Mechanical systems are studied on two bases: their performance for heating only, and their performance for heating and cooling. The reason for this choice is that, for the climate of the region selected for this study, the cooling loads are very small and make air conditioning unnecessary; however, many customers will require air conditioning in their homes to attain their high comfort expectations.
83 Mechanical systems to be studied in this thesis are chosen based on previous research as well as on practical considerations, such as comfort, market penetration and adaptability to different owners’ needs. For space conditioning, electrical baseboards (for heating only) and central conditioning unit with electric resistance coil (for heating and cooling), an air-source heat pump, an air-source heat pump for cold climates and a ground-source heat pump are considered.
A heat recovery ventilator (HRV) is considered on the outside air stream to reduce the energy use for heating of ventilation air.
Domestic hot water is obtained through an electrical hot water tank, with the possibility of adding a flat-plate solar collector or a desuperheater if using a ground-source heat pump. Desuperheaters are not considered for air-source heat pump because no data was available from manufacturers for those systems.
Figure 5.1 presents all the modeled systems which were considered to meet space heating and cooling as well as domestic hot water needs. Systems 1-2 are modeled for the case where only heating is evaluated, while systems 3-4 are the equivalent for the case where both heating and cooling are evaluated. Systems 5 to 11 use the same computer model for both cases.
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Figure 5.1 – Studied combinations of space conditioning and DHW equipment
The following paragraphs explain the reasons for the selection of the mechanical systems. They include the results of previous studies from the scientific literature, as well as performance data taken from technical literature and practical consideration.
Electrical baseboard heating is used as a base case comparison as it is the most common heating system in Québec.
85 Air-to-air heat pumps are a common technology in Canada, as they can provide both heating and cooling, at a lower investment cost than a geothermal system. However, standard air- source heat pumps are not designed to operate at typical winter outside temperatures in Québec; below temperatures of about -10°C, they are usually turned off and a backup heat source is used, consequently reducing the seasonal efficiency of this equipment. For this reason, a new generation air-source heat pump, which can provide better COP at low temperatures, is also investigated here.
It was shown that ground source heat pumps can contribute to GHG emission reduction (Saner et al., 2010 and Zmeureanu and Wu, 2007), as well as being one of the system that uses the least primary energy (Gustavsson and Joelsson, 2010). It is also a technology that is well understood and that is available from most HVAC equipment distributors.
While an electric DWH tank is the most common way to provide hot water in Québec, Biaou and Bernier (2008) demonstrated that solar thermal collectors with an electric backup was the best way to achieve net zero energy consumption for the domestic hot water system in Montréal. However, a desuperheater is also considered in this study even if it was proven not to be optimal for a net zero energy house by Biaou and Bernier. Because the objective of this study is rather to achieve a low LCC and LCE for the whole house, optimal solutions might differ from those for a net zero energy house. Moreover, the extra cost for a desuperheater is significantly lower than a solar thermal collector, when already using a heat pump. For solar collectors, a pre- heat tank is added according to recommendations from specialists (Natural Resources Canada, 2003).
86 Some systems were excluded for practical reasons. For example, a wood pellet heating system could be efficient, but since some houses in the project will be used as secondary homes, it would not be convenient to let the system run without surveillance during the week.
Other design considerations led to the choice of specific features for the space conditioning and domestic hot water systems. For all heating systems except electric baseboards, distribution of heat is achieved through a forced-air system. This choice is made because the project developer wished to use a single system to deliver both heating and cooling, which allows for a house owner to add air conditioning, if it was not included in the initial house, without any major renovations. In order to have comparable cooling systems, electrical heating is coupled to a central air conditioning system, even if a ductless AC system could be less expensive and seems to make more sense. The reason is that heat pumps used for heating will be able to deliver zoned air conditioning, and air dehumidification, something that a single ductless AC unit cannot achieve. Therefore, the author decided that it would not be reasonable to compare costs and energy consumption of systems that do not provide the same service.
In the case of ground-source heat pump, water-to-air or water-to-water heat pumps were both an option. Because the house is to be air conditioned, a forced-air distribution system seems to be a better option than low temperature hydronic radiators or radiant floors, to avoid extra investment costs. Indeed, even if a water-to-water heat pump is usually around $1000 less expensive than a water-to-air, the addition of low temperature hydronic radiators would cost about $4000 (based on prices for Smith’s Environmental Heating Edge radiators, as estimated by the distributor), and radiant floors also require an investment of many thousands of dollars. Furthermore, water-to-air heat pumps available on the Canadian market present better coefficients of performance than water-to-water ones. Typically, a dual stage water-to-air
87 ground source heat pump with an entering source temperature of 4°C and a entering air temperature of 21°C yields a COP of 4 to 4.5, while a dual stage water-to-water ground source heat pump with the same entering source temperature of 4°C an entering water temperature of 38°C (assumed return temperature from the bottom of a stratified storage tank) offers a COP between 3.5 and 4. Dual-stage heat pumps are considered for this application because they offer better comfort and lower cycling rate, which in turn increases the life expectancy of the equipment. Moreover, the cooling loads are much smaller than the heating loads in this study house; using a heat pump sized for heating to cool the house during summer would result in high cycling and temperature variance.