In the context of heat pumps, the climate zone gives you an idea of how closely you need to consider the cold-weather credentials and dehumidification abilities. Higher numbers mean you need to pay more attention to the heating specs. In humid zones, you should pay attention to dehumidification.
Even within these climate zones, there can be big differences in the temperature extremes. Boston rarely drops below 12 degrees Fahrenheit, while you should expect Chicago to get as cold as minus-3 Fahrenheit—even though they’re both in Zone 5A. A heat pump that could work year-round in Boston would have a tough time during Chicago’s bitter winters.
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The best thing to do is look up your town’s design temperatures. The American HVAC industry goes by the 1% cooling and 99% heating temps published by ASHRAE (an engineering guild), based on historical weather data. So about 99% of the hours per year, temperatures in your town will be higher than the heating design temperature, or lower than the cooling design temperature. Those have proven to be reliable cutoffs to design a system that can keep you safe and comfortable, even when temperatures occasionally shoot beyond those extremes.
The latest ASHRAE numbers from 2021 are hidden behind a paywall, but Energy Star published a county-by-county database based on the ASHRAE handbook, and that should be accurate enough for you to form an educated opinion. The NEEP Cold Climate Heat Pump Database also has a tool that can help you find your closest design temperature. Some load calculation software (see the next section) will often incorporate the most accurate design temperatures based on your zip code, too.
Heating and cooling loads
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These numbers, measured in BTU, tell you how much heating or cooling a heat pump needs to crank out to keep up with the weather during those very cold and very hot design temperature days that we covered in the previous section. The industry-standard way to do this load calculation is the Manual J procedure.
It can be tough to perform a totally accurate load calc on your own, but you can probably get a decent ballpark estimate. As we covered in our guide to heat pump sizing, with a bit of patience and some free software like CoolCalc, you can get close enough to the truth.
When you get to the step of picking a heat pump, you’ll be aiming to find a model that can meet the heating and cooling loads at your climate’s design temperatures. More on this below.
Ducted or ductless
Before you pick a heat pump, you’ll need to know whether you’ll install ducted heat pumps or ductless heat pumps (or both!), because it narrows your options a bit.
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If you see a bunch of air vents around your house, your home has ductwork. (Most homes in the US do.) When you get a heat pump, you’ll most likely connect it to those ducts.
If you don’t have ducts, or they don’t connect to certain rooms, it’s usually easiest to go with ductless heat pumps. You’ll mount indoor air handlers onto your walls or into the ceilings in every room that needs it. (You could also add new ductwork, though that’s only practical if you have space in an unfinished attic or basement.)
Plenty of the top-performing heat pumps can be either ducted or ductless systems or even a mix of both at the same time. However some models only work with the central ductwork, and a few are ductless only.
(Some heat pumps can work with hot-water radiators, too. They’re common in Europe, though it can be tough to find an installer in the US, and they’re not very good at cooling. We haven’t spent much time looking into this category yet.)
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