Boiler instead of a pool heater?

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M

mdrejhon

0
Sep 29, 2014
73
Hamilton, Canada
Hello,

With a new purchased pool pump and a new pool heater (still returnable) still in box at pool shop waiting for pickup... me and a trusted installer has discovered a "radical" new solution. Since we also need pool shed heating and a future hot water tank for a pool shed shower.....

...."Why not use a boiler?"

We did the math, and the efficiency would be the same (give or take) for the pool. The boiler is more efficient (160F primary exchanger loop, forced-air operation like the top-efficiency pool pumps, etc), and all the pool chemical wear and tear occurs on a much more inexpensively replaceable off-the-shelf heat exchanger. No pool water goes into the boiler, just to the heat exchanger between primary 160F water and the pool plumbing. Even with the extra heat exchanger interim step, the bottom-line efficiency would end up being roughly the same, given we also need to heat the pool shed too in cold weather! (Better natgas instead of electric...)

Anyone using a general-purpose boiler to do:
-- Pool heating
-- Shower hot water
-- Room heating

The boiler is actually cheaper than doing all 3 separately. And the installer really knows what they are talking about; as an expert in doing all this (having already done this before).

The boiler we are now considering is a variable boiler capable of throttling from ~10K BTU thru 199K BTU, and automatically shutting off when there's no thermal demands. It fires up when there's water flow, and fires harder on bigger demand, all automatically depending on heating demands. (Only showering, only heating room, or only heating pool, whether maintaining it heat or rapidly increasing its temp, or all three, etc), and costs only 25% more than the pool heater we bought. Tons of mature boilers out there with efficiency comparable to pool heaters. Ultimate pool-heating efficiency is roughly the same, assuming fully-thermally-insulated 160F closed primary loop, and a good off-the-stock exchanger unit for the pool heating.

Anyone here using a general-purpose boiler instead of a pool heater, to heat their pool?
 
Users here have toyed with the idea of using tankless water heaters to heat pool water. The problem is, those heaters typically are designed for intermittent operation and fairly low flow rates of water compared to a pool pump.

Your heater idea might work but heat transfer is going to be critical. Remember that, in a pool heater, the heat source (gas or electric) is directly heating the pool water. Therefore, the heating efficiency is very high (BTUs of heat delivered into the water divided by BTUs of the energy source). In your scheme you are adding a heat exchanger between the heat source (160F boiler water) and the pool water. There will be a loss of efficiency due to the heat transfer. So even though your boiler may be able to put out 199kBTU's of heat, there's no way all of that will be transferred to the pool water. This could just mean that your heating scheme will be a lot slower to heat the pool than other traditional heat sources. Also, in all heating applications, the dominant factor is evaporative loss of pool water - if you're not covering your pool (especially in the overnight hours), then it matters little what you use to heat your pool.

Still an intriguing idea. Let us know if you go ahead with the setup and post some pictures. It would be nice to see some data.

Good luck.
 
It appears the bottom line efficiency is supposedly the same as my old heater and the replacement Raypak (still in its box, returnable) because of thermodynamics.

Firstly, the recommended high efficiency boiler, being forced air, it as efficient as the most efficient pool heaters, to the 96 percent efficiency level into the primary exchanger. So more efficient than average pool heaters like Raypak 266.

Conveniently "desired" waste heat: What fails to heat the pool will always heat the pool house. Only 4 percent truly wasted. Law of physics, law of thermodynamics.

That 96 percent primary loop efficiency "makes up" for the later losses in the pool water heat exchanger which loses slightly less than that, netting back to square one in slightly better pool water heating efficiency than the stilll-boxed non-forced-air Raypak I got (assumig properly insulated 160F loop and a good heat exchanger).

Heating speed will be approximately the same, he claims, despite being 199K BTU vs 266K BTU. Instead of hot gases esaping a chimney, cooler exhaust are expelled through a wall side vent (like the newer space-saving efficient forced air pool heaters) -- the forced air 96 percent efficiency means more heat is wicked out of the burning natgas into the primary exchanger, resulting in relatively cool exhaust. Meaning, all 96 percent of the heat is kept inside the building to be expelled either to the pool water or into the pool shed (whether intentionally or as leaked heat from pool exchanger inefficiencies).

