Tankless Gas Water Heater

Tankless Gas Water Heater Systems are usually bigger compared to their electric counterparts, and are usually used to service multiple points of use simultaneously or as a whole house unit.

The image below describes a typical Gas Tankless Hot Water Heater. As can be seen, water is heated as it flows thru piping that runs around a burn chamber. Computer controls the fire’s size to heat the water to the desired temperature.

Natural Gas tankless water heater is very similar to Propane tankless water heater (LP Gas), and the differences are minor. Choosing between the two should be done according to the availability of Propane & Natural Gas where you live, and the type of piping already installed in your home.

Tankless gas hot water heater can be installed outdoor or indoor. An Indoor installation requires proper ventilation to dump the exhaust gas outside. Point-of-Use installation is almost never used.

There are two types of burner lighting systems. The older one uses a constantly burning pilot light, while the new models have an Intermittent Ignition Device (IID), which resembles the spark ignition device on some gas kitchen ranges and ovens.

Since a constantly burning pilot light wastes some energy, prefer a heater with an Intermittent Ignition Device.

Installation requirements consist of:

1. Electrical connection: a minimal electrical connection is required for the ignition system and control circuitry.
2. Proper gas-line: The big unit’s gas requirements may exceed that of your current system and may require a wider gas line to meet its needs.
3. Ventilation: Make sure that you purchase Category III stainless steel (UL1738 certified) venting for your instant gas water heater. Type B venting accessories are not acceptable.

Indoor Tankless Water HeaterIndoor Models:

The interior-mount units can be placed in any room, vent directly to the outside and have an integrated condensate collector. They also have a safety system that shuts down the unit in case of raised levels of carbon monoxide.

Indoor models further differentiate by their venting method, which refers to the way they supply the burner with fresh air while exhausting the flue gases outside:

Non-Direct Ventilation

In non-direct ventilation, a small internal electric fan or blower supplies combustion air intake from the location where the water heater is installed for efficient operation.

At the same time the fan pushes the flue gases to the outside when the unit is operated. Vertical or horizontal venting, which is determined by the direction of the piping as it leaves the unit is possible with this ventilation type.

Direct Ventilation

Tankless heaters with direct ventilation are sealed. Because of this, combustion air is supplied directly from the outside to the gas burner.

Two pipes or a split pipe are used, running from the water heater to a wall or the roof of a home. One pipe is used for fresh air intake, while the other is used for exhaust.

This type of venting should be located at least three feet away from windows and is often used in tight-fitting locations with limited ambient air.

Outdoor Tankless Water HeaterOutdoor Models:

The exterior-mount units do not need a venting system at all. In these models the exhaust vent is built into the front of the unit. That makes installation much easier and cheaper.

The outdoor models are designed to be weatherproof, and most of them incorporate freeze protection technology that will offer protection down to -30÷ F (-34÷ C). This will allow for outdoor installations in most climates.

Typically, outdoor models lack the carbon monoxide shutoff safety system.

In new constructions, homeowners often install two outdoor tankless units, one on either side of the house. They then split the plumbing to avoid long pipe runs.

Outdoor tankless water heaters can be placed in a specially designed metal box which can be recessed into the outer wall of your home. The front of the box can then be painted and textured to match your home’s exterior surface.

Electric Tankless Water Heater

Electric Tankless Water Heater Systems are usually smaller compared to gas fired ones, and are usually installed at the point of use as augmentation to conventional boilers or as the only source of hot water.

The image below describes a typical electric tankless water heater system. As can be seen, water is heated as they flow thru piping sounded by electric heating element. There are no special ventilation requirements.

Electrical requirements change according to the heater’s size. Electrical wiring and circuit breaker should fit the heater’s Amperage requirements, and may require replacement. You should consult a qualified licensed electrician.

There are several reasons for preferring an electric tankless hot water heater system over gas fired one:

• System size – electric tankless hot water heaters are smaller than gas fired ones.
• Ventilation – tankless electric water heater require no ventilation and can be installed indoors, inside a cabinet etc.
• Gas piping – not all homes are equipped with gas, however all homes are connected to the power grid. Electric tankless water heater is suited for both.
• Operating Costs – depending where you live, tankless electric water heaters may be cheaper to operate due to gas vs. electricity pricing differences.
• Environmentalism – while burning gas produces green-house gases, electricity may come from a clean source, making tankless electric hot water heater the preferred choice.

There are four types of tankless water heater electric systems you may come across, and it’s worth getting to know the right terminology.

Single Point:

The most basic type and often the cheapest one, the term “single point” or “single point application” refers to a small tankless electric water heater which is connected to one fixture only. Usually these heaters have only on/off capability.

Thermostatic:

Thermostatic tankless electric water heaters factor in the incoming water temperature and heat only as needed in order to reach the desired outgoing water temperature.


