Matt Sargent and Bruce Landry
The first step in determining if a heat pump is appropriate should be to have an energy audit completed by a certified home performance contractor. The audit will determine costs and savings for recommended efficiency upgrades. Typically upgrading air tightness and insulation will not only improve comfort and provide energy savings but will allow your heat pump to work much more effectively. In Vermont, contact Efficiency Vermont to find a certified contractor in your area.
Once you’ve completed this important step there are a few things to consider prior to selecting a heat pump for your home. What type of fuel do you currently heat with? Oil, electricity, and propane are considered expensive heat, so installing a heat pump to offset these fuel sources is typically a cost-effective upgrade. Natural gas, pellets and cord wood are inexpensive fuels and switching from these fuels to a heat pump may not lower your heating costs.
It may take more than one heat pump to effectively heat your home or business. Check to ensure your electric service (line to your home) has enough electric capacity to run one or more heat pumps. Upgrading the electrical panel can add expense to the installation.
Ductless mini-split heat pumps are basically a “point-source” heating system, meaning they have no distribution system such as ducts or baseboard heat. The best way to take advantage of point-source heating is with an open floor plan. Homes that are divided into smaller rooms may have difficulty maintaining appropriate temperatures throughout the home.
Once you have decided that a heat pump makes sense for you, the design work begins. For best results contact a heat pump contractor in Efficiency Vermont’s Efficiency Excellence Network (EEN). EEN contractors work closely with Efficiency Vermont and have the training and technical ability to ensure your heat pump installation is a success. You can find an EEN heat pump contractor in your area by contacting Efficiency Vermont.
Proper sizing and selecting the right heat pump are critical to the success of an installation. Getting the sizing right is done by performing a heat loss calculation. This is typically done by the contractor or HVAC distributor using energy-modeling software. It’s important to get this step right; if inaccurate assumptions are entered into the energy model software, then the recommended output will be inaccurate.
An example is a project with Central Vermont Habitat for Humanity. This project was enrolled in Efficiency Vermont’s Residential New Construction Program, and their energy consultant created a preliminary energy model from the plans showing a design heat loss of less than 10,000 Btu per hour. For this small, super-efficient home, I felt a single zone heat pump rated at 12,000 Btu per hour might be appropriate; however, sizing and selecting the proper heat pump is ultimately the responsibility of the heating contractor.
When the heating system was put out to bid, Contractor No. 1 came back with a unit rated at 24kBtu per hour. Contractor No. 2 came back with a 42kBtu unit. I believe they both relied on the same HVAC distributor to do their heat load calculation. Keep in mind this is a new high-performance house with no back-up boiler for those cold snaps, and the heating contractor would be responsible if the heating system proves to be inadequate to maintain comfort. Even so, this type of over-sizing can reduce a heat pump’s ability to modulate or turn down to a lower output, which is very important for maintaining efficiency when less heat is required.
For new construction, the Vermont Energy Code, RBES 2015, states that maximum oversizing of a heat pump is 115% of ACCA manual S heating and cooling equipment sizing guide.
Efficiency Vermont has recently learned that in some cases, multi-zone systems are not performing at their expected efficiencies. Multi-zone heat pumps tend to be less efficient than single-zone heat pumps for several reasons and oversizing can exacerbate this performance discrepancy. Installers sometimes make the mistake of selecting a heat pump based on number of zones they want to serve rather than the output capacity of the heat pump.
To run efficiently, modern heat pumps modulate their output to match the heating or cooling load in the building. If they are not able to turn down low enough to match the load, they simply turn off. When a heat pump cycles on and off rather than turning down to a low speed it is called short cycling, which can cause higher-than-expected energy use. You will feel comfortable, so you will think everything is working until you get your next electric bill. In some cases, this short cycling can get expensive.
Design options include floor mounted heads, wall mounted heads, ceiling heads, and compact ducted heads. Each has its own function that should be considered in the design layout.
Compact ducted mini splits are available and can be ducted to nearby rooms. They are not quite as efficient as single heads but can be a better option than multi-heads. Single zone, cold climate heat pumps are most efficient option.
The recommendation is to use one or more single zone heat pumps that are more energy efficient. Never size more than 115% of the heat design load requirements or select an oversized heat pump based on the number of zones required in a home. Oversizing can and often will cause more problems than under-sizing.
The best place to get the right answers is with an EEN contractor or by calling Efficiency Vermont at 888-921-5990, www.efficiencyvermont.com.
Another resource is the Northeast Energy Efficiency Partnership, NEEP guide to sizing and selecting heat pumps https://neep.org/sites/default/files/Sizing%20%26%20Selecting%20ASHPs%20In%20Cold% 20Climates.pdf
Bruce E. Landry is an EEN Energy Consultant. His focus is to remove fossil fuels from our homes and businesses. He also consults with Habitat for Humanity, helping them build their homes to Low Energy Use, High Performance Standards.
Matt Sargent is a Senior Energy Consultant at Efficiency Vermont where he works with builders and architects to help them design and construct energy-efficient buildings. He is passionate about helping Vermont achieve its goal of 100% net-zero new construction by 2030.