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Community and Q&A

Plumbing – home run, remote manifold or other design?

mr_stop | Posted in GBA Pro Help on

I will be remodeling my home which involves relocation of kitchen and baths. I will be running 1″ copper to the main manifold, and then I will split off into PEX tubing. I’m getting nearly instant hot water in my current home config (<25 runs through 1/2″ pipe). However, I will be nearly doubling the length of my longest runs. In the remodeled home, – The longest run will be ~50′ to the kitchen sink/dishwasher from the water heater – Master bath will be about ~20′ from the water heater – Kids/guest Bathroom will be about 40′ from the water heater Questions: 1) Based on the lengths of my runs, will a home run cause a substantial increase in hot water wait time/waste over what I have now? 2) Is a remote sub-manifold design with hot recirculation in each location (e.g. baths & kitchen) a better alternative? 3) If I go the home run route, can you also incorporate hot recirculation back to the water heater to get the best of both worlds?

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  1. krom | | #1

    is it feasible to move the water heater?

  2. srenia | | #2

    That is some long runs. Are you going to some performance loss because of that? Yes. Mini manifolds from the 3/4 inch runs to each location I'm guessing. Will you needs a pump. Maybe. Matters how many people are using the water and how often. You can also use a small water heater at those locations if water is expensive where you live.

    I won't get preachy about having a huge house, though I could. Larger the house the more mechanicals are needed that are inefficient in design. The nature of the beast.

  3. mr_stop | | #3

    It is actually a fairly small house by modern terms. It's a compact two story that is little shy of 2100 sq ft. currently.

    The only reason for the longer plumbing runs is that the kitchen is getting pushed about 15' towards one end of the house. Also, I would like to move the water heater from its current location (center of the house) to clear out space for a more ideal basement layout. I may have some flexibility in water heater placement as I finalize the design. Best case, it would shave off about 15-20' off the longest runs.

    At what point (length of plumbing run) should I consider adding hot water recirculation?

  4. exeric | | #4

    Brad, you did not mention it but I certainly hope you limit the new pex to 1/2 " diameter. The difference in area going from 1/2 to 3/4 moves it from 0.2 square inches to 0.44 square inches. In other words it will more than double the amount of water standing in the pipes that need to be drained before getting to the hot water. 3/4 hot water pipes sounds good but are profligate and unneeded in reality. Since you have near instantaneous hot water now with 25' runs it seems like you should be able to get away with no changes in your hot water heating system provided you keep to 1/2" diameter pex and use direct lines and not home runs. There will be a small penalty in wait time for the 50' line but it shouldn't be too bad. I think the advantage of going direct lines with 1/2" pex is simplicity and less energy use than would be used in an elaborate heating system. Can you wait an extra 15 seconds for hot water on the 50' line?

  5. rocket190 | | #5

    Remember that the interior diameter of 1/2" pex is small than the diameter of copper pipe, so a rule of thumb is to upsize by one pipe diameter. I used a home run pex system in my house and I'm very happy with it, but my longest run is in that 25' range. I think a 50' run with 1/2" pex will have a major pressure loss. The friction loss of pipe is too high, even without using any elbows.

    A 50' run of 1/2" pex holds .52 gallons of water. 3/4" pex will hold 1.14 gallons. Using a standard faucet you'll have to run the water for almost a minute to purge the pipe if the water has gotten cold (3/4" pex). Since the volume of hot water used in a kitchen is very low compared to your bath and laundry areas, I would keep your primary water heater close to those areas and use a small point of use water heater in your kitchen.

  6. exeric | | #6

    Rick, while I agree that 3/4 pex is the more appropriate solution than the 1/2 pex that I suggested I think otherwise you misinterpreted what I was getting at. He is saying that his current 1/2" inner diameter run provides hot water "near instantaneously". That is NOT going to increase to near a minute for a fifty foot run. You are equating pressure drops with time lengthening. They don't equate. While there might be a slight pressure drop because of the increase of 25' over what he has now that pressure drop is much more than compensated by the smaller volume of cold water that has to be drained over the usual 3/4" inner diameter line.

