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Sizing Hot Water Systems

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Posted by kickaha(remove) on January 27, 2004, 8:23 am
Hi, all-

Is there a general rule of thumb for sizing a hot water system for a house?

We have six people living in the house, half shower takers and half
bathtub users.

The bathtubs will not be over-sized and the showers will all have 2.5GPM

The water will be solar heated.  The cold water supply will be from an
above-ground water storage tank.

I've had a system with an 80 gallon tank recommended, but my gut feeling
is that this won't be large enough.

Where we're at now we have a 50 gallon gas fired water heater and it
runs quite a bit in the evenings when baths and showers are taken.

Would a 120 gallon system be ridiculously large?  How about two 80
gallon systems?


Posted by George Ghio on January 27, 2004, 10:28 am

Well we used a 200 Gal (header tank) for the last 16 years or so most of
the time for 4 people but now just two. Pumped water once a day to keep
it full without long pump run times. It also gave up to 4 - 5 days use
if pumping was not able to be done for whatever reason.


Posted by William P.N. Smith on January 27, 2004, 12:56 pm
It's also going to make a difference how long they shower for.  IIRC
showers take alot less water than baths, but as always, your milage
may vary.

Can you split bathing up between morning and evening?  That'll help
with sizing and runtime, but solar of course changes everything...

My experience with low-flow showerheads is that I just shower longer,
as rinsing takes a certain number of gallons, not a certain amount of
time.  I prefer normal showerheads with manual flow control.

I've got an 80-gallon tank with 2 adults and 2 kids, and it's barely
enough for three showers in the morning (but my daughter and I are not
into conserving hot water. 8*)

Solar systems want to have lots of storage capacity to cover showering
at night _and_ in the morning, plus getting through rainy/cloudy days.
I'd probably have a 120-gallon solar preheat with an 80-gallon main
heater (what's your backup heat source?) for six people...

William Smith
ComputerSmiths Consulting, Inc.    www.compusmiths.com

Posted by SQLit on January 27, 2004, 3:24 pm

I have an 80 with one 4x10 collector, year around I think I get something
like 70-80 % of my hot water. You probably have forgotten about dish and
clothes washing. My collector is flat on the roof and is not optimally

If you want all of you heat from solar, again depending on where you live
your going to need 4-6 collectors and I agree with your estimate of 120
gallons.  Talk to some contractors and home owners in the area and see what
works where you are.

Posted by daestrom on January 27, 2004, 10:19 pm

One idea that I've mentioned on these NG's before is a heat-exchanger to
recover some of the heat from the shower drain water.  I have one from
www.endlessshower.com and it works well for us.  By recovering some of the
energy from the drain water, you use less hot water for the same comfort
level in the shower.  We have a 1.5 gpm shower head with pretty good spray
flow (we have low water pressure in this area, I think it's meant to be a
2.5 gpm head, but I measured it at 1.5).  Anyway, a 50 gal tank set for 130F
and four of us can shower without running out of hot water.  I don't use a
lot of *hot*, but my wife and daughter sure do :-)

It only helps for continuous flow things like showers though.  Won't do much
at all for the bathers.  Search this or alt.solar.thermal for some
discussions about using storage tanks and other setups to reclaim some heat
from bathtub water.


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