Fifty-seven minutes, that is the average time my teenager spends in the shower. Yes, the engineer in me compels me to analyse all the energy use in my household. Meanwhile, my youngest takes less than 5 minutes in the shower – but I’ll leave that story for another time. Hot water usage varies from one household to another depending on the number of occupants, the behaviour of the occupants, and an entirely separate, but very important contributing factor: how many teenagers you have.
When designing a custom home, the builder and designer can factor in the behaviours of family members, making a net zero performance-based design more feasible. However, when we are talking about large scale net zero housing in a production setting, builders and designers need to make assumptions about occupant behaviour and design net zero based on typical behaviours. So what should we assume for the amount of hot water used for showering?
Natural Resources Canada’s CanmetENERGY research on hot water energy consumption shows that a typical electrically heated home consumes about 4,300 kWh of electricity for heating domestic hot water with an annual cost of about $470 per household. CanmetENERGY has analysed hundreds of housing data points to determine an average hot water heating load of 143L/day per household based on the average behaviours of 2 adults and 1 child. The latest NRCan voluntary energy efficiency housing programs (R-2000 Net Zero Pilot) currently use this assumption.
As the housing industry moves towards very energy efficient housing design (e.g. net zero), reducing the water heating load becomes very important. Figure 1 summarizes the estimated energy consumption breakdown of a typical 2012 OBC-built single detached home. While over 50% of the estimated energy consumption is attributable to space heating, it is important to note that aside from the NRCan-set standard baseload assumptions for lights and appliances, the next big load is the domestic hot water heating (21% of total energy consumption). Figure 2 summarizes the average estimated energy consumption breakdown across all the single detached net zero home designs from NRCan’s ecoENERGY Innovation Initiative Net Zero Housing Community project (www.zeroenergy.ca). While the builders have reduced space heating loads down to 20% of total energy consumption, the domestic hot water heating load still hovers around 17%. The baseload energy use for lights and appliances makes up over 50% of the energy usage in a net zero house. There’s a huge opportunity to reduce water heating energy consumption. As for baseloads – we’ll leave that for another article.
While a production builder can’t predict the number of teenagers in a family or the duration of their hot showers, efficient water heating technologies are available to reduce consumption. The affordable and readily available hybrid Heat Pump Water Heater (HPWH) is a great example of such a technology.
What is a Heat Pump Water Heater (HPWH) and how does it work?
A HPWH uses the heat from the surrounding air to heat water more efficiently. Instead of heating stored water directly with a conventional electric element, this water heater absorbs available heat from the ambient air, which is then transferred to and intensified by a heat exchanger into the water, a far more cost-effective process. To help better understand this concept, try to imagine a refrigerator that is working in reverse-mode. Instead of removing heat from the inside of a box, the HPWH actually removes heat from the surrounding area and transfers that heat to the water inside an enclosed tank.
How is this different than a regular electric resistance water heater?
A HPWH does not convert electricity into heat like a typical electric water heater. Instead, the HPWH moves heat from the air into the water. A HPWH is more efficient than electric-resistance water heaters. Energy Factor (EF)measures the amount of hot water produced as a result of consuming one unit of energy. Most 100% efficient electric-resistance water heaters only have an EF of 1.0 (or 0.93 when considering standby losses), meaning it uses one unit of energy to produce one unit of energy worth of hot water. The Rheem Hybrid Heat Pump water heater (Figure 3), for example, has an EF of 2.45 which means it consumes one unit of energy and produces approximately two and a half units of energy worth of hot water.
What does a hybrid Heat Pump Water Heater (HPWH) mean?
A hybrid HPWH allows the homeowner to toggle the operation of the heat pump to draw more heat or less heat out of the air. For instance, if you’re finding that there is a lot of solar gain (overheating in rooms with big windows), the unit can be used in Heat Pump Only mode to draw some of that free heat into the water tank (by circulating the furnace fan). Similarly in the summer, the Heat Pump Only mode could provide a measure of air conditioning by removing heat from the indoor air.
Will the location of the HPWH affect whole home comfort?
As most units will be located in a mechanical rooms or unoccupied space, any minor temperature swing in these spaces will generally not compromise whole home comfort. If the temperature swing is unacceptable, the heat pump mode can be toggled.
How does this technology complement Net Zero Energy Home design?
Net zero energy home designs will require a renewable energy generation component like solar photovoltaic panels. Solar PVs produce electricity for the home and the best way to optimize on this renewable is to use electric mechanical equipment (for space heating and water heating). As discussed, a HPWH is almost twice as energy efficient as a standard electric resistance water heater with added cooling benefits in the summer. It is also important to note that most net zero energy home designs also use an electric air source heat pump (ASHP) as the primary space heating equipment. When an ASHP is coupled with a HPWH, it fully optimizes the energy efficiency performance of both pieces of equipment delivering very significant benefits.
Figure 3: Rheem Prestige™ Series Hybrid Heat Pump Water Heater
Case Study: ecoEII OC Net Zero Housing Project – HPWH partner: Rheem
“Hybrid water heaters pair well with heat pumps for space heating – together they deliver enormous energy savings” reports Derek Hickson, the Manager of Sustainable Developments at The Minto Group.
Reid’s Heritage Homes and the Minto Group are two of the selected builders participating in the ecoENERGY Innovation Initiative Owens Corning Net Zero Housing project that will see at least 25 net zero houses built across Canada. Reid’s and Minto are building at least 5 net zero houses in the regions of Guelph and Ottawa respectively.
Both builder design teams analysed readily available technologies and found that the most affordable and most seamless integrated technology was the HPWH.
In April 2013, Rheem debuted an entirely redesigned hybrid water heater with an Energy Factor (EF) of 2.45. “Rheem launched America’s first hybrid electric heat pump water heater in 2009, and now we’ve changed the face of the hybrid market with our new Prestige Series unit,” said Bryan Collar, the Product Manager at the Rheem Water Heating division. “This is one of the most efficient water heaters available today, and it’s something that builders can leverage when trying to educate homebuyers on the energy savings that come with buying a new home.”
Rheem was selected as the exclusive national partner for the HPWH technology for the ecoEII OC Net Zero project. All participating builders, including Reid’s and Minto in Ontario, Mattamy Homes Calgary in Alberta, Construction Voyer in Quebec, and Provident Developments in Halifax will be providing the HPWH as a standard in their net zero homes.
“The Rheem Hybrid Water Heater provides a great option that adds to the overall efficiency of the home. The combination of the heat pump component in the heater provides reduced energy consumption in turn reducing the solar energy required to power the home, making this a cost efficient option with longevity, comfort and peace of mind to the buyers experience,” suggests Jennifer Weatherston, Director of Innovation at Reid’s Heritage Homes.
The ecoEII OC Net Zero project is in its last year and construction of all 5 net zero communities will be complete by March 2016. The first grand opening event will be hosted by Reid’s Heritage Homes in Guelph this Spring.
For more information about the Rheem Prestige™ Series Hybrid Heat Pump, please visit: www.rheem.com