Dual Water Heaters - parallel or series

Had a call from someone that just had there home inspected. There inspector told them

"That the two water heaters could be plumbed in series to improve efficiency. (one tank set low, like 25c, the other set to 56c). He said that they would have been plumbed in series by the builder, and he’s wasn’t not sure why these ones are in parallel.

Now it my understanding that dual water heaters are supposed to be plumbed in parallel. Am I wrong here?

Thoughts … opinions, please

I guess it would all depend if they service two separate circuits or one.

If it is one circuit, they should be in series. If there are two circuits they must remain in parallel.

I had one yesterday where they had one electric water heater and one gas-fired water heater in series. The house was to small to require 100 gallons of hot water. In this set up, they could shut off the gas water heater but not the electric. I recommended that if they did not need 100 gallons of hot water in storage they should have a plumber install a bypass valve so they could use the gas water heater by itself.

With the electric water heater down stream, if you shut it off you get 50 gallons of cold water before the hot water arrives at the faucet.

Turning off the upstream water heater (if they are the same) has no effect on operation, but will save energy loss by not storing unnecessary amounts of hot water.

Try this Greg. :smiley:

Thank you David and Brian!!

Series has an advantage I think. A shower becomes increasingly cooler because the hot water that is used from the water heater is replaced with cold water at a rate faster than the water heater can heat it up. By putting a water heater in between the cold supply and the other hot water heater, you can warm the cold water first so that the final water heater need only heat up already-warm water. This series scenario could potentially be setup where you would never run out of hot water while only paying for the energy to “warm” the water of the upstream tank… and only paying for the energy to “heat” the already-warm water of the downstream tank. So it isn’t quite the cost of keeping two tanks hot like you would have to do if they were parallel.

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One should note that in predominantly colder climates, it doesn’t matter anyway. The energy loss from heating up water on a parallel system that isn’t immediately used doesn’t disappear. It is lost to the air eventually, which someone is paying to heat anyway… and besides, tanks are super insulated these days. Turn off your water heater, go on vacation, come home and feel the water… it’s pretty warm. Most of the heat loss is suffered in the piping to the fixtures.

In some areas where they have ultra lower electric charges in the night use a timer for the middle on the Night .
. This could be a great idea it the electric heater was the upstream and gas down stream .
Nick is correct heat is not lost from water heater for about 8 months when we heat our homes .

If I installed two 50 gallon tanks in parallel, during times of low usage could I not turn off the water to one and just let the pilot burn (or turn off the gas as well). Oh, also turn off the hot water outlet. Then, when I’ve a houseful of people using three showers I could just reconnect and fire up the second of the heaters.

Any problems with this plan? All thoughts are appreciated.

There is a community in this area (Malvern Hunt)
The townhomes all had (2) 75 Gallon Gas Fired Water Heaters

  • Water Heater 1 / Master Bathroom only.
  • Water Heater 2 / Kitchen and remaining Bathrooms & Laundry

Parallel or series makes no difference. It depends upon the design intention.
Obeserve and note accordingly.

You must keep the main water at 120 F if not you will have a chance of building bacteria in the tank. If you plan to use a tank in series this is OK but not parallel.
140 F is recommended in areas critical like Hospitals,Bath areas and Schools.

Water heater temp should be at 140°F to stop Bacteria .
** Max Tap temp is 120° F**

I think that is now Ontario code .
My tap temp is 108°F and tank temp is 140°F



Here is temperature info about the survival of the legionella baterium, which can cause the legion or pontiac fever:

  • 70 to 80 °C (158 to 176 °F): Disinfection range
  • At 66 °C (151 °F): Legionellae die within 2 minutes
  • At 60 °C (140 °F): They die within 32 minutes
  • At 55 °C (131 °F): They die within 5 to 6 hours
  • Above 50 °C (122 °F): They can survive but do not multiply
  • 35 to 46 °C (95 to 115 °F): Ideal growth range
  • 20 to 50 °C (68 to 122 °F): Growth range
  • Below 20 °C (68 °F): They can survive but are dormant

You’ll find pretty much the same info as what Roy posted if you go to this: http://en.wikipedia.org/wiki/Legionellosis

Thank you for your comments. I hadn’t thought of bacteria. What if the “off” tank were drained during periods of non-use? What problems would you anticipate there?

