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Can A Tankless Water Heater Fill A Tub?

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Tim Rhodes

Can a tankless water heater fill a tub?

tankless water heater, also called on-demand water, is more effective and energy-saving than the traditional water heaters on the market.

You can mount it on the wall and supply your entire family with hot water for showers, soaking tap, faucets laundry dishwashers, among others.

The tankless water heater supplies water on demand.

You will get water on your tap when you need it.

The main advantage of this heating type is that you save energy because you don’t heat a large volume continuously and maintain a constant flow of hot water.

Can this type of water heater fill your tub?


Advantages And Disadvantages Of The Tankless Water Heater

  • Endless water supply

The tankless water heart heats water on demand.

You will never run short of hot water.

It is useful for large families or homes with ample water supply.

Homes with a spa system or soaking tubs are ideal for this kind of water heating system.

If you need upgrading your tank size to over 40 gallons, a tankless water heater is right to consider this tankless water heater.

  • Energy efficient

The most efficient traditional gas tanks operate with an efficiency of 62- 67%, lower than the current tankless water heater, which works 82 -96%. You can save a lot of heating costs with this type of water heater.

  • Longer service life

Tankless water heaters have more extended warranties compared to the traditional tank heaters.

You will have the replacement cost of the heating unit half as it could be with a conventional gas tank heater.

  • It saves space

A tankless heater is usually smaller in size which frees up your storage space in your home garage.

Disadvantages of a tankless water heater.

  • Retrofit cost

There is a high cost of shifting from a traditional tank heater to the tankless water heaters.

You will spend a lot of money installing the gas lines, water line charges, and the venting.

The cost of installing a subsequent tankless water heater will not be costly because you won’t incur a retrofit cost.

  • Longer waiting hours at the tap

Most people think that there aren’t long waiting hours at the faucet with a tankless water heater, that isn’t true.

With this type of water heater, you will spend at list 15 extra seconds waiting for hot water to get into the tap.

  • The lower cost of operation is not guaranteed

Since it will be easy to access hot water regularly, most homeowners tend to waste hot water.

The increase in the use of hot water will outdo the low cost of heater resulting from high efficiency.

  • The tankless water heater needs electricity

The tankless water can’t run without electricity. 

Therefore you won’t get hot water during a power outage.

  • Tankless water heater flow

There is no water tank to drain with the tankless water heater. 

Therefore you didn’t remove the water heater with a tall bath.

The issue with the type of water heater is the flow and not the capacity since it does run out of hot water unless water supply flow overpowers the heater.

The tankless water heater will not supply enough hot water in cases where the two showers and the watching machines are running at the same time. 

Water will pass through the tankless water heater without getting heated, consequently producing lukewarm water.

You need to buy a unit that puts out more heat to maintain the supply of hot water regardless of the flow rate and amount of water flowing into the heating unit.

Working Of The Mechanism Of The Tankless Heaters

You need to understand the working mechanism of the water heater before determining whether it can fill the tub.

There are three types of water heater: non-condenser, hybrid condenser, and condensing tankless heater. 

They both differ slightly in functionality but with the same mode of action.

The incoming cold water exchanges heat with the hot coil, making it deliver hot water to your faucet.

The electric heater uses elements, unlike the gas heater that uses burner.

The condensing water heater operates uniquely and more efficiently by introducing a second heat exchanger that utilizes the heat from the exhaust.

Both units of tankless water heater do not produce instant water heater; the water has to take some time to flow from the system to the tap.

Why Choose a Tankless Water Heater For Your Tub

There are other water heaters on the market you suitable for hot your bathtub.

But why run for a tankless water heater?

Water heaters with a storage tank may work effectively with the bathtub, though they may not supply hot water on demand.

The tankless water heater is the best alternative to homes with traditional heater that they need replacement due to less efficiency, age, or leaking.

If you need to lower the water heating bills or want to have an endless hot water supply, try this type a heater. 

The water heater has a wide range of market acceptance both in the global market.

The reason for the popularity of the tankless water heater is its compact size.

It saves the floor space in utility rooms and garage, inability to catch rust, and high efficiency.

The water tank is a gain to both individuals wishing to lower the heating cost or have a positive impact on environmental conservation. 

Some companies can offer you a utility rebate for installing this type of heater.

Installing And Maintaining a Tankless Water Heater

The two primary factors you should look at when installing and maintaining a tankless water heater are location and fuel type.

  • Fuel type

If you currently operate with a gas water heater, you can go for the gas tankless water heater. 

However, you will need to increase your gas line because the tankless water heater needs a more extensive path to meet the high hot water demands.

