Anyone using a tankless water heater to heat a pool or spa?

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Subdave735

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Mar 14, 2013
3
Orlando, FL
Hi folks, I am new here but have really appreciated the info I have found so far, particularly regarding pool chemistry. Apologize for starting a new thread but couldn't find much via search. I want to air out this idea before I start doing something really silly.

I bought a home about 21 months ago with a pool/spa combo built around 1995. So far I have converted to salt water, replaced the pump, replaced the DE filter screens, and made some inroads into removing the copious quantities of scale. During that time the pool heater has been on life support and has actually worked to some extent. When I switched to a new propane supplier, they declared the old pool heater to be a "death trap" and would not connect to it. Sooo... time to redo the heater.

I live in Orlando, FL. The plumbing is set up for one pump to drive the pool and the spa. The spa normally overflows to the pool. The valves have to be manually realigned to just drive the spa jets. The existing heater could heat pool and/or spa. I have no desire to heat the pool.

I am looking for a cost effective way to heat the spa only and I am considering some alternate ideas. Specifically, I would like to use a tankless water heater that typically is used for household water heating and/or hydronic heating. Why, you might ask? I have installed two of them before (in normal applications) and have been very impressed. Currently on eBay there is a re-branded Takagi with remote controller available for about $850 (typically about $1400). Input is 199k BTU/hr. They are condensing units so efficiency is >90%. They are small, about the size of a carry on suitcase. As part of the implementation, I would install a second pump that would only recirculate the spa through the heater and a bypass valve and back to the spa jets. This would eliminate the need for manually realigning valves and would allow 1 button operation for the spa. The bypass is needed as the heater only passes 8 gpm and I think the jets need ~30 gpm.

Two arguments I have seen so far is whether or not a tankless heater can operate for hours at a time and can they handle the chemical environment of pool water. As far as run time goes, they can operate indefinitely and are readily used for hydronic heating. The chemical environment , I'm not so sure.

The unit consists of a standard copper heat exchanger and a stainless steel heat exchanger. The stainless can handle the acidic condensate on the outside so I think they will be fine with pool water on the inside. What I have read about copper says it is pretty resistant to corrosion in salt water environments and with the pool at ~ 3-4k ppm, it is not nearly as salty as the ocean. Of course the ocean isn't running a SWG. I have no idea how aggressive 3-5 ppm free chlorine will be on a copper heat exchanger. But aren't many of the older pool heaters built with copper? Also, there normally won't be any flow through the unit so I think that free chlorine would quickly deplete. For the record, tankless heater manufacturers specifically state they are not so be used with pool water.

There are at least 2 manufacturers making all stainless tankless heaters but their price point is >$2k.

So my questions are:

Has anyone out there done something like this? How did it go?

Does anyone have specific thoughts or data regarding chlorine and copper heat exchangers?

Would it be better for the heater to be wet or dry when not in use? i.e. should I make it drain when there is no flow.

Am I missing something else in this whole exercise?

Thanks in advance for any advice you can offer.

-Dave
 
Re: Anyone using a tankless water heater to heat a pool or s

Welcome to TFP!

Tankless water heaters are designed to produce a large temperature gain on a relatively small volume of water. A swimming pool is best served by a very small temperature gain on a very large volume of water. You already have the plumbing, gas, and electricity in place for a conventional pool heater, which is by far your best choice.
 
Re: Anyone using a tankless water heater to heat a pool or s

Thanks for the reply.

A pool heater would be easiest but it is massive, more than double the cost of the tankless heater, and lower efficiency. With the $$$ and space saved I can buy a second pump to make running the spa easy.

Just to be clear, I don't want to heat the pool - just the spa, roughly 500 gallons. And only just prior to use.
 
Re: Anyone using a tankless water heater to heat a pool or s

I think your best bet is to spend the money on a real pool heater. Output will be far better or intended use and so let's say the other heater only lasts two seasons.....now what are you going to buy?

When I tested your idea using a small point of use heater for a kiddie pool, when I took the thing apart for the season, that heater was full of crud. I was using bleach only and not salt. I can only imagine that a system that is designed to NOT use salt would only deteriorate fast.

Plus if you double down with the real pool heater, you could hit the real pool too, if you wish. Oh, you have 1 1/2 inch lines going through that tankless water heater? That's going to be a serious bottleneck and possible flow problem for your pump to over come.

Bob E.
 
Re: Anyone using a tankless water heater to heat a pool or s

A tankless water heater is not designed to be used in the scenario you want to use it in. Purchase a product that is designed to be used for a spa. In reality you are talking about spending maybe $1600 for a gas pool heater which is only double what you are looking to spend now on a tankless. Of course if you are like most DIYers, you probably can find one on craigslist for much less cost.
 
Re: Anyone using a tankless water heater to heat a pool or s

Another way to look at it is you would be asking your tankless burner to heat 10 or so traditional tank-type water heaters simultaneously.
 
