How and Why Tankless Water Heaters Lose Hot Water

On-demand hot water is a common feature in connection with tankless water heaters. Switching to a tankless water heater seems ideal if your tank water heater struggles to meet your hot water needs. Let’s talk about the question of whether tankless water heaters run out of hot water.

Tankless water heaters provide on-demand hot water; however, during periods of high demand, they may appear to run out of hot water. The unit may need to heat water quickly enough to fulfil demand when there is a heavy demand from several fixtures.

In principle, a tankless water heater should be able to provide unlimited hot water on demand, but you must be aware of certain restrictions. This post will review the fundamentals of a tankless water heater’s operation. Then we’ll talk about why it could have run out of hot water.

What Is the Process of a Tankless Water Heater?

In contrast to conventional tank systems, a tankless water heater does not store hot water.

Instead, it warms the water anytime you open your faucet’s hot water supply. Tankless water heaters for natural gas and propane heat water on demand by burning potent gas. Electric tankless water heaters swiftly heat water using electric heating components.

Although indoor tankless water heaters are also available, tankless water heaters are often put outside. The way they are vented is the sole distinction between indoor and outdoor tankless water heaters.

So, if you turn on the hot water faucet:

  • Your tankless appliance receives cold water, which travels to a heat exchanger.
  • This water flow is detected by a sensor, which activates the burners or heating components.
  • The burners or heating elements create heat, which the heat exchanger transmits to the water.
  • The hot water flows directly to the faucet once it reaches the temperature you’ve specified.
  • The heating process comes to an end when the faucet is shut off.

As a result, on-demand heating uses less energy as a water tank does not need to be maintained at a constant temperature. Even when there isn’t an urgent need for water, a tanked system utilizes energy continuously to heat the stored water.

A tankless system needs much energy to heat the water rapidly enough to satisfy demand. Even so, it’s still more effective than repeatedly boiling water that has been kept.

According to data from, a tankless unit is 24%–34% more energy-efficient than a tank system if you consume up to 41 gallons (155 ltr) of hot water daily. The efficiency gain ranges from 8% to 14% for homes using 86 gallons (325 ltr) daily.

Although it seems straightforward, things might go wrong and leave you without hot water. Let’s look at why that may occur with a tankless system.

How Tankless Water Heaters Lose Hot Water

A tankless water heater may become overwhelmed by demand.

The demand being too great is one of the main reasons your tankless system can cease providing hot water.

If numerous hot water appliances are being used simultaneously, this can occur. For instance, if you simultaneously run a shower, a clothes washer, a dishwasher, and a bath.

If demand exceeds the tankless system’s capacity, some appliances will receive hot water while others won’t. Alternatively, no hot water will reach any of the devices. Even worse, if the tankless system is overwhelmed, it may automatically cut off.

How to Determine Your Maximum Hot Water Demand

Because of this, you must be sure the tankless water heater you choose can provide the hot water you need.

You must determine how much hot water your home uses per minute during peak periods.

Calculating the simultaneous usage of your appliances and the hot water flow rate when they are in use can help you achieve this. You will then be given your maximum GPM or gallons per minute.

For instance, your peak use would be 5GPM if you had two showers running at 2GPM each and one faucet at 1GPM.

The next step is to determine how to raise the entering water’s temperature to the necessary level. This is the increase in temperature.

It would be best to verify the temperature of your water source’s entering water. Groundwater temperature maps might provide a broad indication if you need clarification.

Let’s say the average temperature of your groundwater is 47°F (8.3°C). Your hot water should be heated to 104°F (40°C). As a result, your body temperature has increased by 57°F (31.7°C).

As a result, you must pick a tankless system that can raise the temperature of incoming cold water by 57°F (31.7°C) while the water is flowing at 5GPM.

You may pick a Rinnai RU160iN Sensei system. This results in a flow rate of around 5.6GPM for a 57°F (31.7°C) temperature increase.

