Without going into the thermodynamics the pool is ~ 10K gal, the hot tubs
closer to 1000 gal. (~10:1) Additionally the delta T for the hot tubs is
greater than the delta T for the pool. (a key element is the surface area
of the pool vs the surface area of the hot tub)
For some reason both my system and the neighbor's system are metered
separately than the rest of the house. Additionally we have exceedingly low
NG rates and a 'competitive' electric provider system in Texas (means
electric power users are royally screwed by games the power companies play.)
The exact details don't matter as we (both retired engineers) have compared
numbers over drinks too many times. He'd make the conversion ri NG but the
permitting and other cost are too great to justify the expense.
BTW again without going into the thermodynamics an in-ground pool is very
well insulated especially considering the typical above ground hot tub.
That is a tiny pool and a huge hot tub (like one of those 10 person
My pool is 15,000 gallons with around 500 sq/ft of surface area and
the 8' (OD) octagon tub is about 350 gallons and 45 sq/ft area.
That makes the area to volume ratio a lot worse in the hot tub.
The big difference is it is a lot easier to cover a hot tub and that
cover can have a pretty big R value.
You still run into the problem that unglazed collectors will not get
you a very high delta over ambient air even with a well insulated tub.
If you used glazed collectors you could get the McDonalds coffee hot.
On Tuesday, March 3, 2015 at 7:37:44 PM UTC-5, email@example.com wrote:
I agree, it's not typical that's for sure. And I think he said they
are shared by the same heater? Around here you only see that on
inground pools and not in that size ratio. The 10,000 gallon
pool is like an above ground and many of those are bigger than that.
I would think most home hot tubs are ~500 gallons.
I'm not a hot tup owner nor have I ever been. You leave the same water in
all the time, like a pool? Or, do you fill it like a tub? If you fill it,
like a tub, why couldn't you just tap into a regular HW heater on an on
demand heater (Even better for this application) and fill it up. Then just
use the elec to maintain the temp. I suspect the cost of the water is
nothing compared to the cost of keeping it hot on an ongoing basis.
Thats kind of what I thought - Why not use it like you would a jacuzzi? Fill
it every time? I would assume it has a gravity drain. That coupled with your
favorite NG demand (Tankless) water heater and then you only need 'lectric
to maintain the heat while you are using it. Why am I even commenting I know
sh** about hot tubs.
Because they can take 250 gallons of water and it will take quite
awhile to fill it each time it's used for one. Then there is the
issue of having the correct water chemistry, unless you want to get in
a tub with multiple people and no bromine disinfectant. At least with
the chemicals I use it takes hours to establish the correct level.
They are insulated, so unless it's a long time between uses, it's going
to take more energy to warm up 50 degree water every time you use it,
plus if you use it 4 times a week, that's 48000 gallons of water a year
compared to maybe 1500 gallons. And then there is the problem of what
to do with it in parts of the country that get below freezing in the
winter. Full and with heater maintaining even a lower temp, it won't
freeze. But if you drain it, now you have to fully winterize it, which
means getting all the water completely out, which isn't that easy on
Even a relatively small hot tub holds about 250 gallons. Draining it
is not at all like draining a bathtub, either. It's not hooked up to
the house plumbing, so you need a place for all that water to go.
There will always be a gallon or two of water that must be bailed and
sponged out of the very bottom. It also means taking off an access
cover to drain it, and blowing compressed air through all of the pipes
to clear them so nothing grows in them while dormant. When you refill
the tub, you would also have to bleed the plumbing. It's not that hard
to do a few times a year during normal water changes, but draining and
filling a hot tub between each use would be moronic.
It's not a problem to keep it sanitary, and it's not expensive to keep
You change the water about 4 times a year depending on usage and other
Finally, having used both a jacuzzi and a hottub, I can tell you that
a jacuzzi does not give anywhere near the same experience. It's a
pretty poor second place at best.
The chemicals used in a hot tub, chlorine, bromine, ozone, etc. are
used to kill that bacteria that might otherwise form from keeping a
standing body of warm water for an extended period of time. The human
body sheds a layer of skin while bathing and filtration is necessary
to collect this and the other organic solids that will otherwise
collect in the water.
If you pay for city metered water, the expense of filling up a 250
gallon hot tub every other day would quickly add up. Also to bring
that much water to an operating temperature of 101 F would be high
compared keeping the tub at some standby intermediate temperature and
just applying heat to make up for the losses.
It's not unusual for an in ground spa to have gas heated
water. It's unusual for this heat in a hot tub, however.
Gas heaters are fairly expensive, require piping for the gas
and are large enough that they don't fit inside a hot tub
enclosure. A small gas pool/spa heater is probably around
$1200, where an electric immersion heater is around $200.
That said, because the electric heater is nowhere as
powerful as even a small gas one, it is customary to keep
hot tubs heated 24/7. Otherwise, it'd take a day or so to
reach temperature. With an external gas heater, you could
fire it up a half hour or less before getting in and have
great hot water. That sways things in favor of a gas
heater, if you want to save on operating costs.
At a mountain cabin we owned, I had a hot tub with an
electric heater. Since LP was nearby for our furnace, I
considered getting one of the smallest LP- fired gas heaters
and using it as the primary heat source, with the electric
heater merely being used to keep the water at 40f to prevent
Frankly, in my younger days, I'd have considered buying a
domestic gas-fired water heater in the 30-gallon range and
hooking it to the the hot tub, using a circulating pump.
I'd have built a little enclosure for the water heater to
protect it from snow and rain. I'd have kept the electric
heater for backup or for when I would have shut the gas
system down to prevent freezing when we'd be away for
Pardon me. It's happy Hour here, and I have a 100f in
ground spa awaiting me and my martini.
To handle the concerns about whatever disinfecting chemicals you might have
in your hot tub possibly affecting your water heater, you could design a
closed loop heating system where the water from the water heater goes
through copper pipes in the hot tub, but do not acutally merge their water
with the hot tub water... Presurize this right and you could have water
hotter than 212F going through this loop... Of course, if you're going to go
to all this trouble, you could just design your own heating element out of a
coil of copper pipe in an enclosure with a burner at the bottom... Make it a
multiple layer coil with the cold water coming in a the top, thus it would
pick up some of the heat that would be escaping the flue... The warm water
would come out the lower portion of the tube closest to the flame... Since
copper tubing would not be all that expensive considering the length of life
you would probably get out of it, you wouldn't really be that concerned with
whatever disinfecting chemicals might be in the water, so you wouldn't need
to go the route of a close loop heating system... I designed something
similar to this to provide hot water for showers while camping... It used a
standard commercial fish fryer gas burner with a propane tank for the heat
source... It beat the 'ell out of taking a cold shower...
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