# Water to water heat exchanger for hot tub



## James Ascherl (Jul 2, 2012)

I am going to hook my EKO up to my hot tub, but I am not familiar with heat exchangers. My hot tub will be outside next to my porch. I will have to run pex through the block wall under the porch and then through another block wall into the basement where the heat exchanger will be. I am assuming that I will need to insulate the pex under the porch and maybe install drains at the hot tub in the event that I will want to shut off the hot tub. Assuming my hot tub holds 300 gallons, what type and size heat exchanger do I need? Can I make one using maybe an old hot water tank? Any suggestions and tips would be appreciated.


----------



## ewdudley (Jul 2, 2012)

Here's some suggestions that may help, each situation is unique it seems, so it's hard to recommend much.

A hot tub is a nice load; small, low temperature, and steady, maybe 1000 to 3000 btu per hour, which is like a half a gpm with a 10 degree deltaT.

It looks like you have a small amount of space above your tanks up in the joists. You could tuck a small plate HX or sidearm up in there, or if you are lucky there may be a place under a cabinet or in a closet on the floor above. Then you could use gravity to move hot water from the top of storage to the HX and let the cooled water fall back to the bottom of storage, which eliminates one pump.

Then just run a bronze or SS pump through 1/2" pex over and back to the hot tub. I 'arm' mine with a timer for one hour out of three and use a PID controller to cycle the pump on and off so it can sense the tub temperature and slowly add heat as necessary while it is armed.

Permanently disconnecting the electric heating elements was very satisfying, as was running only 12 ga. wire for just the pumps instead of 6 ga. or whatever for the whole deal.

I just teed into one of the the jet pipes to pull to the HX, and then teed into the drain pipe for the return. You may need a zone valve if running the hot tub would pump water through the HX, or if there is even the smallest amount of elevation that could cause thermo siphon heating.

[Incidentally I ran my hot tub heating circuit as a primary secondary, with the primary circulating from the tub through a UV sterilizer and back to the tub, while the secondary injects heated water as necessary. We adjust for pH and hardness and that's about it.]


----------



## BoilerMan (Jul 2, 2012)

I"m inpressed with that Eliot!  Keep it up!  Gravity feed is the way to do if the elevation is there as EWD stated.  Fewer pumps+fewer watts+ fewer dollars spent ===== more smiles+more $$$ in the wallet+more enjoyment

Taylor


----------



## James Ascherl (Jul 2, 2012)

ewdudley said:


> Here's some suggestions that may help, each situation is unique it seems, so it's hard to recommend much.
> 
> A hot tub is a nice load; small, low temperature, and steady, maybe 1000 to 3000 btu per hour, which is like a half a gpm with a 10 degree deltaT.
> 
> ...


 Thanks ewdudley,
Although I pretty much understand your concept, I guess I was thinking something simpler. My thought was to tee into the hot tub line and run the pex into the basement HX which would be fed by a seperate zone from the eko designated for the hot tub. My thought was that the hot tub pump would circulate the water in and back out of the HX. The electric heater in the hot tub would make up for the times that the eko was down. Forgive me, despite lurking around this site for two years now, I still don't have a complete grasp of hydronic heating.


----------



## ewdudley (Jul 2, 2012)

James Ascherl said:


> Although I pretty much understand your concept, I guess I was thinking something simpler. My thought was to tee into the hot tub line and run the pex into the basement HX which would be fed by a seperate zone from the eko designated for the hot tub. My thought was that the hot tub pump would circulate the water in and back out of the HX. The electric heater in the hot tub would make up for the times that the eko was down.


The primary secondary thing is in order to incorporate the UV sterilizer, which I mentioned just in passing. It seems to be working nicely as a minimum chemical alternative, but I can't recommend it as anything other than an experimental approach.

For your situation, I think feeding a sidearm with thermo siphon directly from storage would be as simple as you please.