The Rinnai RU130iN Sensei, the model after that, is a little less expensive. But this temperature rise only provides a flow rate of 4.6GPM. As a result, you will pay less, but you cannot get enough hot water during busy periods, making you quite frustrated.

If you did this, you would have run out of hot water.

You May Need Two or More Tankless Water Heaters

Tankless water heaters may be required in pairs or more

You’ll need many tankless water heaters to meet the demand for hot water if you have a big family, a property with many rooms, or frequently have house guests around.

We frequently find bathrooms sharing one tankless unit while the kitchens and laundry rooms share another. Even with the largest tankless water heater on the market, large households might still need more hot water than the tankless water heater can supply.

If you have any of the following, having tankless water heater systems is beneficial:

  • A big house where the hot water has a vast distance to go, and the kitchen and bathrooms are on opposite ends of the house. Frequently, tankless water heaters might cause the delivery of hot water to take at most two minutes. This often occurs because the tankless water heater is far from the appliance or fixture.
  • Have a big family or frequently have home visitors who shower simultaneously. If you wash your clothing and take a shower at the same time or run the dishwasher, this is particularly accentuated.

Consider a recirculation pump with a gas or electric storage tank if you want to install something other than two tankless water heaters. See our post on Tankless Water Heater Setups that Need Expansion Tanks.

The Water Is Moving Too Slowly

A slow flow rate develops when a faucet is only slightly opened. Your tankless system will only begin heating if the water moves quickly enough.

Every tankless water heater needs a minimum flow rate to start combustion. For example, the gas Rinnai models require a minimum flow rate of 0.4-0.6GPM to light the burner.

The burner will only light if you fully open your faucet so that there is that much flow through the tankless unit. Thus, the water will flow chilly, giving the impression that you are out of hot water.

A minimum flow rate of 0.26GPM is also required for the Rinnai gas versions to maintain the burner’s operation once it has been lit.

As a result, hot water will be available if you turn on your faucet fully. However, the water will get frigid if the flow rate drops to less than 0.26GPM. This will give the impression that you are out of hot water.

Limescale accumulation restricts flow.

Another typical explanation for your tankless water heater appearing to have run out of hot water is limescale buildup.

Your groundwater’s calcium and magnesium content causes limescale buildup.

The amount of calcium carbonate in the water, expressed as milligrams per litre, determines how hard it is. Calcium carbonate concentrations range from 0 to 60 mg/L in soft water, exceeding 180 mg/L in hard water.

The issue with hard water is that it forms calcium carbonate deposits when heated. These may build up in plumbing and water equipment.

The amount of water flowing through your pipes may be slowed down if these accumulations exist. The rate can drop below the minimum your tankless unit requires to start or keep the burners lit. As you can see above, that may prevent you from receiving hot water.

It will take longer for heat to flow from the burner to the water if the heat exchanger in your unit becomes blocked with scale deposits. It could also imply that the system cannot heat the water to your preferred level. Yet again, it appears that you are out of hot water.

You Have a Tankless Water Heater Error

An error code may also cause your tankless water heater to cease heating the water. There will often be an audible alert if your water heater has malfunctioned.

If your tankless water heater is beeping, look at the digital display’s fault code. You must consult your owner’s handbook to get the problem code. Resetting the tankless water heater can erase the majority of trouble codes. There are certain exceptions, though:

  • A power outage, a lack of gas supply to the unit, or an ignition failure can all result in an ignition failure.
  • It would help if you got your tankless water heater flushed. Tankless water heaters must be cleansed, so yes. Descaling your water heater is a common term for this.
  • The tankless water heater has a malfunctioning burner.
  • The tankless water heater’s thermostat may have failed.

Your tankless water heater will frequently switch down to safeguard the system against problems like these. You’ll often need a plumber to work on the appliance to operate again.


There are several causes for a tankless water heater to appear to run out of hot water. In this post, we’ve examined some of the popular ones.

However, as you can see, the tankless water heater is not out of hot water. Most likely, it just cannot meet demand.

You may prevent the issues mentioned by selecting the appropriate device, utilizing it properly, and maintaining it.