Tuesday, December 19, 2017
An air-to-air heat recovery ventilator (HRV) can be found in most Canadian houses built in the last 10 years. The theory behind their use is that natural air ventilation rates are not sufficient for good indoor air quality, and an air exchanger without heat recovery wastes energy. Although the second part is true, the first part is not. HRVs add $1,500-$2,000 to the cost of a new home, and are often a source of additional heat loss, even when not in use.
For many years I've been saying air tightness is of utmost importance in homes. According to research done in 2007 (pdf), Canadian homes built after 1991 had an average air tightness of 3.6ACH@50Pa. Even with improved construction practices in the past ten years, most new homes being built in Canada today would be better off without a HRV. Most new houses have enough natural air ventilation to maintain good air quality during the coldest parts of winter. Many actually have too much natural ventilation, causing unhealthy low levels of humidity. The supposed need for HRVs is based on the ventilation rates in CSA standard F326. The ventilation levels in F326 seem to be based on bad assumptions and wide margins of error, rather than basic science. That's despite the fact that NRCan published reports analyzing indoor ventilation requirements as far back as 1969.
What I find a bit surprising is that building engineers are aware of this issue. Several years ago a senior ASHRAE member told me, "It is widely acknowledged that continuously ventilating houses at F326 rates can results in the houses being over ventilated". Perhaps what is not as widely known is that even "tight" houses with air infiltration rates of 2ACH@50Pa will have high enough natural ventilation rates during the coldest parts of winter. This is not just based on theory, but also indoor CO2 and humidity testing done by myself and others.
I suspect this is not a concern for most people in the HVAC industry since homeowners can just turn off their HRV in the winter. Besides the unnecessary cost of the HRV, what that ignores is the heating loss from a HRV, even when it is turned off. The ducts installed for the HRV often go in and out of attic spaces, which are sources of air leakage unless they are perfectly sealed. Standard HRV designs use only a single damper to block off either the exhaust or fresh air intake when the HRV is not running. This means the HRV adds a six-inch unobstructed hole to the building penetrations. A thermal infrared scan I recently performed clearly shows the heat loss from an exterior HRV duct.
Since removing HRVs is not a viable option, homeowners should at least turn them off during the winter. To avoid heat loss through the outside vents, I tape over the hood opening. I might even leave the vents taped off all year long, and just use a bathroom exhaust fan. Although I'll loose the benefit of heat recovery, when the outside temperature is only 10-15C different than the inside, that heat loss rather modest.
Sunday, April 2, 2017
It's been over two years since I first wrote about mini-split heat pumps. In that post I explained a bit about how to size a heat pump, and now I'll go into more detail. Most installers will use rather unscientific rules of thumb, so I suggest doing the calculations to get a more accurate estimate of your heating needs. Also, the biggest benefit in heating cost stavings comes from installing one system. When two systems are installed, the second system will rarely provide the same amount of savings as the first. If the first system saves you $800 per year in heating costs, adding a second might only save you $400 more.
One-ton mini-splits appear to be the most popular units for residential installs in Nova Scotia, with 1.5 ton the next most popular. Installed prices for a top brand name (Fujitsu, LG, Mitsubishi) one-ton unit are typically between CAD $3,000 and $4,000 plus sales tax. For the subject house the installation is rather simple, with a short vertical run for the line set from the outside unit to the inside air handler. After searching ads on kijiji and asking for referrals, I obtained a quote for CAD $3000 + tax to install a LG LA120HYV system. It has a maximum heating capacity of 13,720 BTU at -15C and 15,650 BTU at 0C, requiring 1.57kW of power for a COP of 2.92. This means the cost of one BTU from the heat pump is about one third of the cost of one BTU from an electric resistance heater. With electricity costs of 15c/kWh in Nova Scotia, I estimate the LA120HYV system will save $1,500 per year.
While installed prices for a top brand heat pump generally start at $3000 and they are usually only sold as installed packages, systems from Chinese manufacturers Gree and Midea can be found for under $1000. If was buying for my own home where I could install and maintain the system, I'd choose a much cheaper (lower-efficiency) Midea system. For example, a nominal one-ton Rheem-branded mini-split system manufactured by Midea sells for $799 at my local Home Depot.
