is my math correct? (L.G.)

Leo G_99

Customer has a 7 x 7 x 4 foot deep hot tub. Wants to be able to use it during the winter. No cover for personnal reasons. Once maybe twice a week. Would like to be able to fire it up from ambient in about 1/2 an hour. I figure about 40* ambient to 104* dipping temperature would take close to a million BTUH's to accomadate his wishes. Is this right????

Leo G

JimGPE_3

Nope!

7 x 7 x 4 = 196 CuFt of water
196 x 62.4 = 12,230 pounds of water
104 - 40 = 64F
12,230 x 64 = 782,720 BTU
This much heat in 1/2 hr is 1.57 million BTUH OUTput
Assuming heat at 80% efficiency, this is

2 million BTUH input.

I think this guy is somewhat optomistic.....

Leo G_99

OUCH!!!

Jim, thanx, i forgot about the 1/2 hour thing!!!!

Leo G

PJO_5

If there's no cover...

You better consider the heat that leaves this baby as evaporation, too. That will be pretty significant.

In this case, a cover seems like a no brainer...why doesn't he want one? Seems to be costing him a ton of money - both in the boiler selection/heatloss and his fuel bills (unless he's got a monster wood burner near his hot tub).

Take Care, PJO

.

If he only uses it once a week, a cover vs no cover is not a significant difference.

PJO_5

How's that?

Please explain...

With a cover, you would still have at least half your BTUs still in the tub a few days later. I'm assuming this is a "typical" hot tub with insulation...if it's not, then the numbers above will increase even more. My own hot tub (nearly 500 gallons - with an insulated cover) holds the temperature within 5F over 24 hours even when it's 40F outside.

Without a cover, you will be at ambient in a day or two. If you want to use it "once maybe twice" a week you will ALWAYS have to get it up about 80 or 90F.

Maybe I'm missing something here. Take care, PJO

Leo G_99

PJO

, it is for "the look". The hot tub will "waterfall" into the pool and as they are just outside the viewing windows, they do not want the cover in their view.

When the tub is turned on, the level of the water drops enough so that it does not waterfall anymore, and the pool chaps said they could arrange it that the waterfall feature could be disabled during cold weather.

The only solution I can think of right now, is to idle the tub at about 80-90 degrees when not in use, then it shouldn't be that long to get it up to temp.

Still though, the cost of idling will be significant, so I may just leave this one in the pool guys hands.

Leo G

PJO_5

Idling vs. \"Cold Start\"

Leo,

The idling idea will save on the boiler size...now you are at 20F or 30F differential...about 50% or more size savings.

That might work...but you still will lose a lot to evaporation. How about a custom cover that has a "hole near the end where the waterfall is?

Just thinking out loud. Take Care, PJO

joshua van Tol_2

How about a storage tank? Use a very large buffer tank at high temp, and use that to heat the hot tub water.