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I have 1/2" pipes from the water meter throughout the house with two full baths. Every fixture installed is fed by 1/2" line and doesn't have pressure problems.

Soon I will need to replace my hot water tank and would like to go tankless. Every tankless NG water heater I've researched above 4.5 gpm has 3/4" water line connects. My zone needs are a top rise of at least 70 degrees which puts me in around 9gpm.

Can I reduce those 3/4" line connects to 1/2" inlet and outlet and still run that tankless safely and with no loss of performance?

  • Good thing 1/2" is working. Everytime you bump a pipe size, you more than double wait time for hot water, so the larger pipe holds a larger physical inventory of water in the pipe. If you remember your geometry, the area of a circle is proportional to diameter squared. – Harper - Reinstate Monica Nov 7 '19 at 20:13
  • Hello, and welcome to Home Improvement. Good question; if the answer helps you, you can click the checkmark to accept it. And, you should probably take our tour so you'll know how best to participate here. – Daniel Griscom Nov 7 '19 at 22:42
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Yes, you can in most cases. Water heaters have 3/4" connections to allow for increased flow and pressure where it's needed and where the pipes are larger, but that doesn't mean your plumbing has to match. The heater will flow at the same rate (more or less) as a straight run of 1/2" pipe in your case. If anything, the heater will perform better at heating rise since flow is restricted by the pipes, extending time in the heater for each water parcel.

To be sure, read the specifications for the product you're interested in.

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