And perhaps more importantly it should pretty much eliminate the problem of storage mixing that you would have if the HX was fed conventionally with a pump (although this problem could be solved with small enough of a pump or some kind of flow regulator). I mentioned a plate HX earlier, but for thermo siphon feed, low flow, and plenty of approach to work with, a tube-in-tube would probably be the correct type of HX.

Using the hot tub's pump to circulate to the HX seems unworkable to me, first because it's an awful lot of power to spend just to get a few gpm through the HX, and second because it seems prudent not to try to modify the hot tub controls or to override the control logic to work with the HX. Tapping into the hot tub piping and using a tiny circulator that operates independently of the hot tub works quite well it seems to me.

--ewd


----------



## wood thing (Jul 3, 2012)

ewdudley said:


> The primary secondary thing is in order to incorporate the UV sterilizer, which I mentioned just in passing. It seems to be working nicely as a minimum chemical alternative, but I can't recommend it as anything other than an experimental approach.
> 
> For your situation, I think feeding a sidearm with thermo siphon directly from storage would be as simple as you please.
> 
> ...


      I've been wanting to do the hot tub thing for a long time, just have not gotten around to it.  Question for EW, doesn' t the hot tub pump have to run anyway for the jets and general circulation ?


----------



## maple1 (Jul 3, 2012)

I'm thinking there might be times that the hot tub won't be running, but you'd want it to be heating? That is, it might have to run a long time to heat up, before you get in it, if it only heated when running.


----------



## ewdudley (Jul 3, 2012)

wood thing said:


> I've been wanting to do the hot tub thing for a long time, just have not gotten around to it. Question for EW, doesn' t the hot tub pump have to run anyway for the jets and general circulation ?


Yeah, you're right the hot tub pump could do the job in most cases.

If I left my hot tub on all the time it would circulate two hours a day, according to the operator's book. (But I don't because it doesn't need to be on to run the electric heaters that aren't connected, and because I don't believe them when they say it needs to filter for two hours a day, and because I'm cheap.) I gather they require something like 30000 to 60000 btu per day, so that would be maximum 30000 btu per hour for two hours. DeltaT would be about 25 degF from 105 degF to maybe 130 degF, so only 2.5 gpm or so through a small plate HX, tankless coil, or largish sidearm.

I guess the main reason I went with an independent pump was because I was going to be running a little pump for the sterilizer a lot of the time anyways, so I just added another little pump to maintain the temperature.  

However if someone did want to feed the HX by gravity, using a separate small pump would probably be a better plan since the gravity feed would be more of a slow and steady approach.


----------



## BoilerMan (Jul 4, 2012)

How has the UV stabilizer worked as an expierement?  I have thought about this and wondered if it would cut down on some chemicals.  I guess cheap guys think alike, or is it engineering frugality.

Taylor


----------



## ewdudley (Jul 4, 2012)

Taylor Sutherland said:


> How has the UV stabilizer worked as an expierement? I have thought about this and wondered if it would cut down on some chemicals.


 
Well it was one year ago exactly today that it came on line. All I have is homeowner type chemical tests and my anecdotal observations; I need to take some samples in for bacteria tests but have failed to do so. It is used daily after showers by two well scrubbed adults.

We've filled it four times on three month intervals. The first two fills were with rain water from our 'rain water harvesting system', a.k.a. an underground poly-tank cistern. Although the cistern water is filtered pretty well, I think there must be a certain amount of organic matter that causes a tendency towards bacterial cloudiness unless controlled with bromines or chlorites, but it takes much less than the prescribed regimen.

The second two fills were from the old hand-dug well under the side porch. I tested the water and found it was spot-on for pH, hardness, and alkalinity, and it was nice and clear. I gave the well water fills a good dose of Clorox after filling just because, and they both remained very clear. My wife insists on having a 'brominating tab' kicking around the bottom, but the chemical tests never register any bromines, so I'm assuming the effect is negligible.

I run the UV circuit one hour out of three, which is about 3500 gallons a day; I just picked a number and went with it. It's a  Viqua "Sterilight S8Q-PA Silver Series 10 gpm UV System", it's a standard 'whole house' disinfection unit. This particular unit was selected solely because my local water treatment guy sells them and he vouched for its quality and serviceability. It sells for $380 and up retail.

