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Hot Water Recirculating System – Installation Tips

How it Works

When water is heated, it simply gets lighter. It wants to float to the top of cooler water. Cool water wants to fall. If you have a loop that projects upwards, the cool water wants to fall down the loop while the hot water goes up. Gravity fuels the motion.

It works in this fashion in your house. You already have half of the loop in place. This is your hot water piping distribution system which begins at your hot water heater and ends at the farthest fixture which requires hot water. If you were to install copper piping leading back from the farthest point and from other high points in the existing system, you would have a loop. This return loop connects into the bottom of the hot water heater where the current drain valve is located. It is that simple.

The flow of water through the loop is very slow. However, that doesn't matter. The only thing that is important is that hot water is near each fixture. Without a loop there is no movement, so any movement is better than none!

Non-Gravity Systems

Those of you who live in a house on a slab or where a majority of the hot water lines drop below the heater need to use a recirculating water pump. These are simple devices that connect in line in the loop. They are often located near the water heater, however, they can be anywhere in the loop. The pumps circulate water at low pressure and low speeds. Once again, there is no need to have lots of water moving through the loop. It is just important that the water is hot near the fixtures.

If you install one of these pumps, remember that you need to install unions on either side of the pump. Unions are special fittings that allow you to break into the piping system and reconnect without soldering. Water meters are always installed using unions. Look at yours and you will see what I mean.


Once you decide to install a recirculating loop, you need to be concerned with energy loss. The loop will work fantastically without insulation. In fact, it works best without it! But, this can also cause your water heater to cycle on and off more often. Remember, you are bleeding heat from the heater when the loop contains hot water.

There are numerous ways to insulate the pipe. Many insulating materials are made exclusively for water piping. They fit snugly over different sizes of pipe. Some insulation, like the foam types, must be installed as you install the pipe, not after the loop is constructed. When you select your insulation material, ask how and when it should be installed.

Piping installation methods must also be altered. Some people attach water piping to the sides of floor joists. You can't do this with an insulated loop system. The pipe must stand away from floor joists so that the insulation is not crushed. Special inexpensive pipe hangers allow you to do this easily.

All of the hot water lines that lead from the heater must be insulated. They need to be insulated up to where the return loop lines connect and slightly beyond. The return loop also needs to be partially insulated. If you insulate the entire system too well, it may not work! Remember, the water has to cool at some point for the loop to start its gentle movement. I suggest that you leave the final 15 feet of return loop uninsulated.

Beginning the Loop

Houses that do not have a recirculating loop system have hot water pipes that branch off a main line and stretch to each fixture. The ideal loop system would have you start the return loop as close as possible (within 2 feet) to each fixture. The closer the return loop starting point the closer hot water will be to the fixture.

Your existing home can benefit without getting this close to a fixture. As long as you can cut the distance in half or more, you will see a much more rapid hot water access time.

The return loop begins with a simple tee fitting as close to a fixture as you can get. The tee simply creates a right or left turn leading the way back to the water heater.

On your way back to the water heater you simply create other connections as other return loops join the pipe on the way back to the water heater.

Air Locks - BIG PROBLEMS!!

As you construct your loop system, you must be concerned with air traps. What are these? Well, you know the drain traps under your sinks? Imagine if you did this upside down with a water line in a loop system? You would capture air in this trap since air is lighter than water.

Air can get into a plumbing system in any number of ways: a water main break, a repair process in your own home, dissolved air within water, etc. If you create a trap, the air will collect in the trap and BLOCK movement of water within the return loop. Air is not a problem in the regular water piping system. The rapid movement of water through the pipes when you turn a faucet on pushes the air out of the way. Remember, water moves sloooowly through a gravity loop.

Check Valve

Just before the loop enters the bottom of the hot water heater you might need to install a simple check valve. These are one way valves. This valve will prohibit in-rushing cold water from the bottom of the hot water heater from flowing backwards through the loop when you open a hot water faucet somewhere within the system.

Check valves are not always necessary. Some systems need them because of friction loss and other obstructions that make it easier for the hot water to flow backwards through the loop rather than the correct direction - from the top of the heater!

These valves can be installed after the loop is completed. You can install this valve in the vertical - or horizontal - loop pipe just before it enters the hot water heater. The valve should be within 5 feet of the water heater and it needs a 1/8-inch hole drilled in the flapper of the valve. You might want to try installing the loop first without one and see what happens. If you begin to get cold water at a faucet when you should get hot, you know you need a check valve.

Final Connection

As the loop returns to the water heater it connects at the low point of the heater. This is always the location of the heater drain valve. This valve is simply screwed into the heater. Attach a wrench to the valve and turn counterclockwise. It will come out.

Install an insulated nipple in place of the valve. This will minimize corrosion possibilities. Then as soon as possible install a tee fitting with female threads at the tee. If you use the right one, the drain valve will screw right back into the tee. The other end of the fitting allows you to connect the loop to the heater.

