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Series Vs Parallel Water Heaters: What To Install?

Water heaters are a necessity for all households. However, they are great electricity consumers and must be installed properly. Often people debate whether they should be connected in series or parallel to the main output.

Moreover, which is better among series vs parallel water heaters? Series is a good choice for application in residence. Where you might not need a continuous supply of water in a large quantity. But a parallel water heater is more effective when there is a need for a large volume of water in a short period.

Now, there is a lot more to consider when choosing a water heater connection. Below, their differences are added to give a more precise idea of both. So without further ado, let’s dig in!

Table: Comparison Between Series Water Heater And Parallel Water Heater

Below the key differences between a series water heater and a parallel water heater are added.

ParameterSeries water heaterParallel water heater
Storage capacity of heatersCan use a heater with a different storage capacityThe same storage capacity heater needs to be installed
Pipe lengthCan be differentHas to be the same
VolumeLess volume of waterCan supply a large volume of water
Shut offTurning off one water heater results in a shut-offTurning off one water heater still allows the other water heater to heat the water
Wearing offGets worn off fasterGets worn off slowly
Load distributionFirst water heater tank takes the most load of cold waterA load of cold water is distributed equally
Bacterial impactProne to bacterial growthLess prone to bacterial growth
Heat recovery rateThe first water heater has a faster heat recoverySame in both water heater
DependencyBoth the water heater depends on each otherNot dependable on each other

An In-Depth Comparison: Series Vs Parallel Water Heaters

series vs parallel water heaters

The above table shows the key differences between the series and parallel water heaters. In this section, a more brief idea of both of the water heaters is included.

Storage Capacity Of Heaters

Two different-sized water heater tanks can be installed in the series connection.

But in a parallel connection, the water heater tanks have to be the same size. Otherwise, an equal flow of water becomes impossible to maintain.

Pipe Length

In a parallel water heater tank, the outlet pipe’s length should be the same. Similarly, the inlet pipe length should be the same as well. It is because if the length of pipes varies from the cold water distribution line, it would create an imbalance as this can increase pressure in the water tank.

While similar length pipes help to maintain the balance of water flow and heat the water at the same rate. But in series, there is no need to maintain balance. That’s why pipe length does not matter at all.


A parallel water heater supplies water from two different tanks. Where both heaters are heating the water and equally supplying it to the distributor. Thus, it contains a large amount of hot water and ensures its supply.

Whereas, in the series connection, only one heater tank passes the hot water to the distributor. The first tank put complete effort into heating. And the second water tank only stores the heated water and maintains the temperature.

That’s why a series water heater is not suitable for supplying water in a large volume because of the low volume capacity. This water heater would not be able to carry the load and supply enough hot water.

Shut Off

When one of the water heater tanks fails in parallel, the other tank will continue to heat water. There will be no disruption in supplying hot water to households. Also, shutting off one would not affect the other one at all.

In the case of a series water heater, if one tank fails, the other would also stop supplying water. So, the continuous supply of water is disrupted.

Wearing Off

The water heater connected in series wears off faster. The mechanism shows water is heated in the first tank. While the second tank only maintains the temperature and stores the water. It does not require much energy compared to the first water heater tank.

As a result, the load is more at the first tank, which causes faster failure because of continuous use.

On the other hand, in parallel water heaters, water is drawn from both heaters. And the load is distributed equally, resulting in longer stability and life of the heater.

Load Distribution

A load of water is distributed equally in the parallel water heater. And allows the heating of water at the same rate. Whereas the series water heater carries a different load, and the rate of heating is also different.

Bacterial Impact

Series water heaters are more prone to bacterial contamination as the first tank directly gets the cold water and heats it. However, cold water is continuously added to the first tank and thus creates a chance of contamination with microbes.

On the other hand, both water heaters in parallel heat water at the same time and keep it hot. There is no chance of water being exposed to cold water after heating. Thus, it prevents contamination.


The water heater in series is easy to install. Because it takes less amount of pipes to connect the tanks and also while connecting to the distributor. You do not need to keep the pipe length similar either.

But in a parallel water heater, the length of the pipes should be cautiously observed. And requires more pipes to connect the tanks and special supervision. Thus it becomes tough to install the parallel water heater.

Heat Recovery Rate

It is assumed that a parallel water heater has a higher recovery rate in comparison to a series water heater.

In a series water heater, only the first heater works effectively. When water is used from the second tank, it will take some time to pass and reheat the water. Consequently, lower heat recovery is observed.

But in a parallel water heater, two different heaters heat the different tanks. Thus the recovery rate increases in parallel by showing a similar rate of heating.


Well, in a series connection, both the water heaters are dependent on each other. If one water heater disables or fails, the other will not work as well. Consequently, there will be no hot water supply. But it also has the advantage that you can easily identify the failed water heater and fix it.

On the other hand, in a parallel connection, if one heater does not work, it would not affect the function of the other one. The other water heater will continue to do its work. So, the water supply would not stop. But it would also become difficult to know which heater has failed.

Which Is Better Series Vs Parallel Water Heater?

Both series and parallel water heaters have their plus points.

  • Series water heater mainly heats the water in the first tank. While the second tank generally stores the heated water. As the first heater takes the most load, it has a higher chance of failing.
  • At the same time, a series water heater is suitable when the water supply is not required continuously.
  • For storing small volumes of water, a series water heater is the best one.
  • Also, once one of the heaters fails, the other would not work as well. You can easily identify the fault and fix it.
  • On the other hand, in a parallel water heater, both tanks work at the same rate with the same efficiency.
  • Also, it can store a higher volume of water.
  • In big buildings, where several hot water supplies are needed in a short period, a parallel connection is suitable. There will be a continuous supply of hot water with no disruption.

Apart from that, tankless water heaters are also used nowadays. You can also try this after consulting the professionals.

You can follow the below youtube video regarding the connection of parallel and series water heaters.


Below, the most frequently asked questions are included for your further clarification.

Q: Can you put a hot water heater in series?

It is possible to put a water heater in series. But it requires more energy and electricity bills and is more prone to bacterial contamination. Perhaps not the best for using for a long period.

Q: Can you put a tankless hot water heater in parallel?

Yes, you can undoubtedly put a tankless water heater in parallel. It would also increase efficiency. But a tankless water heater might not be recommended by professionals. 

Final Words

Overall, using series vs parallel water heaters has been a topic of debate from the earliest days. Both the water heaters are good in their way. That’s why choosing the right one depends on the consumer. Often parallel water heaters are preferred due to their higher storage capacity and supply of continuous water.

At the same time, parallelly installed water heaters work at the same rate with a lower chance of failure. Whereas, the series water heater keeps the water hot for a long time in the second tank. And suitable when water is required to be supplied at a certain period.