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I have a 1″ riser to the second floor that feeds two long branches in opposite directions. While everything is open I’d like to add a recirc line. Can I add one to each branch and tee them at any point upsteam or would they have to go all the way to the water heater? Oh yea, I’d like to do a gravity loop (no pumps).
TIA,
Jerry
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Replies
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Yes, they have to go all the way back to the water heater. It is heat loss that powers the loop (outbound water is hotter than return water). So if there is no heat source in the loop, it won't work.
Although the loop is powered by heat loss, you should still insulate the loop. An insulated loop will circulate more slowly (no problem) and be at a hotter temperature (which is good) than an uninsualted loop.
If possible, you want the vertical, outgoing portion of the run to be close to the water heater (where hottest). And the return to be at the end on the run (where that water is coldest). So in your case, return each loop along the second floor, parallel to (but the reverse direction of) the outgoing legs. Then bring it down to the HWH. -David
*David,Thanks for the quick and thorough response. A further question: If the line from each remote branch goes all the way back to the HWH they are, in effect, tee'd at that point. Hopefully both loops would still have flow. If instead, I tee'd each remote branch together (say, near the riser) and then brought the one line back to the HWH, it seems the flow in the one pipe would cause flow in both the branches. I just wonder how that flow would be divided by the tee. What do you think?Thanks,Jerry
*There would be slightly higher pressure at the hot, outgoing tee (due to the bouyant effect of the hot water) than at the cooler return tee. Each loop would see that same differential pressure and have a small flow as a result - which is what you want.Could the flow get established in a one leg and that flow preclude flow in the other leg? I don't see how*. Flow implies differential pressure to drive it and both loops would see the same differential pressure between the two tees. Even if one loop is twice as long as the other, they would each have some flow, although more in the short loop.*Caveat: I could probably design two loops in which one circulation pattern precludes the other. But it would involve venturis and flow inducing nozzles. And custom ones at that due to the very low flow rates. Using normal pipe and fittings? Not to worry. The pressure that drives a thermosiphon loop is tiny. A small fraction of an inch water column. Less than 0.01 psi. But you are not trying to move gpm. Just gph. It works in a 3/4" loop. And more height makes it work better. Your 1" riser will work better and 3/4" loops in each direction should work fine. Run the loops close (laterally and vertically) to the 1/2" stub-outs for each fixture to further minimize the time to get hot water.So 1" to the second floor to a 3/4"x3/4"x1" tee would be fine. On both outbound and return legs.Have you figured in a check-valve(s) yet? If all the faucets are connected to both the hot outlet of the HWH and its cold inlet, you'll get some of each. That's not good. So a very low pressure check valve is needed to only allow flow around the loop in one driection. You can NOT use a spring-check - too much pressure is required (typically 2 or 5 psi but even the rare 0.5 psi ones are too much). A swing-check (available at the plumbing supply) or a ball-check (hard for you to find) will work. The swing check has quite low pressure requirements to open when mounted horizontal. But still some. So I would install it (or them) pointing down slightly. Either on the vertical return leg (1" size in your case) or on a short 45 degree offset run in each loop (3/4" in that instance). Then the little flapper inside hangs open due do gravity and the thermosiphon loop doesn't have to open it. When you open a faucet, they be many psi pushing flow in both the outbound and return legs towards the faucet. But the check valve in the return leg will immediately close and the only flow will be from the HWH's hot outlet.If you can find a ball-check, put it anywhere on the return leg (pointed down).I've always thought that a thermosiphon loop is an elegant way to acheive instant-hot at the fixtures. And that instant hot is nice luxury item in a house. Plus you save a few gallons of water each day as a result.Ask me about my thermosiphonig backpacking hot tub sometime. Now that required some careful pipe sizing! -David
*David,You confirm what I suspected as far as the total flow in the loops dividing according to the resistance of each loop. I thought about it in terms of an electrical circuit. I had planned on putting a swing check in the common leg of the return. Swing checks will work installed on the vertical? On the feed side the riser is 1" and both branches are 3/4". I thought 1/2" branches and either a 1/2" or 3/4" riser (and back to the HWH) on the return side would be enough because of the low flow required. I was leaning towards 1/2" for all. Do I need 3/4" and 1" all the way back to the HWH?Thermosiphoning backpacking hot tub - that does sound interesting!Jerry
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I have a 1" riser to the second floor that feeds two long branches in opposite directions. While everything is open I'd like to add a recirc line. Can I add one to each branch and tee them at any point upsteam or would they have to go all the way to the water heater? Oh yea, I'd like to do a gravity loop (no pumps).
TIA,
Jerry