Plumbing, Water Heaters

Can Tankless Water Heaters Keep Up With Hot Water Demand?

Tankless 1 Lg

So, you’re considering moving to a tankless water heater from your traditional water heater system. Well, it’s understandable that you’re wondering if a tankless water heater can keep up with your family’s hot water demands.

Tankless water heaters produce hot water on demand, so they actually can’t run out of hot water. They can appear to run out of hot water when the demand exceeds the unit’s capacity to produce hot water. To ensure you select a tankless water heater that meets your hot water demands, you’ll need to calculate your peak demand gallons per minute and the temperature rise required where you live, then select a model that meets your needs.

Of course, understanding the things you need to know before you make a decision is vital. So, do you want to learn how to choose the right tankless water heater? If so, take five minutes to read the guidance we’ve set out below.

How Does a Tankless Water Heater Produce Hot Water on Demand?

No doubt you’ve already worked out that a tankless water heater doesn’t store hot water.

Instead, what it does is heat the water only when you need it. It springs into action when, for example, you open a faucet or turn on your shower. For this reason, we say a tankless water heater produces hot water on-demand.

As you’ll appreciate, this on-demand business takes a lot of energy. Of course, you expect to get hot water quickly when you turn on your hot water faucet.

A tankless water heater uses a powerful burner in gas models. Electric tankless water heaters use powerful heating elements. Consequently, the water heats up in seconds, giving you hot water very soon after you request it.

In theory, a tankless system can produce an endless supply of hot water. But there’s a caveat. If your tankless water heater can’t keep up with the demand, your tankless water heater can appear to run out of hot water.

A tankless system can only do this if it has the right capacity. What the right capacity is will vary from one home to another. But, it refers to the system’s ability to heat water to your required temperature at the time of your highest usage.

So, you need to consider two things when you’re choosing a tankless water heater, namely:

  • Your peak demand for hot water
  • The difference between the temperature of your incoming water supply and the hot water temperature you want to achieve

Therefore, any tankless water heater you choose must be able to achieve the required temperature increase. But, it has to do that with water flowing at your peak demand rate.

So, let’s look at these factors in more detail.

What Is Your Peak Hot Water Demand?

First, your peak hot water demand is straightforward to work out. You look at how your household uses hot water. In particular, you need to look at times of peak usage. Maybe it’s in the morning when everyone’s getting ready for work or school.

So, at your peak usage, look at the hot water appliances you’re using at the same time. Then measure how much water comes out of each one over a given period. That’s the flow rate for that appliance, and it’s measured in gallons per minute (GPM).

Now you have the flow rate of each appliance used simultaneously at the height of your household’s hot water demand, add them together. That will give you your peak demand flow rate.

Working out the flow rate of your appliances isn’t complicated. Indeed, you can take an empty 5-gallon bucket and time how long it takes to fill it.

You can see how simple this process is in the following video.

So, if it takes five minutes to fill the bucket from a sink tap, the calculation is:

5 gallons/5 minutes = 1GPM

For example, let’s say, at peak demand, your household uses two showers and two sink hot water faucets simultaneously. So, your calculation may look like this:

ApplianceFlow Rate in GPM
Shower #12
Shower #22
Sink faucet #11
Sink faucet #21
Total Peak GPM6

So, now you know at peak times, you need water flowing through your tankless water heater at 6GPM. 

Calculate the Temperature Rise

Next, you need to calculate your temperature rise. In other words, deduct your incoming water temperature from your desired hot water temperature.

Your incoming water is the groundwater supplied by your local water company. You can check with your supplier what the temperature of the groundwater is in your specific location. As a rule, for the sake of accuracy, that’s the best thing to do.

For present purposes, let’s use average groundwater temperatures to illustrate what you need to do.

Example #1 — Warm Climate With Temperature Rise of 38°F

So, let’s say you want your hot water to run at 105°F (41°C), and your groundwater is 67°F (19.4°C). Consequently, your temperature rise is 38°F (3.3°C). 

Therefore, you need a water heater that can increase when water is flowing at your peak flow rate of 6GPM.

Now, let’s see how some tankless water heaters from popular manufacturers stack up against these requirements:

ProductFuelTemperature Rise °FFlow Rate (GPM)Meets Peak Demand?
Rinnai RU130iNNatural Gas/LPG38°F6.2
Rinnai RU160iNNatural Gas/LPG38°F7.8
Rinnai RU180iNNatural Gas/LPG38°F8.8
Rinnai RU199iNNatural Gas/LPG38°F9.8
Rheem RTGH-RH11DVLNatural Gas/LPG38°F11
Rheem RTGH-RH10DVLNatural Gas/LPG38°F9.9
Stiebel Eltron Tempra 36Electric – 240V38°F6

As you can see, all these products can meet your peak hot water demand. So, you have several options. Not only do you have a choice of models, but also fuel.  

But, choosing the Stiebel Eltron may not be the best course. That model only just gives you the water flow you want at the 38°C temperature rise. Often, it’s sensible to future-proof your system in case your peak demand needs were to increase. So, even if you don’t have natural gas, you might consider buying one of the other models and using LPG.

Example #2 — Cool Climate With Temperature Rise of 58°F

Now let’s assume you’re in a colder climate where the groundwater temperature is only 47°F (8.3°C). 

So, to reach your desired temperature of 105°F (41°C), you now need a tankless system that can give a temperature rise of 58°F (14.4°C) at 6GPM.

Let’s see how that affects your choice of models from the same list we used above:

ProductFuelTemperature Rise °FFlow Rate (GPM)Meets Peak Demand?
Rinnai RU130iNNatural Gas/LPG58°F4.2x
Rinnai RU160iNNatural Gas/LPG58°F5.2x
Rinnai RU180iNNatural Gas/LPG58°F5.8x
Rinnai RU199iNNatural Gas/LPG58°F6.4
Rheem RTGH-RH11DVLNatural Gas/LPG58°F6.4
Rheem RTGH-RH10DVLNatural Gas/LPG58°F5.8x
Stiebel Eltron Tempra 36Electric – 240V58°F4.25x

Now, you’re limited to two models only. Of course, you could choose one of the under-spec models. But, you’d find that your tankless water heater won’t keep up with demand at peak usage times.

These examples underline how important it is to match the model you buy to your household’s needs. 

The following video provides an excellent recap of the steps you need to take to do this.

Generally speaking, it’s not wise to go for an under-powered model because it’s cheaper. You’ll find you don’t get the hot water you want at peak times. And many models cut out if you overload them. If you continuously overload the system, you’ll end up shortening its life-span so that it would be a false economy.


So, a tankless water heater is potentially a great way to provide for your family’s hot water needs. 

You must take care to buy a system with a specification level capable of meeting those needs at times of peak demand.

Of course, you should bear in mind that your peak hot water needs might increase in the future. So, you may need to choose a model slightly above your current requirements. Although that might be more expensive in the short term, it’ll be cost-effective in the long run.


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.