HOMEOWNER RESOURCES
Choosing the right size heat pump
With heat pumps, it’s important to get the equipment size just right. In order to maximize comfort, efficiency, and equipment lifespan, a heat pump needs to be sized to meet a home’s heating and cooling demands on the hottest or coldest day, while not providing too much heat or cooling on mild days.
The Goldilocks Principle
The size of your heat pump isn’t about the actual size or weight of the machine; it’s about the maximum amount of heating and cooling capability it has.
HVAC sizes are discussed in both BTUs (British Thermal Units) and tons. A BTU is the amount of energy needed to increase the temperature of one pound of water by one degree Fahrenheit: typically the amount of energy in one match. Heat pumps and air conditioners are typically rated in tons, where one ton = 12,000 BTU. This is how much heat one ton of ice absorbs to completely melt this same ton of ice. This somewhat archaic “tonnage” is a carryover from the old days of keeping things cold with blocks of ice.
In order to maximize performance, efficiency, and comfort it is critical to choose the right-sized heat pump for your home. Too small a unit, and you may not be able to fully cool or heat your home. Too large a unit , and you’ll waste money installing and operating excess heat and cooling capacity. The oversized unit will also turn on and off frequently, which wastes energy.
Why isn’t bigger always better?
If a heat pump is oversized, the system will short-cycle. Short-cycling is when the heat pump turns on and off frequently and repeatedly. This typically occurs because it is oversized for the current condition. As a result, before it is fully fired up, it has fulfilled the home’s needs resulting in inefficient operation.
The charts show the heat-pumps input power over time. You can see the heat pump turn off and turn on rapidly on the left. On the right, it runs in a few steady long stretches.
How are load calculations done?
How much it takes to heat and cool your home at the typical extreme temperatures of your climate is called the heating and cooling load. The process of figuring out how much it takes to do this is called a load calculation.
To calculate heat pump size, the HVAC industry follows a standard sizing method known as Manual J, established by the Air Conditioning Contractors of America, a trade organization.
The main factors considered in Manual J are:
1. The local climate and how many days a year you need active heating and cooling
2. The home’s layout, such as square footage and shape
3. The number and location of windows
4. How much air infiltration occurs
5. How much insulation does the home has
6. How many people live in the home and what appliances generate heat
While considered industry standard, drawbacks of Manual J include the somewhat involved calculations that include several assumptions, as well as the fudge factors that tend to result in oversized equipment.
As an alternative method, using data from energy bills and correlating them with measured temperatures provides a home’s actual heating and cooling loads based on actual measured data. This method can yield more accurate information, which can be referenced when sizing a heat pump.
HOMEOWNER RESOURCES
EDEN Home Basics
Heat pumps vs. ACs: What's the difference?
If you are considering a system to cool your home, both heat pumps and air conditioners will do the job. However, a heat pump will also provide highly efficient heating during the winter as well.
What is a heat pump and why should I upgrade?
A heat pump provides all-in-one heating and cooling for perfect year-round comfort. Not only that, it operates more than 3x efficiently compared with a standard furnace.
Heat pumps vs. furnaces: What’s the difference?
Heat pumps and furnaces heat your home – but in different ways. The most significant difference between furnaces and heat pumps is that furnaces create their own heat.