Washington Heat Exchanger for Spa

Discussion in 'General Discussions' started by Air Locked, Oct 2, 2020.

  1. Air Locked

    Air Locked Member

    My client has a Envision NDW100 that will feed both the PEX floor tubing and also heat a spa tub. Its 119gal Vaughn tank will feed a heat exchanger; the output loop will circulate spa water. The spa is 60+ ft away from the Vaughn tank. There's 2" HDPE pipe between them.

    The 8000 gal spa has a existing 4KW resistive heater that will be secondary to the GSHP.

    The NDW outputs about 26KW; I arbitrarily took 20KW as the available heat for the tub.

    The rub is the heat exchanger. The HVAC contractor installed a stainless Triangle Tube MH-80; it's rated at 26KW. But since the spa uses a salt system vs chlorine, Triangle says we must use a titanium model. (The client previously ran a company that manufactures seagoing hardware; he's well aware of stainless's limitations in salt water & titanium's benefits.)

    Further, Triangle's product line appears to be designed for boiler input, with a maximum working temperature 230F and maximum working pressure: 140 psi (primary and secondary). The NDW will be generating say 110F. So the Delta-T across the exchanger is not much.

    I think we need to find a heat exchanger designed for lower temp/higher flow input; one with far more surface area, and input flow equal to/exceeding the output flow rate.

    I seek pointers to suppliers who make such titanium exchangers.

    (I'm an EE vs ME, so if I've made some mistake in the above, please speak up. I've already reached out to my retired Thermodynamics professor but he's not yet gotten back to me...)
  2. geoxne

    geoxne Active Member Forum Leader

    They Make titanium models-

    But that is not the real problem.
    If you look at the Correction Factor Diagram A in the brochure above. If you want to heat a hot tub to 100F with 110F geo water (10F Delta T) you would need around 1o times the factory rated capacity (110F Delta T of incoming pool and incoming hot water). You would need to use multiple HXers piped in parallel reverse return.
  3. Air Locked

    Air Locked Member

    Thanks for that URL. Their brochure I was reading stopped before that page...

    I knew in my head there would be a performance hit but not how big it would be.

    I've been turning over rocks seeking a HX with a bigger coil, more flow, greater surface area. Not a lot of luck so far.

    What does parallel reverse return mean? I was assuming I'd parallel the inputs for more flow, and trying to model if I'd put the outputs in series.
  4. geoxne

    geoxne Active Member Forum Leader

  5. Air Locked

    Air Locked Member

    Ahh, I know of such but not the term. Thanks...

    On the URL you provided, I see they have a newer MF-350PT. I'm looking into its usability for this use as well.
  6. geoxne

    geoxne Active Member Forum Leader

    I have one experience with Geo W2W heating a pool. It was a retrofit from a dedicated propane gas pool heater it remains as a backup. The homeowner laughs every time the propane delivery guy drags his hose out only to find the propane tank is full. YMMV

    Please keep in mind your application is very different and needs to calculated and designed accordingly. The geo pool heat system mentioned above is powered by qty 2- 5 ton W2W units pumping directly and piped reverse return (at the heat pumps and HXs) to qty 2- MF-350PT HXs. The pool contractor also piped the poolside reverse return with a valved bypass to control pool water flow through the HXs. We did not use a buffer tank because we were confident the heat pumps could easily reject the heat produced to the mass of the pool. All this is possible because of calculated flow rates; properly sized heat pumps, pipes and pumps; calculated heat transfer rates and proper controls.

    You seem to be sizing the spa heating system on the capacity of your power source. This is bad engineering. I strongly suggest you calculate the the heat load of the spa under design conditions and move forward with a system design. You may find you can run your NDW100 (dual compressor) at 50% capacity (one compressor) and satisfy the spa load. This would significantly reduce the flow rate required to design around (think pump, pipe, HX etc.)
  7. Air Locked

    Air Locked Member

    It's not by choice. The 5 GSHP's are already installed and running.
    (The HVAC contractor seems to be of the 'back of an envelope' MO.)