    One more item. That is why I recommend separate runs, especially for long runs. If you had a shower running simultaneously on the same run it WOULD drop the pressure precipitously. It works just like voltage and current. If you are at all worried about one load dropping the voltage too much for another load then you put them on parallel lines, not series lines. It also means that the relatively low volume of hot water used in the kitchen won't cause a precipitous pressure drop over a 50' run.

  7. rocket190 | | #7

    Eric, I think we're on different pages. You advocated for direct lines to each fixture. This is a home run supply the way I understand it. The max flow capacity of 1/2" copper is 3.2 gpm. The max flow for 1/2" pex is 2.3 gpm. The max flow rate of a kitchen faucet is 2.2 gpm. My point is that a 50' run of pex isn't going to flow anywhere near it's max, particularly if there is a barbed fitting that further restricts the flow. A 50' run of 1/2" pex will leave you with a very anemic flow to the issues with the dishwasher since they use small volume of water. So if you use 3/4" pex to the fixture, you need to drain at least 1.14 gallons of water (volume of water supply line), which would take 30 seconds roughly if the fixture flowed 2.2 gpm. If the blending valve at the faucet will not allow. Plus you'll still have some like warm water to flush out of the manifold.

    While I agree that 1/2" pex runs will waste less hot water, I don't think the homeowner will be happy with the result.

  8. exeric | | #8

    Yeah, we were on different pages. I already agreed in my last post about the 3/4 pex being equivalent to 1/2 copper. But the norm, as I understand it, is 3/4 copper and 1" pex. I think 3/4 pex is plenty and will save a lot drain time over 1" pex. It also won't increase the wait time to a minute for hot water.

    I have a physics background so when I hear "homerun" I visualize 1st, 2nd, 3rd, , in series. But apparently that isn't how its used in plumbing. My big point is that there isn't enough of a hit on the 50' run with 3/4" pex to justify the cost and complexity of a whole separate point of access heater there.

  9. user-2890856 | | #9

    Could you post a floor plan of the existing home and addition and where and how the existing water pipe resides at present ?
    You will want to stay away from crimped fitting Pex systems as the fittings are such a restriction . Look at one of the Pex-A manufacturers , Uponor , Rehau , Mr Pex . Use the remote manifold idea for each of these rooms , going by the basic 3 fixture bath , for the hot you should use a third port and run it back to a dedicated recirc line to the water heater . Place On Demand switches at each bathroom door and one by the kitchen sink and use 1 pump to freshen all the lines only when required , a learning pump is another option but will use more electricity . Point of use water heaters that require a #6 wire to instantaneously turn 50* water into 120* water are the most wasteful thing people try to justify .
    Check this out for what will be your best solutions .

    As far as max flows and volumes in pex ,

  10. Expert Member
    MALCOLM TAYLOR | | #10

    I'm not sure that whatever system he decides to use should include any 3/4" Pex except from supply to the water heater and to a manifold if one is used. We use nothing but Pex here and only a combination of 3/8" and 1/2". Including 3/4' isn't going to stop you scalding yourself in the shower if someone uses another tap and you would have to have incredibly low pressure to need it for friction loss.

  11. GBA Editor
    Martin Holladay | | #11

    There are many articles on the GBA website to help you with your problem. The hot-water expert who developed the "structured plumbing" approach, Gary Klein, is the person to listen to on this issue. Klein does not favor home-run plumbing from a single manifold; rather, he suggests using a modified trunk-and-branch system.

    Here is a link to one of Gary Klein's articles -- don't forget to read the articles he links to:
    Inefficient Hot Water Piping Layouts Waste Hot Water

    Here are links to other GBA and Fine Homebuilding articles on the topic:

    Hot Water Circulation Loops

    Waiting for Hot Water

    Fixing a Hot-Water Problem

    GBA Encyclopedia: Efficient Plumbing Supply Layouts

    Hot-Water Distribution Systems – Part III

    Here are some principles to remember:

    1. Locate your water heater near the center of your house.

    2. Use small diameter tubing; often 3/8" is all you need, especially for a lavatory.

    3. In some homes, two water heaters make sense. A small tankless water heater might need to be installed for remote locations.

