAC system do you put in a house that is supposed to, at the end of the year, use ZERO energy?" at one time or another. This article by Gord Cooke, which was originally published in Mechanical Business, September 2014, answers that question in detail…

Well, of course, that may sound like an oxymoron and at least isn't a very fair question. But it is worth considering the optimization of HVAC options for what has been referred up till recently as Net-Zero energy homes. The nomenclature may be changing due to an initiative by the Department of Energy in the US. In looking at better ways to communicate the value proposition of homes that, on average, only use as much as they are able to produce themselves on site, they are considering the term Zero Energy homes. Lets not get caught up in this article in the long debate as to exactly how to define "zero energy"; does it include all energy use, big screen TVs, long showers, should it be carbon neutral, should it include power or fuel transmission losses? Moreover, I am not talking about the in-the-woods, off-grid, earth-berm house that has a little wind mill. I want us to consider the thousands of more mainstream zero-energy homes that are being built across North America this year. In this regard allow me to hint simply at some of the HVAC option considerations that we have been going through generally at our company working with five mainstream builders who are this year building Zero Energy homes in Ontario and specifically decisions I have been wrangling with for my own new modest cottage that is now under construction.

Frankly, in my case the easy part was defining the enclosure elements that can be simply described as deciding on MORE. Whatever you think is enough insulation think again. I settled on an R75 cathedral roof assembly, an R45 wall, an R30 under the slab of the slab on grade construction and of course triple glazed windows and extremely tight construction. With these elements my 2100 square foot, 1.5 storey home, 2 hours northwest of Toronto ends up with a total design day heat loss of just under 20,000 BTUs/hr. So I thought I had it aced until I had the design checked out with the Passive House standard. They recommended a R100 roof, R75 wall, and R50 slab and 0.6 air changes per hour at 50 Pascal air tightness (that's more than twice as tight as an R-2000 home and at least that part my builder sees as pretty common place for him to do). So I did the math and determined it was less expensive to add a few extra solar panels than it was to increase the insulation levels to Passive House standards and I gave up on that label. The house will be R-2000 and LEED for Homes certified however. Lets go through the elements then by starting with what in my mind is the easiest element, ventilation.

Ventilation: It should be apparent to all readers that I chose an ERV, for best latent control in a house that will have very low sensible loads. The most efficient models now have ECM fan motor technology and that is crucial given that I will be doing a full independently ducted installation - fresh air to bedrooms, exhaust air from bathrooms and kitchen - so as to minimize electrical consumption while maintaining proper air quality control. I won't mention a brand name at this time, but you can imagine that it will be the most efficient, Canadian made product available (more accurately, available in the near future).

Cooling: This too, for me was an easy decision. The load will be just 1.25 tons because of the balance of solar gain for south facing windows in winter with allowance of proper overhangs to shade the south facing glass in summer. I used different glazing coatings on different orientations to optimize the heat / cool equipment and overall energy use. With such a small load, but an intermittent load presented by the potential solar gain I wanted flexibility as well as efficiency. I looked at a ducted, zoned system with a two-stage condenser, but decided on a 2 head, variable output Fujitsu system. Many thought I should just do a couple of ductless mini-splits, but I wanted to demonstrate the slim duct air handlers that I think ultimately will be more acceptable to Canadians in mainstream homes. So, I will split the house east - west and have a very efficient, flexible system.

Heating: The cooling decision led me to the larger decision for heating. The slab on grade construction and fully ducted ERV made in-floor heating a "no-brainer". So, yes, 6 loops of in-floor heat on the main floor (remember the R30 insulation underneath and a special thermal break detail in the frost wall, slab connection will enhance the efficiency). However, I also wanted to take advantage of the slim duct units by making them a heat pump version. That gave me a lot of capacity for a small load and I still had to decide on a heat source for the hot water in-floor loops. Taking into account that for the foreseeable future the home will be used only occasionally in the winter, I decided to do an all electric home. This with full respect for the fact that there is natural gas available on my street and hopeful that in the near future there will be a natural gas generator suitable for this type of application. So the main heat will come from the air source, low temperature heat pump with the two slim duct air handlers. The in-floor will be mainly a floor warming system that will be a heat sink for a solar hot water panel system. Peak load will come from a back-up electric coil in the water storage tank - call it emergency heat.

Hot water: As mentioned, I have installed a hot water solar panel with a back up coil in the storage tank. However, I am very interested to see how the Rheem heat pump water heater that we are installing will work. Extensive modeling by NRCan has shown that in very efficient homes, where hot water loads are often larger than space heating loads, heat pump water heaters, over the course of a year, should provide a very good total energy used benefit. Consider that even on a cold, sunny day my south facing windows will provide more heat than I need, so the "cooling" provided by the water heater on that day won't go to waste. I promise I will follow up in future articles and let you know how that is working out.

The final piece is the Dow Powerhouse Solar PV shingles, pictured here. New to Canada as of this summer, I truly believe this concept will be a game changer for solar acceptance. Certainly, they are more expensive, in the order of 40% higher total installed cost than the typical solar panels but they look great. My new neighbourhood is already a-buzz. I was able to cram on an approximately 5.2 kW capacity system. With the optimized roof angle and almost direct south exposure the expected production of this system should come within a light flicker or two of making the home a zero energy home - only using as much over the year as the shingles are able to produce. It won't likely qualify for a Net-Zero designation that is currently being promoted by NRCan and the Canadian Home Builders Association, because the home falls just short of the required energy modeling criteria. There will be two electric meters, one allowing electricity in when I need it, one going back to the grid for others to use. Its uncertain if I will be able to get an OPA MicroFit contract to sell the power back at a preferred rate. If not I will satisfied with simple "net-metering", that I am confident will result in a zero annual bill.

Thanks for allowing me to share this. The real purpose was to ensure you that Zero-Energy homes should be seen as an opportunity for our industry. New equipment, better controls, monitoring, expert installation and thorough commissioning will all be required and thus professional HVAC contractors, ready to learn and embrace new technology will be an integral part of the success of future new homes.

Still have questions? No problem, contact us today and we'll look into the best solution for your build.