has anybody compiled a list of rapid return energy saving measures?

Then, in many ways more than this thing about getting the hot water going, a rapid return energy savings can be obtained by moving to a smaller house.

Not the answer you looked for.... ;-)

Banty

Let's try this again. From my water heater to my primary point of hot water usage, there is 14 feet of pipe. This is not coincidence; I put the water heater directly under the shower and the washing machine for that reason. Now, if it takes 45 feet of 3/4" pipe to make one gallon of volume (someone else's number posted here, which I haven't checked myself), that means I have just about 1/3 of a gallon of water in those pipes. Even to the other end of the house, 35' away, there's only a gallon of water in the pipe between the heater and the faucet. I could calculate the thermal mass of the copper pipe, I suppose, but I don't see it making a lot of difference in practical terms.

Yes, you get _decent_ flow with 1/2" pipe. I get _better_ flow with my

has anybody compiled a list of rapid return energy saving measures?

Hang your clothes out to dry instead of using those wasteful automatic dryers.-Jitney

You're both assuming that the piping itself is the predominent pressure drop. In almost all cases, this just isn't so. The straight length of piping isn't the 'bottleneck'. Keep in mind that an 'average' 90 degree elbow is about the same flow resistance as a straight length of pipe that is

But in both cases, the majority of the pressure drop is right in the faucet/spout. If you figure out the pressure drops through the various portions of a typical lavatory sink run with a flow of about 1.5 gpm and 40 psi source, then replace the piping with a larger 3/4" pipe, the actual flow change is very little. For a bathroom sink with a flow of 1.5 gpm, an equivalent 26 ft of 1/2 pipe only has a pressure drop of about 0.7 psi using nick's formula (Crane TP-410 shows a slightly different number, but pretty close). The rest of the pressure drop is right in the faucet. Even if you eliminated the piping completely, the flow would only increase by a factor of 1.008 (less than 1%).

About the only place 3/4 makes a big difference over 1/2 is if it is feeding several fixtures that may be used at the same time (e.g. the shower and the toilet at the same time is a classic), or when the total flow is not dominated by the faucet (those used to fill a tub comes to mind). So it does make some sense to run a larger pipe to the general area of the bathroom and to the fill spout on a large tub, then split off smaller pipe to individual fixtures.

daestrom

...might use 0.75 gpm of hot water and 0.75 gpm of cold.

And we might close the toilet valve so it takes 5 min vs 30 sec to fill.

Nick