    1 - DHW Production (Small NSW)
    2 - Floor Heat and Pool (NDW100)
    3- - Forced Air Cooling/Heating (7-Series)

    in the Mechanical Room, with another NSW and 7-series in the other end of the house. The client wants to use GSHP heat vs. the integral one, and bought the NDW with that goal. The NDW's 199gal tank feeds the floor manifold and shall feed the HX.

    In other words, I have n equations and n^2 variables.

    The HX the HVAC contractor provided is a MF-80, is stainless, has copper pipe stubs, and thus can't be used on a saline spa. That's my starting point; what HX should we use?

    My only data is: the Endless Pool holds ~8000gal. Its integral heater is 4KW and it requires NLT 18 gpm flow. I can only guess at the actual flow with its 1/12 HP pump. I'll have to spec a boost pump, both for a higher flow rate & to compensate for added head; the Mech Room is ~80 ft. of 2" from the spa.

    I calculated the 4KW will provide 0.2F/hr of temperature rise, the NDW (in theory) up to 1.2F/hr. I'm aiming for somewhere less than the 1.2 but far better than the 0.2F.

    I can't even guess at the heat loss of the pool while idle. It will have an insulated cover, and be in a solorium.

    The rub is: HX folks assume the source temp will be 100+F above the output temp. So they have smaller diameter inputs. I've been looking for a HX with near-equal-sized input and output, and lots of surface area; no joy so far.

    (Another issue: Do I have the HX next to the GSHP in the Mechanical Room, or put it next to the spa? The lines between run though inhabited space. I'm thinking spa water flow will be full of air and noisier than the hotter GSHP water. )
  8. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader


    We have built similar systems, with a NDW doing both floor heat and pool heat. See link above.

    There is nothing wrong with a contractor doing back of the envelope calculations, if he knows what he is doing.

    I have attached a simple pool heat loss calculator from Waterfurnace which you might find helpful.

    Yes, titanium is a must for the heat exchanger.

    For low pressure drop and low delta T you need a much larger heat exchanger than you think. Both will affect your efficiency.
    You simply take the largest model and move on,
    MF-350PT seems to be a good size. At 20F delta T you only have 20% of the rated output, which seems to be 70,000 BTUs/h, which seem to be about right.

    Bypassing the buffer tank with direct to load gives you another 15% efficiency gain.

    2" piping is good, 80' distance one way is not so good for pressure drop. Nor is the 1" port giving you a higher pressure drop. If you run it up to 22 gpm, your pressure drop for the heat exchanger alone should be about 30 ft/hd. Get an ECM pump, not a constant speed.

    Alternatively, take a larger heat exchanger with 2" ports on the heat side, or run only one compressor of the NDW.

    We get ours from "outdoor furnace supply" (eBay shop), they are in Middleport NY. Talk to Kyle.

    They sell outdoor wood boiler equipment, but they have a geo system from us to run their shop and office.....


    What do you mean no joy so far....challenges like this is where the fun starts!

    Attached Files:

  9. Air Locked

    Air Locked Member

    Thank you.

    We differ on this point. He's already made a major error buying a stainless vs titanium HX. Plus I interceded during the well installation to get him to upsize the manifold to house line sizes given the distances involved.

    We'll measure the actual loss once in service, but yes, we SWAG'ed it. The bigger question is start-up time, heating all 4000 gallons from domestic well water temperature.

    Yes, titanium is a must for the HX.

    I'm well aware of the effects of the low DeltaT and low flow rate. I'm still gathering data re: that.

    That puzzles me. I don't see how it would work. As installed, the GSHP heats the 119g tank for both floor and spa use; the thermostat on the tank cycles the GSHP. I don't know the minimum GSHP flow rate but it is clearly more then the floor flow rate. Would I have motor-operated valves isolate the tank when the spa signals demand?

    I'd LOVE to find a titanium HX with 2" input side ports; virtually all I have found assume the input is steam or refrigerant, and thus 0.5-0.75" input size; OK with significant Delta-T but not without.

