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Plumbing system design for high-end house

user-7619767 | Posted in GBA Pro Help on

Hello. We are building a new construction, high-end house and want water distributed to all areas of the house to have consistent hot water. So when the washing machine or dishwasher is receiving hot water, somebody showering doesn’t feel any effect, either from pressure or temperature. We also are planning to use tankless water heaters and a recirculating system. What is the best system design to accomplish these goals?  Thanks in advance!

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  1. etekberg | | #1

    My advice is to do a plumbing plan and specify everything including the brands and model (Uponor insulated aquapex for instance).

    What's the layout look like? What utilities. Lots of ways to do this. I have two cascaded 90% efficient gas fired 200k BTU tankless heaters in a non-central location feeding 3.5 baths including a single 12gpm shower through a 1 inch supply trunk with a 1/2 inch return line connected to a flow switch activated re-circulation pump. The pump runs for two minutes anytime anyone uses hot water, unless the return line is hot. Otherwise it is off. With insulated water lines, hot water is fairly readily available throughout the day. Works good. Without the recirc, it can take 2 or 3 minutes to get hot water at the farthest lav fixture. If I were replumbing I might recirculate through a 6 gallon electric tank heater instead of directly through the tankless units. I would also upsize the return line to 3/4 for lower velocity - right now I restrict recirc flow to 1gpm per Uponor guidelines.

    If doing again I would consider multiple electric tankless units located closer to the source. But I don't know if the electric company would be okay with me having 800amp service. I ran the numbers and electric wouldn't cost me anymore, but I have gas mainly for hot water so the numbers are skewed a bit.

    1. user-2310254 | | #12

      If you are building a large, high-end home and want it to be green(er), consider installing something like an 80 gallon Rheem heat pump water heater (or two 40 or 50 gallon tanks located to minimize the hot water runs).

      Electric tankless units generally require a special electric setup since they draw so much power. I considered this approach on my last home and concluded it wasn't practical (even in a large, high-end house).

      Plus one on using temperature and pressure sensitive valves. They aren't that expensive and make showering much more comfortable.

      1. Expert Member
        Michael Maines | | #13

        Steve, using two 40/50 gallon HPWH is a brilliant idea--I've had a few projects recently where the plumbing is spread out, and I don't know of any recirculating systems that are really efficient. The upcharge for the extra water heater would pay for itself with water saved, I would bet.

    2. Expert Member
      NICK KEENAN | | #19

      Can you describe the flow switch more? Brand or model?


      1. etekberg | | #20

        check out ; it's not a bad solution and you can integrate it with one of the temperature shutoff devices that the pump manufacturers sell

  2. Expert Member

    To properly answer the questions we would need to know which end of the house will be highest.

    1. ohioandy | | #3

      Malcolm's right. Also, given the nature of this forum, which end is green??

    2. tommay | | #9

      If he properly places his water supply he may be able to use a gravity system....

  3. user-7619767 | | #4

    Thanks etekberg, Malcolm and Andy. I'm obviously not a plumber nor am I very versed in plumbing systems. We are the owners and have hired a contractor to build our high end home. In past houses, we have had hot water issues when a washing machine or a dishwasher is running, we get noticeable dips in pressure and temperature when taking a shower at the same time while running these appliances. So we're trying to figure out what is the best system to install that would eliminate this dip in pressure/temp while showering. I have become familiar with trunk and branch, as well as home run systems, just from this website. The green end of the system is our 2 tankless hot water heaters. We will also install circulation pumps to ensure hot water quickly. The house is about 4700sf, 95% 1 level, with the tanks originating in the garage. Thanks for any advice you all might have!

  4. vashonz | | #5

    Getting water from the low end of the home (assuming the garage) to the high-end of the home (cuppola maybe?) should not require a circulation pump, city/well water pressure should do that just fine. Tankless water heater+4700sqft doesnt seem like it fits in the "green" part of the house.

