The efficiency of heat pumps varies, depending on several factors, including the type of heat pump, its size, and, most importantly, the outdoor temperature. Heat pumps are generally efficient heating systems, but their performance is influenced by colder temperatures. Savings over fuel-fired systems depend on multiple factors.
Air Source Heat Pump Efficiency (ASHP)
Air source heat pumps (ASHPs) are the most common type of heat pump in Massachusetts. They are efficient at extracting heat from the outdoor air and transferring it into your home. In extremely chilly weather, some ASHPs may need to use supplemental heating sources. With that said, according to the Mitsubishi Electric Heating & Air Conditioning website, their ‘zero emissions, all electric H2i heat pumps can provide reliable heating even when temperatures outside are below zero.” Similarly, their newest line can supply heat down to –13 F and below.”
Air source heat pumps have a variable efficiency that depends on the outside temperature. In heating mode, the heat pump pushes heat from the cold outdoors into your warmer home.
Think of this as pushing heat up a hill. The colder it is outside, the higher the “hill” you must push up, thus requiring more work. Because there is more work associated with colder outside temperatures, ASHP’s become less efficient as the outdoor temperature drops.
We measure heat pump efficiency by taking the amount of energy or heat produced, divided by the energy used.
This is the coefficient of performance or COP.
An electric resistance heater has a COP of 1 – you put 1,000 Watts of power into an electric resistance heater, and you get 1,000 watts of heat. Mitsubishi air source heat pumps also have a variable COP depending on outdoor temperature that looks like this:
|Outdoor Temperature||Coefficient of Performance (COP)|
What this table means, is that an air source heat pump working at 0° F outdoor temperature, is 2x the efficiency of electric resistance heat, and at 42° F that efficiency is 3.7 times that of electric resistance heat.
To find the most cost-effective method to heat your house, one would need to know the precise cost of electricity, the outdoor temperature, the cost of the alternative fuel source (oil or gas price) and the efficiency of the fuel-fired system.
As an example, an oil boiler that is 85% efficient with oil at $4.50/gallon will cost roughly $42 to produce 1,000,000 BTUs of heat. An air source heat pump operating at 0° F with a COP of 2 using National Grid electric rates of $0.27 /kW-hr, will cost $40 to produce the same 1,000,000 BTUs of heat.
In this case, the air source heat pump is cheaper to run than the oil boiler. As the price of oil, electricity and gas vary, these costs also fluctuate.
Proper Heat Pump Sizing and Home Insulation
Ensuring your heat pump is correctly sized for your home’s heating needs, and your home is well-insulated, is crucial for maximizing energy efficiency in the winter. A thoroughly insulated home retains heat better, reducing the workload of any heating system, especially a heat pump.
Do not forget, a well-insulated home includes airtight doors and windows too.
To ensure your system is sized correctly, your contractor should be performing a Manual-J, or heat load calculation on your home. Manual-J is a standardized method to find the precise heating and cooling needs for your home based on standards developed by ACCA, the trade group for heating and cooling specialists.
Regular maintenance of your heat pump is essential to keep it running efficiently.
Filters should be cleaned or replaced, and the system should be inspected and cleaned to ensure it’s running optimally. Most manufacturers also require regular maintenance to ensure coverage for warranties, too.
To get a more accurate assessment of the efficiency of a heat pump in your specific Massachusetts location, it’s advisable to consult with a local HVAC professional (like Jay Moody HVAC!).
They can consider the local climate, your home’s insulation, and other factors to help you choose the most efficient heating system and provide guidance on how to maximize its performance during the winter months.
Learn more about choosing the best HVAC installer for your project, here.