Heat pump vs gas pool heater in Ohio

A gas heater burns natural gas or propane in a combustion chamber, and the heat passes to pool water flowing through a heat exchanger. The Department of Energy describes gas heaters as the most popular type, ideal for heating a pool quickly and able to hold any temperature regardless of the weather. Outputs range from about 75,000 to 450,000 Btu per hour, and newer models reach 89 to 95 percent efficiency.

A heat pump works like an air conditioner in reverse. A fan pulls outdoor air across a coil, refrigerant absorbs its heat, a compressor concentrates it and the heat passes into the pool water. Because it moves heat instead of making it, a heat pump delivers several units of heat for each unit of electricity. The Department of Energy puts typical coefficients of performance, or COP, at 3.0 to 7.0, measured at 80 degree air and 80 degree water. Standard sizes run from about 75,000 to 125,000 Btu per hour.

A heat pump's output and efficiency depend on the temperature of the air it draws in. The Department of Energy says they work efficiently as long as the outside air stays above the 45 to 50 degree range, and that the cooler the air, the less efficient they become. Brookhaven National Laboratory tested pool heaters and found the same thing: heat pump COP falls as the air cools, and capacity drops measurably at the 60 to 70 degree temperatures common at the start and end of a pool season.

Now look at our weather. NOAA's 1991 to 2020 normals for Akron-Canton Airport show average lows of 50.4 degrees in May, 54.9 in September and 44.0 in October. Daytime highs in those months are 72, 76 and 63. In Chardon, in the Geauga County snowbelt, the May average low is 45.7 degrees, right at the edge of where a heat pump stays efficient. So a heat pump works well here from June through mid-September, works harder and slower in May and late September, and struggles on October nights. That is exactly the stretch of calendar most people buy a heater for.

Usually the heat pump, but by less than the brochures suggest in shoulder weather. You can check it against your own bills with two conversions from the U.S. Energy Information Administration: 1 kilowatthour of electricity equals 3,412 Btu, and 1 therm of natural gas equals 100,000 Btu.

Here is a worked example using the prices the Department of Energy used in its own pool heating tables, $1.09 per therm and $0.1301 per kWh. To put 1 million Btu into the pool:

• An 80 percent efficient gas heater burns 12.5 therms, about $13.60.
• A heat pump running at a COP of 5 uses about 59 kWh, about $7.60.
• The same heat pump at a COP of 3, closer to what you get on a cool evening, uses about 98 kWh, about $12.70.

At warm-weather efficiency the heat pump costs a little over half as much. In cool weather the gap nearly closes. Brookhaven's study found the same pattern once it added the cost of running the circulation pump longer while a slower heat pump works: at its test prices the better heat pump's cost advantage over gas fell to 21 percent at 65 degrees and 6.5 percent at 52 degrees. Plug your own utility rates into the conversions above to see where your numbers land.

Gas, by a wide margin. The Department of Energy compares a gas heater to a muscle car and a heat pump to a bike. Brookhaven noted that the largest heat pumps top out around 120,000 to 140,000 Btu per hour even in ideal conditions, while gas heaters go up to 400,000 Btu per hour and keep nearly the same capacity as the air cools. For a large pool, the study found, a heat pump may need a very long time to raise the temperature, and adding a second unit roughly doubles the purchase price and the electrical work.

That speed difference changes how you use the pool. With gas you can leave the pool cool through the week and heat it for Saturday. With a heat pump you generally set a temperature and hold it, running steadily with the cover on, because catching up after a cold spell takes days rather than hours.

The Department of Energy says heat pumps cost more to buy than gas heaters but typically have much lower annual operating costs, and that with proper maintenance they typically last longer. It says gas heaters typically last five or more years with proper installation and maintenance. Installation costs depend on what your house already has, and that is where Ohio homes vary a lot.

• Gas needs a gas line to the equipment pad, sized for the heater, and a venting location that meets code. If your home has natural gas service, that is usually a straightforward run. On a rural lot without natural gas, it means a propane tank, which changes the fuel cost math.
• A heat pump needs a dedicated electrical circuit. Brookhaven cites a 125,000 Btu unit needing a 220 volt, 60 amp circuit. An older panel without spare capacity may need an upgrade, which can cost more than the distance of the run.

A pool heater is included in every package, and the power run from your panel to the equipment pad is priced after the site visit because it depends on the house.

Choose gas if you want the longest season, plan to swim in May and October, want to heat on short notice for weekends or have a spa you want hot soon after you decide to use it on a cool night. Gas is also the practical choice for large pools and for snowbelt yards where nights stay cool.

Choose a heat pump if you mainly want warm water from June to mid-September, will run the pool at a steady temperature with a cover on every night, have a small to mid-size pool and have the electrical capacity. A smaller pool also plays to a heat pump's strength, because the modest output is enough to keep up.

Some owners with a larger budget install both, using the heat pump for steady summer upkeep and the gas heater for quick boosts and the spa. That is more equipment to buy and maintain, so it only makes sense if you will really use the shoulder seasons.

If you are unsure, start from the calendar. Write down the first and last weeks you would realistically swim, then compare them with the average lows for your area. If those weeks mostly have nights in the high 50s or warmer, a heat pump will keep up with a cover on. If your plan includes May mornings, Labor Day weekend guests in a cool year or a snowbelt yard, gas is the safer bet. Either way, size it from the pool's surface area and your target temperature rather than from the price tag.

A cover. The Department of Energy calls covering a pool when it is not in use the single most effective way to cut heating costs, with savings of 50 to 70 percent possible, because evaporation is the largest source of heat loss. A heat pump without a cover will struggle through every cool night. A gas heater without a cover will burn money. With a cover used every night, either one works far better.

Sizing is the other factor. Both types are sized from the pool's surface area and the temperature rise you want, and windy sites and cool nights call for more capacity. An undersized heater of either type will disappoint in May, whatever the fuel.

Placement helps either heater, too. A pool in full afternoon sun, sheltered from the prevailing wind, needs less help from any heater than one in the shade of the house on an open, breezy lot. Those choices are made before the dig, which is why we look at sun and wind on the site visit.

Both types need an annual check. The Department of Energy notes that scaling in a gas heater's burner or heat exchanger can reduce efficiency over time, which is one reason water balance matters. It notes that heat pumps have more moving and electrical parts and may need periodic service from an air conditioning technician. In Ohio, both must be drained and winterized properly before the first hard freeze, because water left in a heat exchanger can freeze and crack it. That is part of every closing we do.