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Can You Use a Hot Tub Heater for a Pool?

My wife and I plan to get a pool soon, but sometimes it’s a bit chilly for a dip in the pool. So I’ve wondered can you use a hot tub heater for a pool?

A hot tub heater can heat 500 gallons of water between 3°-6° degrees per hour and has 4-5 kilowatts of power. Pool heaters are roughly 4 times as powerful, meaning a hot tub heater would take significantly longer to heat a pool. It also puts significant wear & tear on the hot tub heater. But it IS possible.

But there’s more to know about the crucial differences between pool and hot tub heaters, how long a hot tub heater might take to heat a pool and how many gallons a minute a heater can handle.

So let’s check it out!

How does a hot tub heater work?

Let’s start by examining how a hot tub heater works.

Unlike the hot water heater in your house, a hot tub heater is more like the tankless hot water heaters that have become popular over the past decade. Since they don’t have a holding tank, they simply heat on demand.

The pump pulls the water through the heater tube, warming it as it passes.

Then, between the insulation on the outside of your hot tub shell and the lid you keep closed on your hot tub when not in use, the heat loss is (hopefully minimal).

The actual heater in your hot tub is fairly small.

Not counting the box it comes in which often houses all the electrical connections for your entire system, the heater is only a couple of inches in diameter and maybe a foot long.

While there are a lot of brands of hot tub heaters, and a number of options for each brand, the average hot tub heater is probably between 4-5 kilowatts of power.  They also are typically well under $500 for the complete control pack which includes the heater as well as the electrical system for all the hot tub components.

A pool heater, by comparison, typically runs a minimum of about $1,000, and often well into the thousands.

But to be fair, the pool heaters typically also contain the pump which in hot tub equipment is sold separately.

Unfortunately, for direct comparison’s sake, most pool heaters show BTUs, not kilowatts. But doing a little conversion, I’d say the average pool heater is probably between 15-20 kilowatts.

So pool heaters are roughly 4 times as powerful.

How many gallons per minute can a hot tub heater heat?

The average hot tub holds somewhere between 300-600 gallons of water depending on how many people it’s designed to hold.

As I mentioned above, there are many sizes and types of hot tub heaters. So combined with the various sizes of hot tubs, and of course, a wide variety of weather, it’s hard to say 1 number that fits every situation.

That being said, the average hot tub heater will go up between 3 and 6 degrees each hour.

The other variable is whether you leave your heater on continuously or have it cycle on and off. But if you’re filling up a medium-sized hot tub of 500 gallons from the hose, you’re starting with water that’s likely to be around 70°.

So to get to 100°, it would take about 6 hours.

That amounts to about 1.38 gallons a minute being heater by the average hot tub heater. But again, there are a lot of variables, so just use that as a guide.

How many gallons are in an average pool?

There are even more variations in swimming pools than there are hot tubs!

So for our purposes here, I’m going to give an example of an average in-ground pool as well as an average above-ground pool.

The nice thing about above ground pools is they are a uniform height (usually 48″ or 52″), so it’s easier to gauge how much water is in one. In-ground pools, by comparison, often grade from shallow to deep.

For an average above-ground pool, we’ll go with a 48″ high and 24′ round pool. That pool holds approximately 14,000 gallons of water.

In-ground pools, by comparison, come in a lot more shapes other than round and rectangular. But for purposes of this article, let’s say the average in-ground fiberglass pool is 12’x24′ ranging from 3′ to 6′ deep. That pool would hold about 20,000 gallons.

As you can see, the average pool holds about 34 times what the average hot tub holds. That’s a substantial difference.

How long would it take a hot tub heater to heat a pool?

So we already established a hot tub heater takes 6 hours from tap temperature to 100° in a hot tub.

BUT, most pool owners don’t want their pools at 100°! In fact, most people who use pool heaters, keep their pools between 78° to 82°.

For math purposes, let’s say you have an 18,000-gallon pool and want to get it to 80° from the 70° we said might be the average tap water temperature in your city.

Knowing a hot tub heater can do about 5 degrees an hour per every 500 gallons, it would take almost 3 days to heat your pool to 80°.

The next issue you’ll face, however, is that a lot of pool owners don’t use pool covers the way hot tub owners do. So you’ll lose heat on the surface of the pool. For above-ground pool owners, you’ll also lose heat through the pool walls since they aren’t insulated like the underside of a hot tub.

The other issue you’ll likely face is that since a hot tub heater is designed for a much smaller system and far less water, and often not running 24/7, you would likely wear out the heating element inside your heater tube.

The average electric pool heater will probably only add about 60 cents an hour to your electric bill. So even if you run it 24/7, 365 days a year, you’re looking at $438/month on top of your normal electric bill.

By comparison, a hot tub heater would probably use only about 75% of the electricity of a pool heater, so you’d be looking at an extra $300/month on top of your bill.

But again, that’s IF you run your heater every day all day, which I’m sure you wouldn’t. In most cases, in most places, you wouldn’t use a heater May thru August or September.

You also probably wouldn’t use it in winter as no one wants to walk out to the pool shivering. So that eliminates November-February for most people.

So if we’re talking only using it in March, April, and October, then you’re looking at a somewhat more reasonable increase.

There are also a million variables, so these are just rough estimates.

Are pool heaters much bigger than hot tub heaters?

Yes is the short answer.

As we’ve covered, a hot tub heater (just the heater) is a tube about 2″ around and maybe a foot long. By comparison, while pool heaters vary more in size than hot tub heaters,

The other thing to know is that most hot tub heaters are purely electric, whereas pool heaters can be propane, natural gas, electric, or even solar.

Most experts agree that in terms of power, you want about 50,000 BTU per 10,000 gallons of water.

Going back to our example of an 18,000-gallon pool, you’d want a 90,000 BTU heater. Doing our conversion, that’s about 26 kilowatts (remember the average hot tub heater is 5 kilowatts).

In terms of physical size, those units will be somewhere roughly around 36″ Tall x 36″ Wide x 41″ Deep. So physically, and power-wise, pool heaters are significantly bigger than hot tub heaters.

The most efficient type of pool heater is what’s called a heat pump system which is electric.

Can you use a hot water heater to heat a pool?

This is the million-dollar question.

The short answer is yes, you probably could. Like most hot tub heaters, the average pool heater uses 2″ plumbing connectors.

As I’ve already discussed, though, you’d likely wear out the heater element much faster, and it would take a lot longer to heat your pool. If you’re drawn to using a hot tub heater for your pool due to the much lower cost, I get it!

A few hundred is a whole lot better than a few thousand!

But you may find, over time, that the wear and tear on your hot tub heater and the replacing of the heating element with some frequency aren’t worth it.

If you live somewhere as I do, where it’s warm a lot of the year (Texas), you might get away with it since you’re actual need to heat the pool would be only a few months out of the year.

Without any heating, your pool water is not likely to get much above 75 on blazing hot days. And in spring and fall, it could easily not go above 68. For a lot of folks, that’s pretty chilly.

Also know that by adding a pool cover, you can reduce heat loss by about 50%.

So it’s worth looking into! Solar slat covers even help heat your pool while covered. While they’re pricy, they generally pay for themselves within 2-3 in the energy you’ll save.

So, can you use a hot tub heater on a pool? Yes.  Would I recommend it? Probably not.

Final thoughts

In this article, I took a look at the world of hot tub heaters.

We looked at how they work. But also, how long they take to heat water, and how much bigger pool heaters are compared to hot tub heaters.

Specifically, we answered the question of whether you can use a hot tub heater to heat a swimming pool.

Have you tried a hot tub heater on a pool?

Jeff Campbell