Convertng hot tub to use wood boiler heat

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Gooserider

Mod Emeritus
Nov 20, 2006
6,737
Northeastern MA (near Lowell)
mpilihp said:
Hi what I ment about reversing the flow to flush it out is when I drain the tub to clean and replace the water I could hook a hose to it and blast water through the HX and clean it out.
Might work, I wouldn't put bets on it, as from what I've heard, the crud can take just as much water going backwards to get it out, as it took to get it in there in the first place.

Im trying to do this on the low budget, Im thinking a small HX to start with will work as after looking at how the existing heating of the water works is pretty feable. (see other post I did about how to hook up to the hot tub)

~ Phil

As I said, your call, but I still think you'd be better off w/ a sidearm - but I guess worst case if the FP doesn't work is that you'd need to replace it later...

Gooserider
 

Gooserider

Mod Emeritus
Nov 20, 2006
6,737
Northeastern MA (near Lowell)
I decided that it would be better to have one thread for this project rather than a bunch of different ones, so I have merged the three threads into one - it looks like it might have mixed up the continuity a bit, but it keeps all the discussion of the project in one place, and makes it easier to keep track of...

Gooserider
 

Gooserider

Mod Emeritus
Nov 20, 2006
6,737
Northeastern MA (near Lowell)
mpilihp said:
Ok its a nice day so I figured Id open up the hot tub maint panel and see how I might be able to connect up my lines to heat the hot tub via the wood boiler. What I found kinda shocked me, the only power line leaving the control panel is to the pump, nothing else. Look close at the pump, on the body of the pump there is this curved white plastic thing with two braided clear lines connected to it (can only see one connection) One of the clear braided lines goes to the base of the pump on the left side by the big pipe ive labeled INPUT. The second clear braided line goes to the pump housing as well by the red pipe which is on the OUTPUT side of the pump. (both other ends of the braided lines on the pump body are on the back side so you cant see them)

I think this hot tub is ONLY heated by the excess heat the pump generates and no electric element! No wonder the pump runs all the time and for a long time, it has to get the pump body heated up before it generates heat I think.
hottub-pump-overview.jpg
Sounds like it might be the case, I have heard of tub setups that essentially use the motor heat to warm the tub. Essentially that plastic thing is a water jacket, a small amount of the pumps output water is diverted from the output, sent through that water jacket and back to the input side. The claim is that by getting rid of the separate heating element they save energy, as the pump should be running a good bit of the time anyway to keep the water mixed. However with this kind of setup, you might be limited in how much energy you save by going to wood since you will need to circulate the water anyway. Essentially right now the tub company would tell you that you are getting the tub heat for "Free" as there is no separate heating element, and you need to run the circulation pump anyway...

It MIGHT be possible to put in a second smaller pump that just circulated the water in the tub and HX and spent fewer Watts in moving the water around, but if doing that then you'd want to get a very low flow resistance HX so as to allow the smallest tubside pump possible.

So I have two options I think, the first is to disconnect one of the clear braided lines on the pump body heater and splice in my HX heating loop inline with the pump body heater. Issue with this is I dont know if the pump is NOT running if water will flow through my wood boiler HX heat loop (note there will be a laing plastic pump in the wood boiler hx heat loop) The connections are on the other side of the pump body are are right on the pump, not on the pipes going to it......
hottub-pump-connect-HX-B.jpg
That would sort of work, but you'd have very limited flow, due to both the small line size and the restriction of the plastic HX... It would also only work if the main pump was running, which kind of defeats the purpose of doing the boiler connection.

It doesn't matter what you do on the tub side however, if you are wondering about flow on the boiler side of the HX - the HX is a WALL between the two systems, and while you may throw some BTU's over the wall, the flow on one side won't have ANY influence on the flow on the other... In terms of BTU flow when one side of the HX isn't flowing, unless you have flow from thermosiphoning, the temperature on the two sides will rapidly balance, and you would have no BTU transfer since there would be no temperature differential to drive it.