Operating pressure is listed as 5-125 psig, and max flow rate depends on the minimum UV dose required, nominally a max flow rate of 7.7 to 10 gpm. The Taco 006 is probably pumping 8 gpm or less.

I don't think it could ever pay for itself in chemical costs avoided, it's just a little more pleasant. Just something interesting to try.


----------



## BoilerMan (Jul 5, 2012)

ewdudley said:


> Just something interesting to try.


 
My thoughts exactly.

Taylor


----------



## James Ascherl (Aug 7, 2012)

Ok, after a few weeks off due to a new baby, I am still working on my system. ewdudley, or anyone who can chime in, I was wondering if I could run my hot tub from an unused zone? I currently have 5 zones, but one is capped. The 4 house zones have zone valves (not pumps). I would run the copper across the basement to a sidearm HX (with the HX inside the house but close as possible to the hot tub). I would use a small pump to circulate the water through the HX as well as the hot tub. I would tap into the hot tub on one side of the hot tub pump and then the return on the other side of the pump. My thinking would be that I could bypass the hot tub pump and use the HX pump to circulate the water in the hot tub.
I still want the luxury of using the hot tub during the summer when my WB will not be used, so I don't want to mess with too much in the hot tub.

My hot tub is 350 gal. I plan on using 3/4" copper to the HX, but 1/2" pex from the hot tub. Any recommendations?

I am assuming I would need an aquastat on the HX so it would not kick on the pump if the HX was not up to temp?

Also, what is a PID controller? 

I hope everyone is enjoying their summer. This BB sure is quiet this time of the year!


----------



## webie (Mar 6, 2014)

Hi guys if someone could answer this that would be great . I got a 85000 btu stainless tube and shell heat exchanger . I  thought I had this all planned out till the guy that I got the heat exchanger from suggested that I be useing 1 inch pex for supply and return or at least a minimum of 3/4 .     My supply and return  is about 60 feet each way and the heat exchanger will be under the 500 gal. hot tub . I am planning on running the hot tub circuit as though it was another zone . Do you guys think 1/2 inch pex for supply and return to the heat exchanger  is big enough ? It is almost all inside and will be insulated . This guy thinks I wont get enough btu's and thinks I need  1 inch , I  think he is nuts .  What do you guys think ?


----------



## ewdudley (Mar 6, 2014)

webie said:


> Hi guys if someone could answer this that would be great . I got a 85000 btu stainless tube and shell heat exchanger . I  thought I had this all planned out till the guy that I got the heat exchanger from suggested that I be useing 1 inch pex for supply and return or at least a minimum of 3/4 .     My supply and return  is about 60 feet each way and the heat exchanger will be under the 500 gal. hot tub . I am planning on running the hot tub circuit as though it was another zone . Do you guys think 1/2 inch pex for supply and return to the heat exchanger  is big enough ? It is almost all inside and will be insulated . This guy thinks I wont get enough btu's and thinks I need  1 inch , I  think he is nuts .  What do you guys think ?


Is there a pump on both circuits through the HX, one over to the HX and another from the tub through the HX?

At any rate 120 ft of 1/2" PEX would max out at about 1.2 gpm (maybe 2 gpm) with a Taco 008 or comparable pump.  I would estimate you should be able to get a deltaT of at least 40 degF running 180 degF into your mondo HX with 104 degF entering on the other side, which all works out to a measly 30,000 btu per hour.  It would take about   250,000 btu to bring the tub up to 104 degF from 44 degF over an 8 hour period, but from then on it would only take less (possibly much less) than an hour a day to keep it hot.


----------



## webie (Mar 6, 2014)

Zone pump to feed the HX ( alpha Pump ) is used for all of my zones  . Hot tub pump is used for flow through the HX by way of Tee's before and after the original hot tub heater  . Call for demand is controlled by 120v zone valve with end points operated by original thermostat in hot tub  which also starts hot tub pump .