If you are currently using an electric water heater, you will net to shift to the tankless water heater

Some utility companies offer rebate services when making such a shift.

Electric tankless water heaters are not the best for homes because the standard of their outcomes is undesirable.

  • Locality

Understanding the locality of your tankless water heater is essential because it will help you save your storage and installation cost.

You should talk to your installer about the best installation space for your heater at home.

While installing the heater, put into consideration what to do in case of a power outage. 

Some installers may give you an option of a battery backup option.

How To Select The Right Tankless Water Heater

The rating of the tankless water heater depends on the temperature rise at a particular flow rate.

You need to determine your flow rate and temperature rise demand before buying choosing a suitable hot water system for your domestic use.

You should avoid saving money by under-sizing your water heater.

The following procedure can help you come up with the best water heater for your tub.

Step 1:

Determine the number of devices you need to run and their flow rate requirements.

Sum up all the flow rates to determine the final value you will need for your water heater.

Step 2:

Determine the rise in temperature. You can find it by subtracting the incoming water temp from your required water temp.

People living in warm temperatures may need a higher water temperature.

Step 3:

The final step involves Sizing your water heater.

It will depend on the water temperature rise and the flow rate you need for domestic use.

Reasons For Not Having Hot Water In Your Tub

  • Overusing hot water

Although you may enjoy long hours of hot water, the water heater won’t supply the whole family at the same time.

When everybody is in the bathroom using hot water at the same time, the shower may end up running out of hot water.

The last person will cause shower cold water.

Similarly, running dishwashers, washing machines, and other warm water applications simultaneously will lead to an increase in the flow rate, resulting in inadequate heating of water.

You should not overuse the water heater if you need hot water in your bathtub.

  • Tankless water supply size

The water heater supplies water on demand.

The unit does not have a storage supply, so you need to size your water demand with the heating unit for an endless amount of hot water.

Coldwater from the tankless heater may indicate that its size is not in line with the family water supply needs.

  • Sediment and mineral build-up

Sediments and minerals such as calcium carbonate may still be present in the water despite the filter systems in the heater.

They may accumulate in your water tank, reducing the water heater’s efficiency and, therefore, making you shower cold water.

You can detect the problem by the pop sound coming from the water heater as an indication of sediment and mineral build-up.

  • Worn out or damaged parts

If any part of the heater damages, the functionality will be tempered with resulting in running out of cold water.

Installation Of The Tankless Water Heater

Installing a tankless water heart is a complicated procedure that involves a series of steps. 

A licensed plumber with experience in installing sophisticated home appliances can help in establishing the system.

You will need the following tool before beginning the installation process:

  • Hack saw
  • Drill
  • Spade bit
  • Wrenches
  • Screwdrivers
  • Reciprocating saw
  • Propane torch

If you are installing a propane tankless water heater, you will need additional materials such as 2x four board, stainless steel vent pipe, gas supply, pipe insulation, flux, copper pipe, and a black pipe.

Step 1: Disconnection Of The Preexisting Water Heater

Turn off the heater, disconnect all the gas supply, and drain any water from the tank.

Step 2: Upgrade Or Replace The Gas Line

Add a valve and a tee to the preexisting gas line. 

Install a gas supply or gas-supply line between the attachment point of the tankless heater and the tee.

Step 3: Installation Of The Waterline

Attach the lines for cold and hot water to their respective connection point of the tankless water heater. 

Use a bell hanger for the better control of the pipe at this stage.

Ensure you clean the pipes from oxidation before the application of the soldering. 

With the flux on the end of the tube, join a connector to them and solder together.

Stage 4: Mount The New Tankless Water Heater

Mounting of a propane tankless heater should be about 4 inches from the wall, probably at a place with proper ventilation.

The heater should be sturdy and secure on the wall.

Step 5: Connection Of The Tankless Water Heater

Follow the manufacturer’s manual in installing the heater.

Step 6: Vent Installation

Follow the complicated installation steps in the manual to install the vent.

Step 7: Activation Of The New Tankless Water Heater

With the completion of the installation process, plug the heater into the nearest power nearest outlet.

The Tankless Water Heater Size You Need For Your Tub

To determine the water heater’s right size for your home tub, you must understand the water demand, capacity, and heating capability.

  • Size

The water heater size corresponds to its rating.

  • Rating of the tankless water heater

The water tankless heater’s grade depends on the amount of heat and the volume of water passing through the unit.

  • Flow rate

The tankless water heater flow rate is the amount of water going through the heating element per minute

To get the best tankless water heater for your need, determine the unit size, which can heat water for the combined flow rate of the total output.

