# Best way to parallel pipe two different sized hot water heaters

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#### Dixit

##### New Member
Right now I have two 50gal (one elec and one natural gas) piped up in Series (on one side of the house). I didn't do this, it was done when we bought the house. We are looking to move a 3rd 40gal natural gas water heater (which is on the other side of the house) out of its location and into the area where the two 50s are. But we'd remove the 50gal electric entirely.

So basically in the end we would have just the 50gal gas and 40gal gas water heater. We could just plumb it in series but id like to do it in parallel. I keep hearing about reverse return parallel but not sure how that works for uneven sized tanks. I hear normal parallel wouldn't would since it wouldn't be balanced and the 50 probably would get used more since it has shorter piping. I read in a few places reverse return works on different sizes but not sure how. I've seen diagrams but they don't show how it works for two and uneven sized tanks.

First off is it possible? If it is how would I pipe it up? Or is it fine to do a standard parallel pipe setup? Its all 3/4" copper piping right now. Or must I buy another 50gal tank?

Also we are fine on the exhaust vent. We have enough capacity.

Thanks
Dixit

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#### Sylvan

##### Still learning
One of the primary advantages of having two water heaters is the high recovery rate.

When they are in series because the second water heater does little work until you have depleted the hot water in the first one. If you balance them, all you have is extra hot water storage, with half the recovery rate of a single water heater.

In order to take full advantage of both heaters, you have to hook them up in parallel with all piping equil

#### hj

##### Master Plumber
In a series system, if the first heater keeps up with the demand there is no need for the second one to fire. IF the first one is insufficient, then the second one activates, but it happens LONG before the first one is "depleted", (and ever if it were what difference would it make since both units are now supplying hot water). IF you can perfectly balance a parallel system, which is not an easy accomplishment, NEITHER one will turn on until they have cooled to the activation setting of the thermostats. IF both heaters are activated, the BOTH methods have the same recovery rate.

#### Dana

Twice the water heater, is twice the standby loss, which for gas units is pretty considerable.

A gas water heater in series feeding an electric tank might be cheaper to operate overall due to the much lower standby loss of the electric tank, and would deliver only a slightly slower recovery. The higher output gas burner would be engaged and pre-heating the water being fed to the electric heater long before the electric hit depletion. The electric water heater would always have a smaller temperature lift to handle during the recovery than the gas water heater. Most of the time the gas burner would be doing the lion's share of the heating, with the electric unit only maintaining the (smaller) standby loss. Only when 80-90 gallons are being gulped in one go (say, filling a spa) would the electric water heater be responsible for more than a few percent of the total input BTUs. Unless second water heater is actually being depleted, it's load is essentially ONLY standby loss.

With a pair of gas-burners just the standby losses of the second water heater in series is losing a few tens of percent of the total input BTUs. The standby loss of a newer electric is about 8% of daily usage at ~63 gallons/ day (less if higher daily usage), the standby loss of a gas-burner about 40%. It would take electricity 5x or more as expensive as gas in source-fuel BTUs to make using an electric water heater between the gas-burner and the potable hot water distribution more expensive unless both heaters are hitting depletion daily. Fifteen cent electricity is equivalent to \$4.40/therm, so even buck-a-therm gas is going to have more expensive standby loss.

#### Reach4

##### Well-Known Member
With a pair of gas-burners just the standby losses of the second water heater in series is losing a few tens of percent of the total input BTUs.
Sounds much higher than I was expecting. Would that be convection up the chimney when the burner is off? The ones with a powered vent would not have that, right?

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#### Dana

Look at the EF numbers. At 2015 regulations a legal minimum 40 gallon gas burner is EF 0.59 , a legal minimum 40 gallon electric is EF0.92. That's 41% as the combined combustion efficiency & standby losses- only 20% went up the flue, the other 21% is standby loss. A legal minimum 50 gallon is EF 0.58 gas or EF 0.90 electric. See the chart on page 2.

An EnergyStar 50 gallon gas water heater is EF0.67- so "only" 33% of the source fuel heat is combustion efficiency + standby loss.

Yes, you can do better than that with an Energy Star version with flue dampers etc. For an atmospheric drafted gas burner the convective losses isn't necessarily all up the chimney, but out the center-flue and around the draft hood. Newer-better atmospheric drafted water heaters have active flue dampers interlocked with the ignition. But with gas burners there is always a large gap in the insulation around the burner, a feature that electric tanks don't have.

#### Master Plumber Mark

##### Sensitivity trainer and plumber of mens souls
You are making it too difficult and it will never run correctly with a 40 and 50 in parallell--throw some dialectric unions on the heaters and I guarantee one of them will corrode shut within a few years....

just Run them in series and you will never be in trouble.......
make the 40 be the pre-heater and make the 50 be the hot boy

WE did this one on Monday,,
The one on the left is the pre-heater and the one on the right does all
the work....
We took out 2 crappy bradfords that were in parallell and
of course it never performed properly with the left one
burning out faster than the other one.....
I am pretty sure the Dialectric unions on top of one of
them clogged up in only 4 years and restricted the proper flow

How long these last remains to be seen....
the incoming pressure is too high for the whole house and
we are going to throw in a PRV valve before the water softener next week..

.

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