My second option is to use the drain connection on the PUMP FEED pipe and have that go to my heat loop and connect the return from my heat loop into the plastic pipe for the Jets on the output side. Potential issue with this is when the hot tubs pump runs will its high pressure pump water backwards in my heat loop back to its input or will the running Laing pump prevent the backwards flow? Or would just putting a one way valve in the heat loop prevent this?
hottub-pump-connect-HX-A.jpg
That would NOT work, or at least not when the main pump is running, as you have the two pumps "fighting" each other. You would also get a "short circuit" flow path through the tub pump when it wasn't running. However if you put a one way valve on EACH line, and set up your controls to only allow one of the two pumps to run at a time, then it would be OK, and essentially what I suggested a few lines up...

Which of these two options do you think is the best one? THE first option would be the easiest for me and least disruptive to the hot tub incase we decided to sell it.
I would go with the second option (but using a sidearm HX as mentioned earlier) I don't think the first option would give you enough flow to be worth while...

Also, does anyone think I shoudl also put in an actual electric heating element instead of the plastic pump body suto hx thing? Id think an actual elec element would be more efficent than this for when we are not running the wood boiler.
I would not mess with the pump HX. Remember that at the same time it's heating the tub water it is cooling the pump motor... What I don't know is how important that cooling is to the pump motors life span, and while it may not be the most efficient heater, it is "free" heat that you get whenever the main pump is running - like when you are in the tub and wanting the jets at full power...

It might make sense, but it would take some careful measurements and calculations to be sure, to put an electric element in the same loop as the boiler HX, assuming you were doing a setup like your second choice - then you could run the small pump and the element rather than the big pump... The question though would be which approach would give you the lowest operating cost, and even if it were the small pump, would the payback be worth it?

Gooserider
 

mpilihp

Feeling the Heat
Apr 22, 2008
429
Coastal ME
Hi thanks for the comments, alot to respond to.

Hot water heater idea, my DHW is in an indirect HW tank and when the wood boiler isnt running its heated by oil which I am trying to eliminate burning oil as much as possible, but it does pose an interesting idea if I could limit it from using it in the summer when Im burning oil. A project I am currently working on is hooking up my second indirect DHW tank and second 3-way valve to pre heat the DHW to give us longer times between heating it via the wood boiler during shoulder seasons.

As for the pictures and ideas on how to actually hook into the hot tub, the wood boiler heating loop will have a small low flow pump always running flowing water through the HX or Side arm. In that loop will be a digital Ranco aqustat on the rec side of the hx coming from the hot tub. When that temp drops below say 103 it will kick on the circulator on the heating system side of the hx IF the wood boiler is running. The existing heating control/sensor in the hot tub will be left as is and set to a lower temp, say 101 (not sure of the values) but the idea being if the wood boiler isnt on once it gets down to 101 then the hot tub would heat itself the way it normally does,

Between 1st and 2nd option, I agree I dont think there would be enough flow through the plastic HX on the pump and I had not though that its probably used to cool the pump motor. So I think I will go with option 2 with as you said with one-way valves on both of the external heat lines.

My new question is this, in the picture the piping on top of the red pump connection where I showed to insert a T, what kind of hose is that and how do you make connections to it? It looks like they are GLUED into the T to the pump. Im thinking on that side I would need to Cut in and glue in a T. THe other side would be easy as I can just connect onto the drain.

~ Phil
 

Gooserider

Mod Emeritus
Nov 20, 2006
6,737
Northeastern MA (near Lowell)
I could easily be very wrong, but that looks like the pictures I've seen on Troublefreepool.com of what they call "flexible PVC" hose. Never worked with it, but my understanding is that it takes the same glue and fittings as regular PVC, and is worked the same way... I think I see some markings on the pipe, which might give you a clue as well...

If that doesn't work, I'd take the picture over to troublefreepool, and ask them over there - they are nice folks and I'm sure they would be able to ID the pipe very easily and tell you what you'd need to work it. They are also good folks on dealing with pool chemistry. Since I started following their advice on taking care of our pool, I've never had my water looking so clean, and at the same time I'm spending a LOT less money, and much less effort on taking care of the pool... I would definitely say they are a rather damp equivalent to the hearth...

Gooserider
 

mpilihp

Feeling the Heat
Apr 22, 2008
429
Coastal ME
Hi thanks, I think I have a plan, now I need to prep my system for connecting this to it, good summer project,

~ Phil
 
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