    In all seriousness: Much more detail and research is needed, circulation+tankless doesn't seem like the answer to me. Instead if shower pressure/temp is of prime importance, a thermostatic balanced shower valve/diverter would be what I would use. (I'm being cheap in my build, only installing a balanced shower valve, can upgrade to a thermostatic one if I have issues)

  5. jberks | | #6

    Unamed user,

    Based on what I've read of Gary Klien's work, a loop and branch system is your simplest for a hot water recirculation. in other words, its 1 recirc pump for the whole house.

    I'm a little more brash and enjoy manifold setups, with each hot having it's own return line to a recirc pump (in my case 4 recirc pumps). I did this system for ease of install and that all my fixtures are in wall, so shutoff valves are at the localized manifold.

    Most people say their pressure drops, but I believe is all about flow capacity, ie available water. If you have one small diameter pipe feeding two high flow fixtures, opening them both at the same time will diminish the fixtures outputs. Thus, consider the plan of what all the pipe diameters are going to be. Including incoming water supply from the city. 3/4" is the norm. I did mine in 1" because the jump cost was minimal compared to the jump in performance.

    In terms of water heater setup, most of the time that doesn't effect the issue of loosing "pressure" with a a few fixtures running, it's an issue of not having enough hot water available for the demand of the house. I currently have an on demand water heater with an integral 3.5gal buffer tank. It can output enough for about 6 mixing fixtures like showers, and probably half that for all hot fixtures like washers. But once I start runing the hydronic heating /snow melt system and the hot water recircs, I might find I need to add in a larger secondary buffer tank. I like the tankless+buffer tank approach, I more like high btu tanked units. But those don't look as cool as multiple wall mounted tankless heaters.

    For what it's worth,


  6. Expert Member
    BILL WICHERS | | #7

    Dips in pressure tend to be due to insufficient supply pipe size, especially on long runs. A 1” main should alleviate much of this, and you’ll need that size for the hot AND cold runs. Branch each fixture off of the main with short runs of appropriately sized pipe which will usually mean 1/2”. You can use a 1/2” line for the recirculation side of the loop since that side only carries the relatively low flow of the circulation pump. BE SURE to insulate BOTH sides of that loop well! If you don’t, your recirculation loop is just a stretched out radiator and will have a lot of heat loss. Make sure to put a check valve in the return line to prevent reverse flow on that side of the loop.

    If you have inadequate supply from the city, a small pressure tank such as those used with water wells can help to supply momentary periods of higher than normal water demand.

    I have such a setup in my own home (except my water heater is in the basement so I use a thermosyphon system instead of a circulation pump), and you can flush the toilet and wash your hands in the same bathroom as someone taking a shower and see NO change in the shower water temperature at all.


  7. Jon_R | | #8

    You can add pressure regulators near the outlets to keep output pressure/flow constant as source pressure and dynamic pressure loss changes. This will also help with consistent temperature, but even better would be thermostatically controlled mixing (since it also adjusts for supply temperature variations).

    Larger pipes reduce but do not eliminate fluctuations due to dynamic pressure loss.

    IMO, a circulating system should be on demand (at least during the non-heating season) and rather high powered - so it can act quickly.

  8. walta100 | | #10

    user-7619767 please tell us your name.

    You may find Gary Klein’s web site interesting.
    I like his trunk and twig system.

    The pressure problems you had were likely the internal size your pipes. Sometimes it is the size of the pipe from the street to your house, other times it is the size of the pipes inside your house. If you select PEX piping be aware there are several different systems but all the fitting go inside the pipes so each fitting restricts the flow some brands more than others.

    If pressure changes bother you pay for larger pipes. The down side is longer wait times for hot water.

    Is your plan to run the reticulating pump 27/7? Or are you willing to tolerate an delay in getting hot water?

    I built an on demand controller for my tankless electric heater. To prime the hot water system turn on any hot facet for a few seconds and back off this starts the pump, it runs until the loop is hot about 90 seconds.

    If the goal is a constant hot water temp a large tank style is best.

    Consider upgrading your shower valves to thermostat controlled valves that self adjust for a constant temp.


