Water recirculation loops are very common in homes. They typically have a small pump to recirculate water from the water heater to remote points of use in the home. Waiting for hot water to get to a distant location can consume a lot of water and use of a pump can theoretically be cheaper than the cost of the water that would otherwise be wasted.
I think this is probably true—depending on your water heating costs and the costs of the water itself, but there is a way to do it with almost no extra cost–assuming that both approaches require extra piping.
Thermosiphon
Thermosiphon is the principle where heated water is more buoyant than cold water so it tends to rise in the pipe and then fall as it cools off. If we place this principle in the pipe from the hot side of the water heater to the bottom cooler part of the tank, this thermosiphon will create a loop of continuously circulating hot water without the need for a pump.
The following picture shows how simple it is.
In my system a ¾” supply line was run from the water heater to all the fixtures. At the last fixture the line was reduced to ½” and then run back to the water heater. The lines were well insulated. The effect of the thermosiphon can be greatly improved by leaving the insulation off of the last 10 to 12 feet of pipe before it gets to the water heater, but is not absolutely necessary.
Of course this will only work if the tank can be located lower than the fixtures. Any house with a basement can make this system work for the fixtures located on the upper levels of the home. For fixtures on the same level as the heater you will just have to wait. A check valve was installed to prevent the water from going the wrong way when fixtures at the same level as the heater (in the basement) are being used.
In my own home, this represents the guest bath and the laundry, and since they are located within a few feet of the water heater it is of no consequence.
Our kitchen is really the only fixture that takes considerable time for the hot water to arrive—and the thermosiphon works great and at no cost. It is also VERY quiet—as in absolutely NO noise that is associated with pump systems.
Charles Buell, Real Estate Inspections in Seattle
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Deployed your design in my home during a shower remodel. Converted to all pex running thru the attic w/ drops to each fixture. Works great, reduced wait from 45-50 seconds down to 8. 1 story ranch so WH and fixtures on same level. Connected 1/2″ return in attic at farthest fixture right before at drops in the wall. Only issue found is that if system is static for extended period of time I get warm to hot water out of cold side of the fixtures for several seconds. Closer to the WH the hotter. Supply line is Tee’d just above cold side of WH and I’m thinking hot water must be migrating up WH supply heading to the rest of the house. Would a check valve at the WH supply remedy this and can you think of any issues this might create.
Sounds like you may need an automatic check valve on a timer to keep it from reverse siphoning.
I “invented” this Thermosiphon system 60 years ago when I was 10 years old.
But I decided, when I was 10 years old, that this would be very wasteful of energy, and abandoned the idea because it would be very wasteful of energy ( $ ). “Insulating” the pipes helps, BUT of course, it is the LOSS OF HEAT from the hot water that makes the thermosiphon function. Not so bad in the wintertime, but in the summer, the Air Conditioner needs to run more to remove the EXTRA heat from the thermosiphon (DOUBLES the length of pipe that will lose heat energy). I install 1/2 inch instead of 3/4 inch pipe for hot water lines to reduce the pipe surface area, and substantially reduce the wait for hot water. Your thermosiphon will work almost as well as the re-circulation system with a PUMP, at a LOWER ENERGY COST (not to mention the initial cost of the pump, and the electrical $ connection to make it run. I would rather wait for the hot water, and waste the hot water left in the pipe when I stop using the hot water, then to CONTINUOUSLY waste the energy whether I am using the hot water or not using hot water. We need to avoid doing ANYthing like this that will aggravate “Climate Change”. My Grandchildren will live long enough to see Miami Beach Florida under a foot of sea water. The last hurricane to “hit” New York missed FLOODING the city by only 10-20 miles– imagine all the subways, power tunnels, and building basements flooded with SEAwater, and the electricity OFF for 3 months while they try to fix things. I prefer not to waste the energy to “thermosiphon”, but this is better than a pumped system. Jim in Michigan
Jim, if you did in fact invent this 60 years ago, you also know that it is not temperature loss that drives the thermosiphon but more pressure differentials. The system in fact can cost less to operate than the cost of wasted water waiting for hot to get to various locations. Also, because it keeps the whole tank at more uniform temperatures there is less cycling off and on of the heater to bring water up to maximum temperature—thus the water heater uses less energy. The loop can also be on a timer so that it does not circulate when you are sleeping. The additional cost of piping is minimal and allowing them to travel in the same insulated supply and return insulated duct further supports less loss of energy. This certainly does not do anything to aggravate climate change—in fact it supports conservation. By comparison, allowing construction in coastal areas and rebuilding New Orleans is a much better fish to attempt to catch than our wimpy water heaters. These systems, for me, are not installed to waste energy. They are installed to conserve natural resources and conserve energy.
Its a pretty bold statement to say you “invented” something. Inventing usually means that you were the first to come up with a design & then validate that design. Back when I was a kid (1960’s) my dad’s shop had a coal-fired pot bell stove with a water jacket. It had a loop that fed a series of radiators. I recall asking him where the pump was. He told me that the previous owner’s son worked it out with his high school shop teacher . . . back in the 1930’s (pre WWII). I’m reminded of a college history professor who once told me “Bob, those who don’t learn their history get to retake the class”. Yes, the design works. It’s not new. Flow occurs due to pressure differential that is caused by inconsistent temperature within the system, aka “heat rises”. The good news is that even with insulated pipes a small (sufficient) flow will occur). If the piping is in the basement, any load on the A/C should be inconsequential. Congratulations on your implementations and critical thinking!
Hi Bob, no way I would ever take credit for “inventing” this—just trying to revive its use in a practical way. The idea and system has been around for a long time and operates on basic principles. But thanks for the comment.
Excellent Charles, my name is also Charles. I experimented with radiant heat in my own house. This led to more in depth thought about how to heat, which led to trying to get more heat or more accurate heat out of what was already being consumed. Long story short, I put a pot of water on top of my wood burning stove, took some 1/2″ pex piping and ran it at ceiling level from pot of water and then back to pot, I attached a small aquarium pump downstream from the pot of hot water. Waiting for water to gain a little heat (ie the first bubbles to form on the side of pot), then turning pump on for only a few seconds just to jump start circulation was enough to get thermosiphon working. Finally, above the pex tubing on ceiling, I first stapled up radiant barrier insulation. The reflection of the heat down (by the insulation) was so great that I had to soon remove the pot of hot water from the woodstove top!!
You’re article is spot on! I am on a well with mediocre water pressure and the wait for hot water was excruciating. I installed a 1/2″ return line as you mentioned with a recirc pump per the handyman store suggestion. Some years later I noticed the pump had stopped working but I still had hot water at the ready! I removed the pump from the loop entirely and everything works fine! Now to my question, I am replacing my water heater and noticed the heat traps they install in the nipples these days. I’m assuming this will end my immediately available hot water and am looking for a suggestion.
I ran into this as well and if it is the type with the floating balls, they will not interfere but they will rattle all the time. There are other types of heat traps you can install that would eliminate the need for the ball type. A heat trap can be accomplished by using flexible connectors and just go up and then bend it back down to the tank and then back up again.
My plumbing runs down from my heater thru the crawl space, then back up to appliances. any good suggestions? Spotty.
Put in a return loop on a pump.
I think a 1/4” copper, flex tubing, that returns lower than the supply (even though it has to go back up from the lowest part of your crawl space to the hot water heater – get as much height distance between the two as you can – you might not want to insulate the 1/4” flexible copper tubing, just to get more of a temperature differential) would work.