HX for Hot Tub?

[DenaliChuck]

I have a 250 gallon Snorkel brand wood fired hot tub that I want to heat with my boiler. http://snorkel.com/index.php

How many feet of copper tube would you suggest to load the hot tub? What diameter tube?

Any other HX suggestions (truck radiator etc?)

What diameter pex would you suggest to supply the HX? I've only got a 15' run to make from the Tarm or storage.

Thanks!

 

[BrownianHeatingTech]

I have a 250 gallon Snorkel brand wood fired hot tub that I want to heat with my boiler. http://snorkel.com/index.php

How many feet of copper tube would you suggest to load the hot tub? What diameter tube?

Any other HX suggestions (truck radiator etc?)

What diameter pex would you suggest to supply the HX? I've only got a 15' run to make from the Tarm or storage.

How much do you want to heat it, and how quickly?

For example, if you want to be able to heat it from 40 to 100 degrees, it will require roughly 125k btu's. If you want to do that in an hour, it will take 125kbtuh. If you want to do it in two hours, it will only take 62.5kbtuh. If you are in a hurry and want it heated in half an hour, it will take 250kbtuh (more than your Tarm can supply).

If you plan to heat it up using the submerged woodstove, and then only worry about using the boiler to keep it warm, the requirement will be much lower, as you will only need to offset heat loss - in which case, the location of the tub (indoors or out), and what you have for insulation will determine the amount of energy needed to offset the heat loss.

By the way, how do you like the tub? I've been thinking of getting one of those - a modern fiberglass tub just doesn't seem right in an old farmhouse, for some reason...

Joe

 

[Nofossil]

I have a HX that I made from 6" PVC with 16" of 1/2" copper inside it. I have it plumbed as a zone in my system. It can raise the temp on my 550 gallon tub by about 8 to 10 degrees per hour.

 

[DenaliChuck]

Thanks Joe! I just couldn't get my brain around how to start solving the problem and your post helps a lot!

We'd like to have it go from 40 to 100 in 2 hours, so 62.5 btuh.

Would you recommend a coil of soft copper tube, or possibly fin tube? Can you point me to heat transfer estimates for different sized soft copper?

Do you think having copper in the tub will be a problem since the water will not be ph balanced?

Maybe a plate hx would be a better choice?

How about controls? I'd like to be able to have the Tarm/storage maintain the tub at 40* but we can manually "flip the switch" to bring it up to full temp but would like the controls to automatically stop the heating process when it is up to temp..

Regarding the tub, we love it! We've had it outside and have used it mostly in the winter for the last 11 years. There are no chemicals to buy, add or soak your body in which is one thing we really like. We also love the cedar smell and as long as the bodies that go into the tub are clean, the water will last for a couple of weeks. Cleaning is easy, just drain, squirt all surfaces with a mild bleach solution, rinse and refill.

The only downside has been heating the tub with the wood stove. When we first got it, we were seasonally employed and had the time to fire it up. It takes roughly 5 hours to get it to temp using white spruce or white birch, and that requires tending it every 30 min or so. Now that we're employed year-round, and have one (soon to be two) kids, we just don't have the time to tend it. We're looking forward to the Tarm doing the job!

It is amazing how long it takes for the tub to begin freezing in -20 degree weather with only 2" of blue foam floating on the top! We'll never go fiberglass with the Tarm on the job!

 

[BrownianHeatingTech]

Thanks Joe! I just couldn't get my brain around how to start solving the problem and your post helps a lot!

We'd like to have it go from 40 to 100 in 2 hours, so 62.5 btuh.

Would you recommend a coil of soft copper tube, or possibly fin tube? Can you point me to heat transfer estimates for different sized soft copper?

Do you think having copper in the tub will be a problem since the water will not be ph balanced?

Maybe a plate hx would be a better choice?

Personally, if I was doing an immersed heat exchanger, I'd go with stainless. Corrugated stainless tube (like is often use for gas piping) is available bare (as opposed to the plastic-jacketed gas piping). Reduces the corrosion worries, as copper will dissolve if the pH gets out of whack.

For a pumped heat exchanger, a plate or a shell-and-tube (purchased, or built from scratch, like nofossil's) would be the way to go.

How about controls? I'd like to be able to have the Tarm/storage maintain the tub at 40* but we can manually "flip the switch" to bring it up to full temp but would like the controls to automatically stop the heating process when it is up to temp..