Some pool heaters automatically bypass water so temp differential between intake and outlet is limited in delta, such as 25F difference. With the boiler, if set up accordingly, I may be able to get a bigger temperature delta to permit faster heating of my pool -- at least at the bottom end of heating curve.

Also, the BTU smooth throttling can do wonders with a variable speed pool pump runnig at slow speeds, as the pressure switch wont cut out the way a pool heater does. It will just throttle back on thr BTUs, and keep working even at the lowest possible Hayward Tristar VS speed (apparently cannot do that with many models of pool heaters), only automatically cycling off when under 10K BTUs of heat is being pulled out of the primary exchanger loop. The boiler works at practically all pool pump speeds.

At lower pump speeds -- Jets becoming much hotter much sooner in a cold pool, when sitting in shallow end. (Aaaaaaaahhhhh...). Obviously, the temperature of the jets will need to be thermally clipped at a max temp with the right components (No scalding 110F please! Especially in 50F or 60F water)

And doesn't cycle erratically with the occasional sudden pressure modulations of pool vaccuuming (in sub zero weather, I always heat while vaccuuming).

Plus since any heat loss is simply extra heat for the poolhouse, saving heater electricity anyway! I still save money - Even if you pool heating a bit less efficient! (Which he claims it is not)

We run our pool in winter, after all.....so we heat the poolhouse with electric heaters.
So that tips a bonus to the boiler idea.

Now....that said.... what's the catch?
(Assuming GOOD boiler installer who seems to have a Ph.D in laws of thermodynamics)

In short: The takeaway was, boilers are a mature technology.... the math checks out.... but is this happening to users in actual pool heating practice?

Any users actually doing this?
I know it is done commercially woth large boilers at some nordic spas.
But residential?

Note: We already cover pool when not in use... we ran a 20,000 gallon pool as hottub 95F in winter, outdoors, Canada last January for 8 days near nonstop, with a sub-300-dollar heating bill (not just pool, but also including house heating and gas stove). Click my link below "Cheaply hottubing a pool" in signature for poolside snow. :)
 
The machine the pool heater installer recommended is the Navien NC240E general-purpose boiler. It is not just a lowly or cheap "tankless water heater" but a real bona-fide general purpose boiler too.

He also does Hayward and Raypaks too, but installs boilers too, including for pool heating, commercial and residential, well recommended by others as a good installer, and I truly got the sense he really knows his stuff well.

(I paid attention to class in laws of thermodynamics, and his theory talk is correctly spot on.... I am just concerned about actual practice, reliability, real world experience, etc).

I need to hear more data on real-world use, even if one of you here did a commercial boiler installation. I may be one of the few considering doing the boiler route at the middle-class residential league... as we are a miniature Nordic Spa already of sorts. Looking for the "What's the catch?" angles.

Boiler noise (forced air technique) is probably one of the catches of achieving 96 percent heating efficiency. I presume. But any other catches?
 
Interesting idea, though I don't have any pool experience to offer. I do have a whole-house boiler that also heats our hot water tank. I love it but I can tell you its $15k worth of equipment and even swapping out a failed pump ran $800 on it...so I never thought of a boiler for the pool, though I often wondered why not.

I guess I'd ask your guy if residential boiler components are constructed to withstand higher salinity, as the chlorine etc. in pool water ultimately breaks down into salt that accumulates easily to higher levels than you'd ever see with a water softener...eg my pool was 3000 ppm salt before I went SWG.

The second caveat might be Thermostatting -- if you can't or don't keep your water still, you will not get the energy savings...I need to get my head around how the normal pool pump relates in a boiler set up because low flow conditions when pool pump is off cause wear and corrosion issues for regular pool heaters...which is why you need to turn off heater 20 min. before pump.

Otherwise an interesting idea.

Hubby as has similar thoughts for the air blower heater on our dome, wants to bust out a bypassable loop for it. Haven't asked to our hvac guy yet ;)
 
The secondary exchanger off a main boiler will work, but keep in mind that it's going to be very slow at heating a 20,000 gal pool. It's going to take some specialized valving and thermostating to get it to work properly, but someone with boiler and radiant heating experience can figure it out. The boiler is going to run long enough that you're going to need a way to exhaust the hot ambient air out of the boiler room, and that also should already be in the plans.