Shut Off Valves

While on the subject of valves, let's talk about the shut off valves on top of hot water heaters. I have seen some aggressive homeowners install a shut off valve on both the hot and cold water line. They thought this would help in the event they need to switch out the heater. Well it does help. It also creates a potential BOMB.

If some idiot turns off both valves (happens everyday somewhere), and the pressure relief valve malfunctions or was never installed, and the heater thermostat malfunctions, the heater will explode. It has happened more than once.

Only install a valve on the COLD water line, never on the hot line.

Column B413


20 Responses to Hot Water Recirculating System – Installation Tips

  1. Is there a recommended pipe size for the return line ? My supply line starts at 3/4" copper but does reduce down to 1/2" at the faucets so I was planning on using 1/2". Then I started thinking of the 50" coils of 3/8" or even plastic. Does it work best with copper ? Thanks.

  2. If there is a power failure using a hot water recirculator pump (gas water heater), will hot water still be available as if there were no pump? (or will the pump and valve prevent flow?)

      • I have a ranch with basement, insulated 3/4 inch copper tubing. Takes about 40 sec for hot water to arrive at bath. about a 20 ft run.
        Would Pex work for the return of a gravity water circulation? I would assume copper to heater drain. Typical 40 Gal gas water heater. Is drain 1/2 in or 3/4 male? can see.
        Rog Swanson

        • Pex should work. I've not tried it. The key is the entire line except for the last feet returning to the heater MUST be insulated. Read ALL of my columns about hot water recirculating loops. There are several!

  3. I hope someone can help. Our plumber did a recirculating system but attached the hot and cold pipes together throughout the house so when our hot water runs it gets cold first and the reverese for our cold water. Also our toilets always have hot water running from them. Our gas bill was so high the gas company thought we had a gas leak and it seems that our hot water heater is running alot causing the meter to spin. HELP

  4. Hi my question is if the reciculating pump comes with a check valve in it . do i need to put another one in the cold water supply line?

  5. I have a 75 gallon water heater that I just changed out last week. I also have a 1/2" gravity recirculation pipe that was installed by the builder. My question is that I am not getting hot water at the furthest fixtures for over a minute where I did before the new heater? There is a check just off the tank drain and a ball valve right above it. Not knowing how it was installed could it be air locked? If so will it correct itself at some point? Thanks

  6. Your comment on shutoff valve above the water heater. If the criteria in you senario is the same except you do not put any shutoff valves above the water heater, won't you have the same explosion? If not, where is the pressure relieved?

  7. I have been looking for this, so thank you. I need this for 2 heaters, the gas hot water,because I am on tank water and for the combustion wood heater,which ideally I would like to use with a water jacket behind it as a floor heater to use less wood or even for a hot water supply. Can this be done?

  8. Hi Tim, I liked your little explain on hot water systems. Gravity circulation designs are a simple pump free idea in use many years ago when hot water systems were open vented. On these designs the check valve of choice is a swing check valve installed horozontaly, a) stops water being pulled from the base of storage unit and b) importantly adds very little resistance to the convection flow of the water. Regards.

  9. Hello,

    We have a 75 gal gas hot water heater with a circulating pump that returns below the drain valve. So what do I do to drain the hot water tank to get the sediment out, without draining all the hot water out of the rest of the house through the loop return?



  10. Hey Tim, my house was pre-plumbed for circulation; when I recently changed the heater (it was previously feeding both ends of the hot water loop), I decided to add a recirc pump. I placed the pump at the end of the loop, right before return to the drain tap of the heater. Works fine, BUT, the pump seems to be injecting air into the system. I have NO leaks; to isolate, I just turn pump off, bleed out all air. Is fine until I turn pump back on; a short time later I have air in the system. I have run this isolation step multiple times; air IS getting in when pump is on, no water is leaking from any joint. Have you seen this before?

  11. so after 3 years I finally got around to installing a pump on my return line. added it to the bathroom central fan circuit (with a relay) so that it runs when the fan is turned on. Everything works awesome just one problem: the water runs yellow for the first 10 seconds. What could possibly cause this in the return line over 3 years?

  12. I need some advice on how to correct a problem with a hot water recirculating line.We just had a new hot water heater just like the old one and we have had a hot water recirculating line for years with the old hot water heater which worked like a charm. It has a check valve installed but after some local plumber changed out the old hot water and installed the new one and reconnected the existing hot water recirculating line it will not work properly. I have had the plumbers back twice already and they purged air from the hot water recirculating line and it still will work like it did before. Do you think the line may still have air in it or could it be another problem?

  13. Tim,

    I feel complied to as why don't you show a 'diagram' to aid those thof us that can not visualize what you are providing. For some, such as myself, instructions provided are really helpful, BUT a diagram would add so much in understanding.

    PS What happened to the 'FIRE PIT'? I truly enjoyed reading that.

  14. Just fitted a circulating pump at my home. do i need a branch, with a regulator valve back to head tank,in return hot water line going to geyser?...to avoid any abrupt increase in pressure when circulating pump runs as it's a head tank fitted on the roof at about 15 feet above the geyser.

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