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Community and Q&A

Improving Hot Water Distribution

user-2310254 | Posted in Mechanicals on

My wife and I are buying a house that was built in 2007. It has some issues, such as this one noted by the  inspector:

“The thermostat is set to higher than usual setting to provide 120 F water to kitchen sink. The water pipes are buried in the slab, which serve as a heat sink. Even after several minutes, the water at the sink barely gets hot, so a lot of water is wasted. There is a hot water recirculation system in the Owner’s Bathroom, close to the water heater. Water circulation systems are put at the plumbing fixture, usually a sink, furthest away from the water heater, which in this case would be either bathroom at the front of the house. Water may be recirculating, but it’s benefitting only the Owner’s Bedroom. A plumber can put a hot water circulation control valve on either of the bathroom sinks.”

The owner’s bathroom fixtures are actually in the room closest to the water heater. So, should I replace the current valve, which is located under one of the vanities, with a recirculating pump (see picture), or would it be better to install the pump at the hot water heater (maybe with remote sensors) and simply “push” hot water into the system as needed?


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  1. PBP1 | | #1

    A small on demand electric or a small electric tank under the sink or close by - will guarantee hot water. 2.5 gallon under sink will give over a minute of hot and the on demand is endless and eemax makes them with shutoff at an inlet set point temperature. Circulation is a bit of a pain. If line is not ice cold, a small on demand 3kW or so may work.

    1. user-2310254 | | #3

      Thanks guys.

      I imagine the hot water response in the owner’s bathroom is fine. The kitchen is really slow though, and there are two other bathrooms that are even further away. If I had to choose between efficiency (lower energy cost) and performance (faster access to hot water), I’d go with the latter. I think improving the hot water recirculation makes more sense here.

  2. walta100 | | #2

    If your problem is only at the kitchen sink consider adding a 5 gallon heater under the sink. The second heater will use much less energy than a recirculation system. 5 gallons is plenty of hot water for a kitchen sink.

    With that said I did build a recirculation system I turn in any hot water faucet for a few seconds then back off this starts the pump and the pump continues to run until the loop is up to the set temp.


  3. Expert Member
    BILL WICHERS | | #4

    +1 for a small point of use water heater.

    If that hot water line is buried in the slab and uninsulated (which sounds like it might be the case), then a recirculation system is going to waste massive amounts of energy if it's running all the time. You'd basically be converting your hot water line into a small radiant heating system with that slab. At the very least, be sure you use one of the on-demand type recirculation systems that only runs when you need it. Unfortunately, systems like that don't help as much as a continually operating system in situations like yours.

    A small point of use water heater under the sink will make hot water pretty much instant for things like handwashing. Supplying that small water heater from your hot water line will limit how much heating that small water heater has to do, which will have the effect of appearing to increase the tank capacity of the unit a little (you'll get more hot water until you run out), and by the time you run out of hot water from the little tank, you'll probably have heated up the main run and still be OK.


  4. Deleted | | #5


  5. user-2310254 | | #6

    I appreciate everyone taking the time to noodle this one around. I'm open to using a POU water heater if that's the most cost-effective and efficient way to address the problem.

    FWIW, uninsulated pipes under the slab are standard practice here (CZ3). My first house had polybutylene installed that way, and it worked out about as well you would expect. :-(

    Second time around I used the class-action cash to put in copper.

    In this case, it's CPVC under the slab, which doesn't give me a lot of confidence, either. The rental we are in has CPVC under the slab as well. There must have been an issue with hot water delivery here too since the owner installed a Grundfos pump on the hot water line. With that setup, the most distant fixtures get hot water in about 15 seconds.

    I did something similar with a Grundfos pump on the polybutylene house. In that case, the plumber ran an insulated recirculating line (allowed by local code) in the attic from the furthest fixture back to the hot water heater. Hot water delivery was almost instant, but the pumps only lasted about five years before the bearings would give out.

