Originally Posted by crabbey1
I used half inch all around. Recently though, I read a thread where someone recommended using 3/8" and it made sense to me. My shower is quite a ways from the hot water heater, so there is a a lot of water being wasted into the grey tank waiting for the hot water to make it over there. If I had used 3/8" instead, much less water would be wasted and less space taken up in the grey tank. Plus all the fittings are cheaper at smaller sizes. I see plenty of benefits...
I've actually been thinking about how much water is in the lines lately, because the plumbing lines on my 28' rear-bedroom run from ahead of the wheel well on the street side to the back, around the back curve and back up to the middle of the wheel well on the curbside where the shower is. I'm considering crossing the trailer under the floor to cut out about 15'-18' of pipe.
When I did the calculations, though, the return isn't huge. 15' of 1/2" PEX holds about 0.61 gallons. 15' of 3/8 holds about 0.34 gallons, so the difference between 1/2 and 3/8 is just over a quart. I may still shorten the pipe runs IF it's convenient to do so... there must already be a furnace duct running across there on mine, so I'm still thinking about it. I'm going to stick with 1/2" though.