Sure. Depends upon how fancy you want to get, but a simple switch and two controls is probably easiest. There are "construction thermostats" which are an encapsulated thermostatic switch, permanently set at a given temperature - the intended purpose is to keep contractors from changing the temp setting during construction and wasting energy when there are windows still missing, and such. That would suffice for the 40-degree setting. A hot tub control might be the easiest way to control the temperature accurately, although something like a Tekmar setpoint control could be used. A standard aquastat probably doesn't have accurate-enough control for this application.

You'd just need a switch to disable the higher control - if the higher control is closing its contacts, the minimum-temp control doesn't matter. I'd probably add a manual high-limit control in series with the signal from the others, so a failure could never take the water up to unsafe temperatures.

Regarding the tub, we love it! We've had it outside and have used it mostly in the winter for the last 11 years. There are no chemicals to buy, add or soak your body in which is one thing we really like. We also love the cedar smell and as long as the bodies that go into the tub are clean, the water will last for a couple of weeks. Cleaning is easy, just drain, squirt all surfaces with a mild bleach solution, rinse and refill.

The only downside has been heating the tub with the wood stove. When we first got it, we were seasonally employed and had the time to fire it up. It takes roughly 5 hours to get it to temp using white spruce or white birch, and that requires tending it every 30 min or so. Now that we're employed year-round, and have one (soon to be two) kids, we just don't have the time to tend it. We're looking forward to the Tarm doing the job!

It is amazing how long it takes for the tub to begin freezing in -20 degree weather with only 2" of blue foam floating on the top! We'll never go fiberglass with the Tarm on the job!

Good to know. We're looking at an indoor install, so the wood stove isn't really an option. Spending a grand or more on a chimney just for it would not make sense for us, especially with the option of zoning it off the boiler.

Of course, we need to build the addition (which will house the boiler room and the hot tub) before we can do any of it...

Joe

 

[DenaliChuck]

The corrugated stainless sounds like the way to go since I wouldn't need another pump. How many feet, of what diameter tube would be needed for 60-80 btuh?

An indoor Snorkel would be perfect. Zone it off the boiler and you wouldn't have to buy the stove; I'll probably be removing my stove since the Tarm is much more efficient, easier to operate, and we'll be running the Tarm all summer for DHW anyway. They are beautiful and would be a compliment to the interior to your addition.

 

[Nofossil]

For controls, I simple put a relay with a 22vac coil in place of the original heating element. Draws a LOT less power ;-)

The relay contacts act exactly like a thermostat and open the zone valve for the hot tub HX zone.

 

[DenaliChuck]

Nofo, where is your shell and tube hx located? Near the tub or in the house?

I've been concerned with moving tub water through a plate hx or shell & tube hx since the Snorkel doesn't use chemicals and it would be hard to clean with a squirt bottle of bleach solution.

I'm impressed that only 16" of copper can increase the temp that much! I wonder if a DHW preheat could be done with a loop of copper through a bung cover in one of my propane tank 1 1/4" ports.

 

[DenaliChuck]

For controls, I simple put a relay with a 22vac coil in place of the original heating element. Draws a LOT less power ;-)

The relay contacts act exactly like a thermostat and open the zone valve for the hot tub HX zone.

The Snorkel tub is WAY low tech...just a wood barrel and submerged aluminum wood stove. No heating element here!

 

[Nofossil]

My hot tub hx is right next to the hot tub.

I just drained and refilled my hot tub, and built my first fire since April to heat the thing. Here's some data early in the burn:

Hot tub HX temps -

boiler in: 164
boiler return: 128

Hot tub in: 64
Hot tub return: 85

At this point in the burn, the boiler's recirc valve is cycling to keep the inlet above 135.

This graph shows life from the boiler's point of view - I don't have instrumentation on te hot tub HX.

 

[BrownianHeatingTech]

The corrugated stainless sounds like the way to go since I wouldn't need another pump. How many feet, of what diameter tube would be needed for 60-80 btuh?

Well, it's not just the number of feet, but also the temperature that you want to be able to operate at. Heat transfer in fluids is a logarithmic function, so the closer the storage tank water gets to the temp of the hot tub water, the less heat transfer will occur. The larger the heat exchange surface, the better the transfer. In other words, it never hurts to have a larger heat exchanger.