    I'll get with my plumber on this one. It's likely he's seen this configuration on plenty of production-built homes. I'll need him in any case to replace all those push-pull shutoffs. They are not repairable and are notorious for not working when things go sideways. My last house was a three-story town home, and it had these things on every fixture.

    Tom, that valve setup seems to be SOP around here. I think they are trying to do a thermosiphon loop. I guess. Maybe. You may be right about there being the recirculating line.

    1. Expert Member
      BILL WICHERS | | #7

      Thermosiphons only work if the water heater is at the bottom of the loop, such as in a basement, since you need convection current to make those loops run (that's how my own system works). If you don't have a basement, you probably don't have a thermosiphon.

      I'm not a fan of in-slab plumbing, except for drains where you don't really have much choice. If you have to build that way, PEX is the best choice, but I'd still try to find ways to keep supply lines out of the slab.


      1. user-2310254 | | #8

        I agree with you on the in-slab plumbing. It wasn't supposed to be that way on my first house, but the builder wasn't much into following the spec.

        1. Expert Member
          BILL WICHERS | | #10

          "but the builder wasn't much into following the spec."

          I wouldn't be much into paying such a builder...


          1. user-2310254 | | #11


            The plumbing was the first of many disputes with this builder. For context, we did this house 30 years ago and there really wasn't anyone to turn to for advice. When I talked to our architect, he said, "I didn't give you the name of a builder I would recommend. I gave you the name of a builder you could afford." Well, thanks for that.

            The house turned out fine. (It could have been better, of course.) But the whole process was incredibly soul-sucking.

    2. Deleted | | #9


      1. Expert Member
        Akos | | #12

        I'm pretty sure the valve's MIX output is tied back into the cold water feed bellow the cabinet.

        This works the same way as the comfort valve setup that use the cold water line as recirc return.

        As long as the hot water going to the mixing valve is cold, the valve is open allowing flow back through the cold water pipe. Once the water is up to temperature, the valve shuttles over the cold side and the flow is stopped.

        Its a pretty decent solution to lack of dedicated recirc line. It also has the benefit of only recircing enough to keep just the supply line hot. The drawback is the cold water feed will be lukewarm for the initial draw. This can be an issue for a kitchen sink where you want cold water for drinking.

        Since the house is already set up for this type of recirc, you can also add another mixing valve (comfort valve is easier install) at any of the other faucets where you would like faster hot water.

        1. Deleted | | #13


        2. tommay | | #14

          Just looked a little closer at that valve. It looks to be a Watts mixing valve whereby you are correct that the mix goes out the bottom. Contrary to every mixing valve I've ever installed in the past 40 years. But it still doesn't make much sense to send cold or mixed water back to the hot water heater.
          Most recirculating lines pull a portion of the hot water feed back to the heater which is why a pump and check valve is needed. If there is no check or pump, cold water would enter since the return line is usually tied into the cold water feed of the heater.

          1. Expert Member
            Akos | | #15

            Without a pump, this won't work. You need a pump at the tank inline with the hot water to the house. Pretty much the same as the Watts recirc install.

            P.S. About the only way I can think of getting it to work without a pump is with a venturi tee on the cold side of the water heater.

            This way a hot water draw would create enough pressure to pull the mixed water backwards through the tap cold water feed.

  6. user-2890856 | | #16

    This is the best solution . Smart plug has 2 settings for pump run strategy . You can get rid of the stupid mixing valve in the master . Order a second bypass valve and install them at the furthest fixtures . switches allowing circ to be turned on only on demand can also be employed . This system uses the cold water line as the return back to the heater and they work well . The circ sizing will reduce possible erosion through lower flow rates and protect the slab piping also .

    1. user-2310254 | | #18

      Thanks, Richard. I like your suggestion and will discuss it with the plumber.

  7. GBA Editor
    Kiley Jacques | | #17

    Allison Bailes just wrote about this issue: Why Does Hot Water Take So Long? . Based on his article, I think he would advise you to install a demand-type recirculating pump.

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