    4. If you need a hot-water circulation pump, make sure that it is an on-demand pump, not one that operates continuously or on a timer.

  12. user-2890856 | | #12

    As Martin refers to and Gary Klein wrote "In addition to routing the hot water supply close to each use, the Structured Plumbing approach includes a demand-initiated pump that allows the occupants to prime the trunk line shortly before they want hot water." Pretty similar to my statement .

  13. mr_stop | | #13

    Below are floor plans of my current home & one of our leading options (still needs refinement). I have marked locations of the current water heater which resides in the basement. It's in an ideal location right now from a plumbing standpoint. However, I would like to move it to expand my garage / workshop which is located in the basement.

    The existing pipe is 1/2" copper and is mostly consolidated in a chase between the bathrooms and kitchen. For the most part everything is sharing the same 1/2" input. And yes, you do feel the cold rush in the shower if someone flushes! I'm not trying to save any of the original plumbing.

    I do plan to use Uponor pex for the reasons stated (flexibility, less restriction at fittings) from the manifold on. 1" copper will run to the mechanical room where it will run through a softener then split to the hot/cold lines. Attached are my fixtures flow / supply calcs based on my local code requirements. Granted, this is somewhat pessimistic in it's design. I'm either already running lower flow fixtures/equipment or intend to.

  14. mr_stop | | #14

    Thanks for the article links Martin. I have read some of them, as well as some of Gary Klein's other articles. I'll spend a little more time with them. I'm getting a good handle on the theories and best practices. Unfortunately, design and aesthetics sometimes forces us with less than optimal situations. Otherwise, we end up living in the equivalent of a Pontiac Aztek.

    The big thing for me is understanding trade-offs at this point (e.g. if I go from a 30 ft run to 50 ft run, what is the difference in hot water response time). Also, whether this can be mitigated through a different distribution layout/system.

  15. exeric | | #15

    Yes, I appreciate the links also Martin. Often one doesn't know how much you don't know until you read the material put out by those who do. After reading most of it, it seems like the difference in pex vs copper of the same size is not significant enough to warrant different specifications for the two. There is a slightly smaller volume in the pex which is probably more than made up in straighter runs with much more gradual bends. A wash essentially with the nod going to pex.

    I'm just struggling with whether I could get away with 3/8 pex on my 30' run to shower, bath, and lav or whether it requires 1/2 pex and then 3/8 branch pex. Right now its 3/4 copper and the wait time and water waste is entirely unacceptable. Is 3/8 pex too restrictive for 30' to a shower? Also, if you don't use crimp fittings is there another fitting type that a DIYer can use that doesn't require an expensive machine AND also is a high flow fitting, unlike crimp types? Inquiring minds want to know.

  16. mr_stop | | #16

    I'm no expert, and in the planning stages also, so take my advice with a grain of salt.

    To my knowledge only the Uponor expansion type fittings are the only ones that "don't restrict flow." How much flow is restricted in the other type, I'm not certain. My take is that if you are doing a lot of runs, I would just invest in the $400 for the tool if that is the way you want to go. Once you are done, you could probably recoup much of the cost if you were to sell it on fleabay. If you don't have a lot of plumbing, I would probably go the crimp route and forget about it. There seems to be plenty of persons using that style without too much problem.

    The biggest issue with the Uponor system is the premium cost of the copper fittings and manifolds. From what I understand is that Uponor PEX, can be crimped also. Because of this, I'm thinking about just using crimp style manifolds as a lower cost alternative.

    In terms of the correct size of pipe, you will want to checkout what your plumbing code requires and how it is enforced. Code sets minimum pipe size for fixtures. See my spreadsheet for an example of what is outlined in my local code (1/2" min for shower. Also, you will want to calculate the "water supply fixture units" (WFSU) for the run and then look up the chart that shows the proper ID for your length of run and WSFU. You could also calculate the expected flow based on your fixtures and size the pipe that way.

  17. kevin_in_denver | | #17

    Sharkbite fittings grip the OD of pipe, and therefore don't cause excess restriction. They are available at all big box stores.

