How do I Choose my new Tankless Water Heater?

Proper Sizing

  The number one reason for dissatisfaction with a tankless water heater is improper sizing.    When sizing a tankless water heater, think in terms of flow, not capacity .  Tankless water heaters don't run out of water like a conventional water heater can, but it may not be able to heat water fast enough to serve multiple fixtures at once.

Sizing a tankless water heater is different than sizing a storage tank. Sizing a storage tank requires estimating the volume of hot water you will need within a couple hour time frame. Sizing a tankless water heater requires that you estimate your peak demand in terms of momentary usage; in other words, what is the most you will be running at the same time?

For those with low demand, this could be a single shower. For others, this could be multiple showers plus dishwasher. A tankless water heater is given a BTU (short for British Thermal Unit) input rating and efficiency rating.

These determine its flow rate, typically expressed in gallons per minute (GPM).

Follow this simple task to determine your specific GPM needs:

1. turn on the faucet and time with a watch how long it takes to fill a one gallon container (in seconds)

2. divide 60 by the number of seconds it took to fill the 1 gallon container - ex. a shower that takes 40 seconds to fill a one gallon container has a flow rate of: 60 divided by 40 seconds = 1.5 gallons per minute

One BTU is equal to the amount of energy required to raise the temperature of one pound of water by one degree F. The higher a water heater's BTU rating, the higher the flow rate.

Under normal circumstances, it takes about 31,000 BTUs to deliver 1.2 GPM; 190,000 BTUs typically deliver 5.7 GPM.

Flow rates vary from about 1.2 to 6 GPM. Point-of-use models, such as those that provide hot water to a single bathroom sink, are toward the low end of the flow-rate spectrum.

A 2.6-GPM tankless water heater will handle only one shower at a time. A 4-GPM model will generally supply one shower and one sink at the same time. A 6-GPM tankless heater will handle two showers at once.

Homeowners who are concerned about energy efficiency also need to pay attention to a unit's efficiency ratings.

These ratings, which range from about 78% to 87%, indicate how much of the fuel is converted to heat by the appliance--the higher the better..

Practical Flow Rate

The performance of tankless water heaters are rated in terms of the flow rate (in gallons per minute GPM) they can output while raising the water temperature by some margin (this is called temperature rise). In other words, how many GPM can be output while raising the water temperature by 35°F, 45°F, 60°F, 70°F, etc. The larger the temperature rise, the less the unit can output.

Gas vs Electric

Many customers that come to our store have either gas or electricity available to them, not both. In this case the question is simpler, but not answered. Others have both available and need to decide which a better fit is.    Tankless Product Page

Electric tankless water heaters have a different type of application.  They will usually have a lower output and require more energy (electricity) than Gas units . Each electric heater has different electrical requirements and it is very important that you consult with an electrician prior to making a purchase. In terms of flow rate, most of the electric heaters we carry can only run one major application (shower, dishwasher, washing machine) at a time.  This is commonly referred to as a "Point of Use" tankless water heater.   They are ideal for small homes, cabins, apartments, work shops, boats, motor homes, horse stalls, hair salons and other similar applications.   One should not be fooled into thinking that a typical electric tankless water heater is going to give you 4 -5 gallons per minute or accommodate multiple faucets or outlets at one time. 

Gas tankless water heaters can supply more hot water and are a better choice for those with mid-high demand. Gas powered heaters can be installed indoor or out; though indoor is best in very cold climates to avoid a freezing risk. Electric and Outdoor installations can be less expensive since you eliminate the need for vent pipe.    Tankless Product Page

Need to find an electrician or plumber in your area?  Try Angie’s List.

Check your gas and water supplies

Tankless water heaters can produce three to four times the BTUs a conventional heater produces. Your plumber must verify that your current gas line size, length and even gas meter can provide sufficient gas flow to the unit. Tankless heaters also require minimum water flows for activation; again, your plumber should verify that your supply meets the specifications.

High Flow Applications

Some customers will find that their peak demand is greater than any single unit can accommodate. In these situations, a multi-unit (manifold or parallel) configuration should be considered. This can be a complicated matter and you should consult a professional plumber or electrician before undertaking this endeavor. 

Example below shows (3) Santon HP925M - 220v electric heaters - installed in parallel for 3 bath home. Approximately 9 GPM delivery capacity.

Avoid long <-----> runs between the heater and the faucet.

Because tankless heaters generate hot water only when you turn on the hot water tap, it takes some time (a small delay) to heat cold water to the optimal temperature. Locating the heater far from the tap can result in a more noticeable “sandwich effect” when hot water is used intermittently. One way to overcome this problem is to use a recirculation pump that brings water from the farthest fixture in the plumbing run back to the tankless heater, but because these pumps use a lot of energy keeping water in the loop hot, it’s better to preheat or buffer hot water.

Carefully plan the location of your water heater

Gas Tankless water heaters use an intense flame to heat water on demand. They require more air for combustion and vent more exhaust than conventional water heaters. This affects how and where you install them in two ways:

Venting: For direct venting (through the wall), the vent termination must be at least three feet from any operable window. If you are venting through the roof, the length of the vent is determined by the size of the heater (the BTU output) and the number of elbows, or turns, in the vent. You may not be able to vent the heater through the roof if the vent run is long—for example, if the heater is in the basement of a three-story building.

Combustion air: A gas-fired water heater (tankless or not) requires a source of oxygen for combustion. To avoid back-drafting that combustion air through another appliance’s exhaust pipe, your water heater should be sealed-combustion direct-vent.

 

Example below shows Marey 16L NG 4.5 GPM  (GASEXP16LNG) installed in a new home - with vertical venting.

 

Professionals

Please talk to your local retailer such as Lowe's Home Store  or  Home Depot  about attending a training class on these products. They must be installed properly to work well and installing them improperly may result in damage to the tankless heater, poor performance, premature failure or injury to the homeowner.  Be sure to read and understand the water heater installation instructions - and requirements for installation.   Improper installation can void your warranty.

Additional Information:
Tankless Water Heaters 101 by M. Scott Gregg      An excellent article!