Taking Control of Hybrid Heat Pump Systems
By Gord Cooke
There can be little doubt the focus in 2023 for every professional HVAC contractor across Canada should be to encourage the application of heat pumps in every project, whether it be new buildings or replacement of systems in existing residential or commercial buildings. That is not to say heat pumps can be applied in every circumstance, but that contractors should be looking for reasons, technologies and strategies so their customers are able to say yes to heat pumps as often as is practical.
The compelling reasons for this focus have been well documented in research studies, authoritative technical publications, and government directives and incentives. In the context of greenhouse gas emissions, it is hard to deny the value of a heating strategy which can deliver three to four units of energy to a building for each unit of energy expended.
In the application though, there are inherent challenges to heat pump based heating solutions, starting with the fuel cost advantage natural gas has had over electricity in most parts of Canada. As well, the design day heating capacity requirements in most existing homes and commercial buildings exceed the capacity of practical heat pumps for those applications. Even in a simple residential application, it’s easy for contractors to find reasons to discourage a switch to a heat pump because concerns do exist.
In 2023, we need to find ways to work past challenges such as the duct work is too small to accept the higher airflow requirements for heat pumps; the electrical service to the house isn’t adequate; or if the heat pump is sized to the heating load, the oversized cooling capacity will result in poor humidity control in summer. One opportunity is to consider hybrid or dual fuel applications with a refined control strategy to optimize energy performance.
The hybrid approach
Hybrid or dual fuel applications combine the cost-effective capacity of natural gas or propane furnaces with heat pumps’ dramatic coefficient performance. A hybrid approach allows the application of heat pump technology to a broader range of residential and commercial buildings. The high-efficiency gas furnace provides the back-up heating capacity to allow the heat pump to stay within an appropriate and cost-effective size range in cold climates.
Traditionally, with an all-electric heat pump a “balance-point” control strategy was used. When the heat pump can no longer satisfy the heating load, auxiliary or back-up electric resistance would be turned on. Over the last 30 years, the price of electricity has been much higher than natural gas in much of the country, resulting in higher costs in the coldest weather when the heat pump could no longer keep up. It also created a significant peak electrical load many residential electrical services could not accommodate.
With a hybrid system the heat pump would be turned off in favour of the gas furnace providing the back-up. There have been a variety of control strategies employed to affect this switch over. Most commonly, newer thermostats make the decision to turn off the heat pump in favour of the back-up heat (in this case a gas furnace) when the room temperature falls below a pre-set temperature differential from the thermostat set point. Typically, the default temperature difference to trigger the switch is 1.50°C to 20°C (30°F). When the thermostat finds the heat pump isn’t satisfying the set point in a timely matter it switches to the back-up heat. This function is often enhanced with an outdoor temperature sensor or setting that recognizes the reduced capabilities of the heat pump at lower outdoor temperatures. Generally, these settings are made by the installing contractor and are seldom changed. While this simple strategy has provided reliable heating system control, it misses opportunities now available to the industry and our customers.
New variable refrigerant heat pumps, and specifically cold climate heat pumps, are able to operate down to much lower temperatures, maintaining their output capacity, although at diminishing coefficients of performance. Time-of-use electrical rates are now available in many areas and include seasonal adjustments as well. Propane and natural gas rates are likely to increase significantly over the expected life span of any heat pump you install this year, and there are expectations of electricity rate plans to encourage electricity use at night. These changes are opportunities for optimizing the performance of hybrid heating with respect to cost of operation, greenhouse gas emissions, and peak electrical load management for utilities.
Sophisticated control strategy
New thermostats are able in real time to look at current electricity and natural gas prices as well as outdoor temperatures to make hour by hour decisions on whether to run the heat pump or the furnace.
This new generation of thermostats can also be set to optimize greenhouse gas emissions specific to the electric grid emission factors in different parts of Canada. For example, in provinces where electrical generation comes primarily from hydro or nuclear power, favouring the operation of the heat pump throughout most of the winter can result in greenhouse gas reductions of over 30 per cent. Whereas, in the short term, in provinces with fossil-fuel generated electricity, the control switch choices would be better suited to favouring the operation of the gas furnace. The key is these thermostats can be adjusted through the Cloud in response to changes to energy rates, equipment efficiencies and weather.
Ease of installation
An advantage of the hybrid system is the relative ease of installation to an existing high-efficiency gas furnace and air conditioning configuration. A new heat pump coil, sized to the air conditioning load of the house, can replace the existing air conditioner indoor coil. With the gas furnace providing the back-up heat, electrical service upgrades are avoided, as are significant changes to increase duct sizes. However, with the coil on top of, or downstream of the furnace, it is clear that once the heat pump can no longer meet the entire heating load of the house, it must be turned off in favour of the gas back-up heat. Smart controls take note of the heat pump capacity and make this switch over seamlessly.
As air conditioning season approaches, be reminded that hybrid heating solutions help overcome many of the barriers to fully electric heat pump approaches. By applying new control strategies, you can help your customer make a switch to a heat pump knowing the thermostat will be able to adapt to the projected electric and fuel rate changes to come over the life of that heat pump. Be challenged to make heat pumps your default offering rather than traditional air conditioners.
A different approach
Readers may recall another approach to a hybrid heating system was discussed in a previous article (Mechanical Business Jan/Feb 2023). Jeff Daley of Bryan’s Fuels described how they switch out the furnace for a heat pump air handler and install a hot water coil after the heat pump coil. A high-efficiency water heater, such as a wall hung boiler or tankless water heater, is used to provide the back-up heat or perhaps more appropriately described as the auxiliary heat source. In this case, a high-efficiency, variable refrigerant heat pump can continue to run to much lower temperatures, with the hot water coil boosting discharge air temperatures for comfort and operational optimization. The smart control is configured to optimize the performance of this more creative solution as our industry does its part to reduce greenhouse gas emissions in existing homes.
Gord Cooke is a professional engineer who has spent 35 years helping builders and HVAC contractors implement innovative technologies into high-performance homes. Gord has expertise in IAQ and air flow management in houses and can be contacted at [email protected].