    If you are sure that you will always have 40-50 psi, then 3/8" should work for most fixtures. (It's easy to try out, just hook up 50' of pipe and a faucet) The "home run" piping strategy (not the pipe size) is what prevents the shower temperature from changing when a toilet is flushed or someone turns on a faucet.

  18. Expert Member
    MALCOLM TAYLOR | | #18

    Kevin, Although Sharkbites do attach to the outside they also have a plastic sleeve that fits inside the pipe. And the price!

  19. user-2890856 | | #19

    Cold water supplies that are sized appropriately are and ASSE 1070 pressure balanced Tub & Shower valves are in fact what keeps a person from being scalded . In the late 20th and now in the 21st century there should be nobody with modern plumbing should be suffering from this condition .
    Brad , Later today when I get in from the field i will post a nice solution for this floorplan that will be both functional , acceptable and efficient for the water piping .

  20. kevin_in_denver | | #20

    Response to Richard,

    You're right that the new valves prevent temperature fluctuations. However, they can cause flow fluctuations, which are also annoying. Home run piping will prevent the flow fluctuations.

    To Malcolm,

    Sharkbites are expensive, but when you are using 3/8" pipe, you rarely need an elbow, and with home run piping, you never need a tee.

  21. wjrobinson | | #21

    Richard McGrath, I for one am glad you are posting here now. Great. Post often as to your field I am always learning. I am at present rebuilding a Teledyne Laars unit I installed new for radiant heat over twenty years ago. I may need you to go over my update of it to a new mod con this spring.

    ajbuilderny is a google e mail ... get in touch

    as to this question.... 1/2" PEX home runs for me or 3/4" branching at bathrooms, recirc it if the option is wanted. Most of my homes have had very good stacking set ups so runs have been short. Long runs need recirc or get used to using cold water. At my own house I am used to cold water to rinse hands and I like the idea of small water heaters, they limit over use of hot water. No need to yell at anyone, they scream at themselves for staying in too long!

    In the end I build what the customer wants, recirc or not, huge unlimited hot water supply, or not.

  22. user-2890856 | | #22

    Malcolm ,

    A good quality valve in my experience does not and cannot cause flow fluctuations . The only way this can happen is on a well water system where the pressure fluctuates even if you have a constant pressure set up . That unit must still sort through differing flow characteristics and decipher what it needs to do . If one places a TAMV at the water heater the flow will not fluctuate , it will pass the same amount of water every minute . Col;d flows from the tee to the heater and the mixing valve , upon opening a faucet varying amounts of cold will feed the tank and the valve decided by the valves requirements to maintain a set point temp . If the incoming pressure is constant the valve will not suffer flow fluctuations .

    Brad ,

    You may want hotter water at the kitchen sink than in other locations . My suggestion is this .
    Set the temp on your Phoenix LD to 140* , use a tee at the water heater and feed the KS . Then employ a Taco 5000 series mixing valve set to 120* max at the heater and feed the remainder of the house from that outlet . Find the most common wall between the downstairs and upstairs bath that are on the left of your drawing (size of this would depend upon your preferred usage and how your family lives) 1/2 lines for the first floor powder room would be more than sufficient , continue upward and locate a remote manifold in either of the back to back closets then do homeruns to each fixture in the master bath , add an extra hot oulet to that manifold , this will run back to the heater . Run 3/4 to the other upstairs bath and locate the manifold in the closet behind the lavs , add the extra hot port which will return to the heater . Locate the recirc pump at the heater (Uponor DMand type) . 2 or 3 switches is your choice , tee the 2 return lines together and to the pump . Whenever you go into the bath turn the switch on prior to using hot water , you will figure out how long it takes to move all the cold out of the line , then turn it off . Som4e like to add an aquastat at the pump to shut it off , I find that heat stacking from the heater can at times cause the pump switch to stay open and move no water , timing and knowing each circuit's time on required is often best . KISS principle . The kitchen line could also have one if you prefer that in which case you would just add another tee near the pump and at the fixture to run back to heater location .

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