In my last article I discussed the pros and cons of an electric heat pump pool heater vs. a solar pool heater. While it has little to do with solar pool heating, I thought I would point out the differences between the two traditional heat sources used for residential swimming pool heating, which are heat pumps and gas pool heaters.
Electric Heat Pump Pool Heaters
Electric heat pumps use the latent heat in the air to heat your pool. The efficiency of a heat pump is related to the air temperature vs. the pool water temperature. When the outside temperature is relatively high, the efficiency can by very good. For one unit of electricity (kWH), you can get about 6 times the heating energy (BTU) into your pool water. However, as the ambient temperature drops, the efficiency of an electric heat pump drops. In fact, at a certain temperature, heat pumps will stop working, or will be astronomically inefficient. That means you could be using electricity and barely putting any heat into your pool. I like to joke that you might as well heat your pool with a hairdryer (don’t do that).
Some heat pump manufacturers and dealers tout the “dollar-a-day” cost to operate a heat pump. However, when you read the fine print, the temperatures reached are often not warm enough for many bathers, there are disclaimers about excluding days below certain temperatures, and the cost is an average that includes days when heating is not necessary. If you think a heat pump that runs all the time will cost you $365 a year to operate, think again. You could easily rack up a $365 bill in January alone. The bottom line is that heat pumps cost the most to operate when you need them the most, i.e. when the pool is the coolest. However, if you want to heat your pool year-round, and want fairly reliable temperatures, a heat pump will be more cost effective than a gas heater in Florida.
Electric heat pumps take several hours to heat up a pool, providing at best a couple of degrees of heat rise per hour. If you run your pool pump only during the day, your heat pump will start over each day to recover heat lost overnight. However, for reasons stated above, it does not make sense to attempt to heat your pool overnight unless you absolutely need warm water in the morning (use a cover, and still be ready to pay!)
Gas Pool Heaters
Gas pool heaters work great. Much like a pot of water on a gas stove, you can deliver a lot more heating performance to water with gas than electric. You can raise the temperature by a few to several degrees per hour with the powerful residential pool gas heaters. Some popular heaters sold locally are rated at 400,000 BTU/hr. That is enough (assuming 100% efficiency) to heat a 15,000 gallon pool by about 3 degrees each hour. Imagine how much energy it takes to raise 15,000 one-gallon pots of water by 3 degrees (a lot). The other thing gas heaters do great is heating water in any weather condition. Ignoring losses during the heating process, gas heaters will heat water at the same rate for essentially the same cost no matter what the conditions are. For example, heating water from 70ºF to 80ºF uses the same energy as heating water from 80ºF to 90ºF, and it doesn’t really matter if it’s 50ºF or 90ºF outside.
All that performance comes at a cost, of course. In some areas of the country where natural gas is delivered to homes and is very cheap right now, gas heaters are actually a better alternative to heat pumps, especially in cooler climates. However, most people in Florida rely on delivery of liquid propane (LP) gas to above ground or buried tanks. LP gas is relatively expensive at about $4 per gallon. Propane contains about 91,000 BTU per gallon, so that 400,000 BTU heater consumes about $17.50 in propane per hour! Doing some rough math, that means this gas heater will gobble up $6 in fuel for every degree it heats your pool. If you pool loses 5 degrees overnight and you want to reheat it, that’s $30. Hand it over! More information is available in a the recent post “NOT ENOUGH ROOM FOR SOLAR POOL HEATING! WHAT ARE MY OPTIONS?”
Gas pool heaters can be a good alternative for people who want to heat a spa only, and don’t like to plan ahead. Heating a few hundred gallons of water occasionally at the last minute can be cost effective for some people. Sadly, that hot water is usually dumped back into the cold pool when you are done using the spa, which is one of the downsides of the ubiquitous pool/spa combination in Florida.
Gas pool heaters are the performance leaders, for sure, but perform at a high operating cost. They also require more frequent replacement typically.
A Note About Pool Covers
Nobody I’ve met actually likes pool covers. They love the resulting warm water, but they are a pain to deal with, and some people have safety concerns. However, if you read the information above, you probably realize now that failing to cover your pool, especially at night, is like watching money evaporate from your pool. It’s especially painful when using a gas heater, and pretty wasteful when using a heat pump, too. One more nice thing about solar pool heaters is that you are not losing real money when you don’t cover your pool at night. You are just starting each day from a lower temperature, which reduces the overall speed and performance of solar heating. You can get more enjoyment by using a cover with a solar pool heater, but at least you are not losing money when you choose to leave a solar heated pool uncovered.
How Does Solar Pool Heat Figure Into This Discussion?
As you might imagine, solar pool heaters complement traditional heat sources well. Letting solar do the lion’s share of the daily heating is a good alternative. Some who like to swim early in the day may prefer a gas heater to get temperatures up quickly, and then use solar to maintain temperatures throughout the day.