Plumbing, Water Heaters

Why Would You Need Two Tankless Water Heaters

two tankless water heaters 2 lg

We all need water heaters in our homes. They make sure that the water we use is at the appropriate for which the water is used. The problem, however, is when we have to decide how many water heaters to have in our homes. When deciding how many tankless water heaters to have in your home, you have to factor in many considerations, but it is often recommended to have two or more. So, the question is, why would you need two tankless water heaters in your home?

In larger homes, two tankless water heaters may be necessary to supply enough hot water for your family’s needs. The more hot water your household consumes per day, the more tankless water heaters you will need. Most tankless water heaters will produce 5 to 9 gallons of hot water per minute. You’ll need two tankless water heaters if:

  1. If your household typically exceeds 9 gallons of hot water usage per minute. Taking two showers and running a clothes washer or dishwasher can easily exceed 9 gallons of hot water usage per minute.
  2. If you have bathrooms or your kitchen is on opposite sides of the house. Tankless water heaters produce hot water as needed and it can take up to 150 seconds (or 2.5 minutes) for hot water to travel from one side of the house to the other. Having 2 tankless water heaters, one on each side of the house, will help reduce the response time of hot water and reduce the usage.

We know that the number of tankless water heaters you need is dependent on how much hot water you plan on using, but what are some of the factors that determine how much water you will need? Also, why would you want to use tankless water heaters instead of traditional water heaters in the first place? We explore all of these questions below.

Why Would You Need Two Tankless Water Heaters

Before we look at why you would need more than two tankless water heaters, let us discuss factors that may prompt you to own more than one. Thereafter we discuss why you would need more than two tankless water heaters, and we take a look at why more than one tankless water heater is not going to cost you more than one large traditional water heater.

Factors That Determine How Many Units You Will Require In Your Home

Before you start buying tankless water heaters, first determine how many you would require to suit your household’s needs. Below is a list that you can use to determine how many tankless water heaters you will require to provide you with hot water when needed:

  • The number of fixtures you have in your home.
  • The type of fixtures you have in your home.
  • The duration which these fixtures are used for.
  • The frequency in which these fixtures are used.
  • The temperature of the underground water.

Why Would You Need Two (Or More) Tankless Water Heaters

Now that we have discussed factors that can help you determine how many water heaters you need let us look at why you need two tankless water heaters.

The two most important things to remember when it comes to the capacity of your tankless water heater(s) are the:

  • water consumption you expect (measured in gallons per minute) at the peak demand, and
  • how much the water must be heated with its incoming temperature.

Regarding the first factor, if we want to determine how much water will be required at peak demand, you have to determine the water requirement of every single fixture in your household (in gallons per minute) and add them all together.

After determining the water requirements of your household in gallons per minute, you must determine the temperature requirements during the coldest climate in your area. With the water and temperature rise requirements, you can start calculating how many tankless water heaters you require.

Natural gas or propane tankless water heaters can, in general, handle 5 gallons per minute of water and do this at a 70 F temperature rise. We can use this information to construct a scenario to determine how sufficient this will be for a household.

Let us examine a scenario using a showerhead. A showerhead has 2.5 gallons per minute in water output. If we assume that the incoming groundwater is 50 F and require that water to be 120 F, 2.5 gallons per minute of water must be heated by 70 F. In this scenario, we have already exhausted half of the capacity of the tankless water heater.

Here are some additional fixtures and their flow rates that you should consider:

FixtureFlowrate (gallons per minute)
Kitchen faucet/bath faucet1.5 – 2 gallons
Shower2 – 5 gallons
Tub filler faucet+/- 4 gallons
Dishwasher1 – 2.5 gallons
Washing machine1.5 – 3 gallons

It is easy to see why one water heater will not be enough to power an entire household if the household expects to use more than one shower head at a time or any of the other fixtures listed in the table above.

The general guideline that manufacturers and other knowledgeable sources give is that a single tankless water heater can supply enough hot water to a household that uses 41 gallons of hot water daily. If we use this as a key performance indicator, we can add a tankless water heater for every 41 gallons of hot water used in your household per day.

Accordingly, if you have a large enough household to use 40 gallons of hot water or more per day and you want to use water heaters that are more efficient than traditional water heaters, you will need two tankless water heaters.

The Layout of Your House Matters

The layout of a house matters when it comes to the distribution of hot water. In larger houses with three or more bathrooms, a large kitchen, and a laundry room, it’s not uncommon to find some bathrooms on opposite ends of the house.

When this occurs, hot water has to travel long distances from where the tankless unit is located to reach the fixtures. Most people report that it takes an average of 90 seconds for hot water to reach the fixture from a tankless unit. Because hot water is produced, on-demand, cooled water present in the supply pipes must be expelled before the hot water arrives at the fixture.

However, if your bathroom is located either on a second floor or a long distance from the tankless water heater, the response time can be upwards of 2.5 minutes. If this is the case in your household, you will benefit from having a second tankless water heater installed to supply water to that side of the house. It will improve response time greatly and, except for the installation cost, won’t cost any more money to operate.

two tankless water heaters lg

Why More Than One Tankless Water Heater Does Not Cost More

When it comes to tankless water heaters, the same concerns do not apply compared to those that apply when using a traditional water heater. Some may feel that if you oversize your water heater’s capacity, it may lead to a lot of additional electricity consumption and wasted energy to keep the entire body of the tank warm.

Tankless water heaters, however, do not suffer from this flaw. Tankless water heaters account for the water consumption of the unit and vary their water flow rates accordingly. Thus, just because you have a larger tank with excess capacity (or, in this case, if you have more than one water heater), it does not mean that you will use extra energy to power the water heater(s).

Given that some tankless water heaters perform at efficiency levels of 96%, we can dispel any concerns that one might have for owning more than one tankless water. Tankless water heaters are so efficient that you might save more costs owning two tankless water heaters as opposed to owning one traditional water heater.


In conclusion, the more fixtures you have that increase the hot water consumption of your household, the larger the capacity of your water heater will have to be. If you want water heaters that are more efficient than traditional water heaters, you will have to resort to tankless water heaters.

That said, tankless water heaters have a limited capacity. So, if you have more fixtures that frequently use hot water, you will probably have to resort to getting another tankless water heater, seeing as one will only carry you up to the first 40 gallons of hot water per day.


Hubert Miles | Licensed Home Inspector, CMI, CPI

Hubert Miles is a licensed home inspector (RBI# 2556) with more than two decades of experience in inspection and construction. Since 2008, he has been serving South Carolina through his company, Patriot Home Inspections LLC. As a Certified Master Inspector, Hubert is dedicated to providing his expertise in home inspections, repairs, maintenance, and DIY projects.