The heater can be pictured as a long piece of pipe coiled up and placed over a fire.  As the water flows through the pipe it gets hotter.  In order to reach full temperature it has to stay in the fire for a certain period of time.  It will take longer if the water starts out colder.  If you speed up the flow of water it will get cooler, unless you add more fire.  With the old storage heaters the tank full of hot water is ready to go the moment the faucet is opened so you get your hot water faster. 

American Heat ADK 16 KW Whole House Tankless Water Heater US $269.00

Noritz SC-201-6M Tankless Water Heater System Control US $264.95

Tankless Water Heater Olympia 18 KW Whole House New US $205.00

American Heat AH-18 Whole House Tankless Water Heater US $391.00

TITAN N-160 SCR-3 Electronic Tankless Water Heater BRAND NEW US $309.99

Rheem RTGL74XP Outdoor 7.4 GPM Tankless Water Heater for Liquid Propane US $699.88

Bosch 1600H LP AquaStar 4.3 GPM Indoor Tankless Liquid Propane Water Heater NIB US $580.00

NIB GE INDOOR TANKLESS GAS WATER HEATER 7.5 MODEL GN75DNSRSA US $649.88

NORITZ 8 GPM INDOOR NATURAL GAS TANKLESS WATER HEATER US $1,999.99

NORITZ 8.3 gpm INDOOR HOME TANKLESS WATER HEATER NATURAL GAS NG NR83-DVC-NG US $1,319.99

NEW NATURAL GAS TANKLESS WATER HEATER 4.23 GPM / 16 L US $297.95

American Heat 16 KW Whole House Tankless Water Heater US $269.00

Noritz N-0751M-DVC (LP) - 9.8 GPM - 199,900 BTU/H Tankless Water Heater Propane US $910.00

Rinnai Tankless Water Heater R75LSIN Interior Natl Gas US $1,125.00

8 Valve kits for Tankless Water Heater LEAD FREE (T) US $590.00

8 Valve kits for Tankless Water Heater (S) US $460.00

8 Valve kits for Tankless Water Heater LEAD FREE (S) US $590.00

8 Valve kits for Tankless Water Heater (T) US $460.00

Rinnai Tankless Water Heater R94LSIN Interior Natl Gas US $1,299.00

Rinnai Tankless Water Heater R75LSIP Interior LP Gas US $1,125.00

Marey® 4.2GPM/16L Natural Gas Whole House ON DEMAND Tankless Hot Water Heater US $296.95

Rinnai Tankless Water Heater R75LSe Exterior Natl Gas US $1,090.00

Rinnai Tankless Water Heater R94LSe Exterior Natl Gas US $1,290.00

Eccotemp 40HI 6.3 GPM LP Intdoor Tankless Water Heater US $429.00

Eccotemp 40H 6.3 GPM LPG Outdoor Tankless Water Heater US $429.00

Eccotemp FVI12-LP 3.4 GPM Gas Tankless Water Heater US $269.00

Eccotemp 40H 6.3 GPM NG Outdoor Tankless Water Heater US $429.00

Rinnai Tankless Water Heater R75LSEP Exterior LP Gas US $1,090.00

Rinnai Tankless Water Heater R94LSEP Exterior LP Gas US $1,290.00

Rinnai Tankless Water Heater R94LSIP Interior LP Gas US $1,299.00

Noritz N-132M-ASME (LP) - 13.2 GPM - 380,000 BTU/H Tankless Water Heater Propane US $2,905.00

NIB Rheem Tankless Water Heater US $580.00

Bosch Tankless Water Heater ES VVT Vertical Vent Kit for AquaStar 2700ES 2400ES US $270.00

Marey Whole House On Demand 4.2 GPM Propane LP Gas 16L Tankless Hot Water Heater US $296.95

New Rheem/Paloma Tankless Water Heater 7.4 GPM Propane Gas Power Vent PTG-74PVP US $799.99

NEW Rheem/Paloma 5.3 GPM Tankless Water Heater Power Vent Propane RTG/PTG-53PVP US $529.95

HHP HH19 Single Shower Tankless Water Heater US $434.12

HHP HH28 Tankless Water Heater Whole House US $704.95

HHP HH12 Single Shower Tankless Water Heater US $412.47

Stiebel Eltron Tankless Water Heater Tempra 12B US $435.00

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But you can’t do that with the tankless unit because tankless heaters turn on when they detect water flowing through them.  Usually it takes ½ gallon per minute of flow or more to activate most tankless heaters. Most circulating systems don’t produce that much flow.  If they did, then the water heater would be on all of the time.  Using a standard circulating system with a tankless heater will void the warranty in almost every case anyway.

There is another type of hot water delivery system called a demand system that will work just fine with tankless water heaters.  The way it works is a small pump is installed under the sink furthest from the water heater.  When you want hot water you press a button and the demand system pumps the water at high speed from the water heater to the fixture. The system has a temperature sensor monitoring the temperature of the water, and when it senses an increase in temperature it shuts off the pump.  This prevents hot water from entering the cold water lines.

Since the pump only runs for a few seconds at a time it uses very little energy.  Typically it uses less than $1.00 per year in electricity.  It doesn’t use any more heat energy than a standard plumbing system.  It saves 100% of the water normally run down the drain waiting for the hot to arrive.

Demand systems are suitable for use with tankless water heaters since the pump only runs for a short time, and will not void the warranty of the water heater.  Not all demand systems have strong enough pumps to run tankless water heaters though, so you should check with the manufacturer before you make the purchase. 

Metlund makes a demand system they call a D’mand system.  They have several models, and I think the largest model has a strong enough pump to use with a tankless heater. Metlund’s d’mand pumps range from 1/40 horsepower for the smallest to 1/8 horsepower for the biggest. Chilipepper makes a demand system, and they only make one model, the CP6000.  The CP6000 has the most powerful pump of any system on the market, (approximately 1/3 horsepower) and will work with any tankless water heater.

 Another reason for using a more powerful pump is that the more power the pump has the faster the hot water will arrive at the fixture. 

By combining your energy efficient tankless water heater with a hot water demand system you get the best of both worlds…you are energy efficient and you conserve water.  It’s a great feeling to know that you are doing your part to protect the environment and conserve our natural resources every time you use hot water.