Most thermostatic units are capable of:

• Maintaining +/-0.5°F accuracy.
• Achieving full, pre-selected outlet temperature within 10 seconds after activation.
• Maintaining consistent temperature.

Flow Controlled:

The “Flow Controlled” range of electric water heaters are ideally suited to serve two points like two sinks, or a sink and a shower in close proximity, and are recommended only for cold water feeds.

The “Flow Controlled” feature basically means that once the heater reached its full heating capability, any increase in demand (e.g. another faucet is opened) will trigger the heater to restrict its flow rate so the output temperature does not drop below the desired setting.

This means that instead of providing colder water the system provides less hot water but maintains that water’s temperature.

Type Two:

“Type Two” (nowadays, Type Three is being introduced) simply refers to the bigger, newer more sophisticated units.

They are most likely thermostatically controlled, and often have also a flow control restricting feature as well.

Benefits of Electric Tankless Water Heater

1. Tankless electric water heater use either gas or electricity as heating elements. Gas is more efficient, as it heats at a faster rate, but it is more complicated to install since it needs a gas line and ventilation. A gas-heated device, however, is the obvious choice for a family that uses a fair amount of heat on a daily basis. Electric on-demand types, though much easier to install, are less capable of handling high volume. However, they are more than adequate for a home or apartment of one or two people.

2. These equipments are very useful when it comes to conserving energy. Tankless unit’s efficiency rating is almost 99%.

 3. These tanks heat water as it passes through the system of the appliance. So these appliances continue to provide hot water as long as these tankless heaters get water supply.

 4. Most of these tankless units are designed to run on gas. As a result of this, these are very environment friendly.

 5. The life expectancy is another great benefit of tankless electric water heaters. They are known for lasting longer than the more conventional type of water heater. These heaters can last for up to 20 years. In addition to these wonderful benefits, their ability to be easily repaired is also a major benefit. This makes it easier to replace heating elements when the time comes.

 6. Since tankless water heating is energy efficient, you are not just getting huge savings but you are also helping the environment. Aside from carbon dioxide emission reduction, it decreases overall dependence on non-renewable resources like the natural gas and greatly reduces the number of unrecyclable storage tanks thrown in landfills.

 7. Electric water heaters supply hot water whenever you need it. They are cost effective and efficient. These heaters work as a tankless system. This means there is no need to unnecessarily heat a whole tank of water, since a tank is not required.

For getting more idea, you must read this electric tankless water heater reviews. 

Determining the Required Instant Flow Water Heater Temperature Rise

The second step of sizing your Instant Flow Water Heater is to determine its required temperature rise.

Temperature Rise is the difference between the incoming cold water temperature to the desired hot water temperature. For personal use (bathrooms and sinks) you will want your hot water at 120÷F (49÷C), unless you use a dishwasher without an internal heater, in which case you will want your hot water at 140÷F (60÷C).

If you live in a cold climate, your incoming water is colder. This means that the temperature rise you need is bigger, hence a bigger tank less water heater.


Average ground water temperatures in the USA is between 50÷F (10÷C) and 55÷F (13÷C) winter to summer, so for a basic calculation you may assume that your input water (ground) is 50÷F (10÷C). Your actual input water may be slightly colder (not likely) or very much warmer (likely in warm climates).
If your ground water is colder – you will get less output. If your ground water is warmer – you will get more output.

You should consult the image below to determine the temperature of the incoming cold water.

US climate zone map for temperature rise calculation

To determine the required temperature rise, simply subtract the incoming water temperature from the desired output temperature.

For example, if you live in Illinois than your incoming water temperature is 42÷F (5÷C). Assuming that you need your hot water for a washing machine – you will need it at 140÷F (60÷C).

Your required temperature rise is 98÷F (55÷C)

Most instant flow water heater systems are rated for a variety of inlet temperatures. Typically, a 70÷F (39÷C) water temperature rise is possible at a flow rate of 5 GPM through a gas tankless hot water heater, and 2 GPM through electric ones.

Faster flow rates or cooler inlet temperatures can sometimes reduce the water temperature at the most distant faucet.

Tankless water heater efficiency, also known as Energy Factor (EF) has direct impact on your annual hot water bill and operating costs and should be accounted for as well.

Tankless Water Heater Comparison – Sizing your Tankless Water Heater

Before making a Tankless Water Heater Comparison you need to perform proper sizing of your home’s requirements,

Tankless water heaters are rated by the maximum temperature rise possible at a given flow rate.

In simple words, you will first have to calculate what will be your required water flow rate (based on your application: Single point / Multi point / Whole house), and then you will have to determine the required temperature rise (based on the climate you live in).

These two important numbers are essential for making any tankless water heater comparison.

Determining Flow Rate

Water flow rate is determined in GPM (Gallons Per Minute) or LPM (Liters Per Minute).
According to your application (Single point / Multi point / Whole house), list the number of fixtures that will be attached to the tankless hot water heater, and determine the number of fixtures that will be used simultaneously. Then use the table below to calculate the total maximum required flow rate.