Sizing for immersed coils is never easy, because the coil shape, restrictions to flow, and other factors can all change it dramatically. I'd say that the simplest thing would just be to buy a 50-foot roll, since it tends to be sold that way. 1" would transfer more than 3/4", but not bend as tight, so it would depend upon where you wanted to install the coil. My thought was a coil around the entire tub, against the wall, near the bottom (but a few inches up). I would going to construct benches to go over it, with slats underneath to protect feet from accidental contact with the coil. Nicely spread out like that, the transfer will be better than if it is all bunched in a tight coil, replacing the woodstove and using the "fence" to keep folks away. Of course, it will require more carpentry to protect feet from the hot coil. On a large coil like that, 1" would be fine.

An indoor Snorkel would be perfect. Zone it off the boiler and you wouldn't have to buy the stove; I'll probably be removing my stove since the Tarm is much more efficient, easier to operate, and we'll be running the Tarm all summer for DHW anyway. They are beautiful and would be a compliment to the interior to your addition.

Yeah, there's no place for a wood boiler (or any boiler) in this house. The fieldstone basement is never dry, so I wouldn't put a nice new boiler down there to be rusted-out by the humidity. So, if I'm building space for a boiler next to the house, I might as well put some extra room in there for the hot tub. Just have to build really well, to keep the humidity away from the house and the boiler. The addition will be attached to the sun porch, so that will act as an "airlock" between it and the house, anyway - just have to use all exterior-grade doors and construction.

Joe

 

[Nofossil]

Update: with the hot tub as the only load on the boiler, it gained 14 degrees (64 to 78) in the first hour. That's about 64,000 BTU/hr. Not bad for a 16' heat exchanger. I think part of what makes it work is that the hot tub has a circulator that keep water really moving around the copper pipes.

 

[BrownianHeatingTech]

Update: with the hot tub as the only load on the boiler, it gained 14 degrees (64 to 78) in the first hour. That's about 64,000 BTU/hr. Not bad for a 16' heat exchanger. I think part of what makes it work is that the hot tub has a circulator that keep water really moving around the copper pipes.

Yeah, the pumped flow (instead of natural convection) gives much better performance from a given area of heat exchange surface.

What was the boiler temp at that time?

Joe

 

[Nofossil]

Yeah, the pumped flow (instead of natural convection) gives much better performance from a given area of heat exchange surface.

What was the boiler temp at that time?

Joe

The boiler outlet was fluctuating between 160 and 170, maybe 175 for brief periods. two and a half hours now, and it's gained 30 degrees.

 

[BrownianHeatingTech]

What was the boiler temp at that time?

The boiler outlet was fluctuating between 160 and 170, maybe 175 for brief periods. two and a half hours now, and it's gained 30 degrees.

The heat transfer efficiency should drop dramatically if it were running off storage, at a lower temp difference between the heating system water and the hot tub water.

Joe

 

[Nofossil]

The heat transfer efficiency should drop dramatically if it were running off storage, at a lower temp difference between the heating system water and the hot tub water.

Joe

It does, just as you'd expect. In fact, I rarely even try to heat the hot tub from storage. In general, we use it in the evenings. During winter, the boiler's running most evenings.

I've considered a solar panel for the hot tub to keep it warm during the shoulder seasons, but there's too much freeze damage risk.

 

[DenaliChuck]

Well, it's not just the number of feet, but also the temperature that you want to be able to operate at. Heat transfer in fluids is a logarithmic function, so the closer the storage tank water gets to the temp of the hot tub water, the less heat transfer will occur. The larger the heat exchange surface, the better the transfer. In other words, it never hurts to have a larger heat exchanger.

Sizing for immersed coils is never easy, because the coil shape, restrictions to flow, and other factors can all change it dramatically. I'd say that the simplest thing would just be to buy a 50-foot roll, since it tends to be sold that way. 1" would transfer more than 3/4", but not bend as tight, so it would depend upon where you wanted to install the coil. My thought was a coil around the entire tub, against the wall, near the bottom (but a few inches up). I would going to construct benches to go over it, with slats underneath to protect feet from accidental contact with the coil. Nicely spread out like that, the transfer will be better than if it is all bunched in a tight coil, replacing the woodstove and using the "fence" to keep folks away. Of course, it will require more carpentry to protect feet from the hot coil. On a large coil like that, 1" would be fine.

