What Is One Disadvantage of a Whole House Tankless Water Heater?

Water heaters that don’t have a tank are much more efficient than the traditional units. Moreover, tankless models have a greater lifespan and help save space, but they have one major disadvantage.

The main ‘con’ of on-demand water heaters is their price – the units are often twice as expensive as the models with a tank. Moreover, you might have to pay up to $4.600 for the installation of an on-demand water heater – such units often require additional piping and electrical updates.

How Does a Whole House Tankless Water Heater Work?

Whole-house tankless models work the same way as conventional units with one important difference – they don’t have a tank where hot water is stored.

So, for example, in a tankless gas water heater, cold water gets drawn into the unit only once the hot-water tap gets opened. A special flow sensor activates the burner, the heat exchanger gets warmed up, and the heat is then transferred to the water.

On-demand water heating appliances are much more energy-efficient as they eliminate energy loss during standby operation and literally produce hot water ‘on demand’.

Read: Why Water Heater Is Leaking?

What Is the Disadvantages of Water Heater?

Water heaters have plenty of benefits. However, all of them share a few ‘cons’.

Water heaters take up space

Every type of water heater requires a dedicated space. The units with a tank would, of course, need more square feet.

The supply of hot water is usually limited

If you manage to pick a water heater of the right size, then these issues might not be as noticeable. But all in all, no water heater would be able to produce a never-ending flow of hot water at a high rate.

Water heaters require regular maintenance

Water heating units have to be flushed at least once a year and all units have to be checked by a professional on an annual basis.

What Are the Advantages and Disadvantages of a Tankless Hot Water Heater?


  • Provide hot water almost instantly
  • Have a longer lifespan than the models with a tank
  • Are more energy-efficient (up to 34% more efficient than a conventional unit)
  • Contribute to long-term savings due to energy-efficiency
  • Help save space
  • Qualify for federal tax credits
  • Don’t run out of hot water
  • Offer longer warranties


  • Have a higher initial cost
  • Are more complicated to install
  • Might require additional equipment (like a water softener, for example)
  • It could take 6-12 (or even more) years before the monthly savings cover the unit’s upfront cost

Read: Is Furnace And Water Heater The Same?

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What Is One Disadvantage of a Whole House Tankless Water Heater?

The main ‘con’ of tankless water heaters is the initial cost of the unit together with the installation cost.

Some tankless units cost twice as much as a regular appliance with a tank. 

Moreover, you might have to pay a small fortune for the installation of the appliance. At times, the piping would have to be extended and, in case you have decided to go for a gas unit, a safe vent would have to be added.

Does Water Heater Use a Lot of Electricity?

Water heaters use a lot of electricity. In fact, such systems account for around 18% of your electricity costs.

Even if your water heater runs on gas, the chances are high that it still needs electricity in order to function. The thermostat, the electronic ignition system, and the control board are the elements that are powered by electricity.

Fact: tankless electric water heaters require up to 30.000W. 

Read: The Best Temperature For The Water Heater And Why?

Are Electric Tankless Water Heaters Any Good?

Electric tankless water heaters can become a great option for a house with limited space and for the areas that don’t have a constant gas supply. 

Such units are also far more energy-efficient than conventional water heaters, so you’ll be able to save on your monthly bills.

Finally, tankless models do not run out of hot water as they produce it on demand. If you manage to pick a unit of the right size for your household, then you’ll be able to enjoy the convenience and comfort that tankless water heaters have to offer.

Read: What Is Water Heater Heat Pump? Everything You Need To Know

Can You Run Out of Hot Water with a Tankless Water Heater?

On average, such units can provide around 2-5 gallons of hot water per minute. By the way, gas tankless water heaters can produce higher flow rates.

You can’t exactly ‘run out’ of hot water with a tankless unit, but the water heater might not be able to keep up with the demand if you’ve got too many appliances and taps turned on at once.

To avoid such a situation, you should calculate your peak hot water demand – you might need two or more units to be able to supply your household with enough hot water.

Is Tankless Water Heater Safer?

Overall, tankless units are safer than conventional water heaters as they don’t have a tank that can experience a spike in pressure or overheat. For these reasons, the chances that a tankless system is going to explode or burst are incredibly low.

If you opt for an electric tankless water heater, you are also going to eliminate the risk of a gas leak and carbon monoxide poisoning.

Can Tankless Water Heater Replace My Boiler?

In some cases, a tankless system will be able to replace your boiler. But here are a few things that you would have to bear in mind:

  • A tankless system should be designed for radiant heating if you want to use it instead of your boiler (or it should have a recirculation pump).
  • You might have to raise the hot water production to around 180 F.
  • If you have a big house, two or more tankless units might be required to keep up with the hot water demand. 

Rinnai Tankless Water Heater

Rinnai is one of the companies that make high-output tankless water heaters that are certified by the ASME. This means that the units can be used to provide not only domestic hot water, but also hydronic heat.

Does a Tankless Water Heater Heat a Whole House?

There are special whole-house tankless water heaters that are able to heat your house and provide continuous hot water.

Is One Tankless Water Heater Enough for a House?

One unit should be enough if you have a small household that tends to use up to 40 gallons of hot water per day.

You might also find a large model that would be able to keep up with higher demand.

How Many Tankless Water Heaters Do You Need in a House?

Here’s what you should do to size your demand water heater:

  1. List the number of hot water devices in your household that you plan on using simultaneously.
  2. Add up the devices’ flow rates.
  3. Determine the preferred temperature rise (a 70-degree water temperature rise is the average one).
  4. Choose a unit that is rated for the right flow rate and temperature rise.

In general, a single tankless water heater would be able to supply a household that uses up to 41 gallons of hot water per day.

What Size Tankless Water Heater Do I Need to Replace a 40 Gallon?

Number of peopleWater usageTankless water heater size
1-2Low/regular30 gallons
High40-50 gallons
2-3Low/regular40 gallons
High50 gallons (40-50 for gas-fired units)

Tankless Water Heater Cost

Gas tankless water heaters are pricier than electric ones and cost anywhere between $400 and $2.100. Electric on-demand water heaters cost $300-$1.000.

For comparison, an electric water heater with a 50-gallon tank can be purchased for around $500.

Why Is Tankless Water Heater Installation So Expensive?

Be prepared to pay anywhere between $600 and $2.500 to install an on-demand electric water heater and up to $4.600 to have a tankless gas water heater installed by a professional. 

Some not-so-obvious expenses include:

  • Electrical updates – you might have to update the house’s electrical system, in accordance with the voltage requirements of the new water heater.
  • Additional piping and insulation – in a lot of cases, a tankless unit cannot be installed in the same place as a conventional water heater. So, the plumber and electrician might have to design a new setup.
  • Gas line installation

Installation of a power vent (draws indoor air) or a direct vent (draws outdoor air) – the good news is that tankless gas units can vent through the roof or an exterior sidewall.

Read: How Many Amps Does Water Heater Use?