Re-circulating hot water without a pump

Hot water is less dense than cold water, making it more buoyant. In this sense it will float on the cooler water.  It will displace cooler water which then sinks—because it is denser. This can become an endless loop driven by the hot water in your water heater. All that is required for this loop to function is for the water heater to be below the fixtures that you want hot water at, and for there to be a bit of an “assist” to increase the loop’s performance. This assist consists of having the return pipe from the remote location being smaller than the supply pipe. So if you run a ¾ inch pipe to the remote location, you would then run a ½ inch pipe back to the water heater. The supply side comes off the top hot side of the tank and returns to the bottom of the tank to a “T” installed at the drain outlet of the tank.

Years ago I found some information about how the last 10 feet or so had to be uninsulated in order to make the loop function. You can forget that information now. The loop restarts itself perfectly (immediately) even when all lines are heavily insulated. I am pretty sure it has to do with pressure differentials created by the bigger and smaller pipes. There are probably ways to tweak the system further but for practical purposes we do not have to get all scientific about it. I will leave the nuances to the geeks out there.

This “thermosiphon system” is ideal for when you have a water heater in the basement and you have a remote sink that you want “immediate” hot water at. Sinks and other fixtures that are far away from the water heater can waste a lot of water while one waits for the hot water to get there. Of course to have hot water there immediately it is going to cost you a bit. There are no free lunches.

I have blogged in the past about how simple this can be to do, but this post will clarify some of the information in that post.  It will also show how the system can be set up to provide hot water while at the same time maintaining the water in the loop at a high enough temperature to discourage bacteria growth.

Since the need for hot water is minimal while we are sleeping, wouldn’t it be nice to install a timer on the system so that we could stop the circulation periodically? As long as the water is not allowed to stagnate the bacteria typically are not a problem. Most systems with an actual pump have timers that can be set so that the pump only cycles when you want it to. The same can be done with a passive, or thermosiphon system. This can be done by installing an electronic valve controlled by a timer—of course then it is not quite as passive as it was. These low voltage valves can be operated for pennies a year and are very effective. The valve is normally closed and requires power to open. By reducing the amount of “open time” we can reduce the number of pennies even further.

The timer & transformer, the electronic valve, the sensor

The timer & transformer, the electronic valve, the sensor

Heat trap nipple used as a "check valve" at the return to the tank

Heat trap nipple used as a “check valve” at the return to the tank

One of the difficult problems of a thermosiphon system is when there are fixtures in the home that are not included on the loop. This can happen with fixtures that are at the same level as the water heater (bathrooms and laundries at the basement level etc). When you run hot water at those fixtures you will not only stop the operation of the thermosiphon loop that serves the remote location, but you can even start the flow to reverse. When it reverses you can get very hot water off the bottom of the tank going to the remote location followed by very cold water as the cold water is fed into the bottom of the tank. This problem can be controlled by installing a sensor that can tell when hot water is being drawn by the other fixtures. The sensor is a “open-on-rise” switch that closes as the temperature falls again. When the sensor closes (because the water temperature starts to go down after use) the valve opens and allows the thermosiphon to function again.

I should also note that there should not be any fixtures on the loop either before or after the furthest point. Anything on the loop should come off the end of the loop otherwise those pressure differences I referred to become a problem again.


The general idea of super-insulating the pipes—the reality of unintended consequences.

Another key component to the successful operation of the thermosiphon loop is to super-insulate the pipes that run to the remote location so that the pipes hold the heat for the considerable amount of time that other points of use might keep the valve closed—or when the timer is programmed to let the valve close (such as during the night). (More on my learning curve with super-insulating the pipes later.) This would be hugely beneficial for systems with pumps as well. It also means that the loop does not have to be operational all the time during the day either–perhaps running ½ the time–again further reducing those pennies. The hotter these pipes can be maintained, the less the returning water will cause the water heater to fire to bring it up to temperature.

Any recirculating water system is going to make your water heater run more frequently, but because there is a bigger supply of water at a higher temperature the water heater will not likely run as long—even if it runs a little more frequently. How efficient the system will be depends on water costs, energy costs, the type of water heater and how well insulated you can get everything. There are advantages with gas fired water heaters due to quicker recovery rates but that can easily be offset with an electric water heater that is much easier to heavily insulate. It is not really possible to insulate a gas water heater to the levels that an electric water heater can. I am not talking about adding thin and meaningless fiberglass batt insulation around the tank, I am talking about adding significant 3 to 6 inches of high-R foam around the heater.  Several years ago I added 2″ to mine–the next heater will get more.