    ECM? I have variable speed pumps; is that another term for same?

    They are on my list of resources; I'll be pursuing it when I get back to the office.

    "May you live in interesting times..."
  10. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    Well, indeed not using a titanium heat exchanger for a salt water pool is a major error. Not understanding pressure drop is is also a major error. So obviously this contractor has some deficiencies.
    What I said is that "There is nothing wrong with a contractor doing back of the envelope calculations, if he knows what he is doing." Thus in general it is not wrong to do back of the envelope calculation, if you know what you are doing, which was not the case here.
    I do it all the time. Well, not all the time, today I did not have an envelope...I used a napkin.

    Not sure why the start-up is the big issue. You have a heat pump which has more than 7 times the capacity of the 4KW electric heater.
    It takes 8.33 degree F to heat up a gallon of water by one degree. Lets say you are in the Washington DC area, and you have 60F cold water supply temps coming out of the ground. And you like it to be 105F. Makes 45F delta.
    8.33BTUs x 45F x 4000 galloons=1,499,400

    The NDW100 has roughly 82,000 BTU/h capacity at 32F source temp, so roughly 18 hours, to heat the water up, but not accounting for the heatless to the air during that time.
    But you said it is 8,000 gal above, if that is the case, it is twice as long, plus the heatless during the longer time.

    I was referring to low pressure drop, not the low flow rate. Low delta T means high flow rate, which means more high pumping power if the pressure drop is not low.

    So you run the floors and the spa at the same temperature? That is not very efficient. On warmer days the floors will cycle, being either too warm or cold.
    Also why would you not have a similar max flow rate for floor and HP when all zones are calling? You should be shooting for about 10 delta T which is about 7-8 gpm on the load side for the heat pump.
    I have attached a piping diagram, for conventional piping, is that how it is currently piped for the floors?

    Simple search for "titanium pool heat exchanger 2" " on amazon or e-bay.

    These are the bay specs, attached is a screenshot.

    Product Data:
    Constructed using all Titanium
    Output: 1,200,000 BTU/hr
    Boiler (Tube) Side: 2" FNPT Fittings
    Pool (Shell) Side: 2-1/2" FNPT Fittings
    About $1,500

    Here is the amazon link.

    ECM? I have variable speed pumps; is that another term for same?
    About $1,800

    Attached Files:

    Deuce likes this.
  11. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    Figure 2 is usually a conventional piping, Figure 3 is direct to load piping. That way you do not mix the hottest water in the buffer tank with the return water from the floors, but you send it as hot as possible directly to the floors. That we you can operate your heat pump 10F colder on the load site, which in Theory shall save you about 15% efficiency, in practice it is about 10% for efficient with a multi zone scenario.

    The key is to have a variable speed circulator which serves multiple zones, so your pumping increases when more zones are calling, and automatically more water bypasses the buffer tank. If lesser zones are calling, more water goes automatically through the tank.

    But you are running both the radiant system and the spa at the same temperature. Which reduces efficiency, since the minimum temps for the spa dictate the min temps for the radiant. If you separate the circuits you can use different temp settings, similar as to when you run DHW and radiant of the same heat pump.

    Attached Files:

  12. Air Locked

    Air Locked Member

    I'll look at this; I have a longer post to get out today with more questions.
  13. Air Locked

    Air Locked Member

    But that ~26KW available power will be limited by the spa heat exchanger's capacity, and is shared with the floors.

    One question TBD is: do we heat the spa 24x7 so it's always ready to use, OR do we have a rapid enough recovery time that we don't need to?

    The size of the HX (& its price) will be a significant factor in that question. Bigger input diameter equals less head loss, so smaller pump HP, less noise, lower power consumption. Lots of tradeoffs.

    re: HX
    I'm reluctant to buy a HX from eBay, etc, especially one where I don't have complete specs. (What's the surface area?) I've been looking for similar units from mainstream suppliers but no joy so far.

