All of our drains in the house is tied into a single drain stack in the basement of the house, and a drain stack heat recovery coil was installed in the drain stack. A 6 foot section of 4″ copper pipe sits in the center to handle the waste water, and as the copper picks up the residual heat in the water, the heat transfers to the copper tubing that the incoming water flowing to the hot water tank, thereby preheating the cold municipal water as it enters the tank.
When hot water is being used in the house and drawn out of the hot water tank, replacement cold water effectively flows around the drain stack and picks up the heat from the returning waste hot water.
The process began with the architectural drawings, a specification of materials for insulation, and a designer that reviewed the materials to calculate a heat loss of house at design temperature (-20°C or -4°F in Markham, Ontario). Part of the problem was what R-Value and what air exchanges per hour (how leaky the house was) to use for the calculations, since much of this is unchartered territory, and we didn’t want to buy into the marketing hype from all the product manufacturers in terms of each product’s efficiency. In the end, we took a middle-of-the-road approach for each of the materials for R-Value, and assigned a low air exchange per hour knowing that the house was to be very well sealed. For the purposes of the building permit, we came up with an 80kBTU/h heat loss at design temperature, but in talking with our LEED certifier, he feels a house with the selected material will be far lower, perhaps as low as 50kBTU/h.
Our selection of a heating system had to be based on the 80kBTU/h value as we had to satisfy the building code requirements, but also allowed for flexibility to take advantage of a lower heat loss if it were the case. We considered 3 different types of heating system:
traditional natural gas fired furnace, for the lowest initial cost but the most energy inefficient,
geothermal, for the highest initial cost but also the most energy efficient, and
air-source heat pump, for a middle of the road approach in terms of cost and efficiency.
With traditional natural gas fired forced air furnace, we would achieve between 92-98% efficiency (1 BTU in, 0.92-0.98 BTU transferred into the house), but we would be dependent on fossil fuel. The price of natural gas, however, was (and still is) very attractive as it is at the lowest point in more than 5 years, and about 2/3 less than what it was at the peak. In terms of real costs, based on current Enbridge (local natural gas utility) prices as of October 1, 2010, inclusive of delivery and other charges, natural gas is supplied at $0.2666/m³, or $0.024/kWh as of January 2011.
With geothermal, we would be entirely on electricity, using a resistant heat backup. This would get us off fossil fuel (assuming the electricity isn’t coal or natural gas generated), and we would achieve a COP of 3.6 (1 BTU energy in, 3.6 BTU energy into the house), but the primary drawback was the cost of the geothermal system. Based on our location in suburban Toronto, we did not have enough of a lot size to bury the loops horizontally (trenching), but instead it will need to be drilled and installed vertically. The costs of the equipment and installation costs would have been in excess of $35,000 for a 6-ton (72kBTU) system, with the drilling alone representing over $10,000 of the $35,000 cost, and the rebates that were offered at the time only applied to retrofit homes, which we did not qualify. When we compared it to a typical natural gas system and air conditioner which would have cost well below $10,000, the $25,000 difference was hard to justify. Plus, at our 80kBTU/h, we would have required either a secondary system, or an additional electric resistant heat backup to satisfy the building department’s requirements based on our heat loss calculations, as the largest capacity for a mainstream geothermal system would have been 72kBTU/h. In terms of operating costs, the electricity costs vary between $0.08433/kWh to $0.13233/kWh depending on time of day (inclusive of all regulatory charges), so factoring in the COP of 3.6, we would have been barely break even during off-peak use and it would have been 50% more to operate during peak hours, at least given the current lows of natural gas pricing. To spend $25,000 extra with no cost recovery until (or if) natural gas prices rise, this option seems unpalatable.
