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How is an Air Source Heat Pump Sized?
A heat pump should be sized to meet your home's heating and cooling needs, which depend on factors such as square footage, insulation levels, window types, and local climate. An HVAC professional typically performs a Manual J load calculation to determine the appropriate size, ensuring the unit operates efficiently and maintains comfort without excessive cycling or energy waste.
Does the installing contractor usually do the Manual J Heat Loss calculation?
Many times, yes; many times, no.
If they don't, then what? Who sizes it?
Many contractors only replace equipment with what is already in the home. In new installations, many use a rule-of-thumb calculation that often oversizes the equipment by 50-100% or more. Proper sizing then comes down to you. Find another contractor to handle the heat loss, or contact us.
I hear HPs don't work well in the north. Is this true?
While air-source heat pumps can face challenges in colder northern climates, modern advancements have significantly improved their performance. Cold-climate heat pumps are now specifically designed to operate efficiently even when outdoor temperatures drop below freezing. Proper sizing and installation are crucial to ensure optimal comfort and energy savings, making them a viable option for many northern homes.
How do I know if I have the correct heat pump for northern climates?
Choosing between a cold-climate heat pump and a standard model depends mainly on your region's typical winter temperatures and your home's heating needs. In areas where winter temperatures often drop below freezing, a cold-climate heat pump is best, as these systems are designed for efficiency and comfort even in severe weather. Check your location's average low winter temperature, but avoid using the all-time record low when making your decision. Modern cold-climate models, typically featuring variable-speed inverter technology, provide reliable heating in winter and effective cooling in summer. Make sure any cold-weather heat pump you consider is rated for temperatures close to your area's average winter lows.
Caution: Do not use the All-Time low temperature. The cold-weather heat pump should be rated near the low-temperature rating.
What if I don't live in the north but live further south or in the south?
Complete the Manual-J heat-loss calculation to determine your needs. After completing the Manual-J heat-loss analysis, choose a heat pump that first meets your cooling needs and then your heating needs. You can add a small strip of heat to the air handler section of the heat pump if needed. If you live far enough south that you rarely need heat, a standard heat pump should be OK.
How do I determine how low the heat pump works and how much heat it will supply?
In my area, the average low temperature from November to March is 22F, and the OD design temperature is 14F. The cold-climate heat pump I was considering is rated for temperatures down to 4F. Heating BTUs are derated as temperatures drop and as OD temperature increases. Some kind of backup heat will be required. I will discuss this in more detail HERE.
How much backup heat would be required?
I can't answer this because there are too many variables. For instance, if your heat loss is 60,000 BTUs and the heat pump at 22F OD produces 40,000 BTUs, you would need to make up the 20,000 BTU difference.
What type of unit would I need for the backup heat?
Backup heat can be electric elements in the ductwork or air handler, a gas or oil furnace, a gas or oil boiler, or an electric boiler. These applications, except for the electric elements in the ductwork, will normally turn the heat pump off at a given outdoor temperature.
I have a central air conditioning system now. Would you suggest replacing it with a heat pump?
The biggest question I receive all year round is why does my heating or cooling device run so much, or maybe all the time.
Many people find this situation alarming because they're worried about high fuel or electricity bills. However, when a unit is correctly sized, it will often operate for longer periods or even continuously. We're used to oversized systems that frequently turn on and off with thermostat changes. In fact, the longer a properly sized unit runs to keep the home's temperature steady, the more comfortable the home is. For instance, if your thermostat is set at 70F and the system runs for several hours to maintain that temperature, that's desirable in very cold weather. But if it's set to 70F and it's only 50F outside, there could be an issue. Modern modulating equipment uses less fuel or electricity by adjusting its operation in response to demand, helping reduce energy consumption. So, even if the system runs longer, it may still cost less to operate because it uses resources more efficiently.
My heat pump was sized correctly and still does not heat well. What could be the problem?
If it were properly sized, then I would have to blame a few other possible problems.
- Dirty air filters
- Duct work undersized
- Uninsulated ductwork
- Duct dampers not balanced properly or closed
- Inoperative duct zone valves
- Excessive duct leaks
- too deep of a night/away setting, excessive recovery time
- Fan speed to low
- Excessive fan speed
- Air is being sucked into the air handler after the heat pump coil and air handler air exit
- Incorrect refrigerant charge
- Defrost cycle problems
- backup heat undersized
- During colder temperatures, backup heat is not working