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What You Need to Know Before You Buy

How much hot water do you use?

How much hot water do you use in your home?

Our top models will handle up to two major applications at once, such as a dishwasher and washing machine or two showers running at the same time. The best tankless water heaters will pump out 5-6 gallons per minute, or over 300 gallons per hour - and you never run out. An extra-large capacity 80 gallon tank water heater runs dry in about 15 minutes by comparison.

The trouble with extra-large tank heaters is that whether or not you use the 80 gallons of water stored inside, you pay a high price to keep it hot 24 hours a day. Imagine leaving your car running 24 hours a day!

Once you’ve settled on a heater that will meet your hot water demands while saving you money, you’ll need to carefully consider how it will work within your home.

Gas Tankless Water Heaters

8 things you’ll need to consider before buying your tankless water heater:

1. The appropriate size for your household needs

Above all, anyone buying a tankless unit needs to think about how they use the water in their home—during peak water usage times, how many faucets, shower heads, bath tubs, washing machines, dishwashers and jacuzzis will you be running at once? Tankless water heaters provide endless hot water for up to two major applications at a time and up to 3-4 sinks at a time.

2. Minimum flow rate

Tankless water heaters require a minimum flow of water—typically around a half gallon per minute—through the heating system to heat water. To ensure your tankless water heater will deliver enough hot water, determine the flow rate and temperature rise you'll need. Make sure the unit you purchase can deliver the most common temperature rise of 70 degrees Fahrenheit (39 degrees Celsius) at the desired flow rate. To avoid problems associated with not meeting the minimum flow requirement, consult with a qualified plumber to make sure the flow rates at outlets you’ll connect to the heater exceed the unit’s minimum requirements

3. Gas sizing information

Take a look at the installation manual of the product you’d like to purchase to see the gas sizing requirements. Tankless water heaters utilize three to four times the BTUs of a conventional tank water heater. Ask your plumber to verify that your current gas line, size and length can provide sufficient gas flow to the unit. Consult with your gas company to determine if your gas meter is adequately able to accommodate larger water heaters.

4. Compatibility with water supplies

Local water quality should be considered in areas where the potential for scaling may be increased. Scale control in water heaters requires disconnecting the unit, connecting deliming hoses and recirculating special deliming agents.

5. Combustion Air

Tankless water heaters require combustion air. It’s important to understand how combustion air will be delivered to your water heater and how much combustion air is needed. When placed in an unconfined area, combustion air can simply be the air within your home. Confined areas (such as a closet) may require air ducts or additional air inlets. Whenever possible, it is always better to pull combustion air in from outside the home — it is cleaner and will keep your tankless water heater operating at peak performance. Check the installation manual for more information about combustion air requirements.

6. Clearances

Tankless water heater placement is another important factor to consider. Each tankless water heater will have different distance requirements between the heater and other parts of your home, as seen in the example above.

Some of the clearances you’ll need to consider are:

  • Operable windows, doors and other fresh air openings
  • Adjacent walls or tall bushes
  • Below gutters, sanitary pipework, eaves or overhangs
  • Distance above the ground
  • From a gas meter, gas regulator, electrical box or another heater

Check the installation manual to see what your heater requires and whether it will work with your home layout.

7. Installation issues need to be considered

Special venting and/or the need for increased combustion air may require additional changes because of the unit’s higher input rates. You will also need 120 volt electric power, and in the event of an electrical power outage, the tankless water heater will not operate.

8. Venting

All indoor tankless gas water heaters must be vented vertically or horizontally to the outdoors using Category III stainless steel venting material (No other venting material is permitted). Venting details, including clearances and diagrams, can be found in the "Use & Care Manual" of Rheem indoor tankless water heaters.

NOTE: Rheem outdoor tankless water heaters do not require venting materials.

When choosing an indoor location, you should consider the shortest venting distance to the outdoors. Consult your professional installer or contact us for maximum venting distances. Rheem offers all necessary venting materials, accessories and expertise to make your tankless water heater installation easy - including horizontal vent kits listed below.

Rheem vertical vent kit (RTG20184) for RTG-64DV, RTG-84DV and RTG-95DV series indoor gas models includes:

  • (1) Appliance Adapter
  • (1) Condensation Collector
  • (1) Condensate drain tube
  • (1) Storm collar
  • (1) 6/12 - 12/12 Pitched cone flashing
  • (1) Vertical termination with rain cap

Rheem horizontal direct vent kit RTG20147) for RTG-53DV and RTG-66DV series indoor gas models includes:

  • (1) Vent adapter
  • (1) 94 degree elbow
  • (1) Horizontal termination
  • (1) Wall trim plate

Rheem horizontal vent kit (RTG20155) for RTG-74PV series indoor gas models includes:

  • (1) Appliance Adapter
  • (2) 90 degree elbows
  • (1) 12" length vent pipe
  • (1) Two-piece adjustable length pipe
  • (1) Wall thimble
  • (1) 90 degree termination elbow
  • (1) Pressure relief valve
  • (1) T-Stat wire

Rheem horizontal direct vent kit (RTG20032HHT) for RTG-74DV and GT-199DV series indoor gas models includes:

  • (2) 12-in x 12-in faceplate
  • (1) 12" length concentric pipe
  • (1) Horizontal termination adapter
  • (1) Air intake Tee-Heat Fab
  • (8) Self tapping screws