There is actually a thread here from a year or two ago about someone who did the same thing. I'm sure you can find it if you search around a bit.
 
Thanks for the tips to add to research and decision;

Corrosion: Pool water doesn't enters the boiler -- only into a cheaply-replaced, insulated, external exchanger between 160F primary loop, using off-the-shelf components. As the boiler is protected from pool water, this can give me more worry-free chemical freedom than a pool heater (e.g. Heating shocked winterized water from 35F to 95F over a 60 hour period, once a year, during pool-opening; that's beyond heater recommendations). The entire 160F loop and the exchanger will all be insulated extremely well for our installation. I suspect the insulation will play a huge difference in efficiency. The more efficient the insulation, the better the loop/exchanger will perform at lower natgas burn rate.

Overheating room: Already factored -- it's supposedly similar heating effect of current pool heater. The poolhouse is a single room, split into workshop, change area, future shower area. During warm weather we already have to vent our existing 250K BTU pool heater which can overheat the room. (There's a ceiling fan and windows). I think waste heat emissions will be roughly similar, thanks for the tip about comparisons of that. I'm presuming, given 96% efficiency, the boiler casing will run cooler than the old pool heater, but the 160F loop will be the big determinator of how warm the room gets, how well insulated is.

Homework:
-- Insulation of the primary loop (installer expressed this importance, but will ask more questions).
-- Total heat emission relative to old heater. Determine if it's worse or better than existing status quo (I'm expecting similar -- needing to run the ceiling fan and open windows during warm weather when using heater while pool is still cold, and running boiler 24 hours)
 
Swampwoman said:
I love it but I can tell you its $15k worth of equipment and even swapping out a failed pump ran $800 on it...so I never thought of a boiler for the pool, though I often wondered why not.
Click to expand...
There are multiple models, but the Navien NCB240E (not the "tankless heater" model NPE240, but the more all-purpose boiler NCB240 boiler; they seem to have better reviews and more appropriate for our case) -- runs at $2500-ish, without the labour and extra primary loop components yet factored in (+ possibly one to two thousand).

But it's still much cheaper than buying 3 separate heaters for the same facility, which is our conundrum now. It's much smaller than a house furnace, being only 199K BTU. The cost will actually be cheaper than a natgas heater, water tank, and Raypak 266 heater combined.[/quote]

I guess I'd ask your guy if residential boiler components are constructed to withstand higher salinity, as the chlorine etc. in pool water ultimately breaks down into salt that accumulates easily to higher levels than you'd ever see with a water softener...eg my pool was 3000 ppm salt before I went SWG.
Click to expand...
None of the water ever enters the boiler. The boiler runs a completely separate loop of clean 160F water. It's a heater exchanger between two water pipes (pool water loop & closed primary loop to boiler)

Metaphorically, imagine the boiler providing "electricity" in the form of a closed loop of completely separate (clean) water at exactly 160F that loops out of the boiler and back into the boiler. You add T-connectors into the primary loop where needed to get heating service. Exchangers as needed.
-- A separate exchanger for pool heating
-- A separate exchanger for shower water heating
-- A handle to allow the primary loop water to also flow through a room radiator. (Room heating).

There are external heat exchangers specifically designed for handling pool water.

The primary loop is an independent loop of pipe of 160F water. That part is like a closed loop, none of that water goes to the shower drain or swimming pool. To keep losses low, everything that has 160F primary loop water is insulated. That means all primary loop pipe and primary heat exchangers are to all be insulated, for maximum efficiency (>80% end-to-end efficiency is achievable, that's comparable to my Raypak).

The boiler's only job is bringing the primary loop back to 160F. The more the primary loop is cooled down by its users (shower, or pool, or room heating, or 2 or 3 of the above combined), the more the boiler throttles towards 199,000 BTU in its attempt to keep the primary loop water at exactly 160F.

Users of the primary loop (E.g. Shower exchanger, pool heating, etc) is essentially metaphorically like turning on electric appliances. The boiler automatically works harder, like a generator working harder. The primary loop is a circuit, only it's heated water. And we're talking about BTU's instead of amperes. The more BTUs removed from the primary loop (a.k.a. Primary loop cools down faster), the more BTUs injected back into the loop faster by the boiler.