----------



## webie (Mar 6, 2014)

I also dont want to run into a overshoot problem so while were using it it gets way warm before it shuts down , a nice gradual warm up would be nice . the electric heater raizes the temp 6 degrees per hour  now.
Or would I be better off to run 3/4 and run a flow control ?


----------



## ewdudley (Mar 6, 2014)

webie said:


> I also dont want to run into a overshoot problem so while were using it it gets way warm before it shuts down , a nice gradual warm up would be nice . the electric heater raizes the temp 6 degrees per hour  now.
> Or would I be better off to run 3/4 and run a flow control ?


If my numbers are in the right ballpark, with 1/2" PEX your HX should deliver about the same btu per hour that you electric does now, so it should control well.  The hot tub pump will be moving a lot of water and mixing thoroughly, so the hot tub thermostat should have no problem preventing any sort of overshoot.

(You may want to review the 120 VAC zone valve idea along with any other sources of juice that could find their way to the tub.)


----------



## webie (Mar 6, 2014)

ewdudley said:


> (You may want to review the 120 VAC zone valve idea along with any other sources of juice that could find their way to the tub.)


 
 Good point .


----------



## BoilerMan (Mar 6, 2014)

I was just going to say that!

ANYTHING in contact with that hot tub water needs to be on a GFCI at the very least.  Personally I'd have a 24v ZV and it would be back at the boiler.  Use a RIB or some type of relay to fire it from the original electric hater's thermostat, or better yet make it al low voltage and isolated with plastic pipe.

And ground everything metal with a isolated ground, even the HX.

TS


----------



## webie (Mar 6, 2014)

I have been thinking about this and I think it will be fine the hx will be isolated by PVC pipe from the hot tub  and I will  ground the HX to the hot tub controller  box as  a safety along with put in a fuseable link to the 120v zv . The Hot tub is on a GFCI breaker already . Unless someone sees something seriously wrong  this should be about as safe as my 220 v pumps pumping water and my hot tub controller sitting on top of my hot tub element .


----------



## MarkW (Mar 7, 2014)

webie said:


> I think it will be fine the hx will be isolated by PVC pipe from the hot tub


Not isolated when it's full of water.  The components in the tub are designed to place electricity and water in close proximity while maintaining a good degree of safety.  Is that ZV so designed?  Not saying it's a negative in you're proposed set up but worth a bit of investigation. 
And though you've got a GFCI installed, I've gotten a jolt before as they sometimes do.  Not a fun thought with your "delicates" in that water.


----------



## BoilerMan (Mar 8, 2014)

I assume your hot tub is on a standard 2-pole 50A GFCI.  The threshold of a "household" GFCI is 5mA, this is enough as has been said to let you know it's there. 

Grounding is the absolute best option, the GFCI is just in case. 

Not at all trying to talk you out ofdoing what you are going to do, I'd do the same thing.  Just keep electricity at a minimum and don't trust the GFCI alone. 

TS


----------



## webie (Mar 9, 2014)

I took a look at th zv and its all set up to run a ground to it . Its only 6.5 watts . As a safety I will fuse the circuit with a very lite in line fuse . I super sized the exchanger for ease of plumbing . The unit that I am getting is a 300,000 . The reason is because the hot tub and its tub port are the same size at 2 inches . I originally was going to run all the water through the HX but was a little worried about flow restrictions so instead I am going to to TEE in and out of my plumbing and some flow should go through th HX.  PVC fittings should be minimal and a few feet of 2 inch PVC flex hose . Just wondering if there is a spcial cement that they use on this flex hose . I havent checked that out yet . I am going to run 3/4 for my boiler supply and return to near the hot tub but then reduce down to 1/2 because unfortunately I have to have the pex going on top of my tile floor . If after I have this going I dont have enough heat  I can alway up size the rest of the line easy enough . The other reason for the the increase in size of the HX is for use with my storage . We are planing on heating th tub all year round with this and will be useing storage for the heat source most times so supply temps could be way down to 130 . Were not worried about heatting the tub up at that point just maintaining the tub temps .


----------

