moving electric water heater to old shower cove

doing extensive remodeling of a small (1400 sq ft) house. I want to move the electric water heater to an old shower cove. Getting the wiring there will be easy (its about 5 feet away and all the walls are currently open).

I would use the old shower cold water pipe to supply the water heater and use the old shower hot water pipe to supply the rest of the house from the water heater. Those connections would be simple to do.

I realize the 1/2" shower lines are smaller than recommended for a water heater, however at the old water heater site the 3/4" output line is immediately reduced to 1/2" BEFORE any branches - so the entire house is currently being served by 1/2' trunk lines. The street supply to the water heater in its current location is 3/4', so that would be a significant change in the new location.

It seems to me that, physics being a constant, if the output trunk is 1/2", then having a 3/4" supply to the water heater doesn't matter, since the water can't come in faster than its going out. Therefore, there should be no change to the pressure or flow rate throughout the house with the water heater relocated to the old shower.

Finally, the new (old shower) location will have a floor drain (concrete slab, single story house) so that's a plus for the relocation of the water heater.

My question : is there anything I'm missing about connecting the old shower pipes to a water heater?

I will throw in here that putting in an electric tankless heater in the current water heater's location is an option, but I am not thrilled with that plan - tankless electric water heaters seem to be less powerful than their gas fired cousins.

• It's not just the narrowest pipe which limits flow; there's additional drag along the whole of the narrower pipe. So, reducing the length of pipe that's 1/2"D will (somewhat) help the flow rate. Jun 24, 2016 at 16:20
• all of the pipes in this house are 1/2 inch - no changing that
– Tom
Jun 25, 2016 at 1:23

You can move the water heater to just about anywhere that would be convenient. However, it is not a good idea to reduce the size down from 3/4" to 1/2" directly off of the tank. You will likely run into issues with water pressure.

In branch plumbing, it is best to keep the size of the pipe at its maximum until it branches off to a specific fixture. Most fixtures, with the exception of a toilet takes 1/2". A toilet is actually reduced to 5/8" by the time it gets to the fill valve. 5/8" is an odd size in plumbing, so most plumbers typically run 1/2" anyway.

You should try to extend the 3/4" pipe as far as you can go, and then tee off of it using multiple 3/4 x 3/4 x 1/2 tees. When you tee off, try to make that run directly to a fixture without teeing off again. This will greatly help keep the water pressure balanced. If all of the plumbing is 1/2", then the fixtures will compete against each other for water because there is not enough volume of water coming through the pipe.

• I should have added that this is a concrete slab, so changing the existing pipes is not an option.
– Tom
Jun 25, 2016 at 1:04

Alternate opinion here.

People do not use hot water continuously. They have moments of activity when they shower, wash clothes or do dishes. The rest of the time, the water in the pipes cools off. You know the drill: How long the hot water takes to get hot, depends on the inventory (volume) of water in the pipe.

Pipe volume is the square of diameter. Go from 1/2 to 3/4, increase volume to 225%. More than double the water; double the wait time.

Pressure is not that big a deal; faucets intentionally slow it down to non-hazardous pressures. Flow is not so critical either: not least, give people too much flow and they'll complain that hot water "doesn't last very long".

Appliances like washing machines can be thrown off by that huge plug of cool water in the pipe. I've seen plenty where "warm" meant cold and "hot" meant warm. Do you really need a 3/4" pipe to sustain a 1.2 to 2.5 GPM low-flow shower head? Of course not.

You do, however, want pipes to be reasonably of equal size, otherwise machines with mixing valves will not mix well due to dissimilar pressures. If a washing machine's cold supply blasts, but its hot trickles, it will mis-mix "warm".

That's my minority report.

As far as a narrow pipe making a fat pipe irrelevant, somewhat yes and somewhat no. Yes, the 1/2" "orifice" will restrict flow. But even so, a long run of 3/4" pipe will flow better than 1/2" pipe because the water will be able to move more slowly with less friction against pipe walls. This is similar to the way voltage drop on a fat wire is always less than on a thin wire, even if the flow squeezes down to a thinner wire for a short distance. (of course the breaker must be set for the thinnest wire.)