Instant Hot Water

Introduction: Instant Hot Water

When the faucet is a long distance from the hot water tank, it can take quite a while for hot water to arrive. This wastes water, sewer capacity, and time. There are various devices available which utilize pumps, check valves, and additional piping to deliver hot water faster. If your hot water heater is at a lower level than the faucet, there is a much simpler and less expensive way. Simply connect a needle valve between the hot and cold water lines near the faucet. Cold water is denser than hot water. The connection via the needle valve creates circulation as the denser cold water falls and "pushes" hot water from the hot water tank. The needle valve allows adjustment of the circulation so that the cold water is not too hot and the hot water is not too cold. The savings in water, sewer, and time are somewhat offset by higher water heating costs as the hot water line is always full of hot water, resulting in more heat loss, and cold water, which needs to be heated, is always flowing into the hot water tank. This flow however, can be minimized via the needle valve. This "system" can be shut off by simply closing the needle valve.

Step 1: Install Cross Connection

The key component is the needle valve, which allows for fine adjustment of low flow. I used a 3/8" needle valve with compression fittings from Home Depot for about $10. The additional fittings depend on the type of plumbing feeding the faucet. I uses 3/8" copper tubing, tees, and compression fittings. I estimate total cost at about $40.

Step 2: Check for Leaks

With faucet closed, open shutoff valves and check for leaks.

Step 3: Prime the System

Close the needle valve and let the hot water run until the water is hot at the faucet.

Step 4: Open the Needle Valve

Open the needle valve about 1/2 turn. Wait an hour or two.

Step 5: Fine Tune the Needle Valve

Check the temperature of the hot and cold water. If the cold water is too hot, there is too much circulation. Close the needle valve some. If the hot water is too cold, there is too little circulation. Open the needle valve some. Repeat this adjustment until the temperatures of the hot and cold water are acceptable. Further adjustment should be needed only if something changes.

Step 6: Conclusion

The end result will be hot water that is not quite as hot as it was originally since a small amount of cold water is flowing through the needle valve to the hot side. Similarly, the cold water will not be as cold as it was originally since a small amount of hot water flowing through the needle valve to the cold side.

My goal was to wash my face and hands and brush my teeth in warm, not cold, water without waiting. This system has met this objective.

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    4 Discussions


    Question 1 year ago

    How can I do this if my sink is lower than the tank by 1 story? I really want to do something like this but can't afford the $300 system. I have circulation pumps on hand including a tiny 12v hot water pump for a diy solar heater using black sprinkler poly but it doesn't have much pressure. Any suggestions?


    2 years ago

    I find the biggest advantage to this project is that the shower water is hot within about 10 seconds of turning it on, even though the "bypass" is located at a nearby sink.


    2 years ago

    Interesting. Not sure it has broad appeal, but it solves your concern.