How effective are heat pumps for spas?

citivasMarch 7, 2011

Signing a contract for an in-ground pool and spa in Central New Jersey. Pool will be approximately 700 sf (20x40, 6 ft deep), the spa 49 sf. of surface area. The contractor is recommending an AquaCal Heatwave H120 (110K BTU) heat pump instead of a propane-based heater (I don't have access to natural gas).

My primary concern is the spa. It would be nice to "extend my season" but I want to make sure I can get the spa up to 103 degrees within a reasonable time when I want to use it. By default it will be circulating with the pool using the same equipment so it will have pool temp water except when the system is changed to circulate only the spa.

I have read completely conflicting things about how effective heat pumps are at raising temperatures (including the mega-post here that turned into a debate about the physics of how they work). My focus is, will it get my spa to 103 if the outside temp is say 70 degrees and how long would it probably take? The pool guy says 45 minutes tops, but I am dubious.

Also, how long would it really extend my pool season in Central NJ? He said April to November but that also seems aggressive, given temps could be in the 50-60's. I have a friend who got a pool and heat pump last year in Long Island who says he couldn't get the pool to 82 degrees if the temp was below 70 degrees outside. He didn�t have a hot tub though so I couldn�t ask how that went.


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Assuming your spa is around 1000 gallons, the best you could expect from 110k BTU would be about 12 degree rise per hour.

And yes, heat pumps don't work very well at low temps. If you want to heat the spa quickly when it is cold out, you need a gas heater and preferably a large one.

I have a 400k NG heater and when the water is in the 50s, to raise the water temp 50 degrees can take close to an hour in a 700 gallon spa.

    Bookmark   March 7, 2011 at 3:16PM
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Lets look at the math:

1 BTU is needed to raise one pound of water, one degree F.

1 gallon of water is 8.345404 pounds

mas985's spa is about average sized.

To raise his spa one degree with take about 5842 BTUs

To go from 70 to 100 will take 175253 BTUs.

Assuming ideal conditions, 80 degrees, 80% humidity, a 125K BTU heat pump will still take well over an hour to heat it up.


    Bookmark   March 7, 2011 at 4:05PM
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Rack Etear

It's not that simple of a calculation.

The 1 btu per pound is only good in a vacuum. You have other factors, such as wind and the delta between the water and air temperature to combat.

First if you are getting an aquacal, I'd go with the sq155, or 175. The sq156r is good if you plan on heating ( the pool) in colder weather.

I'd use a gas backup for the spa, because it's really hard for a heat pump to heat more than 30 degrees above the air temperature which often happens with a spa.

    Bookmark   March 8, 2011 at 12:25AM
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Your added factors usually make it take longer, furthering the point mas985 and I were making.

A gas fired 400K heater will do the job in about 20 minutes without losing it's ability to inject BTUs into the water when it gets cooler, like at night.

Heat pumps lose efficiency as air temps drop, like at night, making heat up times even worse. If it's 65 degrees out, he'd be looking at over two hours to heat the spa.

That to me, when compared to the 20 minutes or so with gas is a no-brainer.


    Bookmark   March 8, 2011 at 3:37AM
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I have the AquaCal 120 also. My pool is slightly smaller than yours, and I'm in suburban Phila. I would echo what the experts here are saying - it's probably not the way to go if your goal is to extend your season. I close everything (including the spa) by the end of September. Up to that point, on most nights I can raise the spa temp within an hour. I keep the pool around 80, so I'm only looking at 20 degrees or so. I would also mention that the unit has not been the most reliable piece of equipment I've ever owned. I put it in new 4 years ago (along with the entire pool), and have had to have it serviced every year. AquaCal has been great about it, saying mine is the exception, but it's info you should have. I love the heat pump as a slow-and-steady source. With my wife's health, we need to keep the water warm all the time, and I just set it and it's always 80 (assuming the unit's working!).

    Bookmark   March 8, 2011 at 8:36AM
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