How many of you commenting here
have a thermosiphon system and have lived with it? Or are you just relying on a bunch of various internet articles? I had such a system as I have repeatedly described here for 27 years.
Jamie: Yep, that's me! Your experience is actual data, my words are my analysis from various, sometimes conflicting, Internet articles that mostly don't exactly say how a plumber should do it, or why. I am curious and a future DIY installer for my house, so I want to know what really makes it work. I do try to back up what I say with math or references. I love finding out I am wrong on something, because then I am smarter for it.
The first expert (jlconline) shows a diagram
w/o a check valve at the tank...that’s one of the essential requirements. Now, understand that all check valves are not created equal. There are spring check valves, and these have a "cracking pressure" which is the force required to open the valve. They also have gaskets and seals. These will not work in this application. You just want a swing check valve, and ensure it's mounted properly: that is, horizontal with the flow towards the bottom of the tank. A swing check has no cracking pressure, and generally no wear parts either. Mine was in and functioning for 27 years, no issues.
Jamie: I saw an example read here where some installations without check valves kind of work, but not at the last fixture, as it has so much backward flow in the return line it never gets hot enough.
The "supply pipes" from the water heater to ALL the fixtures in the house were 3/4" copper. Branch lines off the supply lines to the fixtures were 1/2" copper. The return line started at the furthest fixture on the second floor and was solely 1/2" copper.
It’s not “instant” hot water; nothing is. But, it reduces the wait time to seconds.
Are you saying the water starts warm and then rises a bit as it runs? I think that means heat loss in the feed side is higher than the velocity of flow and you would benefit if the water either flowed a bit faster or the feed side was insulated better.
Regardless of whether you insulate or not, water at the hot water tank exit will
always be hotter than water at the end of the return line. With a thermosiphon system as I've repeated, the flow is low and slow. Hot water at your tank tempurature slowing rises to the furthest fixture. It cools off a bit, particularly if there's no use in the supply lines--meaning just the recirculation system flowing. It's still warm as it gets back to the tank, and it takes a very long period of inactivity (meaning no hot water usage) before MY hot water tank turned on again. Remember I had 27 years of experience in my own home (not a customer's home) so I know exactly how it worked.
The return line is just static piping. If you put such a system in and it doesn’t work to your satisfaction you can always add a pump w/o re-piping your system.
Jamie: And, if a circulator is used, it would be the most energy efficient if it were variable speed circulators with a delta-t control having a temperature sensor on the water tank's output and on the return line perhaps 2 feet from the tank. I think this Taco variable speed Delta-T circulator set to move the water just fast enough to keep the return water within 5 degrees of the feed water would draw the least possible energy and would deliver instant hot water at every fixture in the loop. With that, insulating the whole loop would help in climates where heat loss is not beneficial.
To make your static system flow a bit faster, you could swap out a section of your return line with baseboard radiator (as high above the water heater as possible) so the return water cools down more and at a higher elevation. That return line filled with denser water at a longer height will elevate the PSI at the point it enters the bottom of the heater. That higher the PSI differential at the bottom port of the heater, the faster the flow.
My home with this system was in a climate where we heat September-May, so these standby losses were throwing off unmeasurable amounts of heat within the home’s building envelope thus were for all intents and purposes, irrelevant.
Jamie: Yes! Heat loss is not "heat wasted" if that heat is beneficial.
This worked so well, and was so simple, that I helped a friend add a kluged system to his house. In his case the plumber did not add it to start. What we discovered was that all the hot and cold water supplies to his second floor were in a wall at the back of a closet on the first floor. At the ceiling point in the closet we opened up a small area of drywall, added a "Tee", and dropped a return line down to the basement. This was not as effective as starting the return line at the furthest fixture, but it made a measurable difference in their wait for hot water upstairs. The work was minimally invasive; just a small drywall patch.