I'll note that making technical comparisons of mini-split heat pumps is a very difficult process. Finding engineering manuals for cheap systems from Midea is almost impossible. For the high-end brands, despite marketing about their quality and efficiency, many don't publish the engineering data on their web sites to back up those claims. Between Fujitsu, LG, and Mitsubishi, LG is the only one (as far as I could tell) that makes their engineering manuals readily available on their web site. For Fujitsu, I had to call their technical support to request an engineering manual for their RLS3 series. A local Fujitsu dealer I spoke to justified the higher prices of the Fujitsu units by claiming they are more efficient than LG, but he didn't have any numbers to back up the claim. The specs on the LA120HYV1 show that it is slightly more efficient than the Fujitsu 9RLS3. The Fujitsu does have a bit higher output than the LG (15.4 vs 13.7k BTU @ 5F), so at 13.7k BTU of output the Fujitsu might be a bit more efficient. Given the installed cost of the LG is much less than the Fujitsu, I think the certainly of lower capital costs trumps the possibility of slightly lower operating costs.
If you are doing your own comparisons, take note of the difference between the actual and rated (advertised) capacities. Industry standards require that a heat pump advertised as a 12,000 BTU model must be able to output at least 12,000 BTU at an outdoor temperature of 47F (8.3C). System's like Fujitsu's RLS3 series and LG's HYV1 will output much more than their rated capacity, and even output their rated capacity down to -15C or colder. LG's lower-cost HSV4 series however only outputs 69% of their rated capacity at -15C.
Addendum - beyond the numbersAvoid installing too much capacity, such as putting two 1.5 ton systems in a small house (something I have seen in my own neighborhood). Modern split heat pump systems are variable capacity, meaning they will reduce their heat output before they shut off. A system that has an output of 16,000 BTU at 0C may reduce it's output to 8,000 BTU before shutting off. However if it only stays on for 15-20 minutes before turning off, and then turns back on 5 minutes later, it might never enter a defrost cycle. Depending on how intelligent the controls are, this may not be a problem for some units, but I have heard of this happening with Daikin, Fujitsu, and LG units.
In snowy climates the outdoor units should be sheltered from blowing snow, or raised high off the ground. When snow blocks the fan blades on the compressor unit, it can blow a fuse on the fan controller board, or fry the control board if it has no fuse.
Saturday, January 14, 2017
Twenty years ago, natural gas exploration near Sable Island promised economic development as well as cheap, clean (compared to oil and coal) energy. For about a decade, those promises came true, but things changed about five years ago. Sable Island gas production was dropping, and new wells like Deep Panuke were not producing as expected. So while gas production in the eastern US has boomed, keeping prices below CAD$4/GJ, prices in Nova Scotia have more than doubled.
In addition to the high market prices for natural gas, Heritage Gas charges a delivery fee (BEC) of $8.18/GJ. That is quadruple the ~2/GJ delivery fee charged by ATCO. A year ago when the total cost for a residential customer was over $20/GJ, heating with oil was significantly cheaper than gas. Although prices in January of 2017 are lower than they were a year ago, natural gas is not a cheaper option than oil for home heating.
One liter of heating oil produces about 0.038GJ when burned. With current prices of around 75c per liter, heating oil costs about $19.74/GJ. While that is slightly more than the variable cost of natural gas, when the fixed cost of $21.87/mth is factored in, natural gas becomes much more expensive. For a three-person residence constructed in the last 30 years that uses natural gas for heat and hot water, annual consumption should be around 75GJ. With 75GJ/yr of consumption, after adding the fixed monthly cost, the total cost for gas comes to $21.50.
Unless Heritage significantly reduces the delivery fee, I think natural gas is likely to remain noncompetitive compared to oil. LNG deliveries to Canaport will likely keep prices below $15/GJ, but the days of cheap natural gas in the Maritimes are now long gone.