Tony wrote:

Theo wrote:

I've tried to convince the installers that 40 gallons of water expanding
from 55F to 140F is going to strain the pipes in this small house...

With a density 62.46-0.01(T-68) lb/ft^3 at T (F), 40 gallons at 55 F weighs
62.59 pounds per ft^3, ie 62.59x40/7.48 = 334.7 pounds. Heating it to 140 F
raises the volume from 5.348 to 5.421 ft^3, an increase of 0.073 ft^3, or
0.55 gallons. In a large house with a working check valve on a city water
supply, those 2 quarts might expand the pipes elastically with no damage at
say, 60 psi, but that seems unlikely, since copper doesn't stretch much
at that pressure.

If there is an inlet supplying pressure at 30psi, the pressure rise due
to expansion of heated water will just push a tiny amount of water back
up the pipe,

"Tiny" as in 2 quarts :-) But we can't push water back through a check
valve, which is often a safety requirement with city water supplies...

This could be a non-problem if the water heater were a more elastic $60
1"x300' 13-gallon black plastic HDPE pipe coil in 140 F solar-heated water
in a 4'x8'x3' deep plywood box tank with a folded 10'x14' EPDM rubber liner.

Nick