Joe

Based on Nofo's advice I'll plan to heat the tub with the boiler, so there would be a good temp differential (~70*) and for now I think I'll try a 3/4" x 50 section broken down into 2 or 3 coils behind the fence. The tub develops a lot of stratification and by supplying the hot water to the bottom of the tub on the one side, I might be able to generate an "upwelling" that would mix the tub.

Thanks for the advice!!

 

[steviep]

Nofossil could you show us how you built your hot tub heat exchanger. I have a hot tub that has been sitting idial for a couple of years . You guys have got me thinking about hooking it up again.

 

[Nofossil]

Nofossil could you show us how you built your hot tub heat exchanger. I have a hot tub that has been sitting idial for a couple of years . You guys have got me thinking about hooking it up again.

I didn't take pictures as I was building it. No picture, so a a thousand words will have to do...

I used a section of 6" PVC about 30" long. I solvent-glued a cap on one end and a threaded cap on the other. I drilled and tapped through the sides top and bottom for 3/4" npt fittings for the hot tub connections.

I drilled and tapped two holes through the end of the removable cap for 1/2" npt. I took two 1/2" npt copper sweat fittings and drilled them out so that 1/2" copper would slide through.

I slid 36" pieces of 1/2" copper pipe through the fittings about 26" and soldered them together. I then threaded these into the removable cap. I took 6 pieces of 1/2" copper 24" long with a pile of 1/2" sweat ells and street ells, and soldered together a heat exchanger connected to the two original pipes. Something like this: Pipe comes through cap, continues for 26". Elbow and street elbow makes 180 degree turn, another pipe goes back up 24", another 180, back down 24" and so on.

The whole exchanger fits inside the 30" PVC, and the cap screws in and makes a hopefully watertight seal.

Connect 3/4" brass hose barbs to the sides and plumb in series with the little bitty circulator that runs 24/7 so that hot tub water at very low pressure is circulating through the PVC pipe.

Connect the 1/2" copper to a heating zone valve and the boiler return. Life is good. Almost good. The 6" plastic pipe threads that are supposed to seal don't do that as well as you might hope. Lots of goop solves the problem, but makes disassembly with anything less than a sawzall a total impossibility.

Here's a photo of the assembled unit.

 

[steviep]

Guys my hot tub is outside about 5 feet from the house. I have seen some people install thier exchanger in the cabnet of the hot tub, and some people move the heater inside the house. What idea do you guys like.

 

[Nofossil]

Guys my hot tub is outside about 5 feet from the house. I have seen some people install thier exchanger in the cabnet of the hot tub, and some people move the heater inside the house. What idea do you guys like.

Heater inside the house. On mine at least, theres a small circ that runs 24/7 in the hot tub. That would keep water moving through the HX lines and prevent freezing.

 

[BrownianHeatingTech]

Guys my hot tub is outside about 5 feet from the house. I have seen some people install thier exchanger in the cabnet of the hot tub, and some people move the heater inside the house. What idea do you guys like.

The decision depends upon the type of heat exchanger you use.

If it's a full-flow, low-head heat exchanger (like a shell-and-tube), you won't want the heat exchanger in the house, since that means running full-diameter PVC back and forth.

If it's a flat plate or other bypass-type heat exchanger (where you only siphon off part of the flow, heat it, and return it), then you'll be talking about smaller pipes, and location will not matter much.

Putting the heat exchanger outside runs the risk of freezing your heating system piping (can be mitigated by proper insulation and burial of the pipes, or use of non-toxic antifreeze). Putting the heat exchanger inside runs the risk of a leak siphoning your hot tub's contents into your basement (can be mitigated by locating the heat exchanger higher than the tub and/or using anti-siphon devices).

There's no one right answer, and you'll have to balance the benefits/drawbacks for your personal situation.

Joe

 

[603doug]

I had a snorkel stove in a my tub of about 450 gals and would drain it at least once a week and it would take 8 hrs to go from 49 f to 105 f. Bought a wooddoctor converter and hooked everything up last week. I used a shell and tube 70000 btu exchanger and it took about 8 hrs to heat. Now I hooked up my fast fill so it goes through the plate exchanger then the shell and the water was 95 f in the tub. Cost about $12000 to lose the snorkel stove but I guess I will heat the house too

 

[sdrobertson]

Bought a wooddoctor converter and hooked everything up last week. -)

Let us know how you like that stove in a seperate post as I don't want to hyjack this - I looked at the pictures but I have never seen one up in person.