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Old 12-01-2008, 11:01 PM   #1
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tankless water heater installation

I am planning on getting a tankless water heater but it says I need 3/4 inch water supply. My pipes coming out of my basement floor, and throughout my house, are 1/2 inch. What is the best way to upgrade? Can I increase the size from the meter on, or do I have to increase it from the supply line at the curb and tear up cement in my basement?


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Old 12-02-2008, 01:16 PM   #2
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I think you're making this more complicated than it is. I'm 99.9% certain you just need a $2.00 fitting that goes from 1/2" to 3/4" before going to the tankless heater and then another to turn it back to 1/2" after the tankless.

By code, a house has to be built with worse case scenario in mind which in a nutshell you calculate if every fixture in the house is turned on will the waters velocity in the pipe attached to your meter be going so fast it will exceed 8 fps, which at that velocity causes severe pipe erosion (and likelyhood of pinhole leaks). If yes, then you do the same calculations but for 3/4" pipe (which supplies the same volume but with less velocity) and recalcualte until you get a pipe big enough that when everything is turned on the velocity of the line attached to the meter stays under 8 fps. Someone did the calculations to your house and determined there isn't enough fixtures to exceed 8 fps with 1/2" pipe when everythings on. The reason the tankless wants 3/4" is because the velocity of the water slows down in bigger pipe which gives your tankless twice the amount of time to heat it... you only need to get the converter fitting to upsize it to 3/4" shortly before the heater and then downsize it back to 1/2" after the heater.

Are you certain the pipe coming to and out of your water meter is 1/2"? Unless you have ultra-low flow fixtures it only takes 4 fixtures to require at least 3/4" pipe coming off the meter. 1/2" pipe can only typically supply 3 fixtures before reaching 8 fps. I mean, your house would have to have just one bathroom and kitchen (shower, bathroom sink, kitchen sink = 3 fixtures) to have 1/2" pipe suffice for the whole house. Almost all houses I've seen the pipe off the meter is 3/4" and every "branch" to sinks, bathrooms, etc is 1/2" which is fine... as long as each 1/2" branch doesn't supply more than 3 fixtures (you wouldn't run 1/2" pipe as a branch to the 2nd story of a house to feed two bathrooms w/showers up there that's 4 fixtures).


Last edited by Piedmont; 12-02-2008 at 03:55 PM.
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Old 12-02-2008, 03:24 PM   #3
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You may not have a water supply problem but a gas supply problem if the unit is gas fired. If so, you need to check the size of the gas line from the meter to your house. Also consider that tankless systems are supposed to have stainless vent piping so if you have a long vent run, it could get pretty expensive.

If the unit is electric, then.... never mind.
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Old 12-02-2008, 11:29 PM   #4
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I have to agree with Piedmont on the supply piping from the meter. I have not seen a 1/2" water meter. They are called 5/8' meters but have a 3/4" connector on the discharge end. I also think the adapters on each side of the heater would work fine.
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