I was recently contacted by a friend from high school who is having a pool built at her Naples, FL home. She was interested in a solar pool heating system to make the pool more enjoyable for her children. Initially I was excited to show a friend what we can do, but I took one look at her roof and saw that it just wasn’t going to work. There were too many small roof sections without a good orientation. Solar pool heaters require quite a bit of roof space, and her roof just wasn’t going to work.
Most roofs in Southwest Florida are hip roofs, which are better for wind mitigation. That means we are trying to fir rectangular panels on triangular roofs. Ideally we have a good south, west, or east roof, or a combination of two adjacent roofs. In this case, all of the roofs were too small to fit a suitable number of panels. Adding to the complexity was the split-story arrangement of these roofs.
So what do you do if the isn’t enough room for a solar pool heating system? Well, get ready to pay. The initial cost of a quality gas or electric heater (heat pump) is about the same as a solar pool heating system. Sadly, the costs keep coming, as using one of these heat sources will require ample amounts of gas or electricity. Heating water is hard work. Warning… I’m about to get all mathematic and scientific on you.
I don’t recall if my friend shared my Physics class with Mr. Johnson, but we learned that heating one pound of water by one degree is one BTU. A typical pool in Southwest Florida has about 100,000 pounds of water in it, meaning you need 1 million BTU to raise pool water 10 degrees, which is typical of a solar heated pool. Under idea conditions, a heat pump with a coefficient of performance (COP) of 5.0 would require 200,000 BTU of electricity, which equates to about 58 kilowatt-hours of electricity. At today’s prices, that’s about $6 every time you want to heat your pool by 10 degrees. However, it gets worse. Typically you need to heat your pool the most when conditions are not ideal, i.e. when the outside air temperature is relatively cool. Under these conditions the COP of a heat pump drops and the cost increases. At a certain point when the air temperature drops, heat pumps become completely ineffective, unable to heat the pool at all. You might as well heat your pool by blowing on the surface with a hair dryer (don’t do that).
That’s the problem with heat pumps – they cost the most to operate when you need it most because they rely on the heat in the ambient air to “pump” heat into the pool via a compressor. The colder the air is, the longer the pool takes to heat up. Don’t fall for the $1/day nonsense that heat pump dealers try to tell you. It never works out that way. However, a heat pump is often the next best choice after solar if you want a warm pool on every day on which you are likely to swim.
The other choice is a gas heater. Heating water is more expensive with propane gas, without question. A pound of LP fuel contains about 21,500 BTU per pound, and gas heaters are around 80% efficient, meaning you will get about 17,000 BTU of heat into the pool from a pound of fuel. That means you need about 58 pounds of propane to heat 100,000 pounds of pool water by 10 degrees. At current prices for LP delivery in Southwest Florida, that’s over $50 in LP gas! Let me put it another way — when using a 400kBTU gas heater, you will consume a full 20 pound (BBQ size) propane tank EVERY HOUR it runs! Now the good news — a gas pool heater heats a pool very quickly. Unlike a heat pump, you can get several degrees of temperature rise each hour of operation, and the heating speed is not related to the outside temperature.
Since your pool loses heat every night (although not all 10 degrees) you will pay every day to maintain the temperature. It’s not uncommon for people to call us complaining of several-hundred dollar bills after running their heat pump or gas heater in the cooler months. Pool covers will save tons of money — if you don’t lose heat overnight, that’s money that is not evaporating from your pool!
So what heat source do you choose? That depends greatly on how you plan to use your pool. If you are the kind of person that wants the pool to always be warm so you can get in spontaneously, you should choose a heat pump. If you don’t use your pool frequently and are willing to plan well ahead to heat your pool, a heat pump is also suitable. If you are not a planner, or use your pool very infrequently, a gas heater is probably best for you. If you have a pool/spa combo, and are likely to only heat the spa for impromptu enjoyment, a gas heater is definitely the way to go.
I’m sorry to be the bearer of bad news, C.G. If you want a heated pool, there will be an initial cost and an ongoing cost. It’s up to you to decide how you will likely use your pool. If I can speculate on your ideal scenario, imagine you are on the boat, making a run home on a cool spring afternoon, and you are dreaming of jumping in a steaming hot spa when you get home (while D.G. cleans the boat). You open your iPhone app and tell your automation system to turn on your gas heater and put your system in spa mode. Your 600 gallon spa needs to heat from 70 to 100 degrees in 30 minutes, and it costs less than a glass of wine to do so. How does that sound?
If you’re not sure a solar pool heater will work for you, we are happy to help. We can get pretty creative with panel placement, but some homes are just not built for solar. We’re happy to help you make an informed decision on an alternative.
Note: calculations are rounded and simplified above, and ignore losses during the heating cycle.