  9. gusfhb | | #11

    not a pro,but:

    google 'thermostatically balanced shower valve'
    I have nearly 40 feet of actual pipe length between my tank and shower, and can barely brush my teeth in the time it takes to get full hot water. They react much more quickly, are much more comfortable, than the pressure balanced valve your plumber probably prefers

    If I found a pump necessary, I would probably wire it to a timeout relay[one that shuts off after X seconds] and wire it to the shower light.

  10. Expert Member
    Peter Engle | | #14

    ON-demand recirculation is easy to install, and it saves a ton of energy. It will take a bit of tweaking to work with on-demand hot water, because you don't want the water flow to stop once the hot water reaches your fixtures. Walter had one approach above, where the recirc pump starts when the water flow starts. That's a pretty simple setup and should work well.

    With a large house, you will need multiple on-demand heaters rather than one really big one. The lower flow limit on the big ones is often greater than a single fixture demands, so they won't even turn on if you only need a trickle of water. Since you're installing multiples, place them at multiple locations in the house, as close to the demand as possible. The shorter the pipe runs, the better and more efficiently they work. Don't even think of putting them in the garage. That completely defeats their benefits.

    With 4500 square feet to work with, you can afford to put in some small mechanical closets really close to your bathroom groups. And, you should group your bathrooms as much as possible. Putting a tankless in a closet between two bathrooms is ideal.

    1. Expert Member
      Peter Engle | | #15

      You should also know that on-demand water heaters don't play well with high-efficiency washing machines and dishwashers. The washing machines take water in spurts rather than continuously, and this causes havoc with the flow switch of the water heater. In effect, it never turns on. If you have HE laundry equipment, you will need a buffer tank at least as large as the W/M demand, and it still can cause some screwy behavior.

      For dishwashers, I've seen some pretty convincing papers suggesting that modern dishwashers should connected to the cold water line rather than hot. The argument goes that they use so little hot water now, that the demand is often less than the volume of (cooled) water sitting in the hot water pipes. So in effect, the dishwasher never sees hot water anyhow. All modern dishwashers have their own heating elements to get the water hot enough to do its work, so let the internal element just heat the cold water to working temperature.

      Of course, a 24/7 recirculating loop that passes very close the D/W would solve the hot water problem, but at a very high energy cost.

    2. etekberg | | #21

      Can you point me to a gas fired tankless manufacturer that specs a smaller unit with a lower flow activation than their larger units? Every brand I've looked at are the same across their line-up. For the record, I pretty much have everything you say that won't work. And... it works. Not ideal from a "make everything perfect" Engineering perspective, and I would design differently given the opportunity, but works just fine.

  11. Expert Member
    BILL WICHERS | | #16

    Note that you can get recirculating pumps with timers that can be set to only run the pump during times of likely need. I like the idea of wiring the pump in with bathroom fans/lights. If you use some relays, you can set up a simple system where if ANY bathroom light is activated it will also turn the circulation pump on. This way the system is automatic, activated on expected demand, but no extra steps are required of the occupants to use the system.

    I agree a beefier than normal circulation pump will let the system start up more quickly. You could even put a temperature switch on the return line near the water heater to shut off the pump once the entire loop is warmed up sufficiently. If someone is actually using hot water somewhere, the pump isn't really needed unless the hot water user is very close to the start of the loop so that the rest of the loop doesn't stay warm.

    Be sure to use a circulation pump rated for potable water here. Many of the circulation pumps intended for use with hydronic heating systems are not rated for use on potable water systems.


    1. Expert Member
      Peter Engle | | #18

      Metlund produces the D'Mand pump system. The pump itself has a thermostat on it, so when warm water returns to the pump, it shuts off. They also make a bunch of different actuators. I've got their system in my house, with a simple pushbutton (looks like a doorbell) on each of our vanities that starts the pump when you want to. They also make interconnects for the light switches. Both hardwired and wireless. Pretty much a complete solution. The pump flanges are the same as standard TACO and other pumps, so pretty much interchangeable with other pumping solutions.

  12. user-7619767 | | #17

    Thank you all! I'm sorry, I just joined so wasn't able to update my name yet. My name is Spencer. It's going to take me some time to not only review these responses but also research what each point is that you are making, to understand how it might work in my specific situation/config. I'll be back with specific questions soon. Thank you!!! Spencer

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