Gas vs. Heat Pump for Pool/Spa

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IzzyB

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Jul 29, 2015
41
Orlando, FL
Trying to figure out if we should go thru the trouble of burying a propane tank to do gas for our pool/spa. We are looking to heat the pool in the winter for weekends that are in the 80s. So let's say the temps have been in the high 60s to mid 70s and the upcoming weekend is in the 80s. We want to be able to turn the heat on (either gas or electric) and warm the pool up to be able to swim in it over the weekend. Would electric be able to do this? The one pool guy said sure and the other one said no. One seemed to prefer electric and the other gas. So I wanted to get opinions of others in warmer climates that can swim in the winter time.

Also, for your spa, I was told electric could heat it in 3 hours, is this true? Or is it longer. I was told gas would be around 10 to 20 minutes.
 
IzzyB said:
Trying to figure out if we should go thru the trouble of burying a propane tank to do gas for our pool/spa. We are looking to heat the pool in the winter for weekends that are in the 80s. So let's say the temps have been in the high 60s to mid 70s and the upcoming weekend is in the 80s. We want to be able to turn the heat on (either gas or electric) and warm the pool up to be able to swim in it over the weekend. Would electric be able to do this? The one pool guy said sure and the other one said no. One seemed to prefer electric and the other gas. So I wanted to get opinions of others in warmer climates that can swim in the winter time.

Also, for your spa, I was told electric could heat it in 3 hours, is this true? Or is it longer. I was told gas would be around 10 to 20 minutes.
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This topic comes up a fair bit. It really depends more on the temperature of your pool/spa water in the winter. Here in Tucson, we can sometimes get 75-80F days in Jan or Feb but my pool water temp will be high 50's or low 60's at best. You have to realize that water is a HUGE thermal mass which means it takes A LOT of energy to raise it's temperature even a few degrees F.

By way of example, last fall in November my pool water hit ~75F on a day where we we trying to squeeze in one last swim lesson for my oldest son. The air temp was 82F and the lesson wasn't until 2PM so I decided to kick on the gas heater at 9AM to get the water warmed up faster. It ran for almost 4 hours straight (400,000 BTU/hr nat gas heater) before breaking 80F on the water. We had a noticeable spike in our gas bill that month. After turning off the heater, the water temperature dropped right back down by the next day (within a few hours really).

So, at the end of the day I'll say that I agree with the pool guy that said "no way", at least for the pool. When the water temperature drops below 75F, it just uses way too much energy to try to get it up to a swimmable temperature (most people balk at pool water less than 80F). I definitely don't think an electric heat pump will work well in the winter and gas heat without a decent pool cover to keep the heat in will just waste gas.

For a spa, if you have a decent cover on it while you heat it up, then a gas heater will do the trick for you.
 
If you want to swim in our winter months, get the gas. Especially of you want to be able to turn the heater on a day or so before the weekend Gas is best for quick heat ups. Heat pump for longer heat up times and for keeping it warm over longer periods of time.
 
I recommend the heat pump based upon my experience so far.

Cost of heat pump/chiller = $4,600 in 2007
Cost of electricity to *run HP = about $30.00 to $40 more per month (*only several days)
Cost of repair of Jandy Heat Pump = about $400 since installed 2007
Time to heat pool = about 1 to 1.5 degrees per hour for MY 10,000 gallon inground.
Time to heat 20,000 gallon inground pool by extrapolating data would be about 1 to 1.5 degrees every 2 hours?

Obviously there are some variables that must be considered.

My experience with a pool started May 2007, when it was installed here in Casa Grand, AZ. I did quite a bit of research by asking pool owners, pool builders, and pool service companies - before building the pool. I ended up spending the extra cost of a heat pump at about $4600. Since then I have kept records on using it.

Gas of any kind is very expensive here and learning from friends with gas heaters - $300 to $500 to heat a pool up to a comfortable temp for one day - was outside of my budget. With no natural gas option in our subdivision and propane too costly per gallon and cost of delivery, I went with a heat pump. The Jandy HP is 115,000 BTU/HR, the pool is 10,000 gallons.

After carefully tracking the ambient temp, pool temp, and relative humidity when starting the HP, the average pool temp increase is about 1.0 to 1.5 degrees per hour. Several years ago, we were in the pool on Thanksgiving Day! In the past year when Easter fell on March 23, we were in the pool.

