# How to Size Hot Tub Heat Exchanger



## CT-Mike (Dec 27, 2010)

So we have a 525 gallon ot tub that is typically kept at 104 F during the winter. How do i figure out what size heat exchanger I need? What other items do I need for controls, etc?

Thanks,

Mike


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## juddspaintballs (Dec 27, 2010)

I'm sure there's fancy calculations to figure it out, but I can get water temps above 104 degrees out of my water heater's side arm exchanger using a Taco 003 stainless circulator on the domestic side.  I have a ceiling fan control to keep the speed turned down so it's flowing about 1-2 GPM.  I have unlimited hot water through the sidearm exchanger like this.  I would imagine that a fairly small shell and tube exchanger would keep your hot tub at 104 degrees so long as the hot tub's pump is moving water through the heat exchanger to speed up the process.


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## varna (Dec 27, 2010)

CT-Mike,
I heat my hot tub and love it. To save time, go here    http://outdoorwoodfurnaceinfo.com/forum/index.php?topic=177.0
and see my post .....reply #5.


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## bigburner (Dec 27, 2010)

I got a SS shell and tube for my 12,000 gal indoor pool. it's set up where the 1.5 inch pool pump lines run threw the middle and the 3/4 inch side ports are connected to a secondary [pump 007] of the primary loop. auquasat cycles on temp if the pool pump in running. I only run it 4 hours a day and pool is in the 80's. Works good!!


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## Bad Wolf (Dec 27, 2010)

CT-Mike  I shamelessly stole my design from nofossil.  I used about 14' of 1/2" copper pipe folded back and forth in a 2 foot section of 4" PVC pipe that is inline with my electric heater.  
The relay that once turned on the heater now turns on a circ pump just like an extra zone. When the boiler is at temp (170) it only comes on for a couple of minutes each time.  If I'm running on storage it take longer but I have no trouble keepin the tub hot. I keep the tub at 103 and I figure it saves me a good $600 - $800 per year.
Matter of fact I'm going to go shovel it off when the snow stops and we'll go sit in and admire the snow we just got.  Or maybe not, its pretty windy out there.


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## lawandorder (Dec 27, 2010)

Maybe a stupid question but does adding a hot tub to your boiler zones use a significant more wood?? I was wondering how much more is needed to add a hot tub to a system and wether its cost effective


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## Nofossil (Dec 27, 2010)

lawandorder said:
			
		

> Maybe a stupid question but does adding a hot tub to your boiler zones use a significant more wood?? I was wondering how much more is needed to add a hot tub to a system and wether its cost effective



I'd say it varies a lot based on how big a hot tub, how exposed to the wind, how good the cover is, and so on.

But if you wouldn't want to heat your house with electricity, why would you want to heat your hot tub that way?

Wood is MUCH cheaper than electricity most places. Heating your hot tub with wood has a fantastic payback in terms of $ per log.

I seem to remember that ours runs around 30,000 BTU per day, but don't quote me on that.


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## lawandorder (Dec 27, 2010)

So next years project was to look for a hot tub to add to our system.  Anyone have suggestions on what to look for as far as efficiency is concerned and some ideas on critical options to have that make it more wood boiler friendly?? Wood a Tarm Solo 40 have enough output to handle the extra btus??


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## leaddog (Dec 27, 2010)

I have my tub hooked to the boiler. SAVES alot. also have a GOOD cover (4in foam) and also make or buy a foil/bubble/bubble/foil floating cover as that will really help keep the water warm.. I made mine using fbbf I got at menards. I've also used swimming pool cover cut to fit but it would only last about 6 months as the heat would make it breakdown. But it really saves on evaporation. 
I hooked up the heater sw to turn on a pump when it needed heat. 
leaddog


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## Nofossil (Dec 27, 2010)

Just based on my experience:

I think it's a lot easier to bypass the built-in heater if that heater is a separate device that has simple plumbing connections. Apparently some hot tubs integrate the heater with a circulator or some such.

Ours has a very small (1/20 hp) pump that runs 24/7. That's perfect - we just plumbed it to draw through our HX. We wired a relay with a 220vac coil in place of the heating element. The relay contacts drive a zone valve which circulates hot water through the HX. That's all there is to it in our case.


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## bpirger (Dec 28, 2010)

Leaddog...a quick question.  Does your floating cover literally just float on the surface of the water to reduce evaporation?  What do you do with this when you jump in the tub on a cold, windy night?  Do you just set it on the cover and then through it back on the surface of the water when you get out?


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## in hot water (Dec 28, 2010)

Insulation and more insulation around the tub.  My sister had one of those Coleman hot tubs, outside in SLC, Utah.  .  I recall a 6" foam cover and at least that all around the tubs.  The energy consumption was very low, electric only on that tub.  The cost to run it varied depended on how many hours it was uncovered and in-use.  The bubblers would cool it off quickly also as it forced cold outside air into the tub, and increased the evaporative loss.

Foam under the tub is another place to watch the details, basically you want a foam block with a puddle of water inside, to limit heat loss.

hr


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## Justburnit (Dec 28, 2010)

I have a Sundance Cameo that is heated by the boiler during the winter. 550 gallons or so. 
I use a Grundfos 220V stainless circ that pushes tub water through a 1/2 pex loop in the storage tank. The loop is about 80' in the tank and about 140' run to the tub that is exposed in the basement. I used a DPDT switch to select the electric heater or the circ.
I quickly learned I had to add a flow check in this loop so the tub would not overheat since the tubs little 3rd pump was moving water through the loop. Thermosiphoning I expect. I am surprised how short the run cycles are, This is my first season with this setup, the system works great. At this time I am seeing a $30-50/ month reduction in the power bill and we are not fully into the cold season yet.

As for the original question about sizing the heat exchanger, my opinion is use what you can. The duty cycle will vary depending on the efficiency of what you build but if people are having good luck with sidearm exchangers than most anything will get the job done.
Let the tub control the heat as it would with it's electric element.

I had a bubble cover on the tub for awhile, the ozonator kills them.


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