That could put the temp about
35 to 46 °C (95 to 115 °F): Ideal growth range
20 to 50 °C (68 to 122 °F): Growth range

If I was going to leave water in the tank then I would add about a pint of Chlorine … Roy

As a non-member I want to thank you all for allowing me to use this forum. I received valuable advice and information.

Not necessarily.

Some electric tanks have EF’s (Energy Factor = efficiency rating) as low as .81 and gas tanks are as low as .56-.57 . The R values of today’s foam tank insulation are about 10-13, hardly super insulated, but much better than the fiberglass of past with only R 5-6.

If your electric tank is in a cool spot such as a storage room or not fully or directly heated basement, extra tank and pipe insulation, heat traps (some are now internal to the tanks) plus bottom insulation has a short payback period of probably less than a year. The extra 20 or so degrees lower temp. in these spaces certainly warrant the above.

Here is what the DOE has to say:

"Insulate Your Water Heater Tank for Energy Savings

Unless your water heater’s storage tank already has a high R-value of insulation (at least R-24), adding insulation to it can reduce standby heat losses by 25%–45%. This will save you around 4%–9% in water heating costs.
If you don’t know your water heater tank’s R-value, touch it. A tank that’s warm to the touch needs additional insulation. (this is a poor statement-HI)
Insulating your storage water heater tank is fairly simple and inexpensive, and it will pay for itself in about a year. You can find pre-cut jackets or blankets available from around $10–$20. Choose one with an insulating value of at least R-8. Some utilities sell them at low prices, offer rebates, and even install them at a low or no cost.

Insulating an Electric Water Heater Tank

You can probably install an insulating pre-cut jacket or blanket on your electric water heater tank yourself. Read and follow the directions carefully. Leave the thermostat access panel(s) uncovered. Don’t set the thermostat above 130ºF on electric water heater with an insulating jacket or blanket—the wiring may overheat.
You may want to see our instructions for installing an insulation blanket on an electric water heater.
You also might consider placing a piece of rigid insulation—a bottom board— under the tank of your electric water heater. This will help prevent heat loss into the floor, saving another 4%–9% of water heating energy. It’s best done when installing a new water heater.


Download high-resolution diagram:
JPG (ZIP 86 KB) | EPS (ZIP 480 KB)

Insulating a Gas Water Heater Tank

The installation of insulating blankets or jackets on gas and oil-fired water heater tanks is more difficult than those for electric water heater tanks. It’s best to have a qualified plumbing and heating contractor add the insulation. If you want to install it yourself, read and follow the directions very carefully. Keep the jacket or blanket away from the drain at the bottom and the flue at the top. Make sure the airflow to the burner isn’t obstructed. Leave the thermostat uncovered, and don’t insulate the top of a gas water heater tank—the insulation is combustible and can interfere with the draft diverter.
Related Information

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Good Info HI!

I have a 75 gallon gas powered hot water heater and sometimes in the winter when many are showering we run out of hot water. I’ve been told that my gas line is too small to add another hot water heater (considering everything else I have on the system) and changing the line is too costly. I am considering adding a 75 gallon electric hot water heater to be used only during peak water usage times. Should I install them in series - using the electric as upstream and leaving it off when not needed, or should I install in parallel- turning electric off when not needed and closing valve that attaches to plumbing. (Also, Do I need to worry about bacteria issue with either of these plans? I assume in plan A (series), that if any bacteria is caused by leaving electric off, the gas hot water heater will kill the bacteria before it enters the pipes and in plan B (parallel), the bacteria would be killed when tank is turned on?). Thanks for any feedback.

Instead of the extra utility demand why not go with an on demand system instead. The cost to purchase would be close to the same as two water heaters and would likely save you money in the long run. You can still use two water heaters in series and keep the second one at a lower temperature. Once the bacteria gets above 50C it will kill off any bacteria from the other tank.

Water heaters in parallel are for separate zones.

Water heaters in series are to increase capacity.

As said above, install the electric water heater downstream of the gas and lower its temperature set point below the gas heater so it doesn’t come on as much. (You still will have significant standby loss. Because you shut off a valve does not mean the water heater is not operating).

Install a bypass loop and valve if you ever want to take it out of service for extended periods of time and shut it down.

Do you have a circulation pump? You have significant drawdown to get hot water to the showers? If so, this is not the way to fix it.