Lo,

I'm trying to figure out a way to heat an outdoor hot tub quickly. My goal is a 30 deg change in under 20 min for 300-400 gallons. I calculate I need about 90,000 BTU's. (Or 270,000 BTU's/hour or more.)

I think my only option is a gas pool/spa heater (BUT I'M OPEN TO SUGGESTIONS.) Problem is, it has to go in my basement garage.

I think I've resolved the combustable air issue with a Field Controls CAS-4.
http://www.fieldcontrols.com/cas.php#cas3_4

But now I've found out any heater over 200,000 BTU's is classified (code wise) as boiler. How does this change the game? (It's not like it's running at 30 PSI or anything.)

Last I want to vent it out my chimney that was previously used for my oil furnace (unlined 6" x 12").

Thanks! Kyle

I've installed pool heaters indoors similar to this install, and used the CAS system as described. Fwiw, about half the world calls a pool heater a boiler. That said, the heater is not under static pressure effects similar to a conventional water heater/boiler, and only heats when water is flowing.

Don't know if your proposed vent flue would be code compliant though... it would probably work fine though.

Now, whatever heater you decide on, you should not use the built in pressure switch on the heater, and install a flow switch in line with the plumbing because the weight of the water in the piping (from an overhead tub) will cause the switch to bounce on and off... or errantly trip on - which is something you never want to have happen.

For a 30 degree temp shift you're really asking for a lot in 20 minutes - but your best weapon in this dog fight is going to be a cover on the tub while it's not used and especially during the heat up period as surface and shell exposure have the greatest influence on temperature stability. Additionally, your pipe runs are a factors to be included in this equation as the water ends up heating the environment around them too.... more waste.

You can get close with the calculations, however you'll never get a perfect match - as ambient temperatures affect EVERYTHING - ie, you'll heat up 30 degrees a lot faster in the summer than you will in the winter etc.. you get the picture. Then again, your water temp in the summer (depending upon your location) may not drop below 75-80 F anyway.

Short of dropping a piece of weapons grade plutonium in the water, gas is about the only thing that'll give you the rate of rise you seek.

Any compliance/code probelms? It appears that this device removes all confined space requirements like taping into other rooms or the outdoors.

I'm after code compliant. You think just lining the chimney would do?

GOOD TIP!

I'll just try Googleing "I need weapons grade plutonium"... (Just kidding homeland security!) No, really. Hey! (Bang, Bang) help! OW!

I don't think you'll have any issues with code compliance.
And the vent flue may work fine without lining (no reason why it wouldn't). It all depends on whether or not you're going to have some $%@#$ inspector come look at it.

My rule of thumb on that is... don't do it unless it's either unsafe, or some jerk says you got to just because he thinks he wrote the code himself.