With an air source heat pump, it works very similar to a ground source heat pump, except that it extracts energy from the air rather than from the ground. For the pump, heat exchanger, and air handler equipment, the costs were similar when compared to geothermal, but because there were no drilling costs involved, the cost of an air source heat pump would be far lower than a ground source. The primary drawback was that the efficiency of the air source heat pump tends to be lower than a ground source, and the output and efficiency of the air source heat pump declines as the temperature drops. In addition, for an air source heat pump, at least for the climate of Toronto, it was very unlikely to have enough output at design temperature unless it was for a small and highly energy efficient house, and as such the cost of the backup system needs to be factored in.
In the end, we chose a Daikin Altherma air source heat pump with a Navien tankless natural gas water heater as backup heat, fed into an in-floor radiant basement heating system and to a Lifebreath Clean Air Furnace with a built-in HRV which acts as the air handler to deliver warm forced air to the main and second floors. A Daikin domestic hot water tank was also installed so that the heat pump can be used for hot water for the house. This hybrid system approach allows us to adapt to a changing energy price market, by giving us the choice to select how low of an ambient temperature to run the heat pump to before switching over to natural gas. We also plan on adding the solar water kit that would allow us to use the sun to heat the domestic hot water to further increase efficiency.
In our case where the goal is net-zero energy, we would use the Altherma to as low an ambient temperature as possible. But, if our choice was for dollar efficiency, we could choose to run the Altherma when the ambient temperature was around 0°C, where the Altherma was more cost effective with the higher COP, and when the temperature falls below 0°C we would switch to the Navien tankless, with the logic being programmed into the Altherma Hydrobox, with no user invention required.
We chose their ERLQ split system that was rated at 54kBTU, but for our purposes of heating in a Toronto winter, it would only be capable of generating about 25kBTU/h at design temperature of -20°C, and about 37kBTU/h at freezing, which would allow us to be on the heat pump until the temperature dropped well below freezing. Even at -20°C, the Altherma would deliver a COP of 2.5, and at 0°C it would deliver a COP of about 3.0, so while it doesn’t deliver quite the efficiency of a ground source heat pump, the lower initial costs far outweights the reduced efficiency, and it contributes significantly to our goal of becoming a net-zero energy house. In the summer, the Altherma would deliver chilled water to the system for cooling.
When the heat loss of the house becomes greater than the Altherma can handle, the Navien tankless water heater would kick in, supplying up to 160kBTU/h, more than sufficient for any Toronto weather, and also satisfies the building department in terms of meeting the design heat loss. The Navien tankless unit offers efficiency of up to 98%, which is one of the highest (if not the highest) efficiency available on the market today.
The Lifebreath Clean Air Furnace is a water coiled based air handler that would take the warm water from the Altherma or the Navien tankless, and blows air over the coil to extract the heat into the house. A built-in HRV is part of the Clean Air Furnace, which we needed because of the air tightness of the house.
Total system cost is estimated to be about $10,000 to $15,000 above a traditional forced air natural gas and an air conditioner, perhaps less as we had an in-floor radiant heating installed for the basement. Cost recovery as compared to current natural gas rates would be fairly long, but in temperatures of above freezing, the higher COP would be more cost efficient than natural gas, and in the summer the system should be more efficient than a standard air conditioner. In operating costs, it would be cheaper to operate the Altherma over natural gas when the ambient temperature is above freezing, and would cost slightly more when the temperature drops below freezing, based on the current utility rates. But the balance point would change should natural gas prices begins to rise from these very low levels.
CaGBC LEED for Homes – Points can be acheived in Energy and Atmosphere, in HVAC (EA 6), or exceptional energy performance (EA 1.2) via the ERS/HERS method.
Update (March 2012) – Through the 2011-2012 winter, we have observed that we were able to continue to run the ASHP and maintain house temperature to as low as an outdoor temperature of -10°C, consistent with the 50kBTU heat loss projection as opposed to the 80kBTU design calculation.
Update (July 2012) – The continuing decline in natural gas pricing has skewed the economic benefits towards natural gas over ASHP or GSHP, so please bear this in mind in considering the cost efficiencies of ASHP or GSHP.