Re: Anyone using a tankless water heater to heat a pool or s

Safetybob, what kind of crud did you find? Curious as to whether or not the water was filtered before it hit the heater. Any recollection as to how tight the internal passages were on the heater?

There are some manufacturers pushing electric tankless heaters for smaller pool heating. I have no idea how different that design is internally compared to a gas fired tankless heater.
 
Re: Anyone using a tankless water heater to heat a pool or s

It was a hard crusty brown to rusty brown HARD Crud. Not much of a filter system was used, but there really was nothing but clean water being pumped through it. It had closed down the 3/4 inch hole around 50%. Please don't ruin that expensive tankless unit doing this.

I think there are around two or three people manufacturing electric pool heaters with the largest I think around 11KW, that's what I have. The Coats brand is the same as Raypac and another as I recall. Mine was from Little Giant who also makes baptismal heaters too but with a slightly different design. Mine has two 5500 watt submersable heating elements that just sit in the tube and heat the water.

Performance. My little pool I had this attached to was right at 1500 gallons. My side walls were insulated and I had a DIY 1 inch insulated top/cover which gave me minimal heat loss. Once uncovered, if it was around 60 to 65 degrees, around an hour or so you would start noticing that it wasn't quite as warm as it was before (generally 90 degrees). If your spa is bigger or isn't covered, I urge you to bit the bullet and get a gas heater. You will need the additional output to keep it warm. As mentioned before, look on Craigslist. Someone all the time is getting tired of feeding their pool and will put a heater on sale.

Bob E.
 
Re: Anyone using a tankless water heater to heat a pool or s

The things I would be concerned with:

At 8pgm, it would take about 68 minutes to completely turn over your spa at 550 gallons. How long will it take to heat the spa at that low flow rate? Our spa is 7'x7', so approx 1000 gallons (rough guess) and it takes about 30-40 minutes to heat it up from 60 to 100, that's with a 400K heater.

Lets say you get this tankless heater for $850, add in the extra costs for another pump, misc plumbing and vavles and you are looking at $1000 - $1100 cost. Why spend that much when you could get a 200K spa / pool heater that is designed for the application for around $1500 -$1600 or less if bought used. The pool heater will last 10 yrs (guess?) while you don't know if the tankless system will last more than a year or two with pool water (could work longer - unknown). Factor in the risk of losing $1100 if it fails quickly, the extra time and $ (propane) to heat the spa, and it doesn't seem worth it to me to save $500.

Steve
 

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I am in Orlando too and solar for a spa is not reliable heat year round. No heat is going to occur on cloudy days.

I am sure using the tankless heater would be a fun and unique project but it has not been tested by enough people to reduce the risk of losing $1000 and still having to buy a heater.
 
Tankless water heaters sold for residential hot water heating are designed to produce a relatively large temperature gain on water at a relatively low flow rate. A spa (or pool) needs a relatively small temperature gain on a relatively large flow rate. The design criteria are very very different. Attempting to use either one in the other context will cause problems.
 
JasonLion said:
Tankless water heaters sold for residential hot water heating are designed to produce a relatively large temperature gain on water at a relatively low flow rate. A spa (or pool) needs a relatively small temperature gain on a relatively large flow rate. The design criteria are very very different. Attempting to use either one in the other context will cause problems.
Click to expand...
^^ This illustrates the main point regarding the design difference between these two types of heaters as simply as it can be stated. I'm with the consensus on getting a heater that is designed for pools/spas. In this application, a proper pool/spa heater will likely outlast the tankless heater several times over. My pool heater is over 20 years old and still works great.
 
I want to do the exact same thing. Been contemplating it all summer. I'm about to take the plunge just wondering if you had any luck. These other members are missing the whole point. If saving a thousand dollars can be accomplished by some simple plumbing and a little innovation I think its crazy not to. Besides, if we are right and it does work the rest of the world will be doing the same in a matter of time. I give it ten years and people wil lbe saying "hey remember those old clunky pool heaters we used to see" lol
 
I had to go back to the top to realize that the original post was over 2 years old. This is an off-topic comment but what's the policy for closing threads? I have seen a number of very old threads lately getting bumped up because a new post was added. Perhaps it would be a good idea, if possible from the administrative software side, to automatically close posts older than 18 months. If it's a topic of interest to someone, they could always copy the posts URL into a new post and reference the subject matter that way.

So as to avoid being off-topic, I too would add that this is bad idea. Not only are tankless water heaters (hey, pool heaters are "tankless" too!!) not designed for spa flow rates, but they are probably not made with materials compatible with pool water salinity levels. SWG or not, pool water typically has salinity and mineral levels much higher than what you find in residential water supplies. Therefore, low-grade stainless steel components will easily corrode. This is why pool heaters only use copper, cupronickel or titanium alloys for internal heat exchange components.
 

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