 Never take a water heater that does not meet your demand.

Finding the most suitable size of the tankless water heater for your tub involves the following series of steps.

  • Count the output

Determine the number of outlets you need to connect to the tankless water heater. 

Find out if the system will be supplying hot water to your kitchen bathroom or any other point of use.

Determination Of The Total Water Flow Rate

Find out the overall flow rate of the output you are planning to connect to the heater. 

If you have several outlets, think of the possibility of using all the hot water points simultaneously.

The tankless water heater you purchase should exceed all your output flow rate to maintain a regular supply of hot water.

Take note of the incoming water temperature.

You can use the thermometer to take the water temperature of the incoming water.

It is usually affected by the outdoor temp of the area.

Determine the temperature you need for your tub.

Find out the temperate of water your intent to have in your bathtub.

Find the difference between the incoming water temperature and the intent temperature.

What If You Want To Fill The Water Tub Quickly?

A tankless water bath can fill the water bath with hot water. 

However, you must ensure that the bathtub flow rate matches the water heater’s rate.

If your bathtub has a high flow rate, you will need a high heating capacity to meet the need.

If the tankless heater has a low water flow rate, its supply will be inadequate for the bathtub.

A storage tank heater will be the best alternative to the quick filling of the water bath with the hot water.

A tankless water heater can provide a continuous supply of hot water.

However, if the flow rate exceeds the heating unit’s capacity, the water temperature will fluctuate.

You can keep the flow rate low to maintain the water temperature.

The challenge with a low flow rate is that it will take longer to fill the tub.

Such a challenge will be to make the water temperature low before you take a shower, which will make you have a cold bath.

Tankless water heater takes time to sense the flow rate of water. 

When you switch it on, you will realize a flow of cold water before hot water starts flowing – a scenario called cold water sandwich.

With this cold water flowing in the tub, the overall water temperature will go low that will make you take a discomfort cold shower.

As you reduce the heat flow rate, your tub’s water temperature will keep on growing cold.

By the time you shut off the heater, the water temperature of your tub will be colder and not comfortable for a shower.

For that reason, the storage heater is more commendable for filling tub compared to the tankless counterpart.


If you are looking for a tankless water heater to fill your tub with hot water, try this type of the heater.

The 6kw/240 volt electric tankless water heater will supply your bathtub with a continuous flow of hot water for a comfortable shower.

The heater is ideal in supply hot water to individuals living in tropical regions.

The tankless heater design is compact and with an excellent shape that is easy to install and consumes less space.

Once you install the heater successfully, the ATMOR top is ready to release hot water within a few seconds.

It has a design compatible with a single shower, 6kw/240 volts, 30 Amp double pole breaker,30 Amp double pole,23 Amp single phase.

The product comes in a dimension of 8.46 x 3.03 x 15 inches and a weight of 4.41 pounds you will also enjoy a warranty of 2 years for the parts only and seven years’ linkage free.

The water heater has an IP25 splash-proof certification for high-quality standards and performance.

It has a good shape and design for wall mounting that saves your space while serving you with hot water without delays.

Why Select This Type Of Heater For Filling Your Tub?

The design has a high flow rate for supplying several hot water output simultaneously.

It cuts the cost of heating water.

You only heat water when in need

The water heater enables precise temperature control.

It comes in a compact design that saves the installation space.

The tankless water heater on the other side is energy-efficient and saves your storage space by providing an on-demand water supply.

The main challenge of using a tankless water heater is their ability to fill the tubs with hot water.

To use this type of heater in filling your water tubs, you need a specialized plumber to calculate your flow rate and recommend the best heater to meet your needs.

Remember, the tankless water heater does not sense the water flow instantaneously, the faucet may begin by producing cold water before hot water starts flowing.

Filling your bathtub with this cold water can reduce the overall temperature of the water, causing undesirable effects.

Pros And Advantages

The heater comes with additional accessories such as water heater, handheld shower rise rail, and a hose.

It has a compact design that saves your space.

It is easy to install and maintain.

It allows for constant water supply.

Final Thoughts

The question remains, can a tankless water heater fill a tub?

To enjoy a hot shower in a bathtub, you need a constant supply of hot water.

You, therefore, need a suitable heater that will supply you with regular hot water for your bathtub.

Though storage water tank may be a good alternative for supplying hot water to your tub, they may not be very energy efficient or provide an on demand-supply.

You must be able to balance the tubs flow rate with that of the tankless water heater.

Running a large flow rate bathtub with a water heater having a low rate will result in under heating of water that will result in cold water.

If you do not balance the two, you will have cold water flowing into your bathtub, making a shower unfordable.

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