Typical Flow Rates

Please note

1. These numbers may not reflect your appliances or fixtures due to:
2. Differences in water pressure from location to location.
3. Efficiency of appliances and fixtures.
4. Installing low-flow showerheads and faucets may reduce these numbers by 30% – 50%, resulting in a smaller unit.
5. If you have a multi shower head system – you need to find out exactly how much water all the shower heads will consume.

Example 1: A bathroom single point application (only shower)

Depending on your showerhead, you need a flow rate of 1.5-3.0 GPM (5.5–11 LPM)

Example 2: A bathroom multi point application (Bathroom Faucet + Shower + Bathtub)

You can assume that you will not use the shower and the bath simultaneously, so use the higher flow rate fixture (Bathtub).

You may also assume that you may use the Bathtub and the Bathroom Faucet simultaneously, so add the Bathroom Faucet’s rate of flow (0.5 GPM / 2 LPM) to the Bathtub’s flow rate (2.0-5.0 GPM / 7.5-19 LPM).

You need a heater that supports flow rate of 2.5-5.5 GPM / 9.5-21 LPM.

Example 3: A whole house application (2 Bathrooms and a Kitchen)

In this example you have a few options. You can assume simultaneous use of two bathrooms, dish-washer and kitchen sink – resulting in flow rate of 6.5-12.0 GPM (24.5–45.5 LPM). This will require a large unit, which might be to expensive.

Another option would be to assume simultaneous use of either two bathrooms or one bathroom and a kitchen. That will result in a smaller and cheaper unit that needs to support only 4.5-9.0 GPM (17–34 LPM)

Note: You can install low-flow showerheads/faucets or water aerators to reduce the demand flow rate. This will enable you buy a smaller heater.

After sizing it properly, you will need to find the required Temperature Rise for your instant flow water heater.

Introduction to Instant Hot Water Heaters

Instant Hot Water Heaters can help reducing your hot water heating costs. This step by step tutorial describes how they work, the different types and fuel sources, how to size the unit and issues associated with this technology.

What are Instant Hot Water Heaters?

Often called Tankless Water Heaters / Demand Water Heater / Instantaneous Water Heater / On-Demand Water Heater / Continuous Flow Water Heater / Point-of-Use Water Heater / Inline Water Heater / Instant-On Water Heater/ Tank Less Water Heater / Tankless Hot Water Heaters – these devices produce hot water only when there is a demand for it.

Unlike conventional water heater who pre-heats the water and then stores it for later use, a tankless water heater has no tank to store hot water. It heats the water at the moment they are needed.

How Does Instant Hot Water Heaters Save You Money?

On demand water heater saves you money in three ways:

1. With instant water heater you pay only for what you use.

Conventional storage tank water heaters pre-heat the water and store it in a tank for later use. Obviously, you always heat more water than you need because you never know in advance exactly how much water you will use, and you don’t want your hot water to run out in the middle of the shower.

With tankless hot water heater however there is no pre-heating. As water is heated on-demand, you heat exactly the amount you need. Thus you save a lot of money because you heat less water.

2. With on demand water heater there is no standby heat loss.

Conventional hot water heaters with storage tank loose energy on standby heat. Since water is pre-heated, they loose some of the heat until you use them. More so, the hot water that was not used loose even more heat until the next day.

With tankless water heater there is no standby heat loss because the water is used immediately.

3. Tankless water heaters have an average life expectancy of 20 years, twice as that of a conventional storage tank system.

Although most tankless hot water heaters cost more than conventional storage tank heaters, the return of investment period (for an average sized heater) is 2-3 years, and 5-6 years for the bigger units. This leaves you with almost 15 years of pure saving!

How much money can you save by switching to On Demand Water Heater?

That depends on how much water you use and the installation configuration. Contrary to what most people think – the more hot water you use – the lesser the saving. The reason for this is that most of the saving is on the amount of water you DO NOT heat. If you use a lot of water – you save less.

• Homes that use up to 41 Gallons daily (155 L) can expect to save 24%–34% of its water heating costs compared to conventional storage tank water heaters.
• Homes that use up to 86 Gallons daily (325 L) can expect to save only 8%–14% of its water heating costs compared to conventional storage tank water heaters.
• Homes that install an Instant Water Heater at each hot water outlet may expect bigger savings, 27%–50%.

What do these numbers mean?

• The percent annual savings of on demand water heater over a traditional tank system is (on average) 25% or $200.
• This amount ($200) is equal to the half the cost of an average mid-size tankless water heater.
  Please note: Installation costs were found to be comparable for both tank and tankless systems, ~$150.
• In addition there is the potential for Federal Water Heater Tax Credit on tankless water heaters.

Given the longer life of instant hot water heaters at 20 years versus a tank system 10 years there is the potential of saving over $2,500 over the long term.