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My question is similar to the one asked in the following link.

What if you reverse the pipe sizes and go from a larger pipe to a smaller and then back again?

I am installing a point of use small hot water heater, I have to go from 3/4" house pipe to 1/2" for hot water tank, then back to 3/4". I would just leave it at 1/2" after hot water heater except the 3/4" goes up into the wall and branches off to shower somewhere.

Effect of changing the size of a pipe

  • Why would you route water through the tank for both that bathroom and the other one? Why not split before? Will the heater serve both locations? – isherwood Jan 17 '18 at 16:59
  • Basically what you are doing is restricting flow. Your flow rate will be affected Gals/m – Ken Jan 18 '18 at 6:42
  • The plan was to just have hot water at these points until main hot water arrived, I have the hot house water going into the cold side of tank. It's is a 12 Gallon so I would think that there would be plenty of hot immediate hot water til the house picks up. – Rich Jan 18 '18 at 23:14
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Changing from 3/4 to 1/2 and back will not cause any problems, the point of use heater will have a lower flow rate than the 1/2" pipe is capable of and the higher you need to raise the temp will be the restricting factor. Or to say it differently if you need to raise the temp 40 degrees you may only have 1 gallon per minute flow, shower if you only need to raise the temp 10 degrees you may have 3 or 4 gallons of flow. There should be heating curves with the unit you install. But the pipe size won't be a problem in this case.

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