    Looking at your piping diagrams, I now see what you mean. But there's no guarantee that the GSHP will be running whenever the spa is being heated. If it's less load than the the GSHP output, the latter will cycle on/off.
  14. ChrisJ

    ChrisJ Active Member Forum Leader

    "Looking at your piping diagrams, I now see what you mean. But there's no guarantee that the GSHP will be running whenever the spa is being heated. If it's less load than the the GSHP output, the latter will cycle on/off."

    If the spa calls for reheat, it will draw from the tank until the temp lowers enough to turn on the heat pump.
  15. Air Locked

    Air Locked Member

    Yes, that was obvious, but the HX input temp will vary over time. Not a big deal; we plan to have multiple 1-Wire DS18B20 sensors on the whole system.

    That actually brings up a related question. I'm seeking sources of thermowalls for the 2" piping. I've found some PVC ones in 90's, and some stainless that will need a T to go inline.

    Any suggestions?
  16. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    You can prioritize on the spa for 1/2 hour, or one hour, the house usually has enough thermal mass not to significantly cool down.

    Can't help you with that one....

    Yes, with heat exchangers, bigger is better.

    Nothing wrong with eBay. They have a large distribution center in Lockport NY, and use eBay as a distribution channel. You can go to their website too. Same thing.


    The heat exchangers come from China, and are no name, but very reasonable priced, high quality, and do not cause any issues in the 8-10+ years we have used them. You expressed frustration that you could not find any with the large sports you felt you need, which I agreed on. You want more specs, just give them a call.

    If you have a larger heat pump, and an even larger heat exchanger, the heat will be transferred in the spa, as long as the spa's set point is not satisfied.
    There is no cycling as long as the heat exchanger transfers more heat that the heat pump can supply. With a 82,000 BTU/H heat pump, and a 1,200,000 heat exchanger, that is pretty much a given...
  17. Air Locked

    Air Locked Member

    That 1.2E6 BTUH/ rating is only when you have the spec's deltaT and flow rate. I found the enclosed spec sheet somewhere on their site. It says:

    *Nominal values are based on 60°C (140°F) temperature difference between incoming heating and heated water.

    But of course, we will have say 10°F between input and outout, and my SWAG is that factor alone will get us ~10% of the rated capacity. Further, it is rated at 30 gpm input flow & 224 gpm output. I may be able to get 30 GPM on the input, but the pump needed to get 224 gpm through the tub won't be practical. Further, that flow blasting on the user's leg will not be well received, to put it lightly.

    I'm not sure what gpm will be acceptable so my thinking now is a variable speed pump such as a Pentair IntelliFlo VS Pool Pump; set it at whatever flow is found to be acceptable.

    I'm reassured you have experience with those HX. I was actually looking at the other line they offer: https://www.outdoorfurnacesupply.co...at-exchanger-outdoor-wood-furnace-boiler.html as they can be opened and cleaned. Have you used any of them?

    Update: I got the data sheet on the above line of HX and attach it. It has specs for lower DeltaT, that's reassuring. But it expects 100 gpm though a 2" port. That's in the 12f/s velocity range; again the issue of making the user uncomfortable.

    Attached Files:

    Last edited: Oct 23, 2020
  18. Air Locked

    Air Locked Member

  19. Air Locked

    Air Locked Member

    Chatted with E.J. Bowman.
    [Side note: it now costs me 1¢ per minute to call them. I know I'm old, as I recall $5-10/minute!]

    Their response is attached.
    EC160-5113-5C-T HX <https://ej-bowman.com/products/swimming-pool-heat-exchangers/>

    It calls for a flow rate exceeding 50 gpm/5 fps; somewhere I read that is undersirable. It's a short loop, the pump & HX will be 10 ft from the spa. But I also wonder about that rapid a flow disturbing the tub occupant.

    Attached Files:

    Last edited: Nov 8, 2020
  20. Air Locked

    Air Locked Member


    Worrying about that flow rate, I asked EJW about upsizing to the next larger HX. His surprising response was that it would need more than 50gpm to deliver the same KW out. That,I'm having a hard time grasping....

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