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. 

Tankless Water Heater Olympia 24 KW Whole House New US $225.00

Tankless Water Heater Olympia 21 KW Whole House New free shipping US $215.00

Tankless Water Heater Olympia 27 KW Whole House New Free Shipping US $235.00

GAS TANKLESS WATER HEATER - DIRECT VENT - WHOLE HOME US $394.95

American Heat AH-18 Whole House Tankless Water Heater US $390.00

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

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

NEW NORITZ LP/PROPANE 9.3 GAL./MINUTE TANKLESS WATER HEATER* US $939.00

New Ruud Propane tankless water heater RUTG-66DVP LP US $629.99

New Rheem/Richmond tankless water heater RMTG-66DVP LP US $629.99

New Rheem/Ruud tankless water heater RUTG-66DVP LP US $629.99

4.3 GPM MAREY HEATER ON-DEMAND PROPANE GAS TANKLESS HOT WATER HEATER US $296.95

240 208 VOLT ELECTRIC TANKLESS HOT WATER HEATER NEW US $259.95

16L Tankless HOT Water Heater - LPG US $296.95

10L Tankless Hot Water Heater - Liquid Propane Gas US $209.90

MAREY ON-DEMAND NATURAL GAS TANKLESS WATER HEATER 2.7 GPM US $209.90

Marey® GA10NG Natural Gas Tankless ON DEMAND Hot Water Heater 2.7GPM / 10LPM US $209.90

Quietside 180k BTU Tankless Water Heater ODW-180A US $1,050.00

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

Girard Tankless RV Water Heater 1GWH NEW US $656.50

ADK-12 - American Heat Electric 12KW Tankless Water Heater US $208.50

ADK-9 - American Heat Electric 9KW Tankless Water Heater US $208.50

Ecosmart ECO 27 kW at 240 Volt Electric Tankless Hot Water Heater US $544.50

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

RA18 POU seisco electric tankless hot water heater RA-18 On Demand Electronic US $300.00

Rinnai Tankless Water Heater Internal Propane - RL94IP US $1,089.99

NEW NAVIEN NR-240 NATURAL GAS TANKLESS WATER HEATER US $1,359.88

TANKLESS HOT WATER HEATER LP NEW US $425.00

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

Aquastar Tankless Water Heater US $450.00

New Rheem/Paloma Tankless Water Heater Natural Gas Direct Vent 7.4 GPM PTG-74DVP US $719.99

New Rheem/Paloma Tankless Water Heater 7.4GPM High Altitude Propane PTG-74PVPH US $719.99

New Rheem/Paloma Tankless Water Heater 5.3 GPM Propane Gas Power Vent PTG-53PVP US $539.99

New Rheem Tankless Water Heater 7.4 GPM Propane Gas Power Vent RTG-74PVP US $719.99

New Rheem 7.4 GPM Tankless Natural Gas Power Vent Water Heater RTGL-74PVN US $719.99

"BOSCH" POWER VENT TANKLESS WATER HEATER, 130,000 BTUs, NEW US $329.00

Rheem Tankless Hot Water Heater US $990.00

NORITZ VK4-V1 N-VENT STAINLESS VERTICAL 4" VENT KIT FOR TANKLESS WATER HEATERS US $329.99

NORITZ 7.1 GPM RESIDENTIAL NATURAL GAS TANKLESS HOME HOT WATER HEATER NR71-SV-NG US $889.99

2400 NG Bosch Tankless Water Heater US $700.00

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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.