Okay, so you've decided you want a bigger shower
with multiple heads, a bigger bathtub or a jetted
tub, and you suspect your old hot water heater
might not be up to the task of providing enough
hot water for you. I get asked these two questions
all the time:
we upgrade our existing hot water heater?
we add another hot water heater?
Well, first off you have to figure out if you are
going to need more hot water then you current
system is going to be able to supply. There is
always the wait and see method and there is the do
some simple math and make a guestimate method.
you need to figure out how much water you need
then figure out how much water your tank is going
shower heads, they all will deliver a maximum of
2.5 gallons per minute or less. So 4 heads equals
10 gallons per minute. Multiply that by the length
of time you plan on showering, let's say 20
minutes. That would equal 200 gallons.
there are a lot of variables that go into figuring
out how much hot water flow you are going to get.
The temperature of the ground water feeding the
tank is an element of the equation. Average ground
water temperature is 50 degrees, and most people
like to take showers with water somewhere between
105 and 120 degrees. The age of your tank, whether
it is gas or electric, the recovery time of your
tank and even the insulation or lack of on your
So using our example of our 4 head shower we need
200 gallons of water, now not all of that amount
will be coming out of the hot water tank, you will
be mixing cold water with it. So a good rule of
thumb is figure somewhere around 60-70% of the
water will be coming from the hot water tank on
average. Then you have to figure in that as hot
water comes out of the tank, fresh water will be
coming in and it will be heated so you should end
up somewhere around a figure of 40% of the water
needed (200 gallons) * 40% = 80 gallons. So your
40 gallon tank probably won't be doing what you
a jetted or larger soaking tub figure about a 70%
number to figure what your tank can deliver. So a
40 gallon tank can deliver about 28 gallons of
hot-hot water then you are also mixing in some
cold so a 40 gallon hot water heater should be
able to deliver close to 40 gallons of hot water
give or take some. If your tub is a 65 gallon tub,
which is about average for a good sized one, your
40 gallon hot water heater probably won't be doing
what you want.
what to do? Upgrade the existing tank to a bigger
one or add a second one?
definitive answer is - It depends.
you have room for a second tank? If not then
upgrading the existing tank is the only answer.
you have room there are other issues such as
feeding power to the tank such as gas, there could
be a slight chance you might be maxing out your
gas flow with your current sized gas line, it's a
possibility, a slight one, but a possibility
depending on a lot of factors. Have you added a
gas fireplace? Converted an electric stove to gas?
This could cause you a problem now with adding
another gas hot water heater. An upgrade to the
gas line from your meter might be required.
There is the flue issue. You might now be required
to upgrade the size of the flue to accommodate the
How old is your tank? If it's on it's last
legs then adding a 2nd tank just means you will be
replacing the first tank pretty soon too.
What would I do?
I would always try for the 2nd tank if it's
possible. Some of the reasons are because two
tanks are always going to give you more hot water
then one large one. A 40 and a 50 is bigger than a
single 60. A second tank if we have the plumber do
it right means you are never going to be out of
hot water due to a tank dieing on you. You simply
turn a couple of valves and temporarily take the
dead hot water heater out of the loop and by pass
it, you run on the single tank until at your leisure
you replace the bad tank.
What we like to do is hook them up in series, not
parallel, you use the old tank to feed the new
tank. The new tank would be a quick recovery hot
water heater, somewhere around 50 gallons. You
turn the old tank temperature down, and it feeds
preheated water to the new hot water heater, the
old tank's life gets extended by a large amount
since it isn't working so hard anymore and the new
tanks life gets extended since it isn't either
since the water being fed to it is already pretty
warm, it might only be raising the water being fed
to it by 40 degrees now instead of 60 or 70 as
You can dial them in over time, fooling about with
the temperature settings on the feeder tank and
seeing what works the best. If you want to take a
hour long shower some time, you might need to dial
up the first tank. Regardless you are going to get
a ridiculous amount of hot water using the two hot
water heaters in series method, a very long life
to your heaters and peace of mind of never being
out of hot water if one dies on you.
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