Rigid foam insulation fitted around the tank

Rigid foam insulation fitted around the tank

Regarding the argument that some manufacturer’s void the warranty if you add insulation, this may be a risk that is worth taking. Who cares if adding insulation shortens the life of the water heater if the insulation has saved you hundreds of dollars in energy costs. I for one, would like to be enlightened as to why adding insulation would shorten the life of the heater. I am assuming that all access points, data plate, warning labels, drain, TPRV etc remain accessible.  Not insulating the tanks is probably more about these later issues than it is about the life of the tank.

Most water heaters, even ones meeting modern energy requirements, actually have insignificant amounts of insulation around them—typically no more than R-8 to R-14.

This is where it starts to get even more complicated. We want to keep our water at the taps controlled to below 120 degrees Fahrenheit to prevent scalding injuries. The problem with that temperature is that it is ideal for the growth of bacteria—including Legionella Bacteria. It is typically recommended that tank temperatures be maintained at 140 degrees Fahrenheit to control bacteria growth in the heater. If your tank is at 120 degrees Fahrenheit for the week you are gone on vacation you have actually created a potential incubator. Some authorities consider bacterial infections caused by water heaters to be hugely under reported.

The solution is to install a thermostatic mixing valve.

I think it is a good idea to install one at the water heater so that all delivered water after that point is at a safe temperature. The valve dilutes the hot water to whatever you adjust the control valve–anywhere between 112 and 120 degrees Fahrenheit will generally be satisfactory. Some people insist on having water hotter than that to the dishwasher and in that case you might want to tap into the hot water prior to the mixing valve for that appliance. However, most modern dishwashers boost the water temperature at the dishwasher and therefore this may not be as necessary as one might think.

Temperature gauge and Thermostatic Mixing Valve

Temperature gauge and Thermostatic Mixing Valve

In my own house I have a mixing valve at the water heater that covers both bathrooms (upper and lower) and the laundry (lower). There is another mixing valve at the end of the long recirculation loop for the kitchen.

In a way you can think of the recirculation loop as merely an extension of the water heater–both are very hot water to control bacteria growth, while the two mixing valves keep the water safe to prevent scalding.

But what about the costs of storing all that water at such high temperatures?

Keep in mind what I said before about there being no free lunches. Additional costs associated with energy consumption to maintain the heaters at a higher temperature can easily be offset by super-insulating the storage tanks, perhaps even reducing the size of the tank needed or at least having more available hot water to dilute for use at fixtures. Is it more costly to keep an 80 gallon tank at 112-120 degrees Fahrenheit or a 50 gallon tank at 135-140 degrees Fahrenheit? Again, I will let the geeks look into that question. But the bottom line is that the tank is safe from bacteria growth and appropriate temperature at fixtures is achieved by mixing valves.

Expecting to save a lot of energy and having safe hot water in our homes may at best be a compromise.

A note on super-insulating the hot water pipes of the recirculation loop.

In my case I decided to run both the supply and return loops close together, with each line conventionally wrapped with foam pipe insulation–about R-4. The two pipes were run inside a 7” metal duct that I then filled with spray foam—like “Great Stuff.” I assembled the pipe in 3 foot sections and sprayed the foam inside the pipe as I assembled it. Each pipe length had four ¼” holes drilled along its length to spray the foam into the pipe through. I could look in the open end of the pipe to keep track of how it was filling up. My run of pipe is 44 feet. THIS TAKES A LOT OF CANS OF FOAM! And now we are at the “learning curve.” Basically it amounts to not knowing that spray foam in a can requires air and humidity to cure–without those ingredients the spray foam all RETURNS TO LIQUID!

The pipe opened up to reveal a hollow mess

The pipe opened up to reveal a mess

Whodathunkit! So now I had 44 feet of duct with a liquid mess in the bottom and I had to start over. Using all those ¼” holes as a pilot hole I drilled 2-1/8” holes with a hole saw over the entire length. Through these bigger holes I could then layer the spray foam in over the entire length adding a little each day. This gave the foam a chance to cure with air around it. It makes me wonder how often this has happened to other people when they think they are successfully insulating inside a cavity, when really it is just turning back into a liquid again.

If I were to do it all over again I would build a bigger box around the pipes and insulate it with cellulose fiber for a fraction of the cost.

Charles Buell, Real Estate Inspections in Seattle


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