Sunday, March 6, 2016

Automatic RV Hot Water Recirculation System

To minimize waste of water while waiting for the hot water in the sink or shower, I installed an automatic hot water recirculation system in my Agile. I can also use it to prevent the water line from freezing when camping below freezing temperature. I'm already thinking of re-plumbing the hot water line under the RV to provide heat into the fresh and holding tanks via a DIY heat transfer plates which will be installed under each tanks. A manual diverter valve will be used to divert hot water away from the heat transfer plates when it is not needed, that would be my near future project.

The DIY hot water recirculation system is installed into the galley faucet hot and cold water line which is the point farthest from the water heater. It is fully automatic with manual switch override. A digital temperature controller is used to switch the pump on and off to automatically recirculate hot water back into the cold water line. The adjustable fan-control thermostat switch (see below) monitors the water temperature in the water heater tank and the system only becomes active when set temperature is achieved. It will shut the system off when water temperature falls below the set temp, doing this will prevent the recirculation pump to continuously run when water heater is shut-off and hot water falls below a certain temp.

Hot water recirculation system wiring diagram.

Parts used:
  1. PEX tubing and fittings: (2 pcs.) tee fittings, (2 pcs.) PEX to 1/2-inch NPT female elbow adapters
  2. Acrylic plastic for the electronic temperature controller enclosure (optional)

The electronic temperature controller with fabricated acrylic enclosure and a manual override push button switch.
Tee fittings connected to PEX to 1/2 inch 90 º male thread adapter. Crimping the brass tee fittings into the hot and cold water lines is almost impossible due to very limited space under the sink. It would be easier if I use Sharkbite PEX push-to-connect fittings instead, they are little expensive but it will save you time and sanity.
The electronic temperature controller temp probe is epoxied into the brass PEX elbow fitting of the cold water return line (or to the hot water line, if you prefer), and then it's wrapped with an insulation material.
An inline brass check valve was installed into the cold water return line. The check valve prevents cold water from flowing back into the hot water line. The check valve cracking pressure also prevents free flow of hot water to cold water line when system is off.
A pair of 12-inch flexible stainless steel braided water supply hoses is connected to the recirculating pump inlet and outlet ports and to the hot and cold water lines via the 90º elbow fittings.
The electronic temperature controller is mounted under the sink using 3M plastic hoop-n-loop fasteners.
Made an insulation cover for the recirculating pump. I tried to directly mount the recirculating pump into the base of the cabinet but it was running loud. 
 The recirculating pump inside the insulated DIY pouch.
The recirculating pump is then tucked behind the microwave.
Hot water recirculation switch is installed into the factory switch panel above the RT side sliding door. The positive 12-volt line was tapped from the now unused Carbon Monoxide (CO) detector circuit in the AC/DC Distribution/ Fuse panel.
I run the ground wire to the adjustable fan-control thermostat switch then wire terminates into the chassis ground. 
A close-up view of the adjustable fan-control thermostat switch. The thermostat switch is mounted directly and in contact with the steel water tank, a small piece of  the tank insulation was removed. The thermostat switch is secured in place by a metal strap.

  The adjustable fan-control thermostat switch is adjusted to turn on at about 105ºF and should turn off at around 95ºF ( a  Delta T of 10ºF). I set the electronic temperature controller to 32ºC (89ºF) with Delta T of 2ºC (controller only displays ºC). The recirculating pump runs until the remote temp probe senses water temperature rise to 32ºC and above then it shuts off, pump runs again when temp drops to 30ºC and cycle repeats.
Note: Only set the electronic temperature controller activation temp below the fan-control thermostat switch shut-off point.

The controller LED display is visible through a small gap on top of the modified waste bin slider (originally a slide out pantry).

Although not necessary, a water leak alarm is installed under the sink.


  1. Impressive and clever work! I was shopping for the same thermostat controller that you used here, and found your project from your review. I'm also using the thermostat to help with freezing pipes, by controlling an electric heater in an insulated coroplast box around the pipe. I noticed that you seem to favor working with acrylic, which i've never gotten into. Is it pretty easy to work with? Also, what do you use to create your very tidy labels?

    1. Thanks for the kind words. I apologize for the late reply; I was out for the memorial weekend with no connection.

      Acrylic is easy to work with; you can cut it with circular saw, router or table saw. I use a water thin Acrylic glue like this: ( ) to bond the pieces together. Here is a Youtube link how to glue Acrylic plastics:

    2. For the label, I use DYMO portable labeler with white/clear label cassette.

  2. Thanks for the info, now I'm going to be on the lookout for projects I can try using acrylic for! I'm afraid what will happen if I ever get a label maker, it will be labels everywhere!

  3. Will it work when you are on city water or when the water pump is turned on?

  4. Also will it work with an on demand tankless system

    1. It should work...but run time could take a little bit longer as the on-demand tankless water system fires up and heat the water. It could be annoying as every time the recirculation system goes off your on-demand water heater also fires up.

      Greatest benefit of the system is saving water when bondocking and instant warm water at the sink or shower during cold days/nights.

  5. Hello! I've been thinking of a way to recirculate the cool water while waiting for hot in my 2016 Zion SRT with Girard on-demand heater. How about a thermal valve at the kitchen sink which diverts its output back to the fresh water tank until it hits the temperature setpoint? No additional pump but need to run a return line to the fresh tank. I don't know how hard to run a line back to the tank but if feasible, that would be but otherwise a relatively simple solution. What do you think, my guru?

    1. Sorry for the late reply. Your idea is a good option, I actually thought of doing that...but running a return line to the Agile fresh tank is not that easy, if you can figure out how to do that on your Zion, I'm interested. Thank you and happy trekking.

      On the side note, I don't know the advantage/disadvantages of doing it with an on-demand water heater. I can imagine the water heater firing every time it need to recirculate hot water???

    2. Thanks for the reply.
      I don't know how difficult it would be to run a cold return to the fresh tank since I can't find a diagram anywhere on how /where the plumbing is run on the Zion. A simple but not elegant way would be to simply run a hose from the shower head (for example)out of the bath closet and simply dump back into the filler hole at the driver door. When it turns hot, stop the diversion to the temporary hose and enjoy the shower.
      With respect to your second paragraph: on my Girard, once the hot flow is going and is continuous, the heater stays on so there is no issue it the example of a shower where the flows is continuous for a few minutes. I set the temp at about 102F so no cold water needs to mix and the full flow is through the Girard. The burner stays on continuously that way. However, in the case of a submariner shower (water on to wet, water off to soap, water on to rinse), or in the case of washing dishes (hot water on for short bursts ), this scheme is not very practical or useful.
      For now, we try to plan ahead and define some use for the initial gallon of cold water we would otherwise waste waiting for the hot water to arrive at the indoor or outdoor shower. An automated diversion of cold to the fresh tank would be ideal so I will keep thinking about how to do that.
      Thanks again for your response... I love your projects.

  6. Does the temperature controller hold its settings when power is cut and powered back on?

    1. Is the pump so loud that it would wake someone up?

    2. NO not at all. You can barely hear it running even in the dead of night.


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