So, I've calculated the cost per daily use of the pool, total cost pool installation, and adding the cost of the HP. It turns out that by being able to extend the "use time" - for me, the cost of the HP, maintenance on the HP, and the cost of electricity is definitely worth it. With a high use family - like active pool lovers and kids - it's a justifiable investment. If you aren't going to use the pool much, it may be considered an expensive luxury.

The cost per month when I heated the pool was only about $1 per day more than our regular utility bill. So the first time I heated the pool, my electric bill was about $30.00 higher than average for that month.

There have been a number of times when I've heated the pool from a "start heater" ambient air temp of 75 degrees, and pool temp 65 degrees. I started the HP on Thursday night (off-peak), running for 6 hours. Started HP on Friday night, running about 6 hours. On Saturday (off-peak) turned on HP at 6am, by noon on Saturday pool temp was 88 degrees. With the ambient air temp around 75d and the pool temp much higher, the pool feels REALLY good! We have a propane fire pit, so getting out of the pool, drying off quickly, and sitting around the fire pit is a nice way to spend Thanksgiving Day at home!

An added benefit -- One 4th of July weekend (ambient air temp at noon was 105d), we had a pool party and the pool temp was a toasty 92 degrees – sunny Arizona! I turned on the HP in chiller mode. In about 4 hours the pool temp dropped to 88 degrees.

For a spa, gas might be more efficient, quicker heat up, and still affordable?

Heat pump efficiently is effected by the relative humidity, so depending on your climate and what time of the year you want to heat the pool, size of pool you are heating, cost of electricity, and how much time you have to heat up the pool, you can decide what works best for you.

Here's a link to the heat pump I have: Jandy AE-Ti Heat pumps

You can see Jandy's data on temp vs. time chart.

That's just my worthless opinion.

Randy
 
Wow Randy! Thank you so much. This is the information I am looking for. We probably live in similar climates. Our pool will probably be around 13,000 gallons so also not that far off. But we are looking at 140k BTU Jandy Heater, so I am thinking the extra BTU should make up for the extra gallons in my pool. Your Thanksgiving example is pretty much the example I was looking for. Turning the heater on Thursday for a weekend swim at those temps. The chill mode is also very nice because in the summer time I know pools around here get a little too hot.

Thanks for all this great information!!!!
 
Hi Izzyb,

It's my pleasure!

http://www.veraahdyheatpumps.com/heat-pump-efficiency.html#sthash.x4011h8M.dpbs

Here in Arizona our average yearly relative humidity is about 37% Orlando’s average RH is about 47%.

Given the fact that heat pump efficiency increases with the increase in Relative Humidity, I’d assume your heat pump will produce of higher COP (Co-efficient Of Performance).

Here’s my original unscientific analysis before and after adding the heat pump to the pool build:

I took the entire cost of the pool = $30,000.
I estimated the number of actual days I could theoretically use the pool with just the ambient air temperatures heating it = May 1t to September 30 = 152 days. Cost per day of use investment = 30,000 divided by 152 = $197.37 per day use cost/investment.

Then I did the math and added the cost of the heat pump and extra energy consumption/electricity.

Estimated number of days I could use the pool… ambient air…
Adding only 2 months to make the use time extended = April through October = 213 days at 30,000 plus the cost of the heat pump $4,600 = $162.24 per day use cost/investment.

Oh.. but we have to add the electricity right? I doubled the highest average cost of running the HP in any given month to $80 extra dollars (over my normal electric bill for those months) for the extended use months of April and October = $160. Add that to the total cost/investment figure of $34,600 + $160 = 163.19 or approximately 18% difference in cost per day use when you add a heat pump to our total cost/investment.

I left out the repair cost of the Jandy HP because so far it's rather insignificant.

So, that’s how I justified spending the extra money when we decided to have the pool built.

Total investment - $197.37 per day cost without the heat pump
Total investment - $163.19 per day cost WITH the heat pump

You can see that my numbers are based on estimates and theoretical pool use.

One of these Christmas holidays I'm going to heat the pool up to 92 degrees, get in wearing a Christmas hat, holding a tall glass of eggnog and take a photo and send it to my friends in Minnesota!!!

I have not calculated the actual energy consumption (watts per hour) of the Jandy HP, but I’d be surprised if that data is not readily available.

I’d be very interested to read about your data, if you have one installed and decide to track the data when heating up your new pool.

When I track my data, I simply record the outside ambient air temperature in the shade of the patio, and the pool temperature read at the heat pump at the start up of the heat pump. And, I get on the local weather site and check the reported RH for my area. Then every hour I record the increase in pool temp, ambient air temp, about every 2 hours or so, I check local weather site to see if it reports any change in the RH.

Randy
 
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