Pool pump runs slow at the moment? In that case, less heat is being wicked out of the primary loop. The primary loop water is easily brought to 160F using far less than 199,000BTU. Heater will just merrily automatically reheat the primary loop water back to 160F at really low settings.

Nothing turned on? (Pool off, shower off, no room heating). The primary loop water stays at 160F comfortably in its insulated loop, for a very long time, like a styrofoam cup of coffee -- takes a long time to cool down! The boiler automatically shuts off, only firing intermittently very briefly to keep the primary loop at 160F as the only thing that needs to stay hot.

Full pump speed? The primary loop return back into the boiler will always be far below 160F, and the heater throttles to the maximum, trying to keep the water at 160F -- so it then goes full tilt at 199,000 BTU, injecting heat into the primary loop at 96% efficiency continuously, but the primary loop doesn't go back to 160F (Because of full demand pool heating).

I turn off the pool pump? No problem; heater automatically throttles down or shuts off because it notices the primary loop goes back to 160F and stays there, since no heat user is wicking the heat away from the primary loop. It's not a pressure switch at all (like a pool heater). It doesn't know what the pool heat exchanger is doing; the boiler is simply trying to bring the primary loop water temperature back to 160F efficiently and intelligently -- and if there is no water being circulated through the pool heat exchanger, the primary loop temperature doesn't drop below 160F fast. I can also manually shut off the heater separately, but it will now just do intermittent bursts of heating just to keep the primary loop hot, so that pool heating can resume immediately later (whereupon the boiler will throttle up automatically again, when it notices primary loop water rocketing below 160F).

It is not a simple on/off cycling for this type of boiler, so there will be a continuum of efficiency-to-less-efficiency at all the different pool pump speeds. Unlike most pool heaters, it's a continuum rather than thermostat cycling. There will be an optimium setting (e.g. Max pump speed, max heating speed) which I would use whenever I'm doing long bursts of pool heating.

I will still need to keep the pool still when I'm not wanting to increase its temperature (e.g. I've reached target temperature, or if I'm not using pool). For that situation, I simply turn off the pool pump. The boiler automatically throttles down and/or shuts off (with intermittent cycling to minimum 10K BTU, the 'boosts' to keep the short currently unused 160F "primary loop" hot). If I am not using the boiler for an extended period (no shower/shed/pool for a few days) I can turn off the boiler. But I don't have to for the day-to-day operations, as those minor heating boosts of of an unused primary loop, allows quick response.

The achille's heel will be how efficiently the 160F primary loop is insulated, so I can get the ~80% pool water heating efficiency (after primary loop heating and exchanger losses). If I get that 80% efficiency, it's possible to heat pool water at approximately 2F per hour at the bottom end using this type of boiler. Which is roughly the same speed as my current heater -- I can pretty much live with that. 2F per hour for 20,000 gallons cold water and 1F per hour for 20,000 gallons for warm/hotter water. About the same speed as today. Might be a bit less, a bit more -- but this is the ballpark. The better insulated the primary loop is, the more efficient the exchanger is, and exchangers can hit the eighties (80% efficiency). I would pay roughly the same per degree F rise in pool temperature, if everything is made that efficient and well-insulated.

It can be much worse than a pool heater if poorly installed and set up. But it doesn't have to be that way; a well-done install can perform better than the non-forced-air pool heaters (despite the overhead of separate loops of water).

There will probably be an optimal pump speed for maximum pool heating efficiency, and it will probably be one of the higher speeds, but the slower speeds will not be much less efficient (it will simply be a function of how rapidly cold ground cools down water in underground pool pipes.... slower flowing water means more time for the water in cold ground....but at least this is heated water; much better than circulating the water while the pump is turned off)

The installer seems to know his stuff, so I'm researching given the triple-heating needs of the shed/shower/pool. Keep tuned.
 
I very much doubt your 199kBTU/hr hot water tank is going to heat the pool faster than a 266kBTU/hr pool heater especially considering there will be loses associated with pulling water out of the primary heating loop to exchange heat in a secondary, external heat exchanger. Of course, all that means is slower heating of the pool which may or may not be a bad thing.

