"I hate to split hairs but the energy savings from the tankless is that
you don't keep 40 gallons of water hot and on standby. The energy used
to fill flush the pipe is the same with either, unless you get the tankless close to the tap. Maybe that is what you are talking about. "
Yes, that is what I pictured in my mind, because the tankless ones I've seen have been close to the point of use. If you just replace a regular water heater with a tankless, then I agree, the same amount of energy is wasted due to the pipe runs.
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
3/4" pipe, and given that I never use straight hot water, running a gallon of cooler water through the hot pipes isn't a loss anyway. To me, the extra 20 bucks for pipe when I built the house is worth not having to worry about what else is happening in the house when I want to take a shower.
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
30 'pipe diameters' long. And a typical globe valve is about the same as
140 'pipe diameters'. So a 14 foot run of 1/2" pipe, with just five 'elbows' and one globe valve under the sink is really equivalent to 14' + (5*30 + 140)*0.5/12 = 26 feet. If it is 3/4" pipe, then the same number of elbows and globe valve is equivalent to 14' + (5*30+140)*0.75/12 = 32 feet of straight pipe.
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.
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