As for bypass flow mixing, thermstating and water temperatures, you need to do more homework here. PVC is not designed to carry water at high temperatures. This is one of the reasons why pool heaters use a high-flow bypass - to keep the pool water temperatures from going above 120F. At too high a temperature several bad things can happen. Two of which are (1) PVC will start to leach vinyl chloride into the water at temperatures above 140F and thus become a source of organic contamination (this is why you want to use CPVC if you can to carry hot water) and (2) at higher temperatures your water is more like to become imbalanced relative to hardness and result in calcium scale and fouling of the heat exchanger. High temperature water can also put a lot of thermal cycling stress on PVC glue joints which may result in a leak developing. Finally, hot water can scald people, at an exit temperature of 120F, no one is likely to be injured by a pool heater turning on.

You certainly have an interesting concept here and I did read your previous thread on the winter pool operation. But I would caution you to be more careful in your analysis as there are good reasons why pool water equipment is designed to operate differently from residential potable water equipment.
 
Ok, now i get how youd be using it -- the boiler you're talking about is one of those condenser type efficient boilers.
(Mine is not...its old school with an indirect hwt.)

The only thing I know about condensing boilers is that hvac guys will warn they have about half the lifespan of a standard boiler. But at half of 25-30 years, if that brand is decent, that would still be a comparable longevity to the ED2, which lasted me 15 years ;)

At one point, I'd asked my hvac pal to work up a scheme to do whole house and pool geothermal, integrated, but we couldn't find a way to close the loop due to some features/obstacles of my lot (I have a few acres, but most of it is forest up a hill.) We'd talked about swapping out the boiler with a condenser type but he'd advised against it for our articulate setup given existing is working great.
 

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Sizing of the water-to-water heat exchanger is going to be important, and you're probably all over this aspect. It seems to me that the heat exchanger should be rated about the same as the boiler so you can transfer the full potential heat energy.

FWIW, I've had a fair number of years experience with centralized large boilers, small (house-size) boilers and small (house-size) condensing boilers. The small condensing boilers were scrapped after a few years due to higher maintenance costs and efficiency improvement less than what was suggested by the marketing. Under certain test conditions I'm sure they can achieve the gains, but with varying ambient conditions and heating loads the improvement did not measure up to the forecast. This was from greenhouse heating in a reasonably wide variety of configurations. Obviously anyone selling them will disagree, but that was my experience as of about 5 years ago, so I would also hope they're getting better. We were disappointed after making substantial investments back when natural gas prices were high.

I hope someone with this type of installation pops up so you get the feedback you're after.
 
I spent some time doing some math on pool heating. The pool is 20,000 gallon, so it requires 166,000 BTU to raise it 1F

We've been able to achieve between 1.5F-2F/hour when heating 35F to 55F and about 1F/hour when heating 75F to 95F.

I am now asking about upsizing the boiler, to ensure we have sufficient BTU's, if we go the boiler route.
 
my pool and tub use a boiler. nat gas fired one with a heart exchanger for pool and tub works real good. I run water temp at 170. 20 psi in the lines. I just run plain water. uses a 5 micron sidestream filter. heats the tub real quick and temp fluctuates only about 1/2 a degree if that. I use zone valves and temp probes to regulate water flow.
 
mdrejhon said:
I spent some time doing some math on pool heating. The pool is 20,000 gallon, so it requires 166,000 BTU to raise it 1F

We've been able to achieve between 1.5F-2F/hour when heating 35F to 55F and about 1F/hour when heating 75F to 95F.

I am now asking about upsizing the boiler, to ensure we have sufficient BTU's, if we go the boiler route.
Click to expand...
mdrejhon, your pool heater is very interesting. What kind of heat exchanger did you use? I was thinking of making a heat exchanger with CPVC and 1” copper pipe. Heating would be by a heated loop like you mentioned
 
abycat said:
my pool and tub use a boiler. nat gas fired one with a heart exchanger for pool and tub works real good. I run water temp at 170. 20 psi in the lines. I just run plain water. uses a 5 micron sidestream filter. heats the tub real quick and temp fluctuates only about 1/2 a degree if that. I use zone valves and temp probes to regulate water flow.
Click to expand...
Abycat,
What size heater exchanger did you use?
 
There are quite a few heat exchangers made that accept large pipe like pools. They have their place, and are readily uses in Europe. If you already have a boiler and its convenient amd close it could be ideal for this. Usually a 2 to 3k pool heater is easier than having to retrofit ad add piping and zones to an existing boiler, considering material cost then the additional extensive labor
 

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