Hot Water Recirculation Design - Remodel

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by Rossn, Jun 13, 2018.

  1. Rossn

    Rossn Member

    Joined:
    Dec 12, 2017
    Location:
    Denver, CO
    Hello,

    I am looking for some recommendations on a challenging hot water recirculation design issue. Apologies in advance for the wordy post, as it's a complicated situation:

    I'm remodeling a good portion of my home and am adding a bedroom/bath above the garage, which requires about 100' on the hot water line. Additionally, I have a bath about mid-way (50') , which historically has taken quite a long time to get hot water to the lavatory. On the attached diagram, these are on the right/east end. The room above the garage may sometime have a roommate and get used regularly, and at other times it may be empty with virtually no use. The other East bath is intended for office/guest room use, and is planned for intermittent use, but if we have another child, could be used more frequently.

    The consistently used area of the house would be the kitchen and 2.5 baths on the west end of the home. Nothing here has a major hot water issue, though the far west lavatory and shower do take around 30 seconds to get hot water.

    The boiler and side arm are between the two areas, and shown on the diagram.

    I'm interested in installing a dedicated hot water recirculation return line (house is gutted currently), as I'm not keen on the nature of pushing hot water from the cold to hot side because a) the nature of warming the cold water side b) we drink water from the tap c) the fact that in Colorado, ground water can be quite cold in the winter, and this could draw colder water into the side arm, thus being more likely to trip on the boiler

    One of my primary concerns is wasting energy from hot water sitting in the line, going cold, which is why I'm gravitating towards an on-demand/push button type system like D'MAND (http://www.gothotwater.com/), and installing the pump on the 'far end' so as to not also have hot water sitting in the recirculation line. Along with this goes the fact that I don't want to kick on the boiler more than absolutely necessary.

    I haven't been able to come up with a solution that meets my energy desires, provides hot water, and also sources both the frequently used areas and the intermittently used areas, while using a single pump.


    Summary of design goals:
    - Must have DHW recirculation on two east end bathrooms, which may see intermittent or regular use
    - Prefer DWH recirculation on frequently used W end areas, though not mandatory (1 shower and lav are slow to get hot water ~30 seconds). Single pump would be nice, but not if it dramatically increases energy waste.
    - Boiler is approximately between the two areas
    - Want to be very mindful of energy use from both hot water sitting in the hot water line and the boiler kicking on more frequently
    - Want to be mindful of maintenance and pipe longevity

    Questions:
    1) What is the design recommendation for pump placement and recirculation line(s)
    2) I have read that higher velocities can cause pin-hole leaks. Is this real? I was intending to use the D'MAND pump on the highest setting, to reduce the time to getting water to the far end. If real, would I be better off using PEX on the hot water side (was intending to use copper throughout)
    3) How practical is the issue of kicking on the boiler much more frequently?
    4) Are there other solutions I should be looking to?

    Thanks in advance for all the help!

    Some tech info:
    Approx. ONE WAY distances and volumes for DHW line of varying options based on 3/4" copper:
    • W End Loop, no Kitchen = 40’ (0.9 gal 1-way)
    • W End Loop, w/ Kitchen = 48’ (1.1 gal 1-way)
    • E Bath Loop (no Garage) = 54’ (1.2 gal 1-way)
    • Garage/E Bath Loop w/ pump under vanity = 102’ (2.3 gal 1-way)
    • Garage/E Bath Loop w/ pump under kitchen = 108’ (2.4 gal 1-way)
    • Garage/E Bath Loop w/ pump under stairs = 112’ (2.5 gal 1-way)
    • Main House Loop = 106’ (2.4 gal 1-way)
    • Single Combined loop = 143’/173’ (3.2/3.9 gal 1-way; to kitchen vs to bath)
    Upstairs:

    remodel_recirculation_upper.jpg

    Downstairs:
    remodel_recirculation_lower.jpg
     
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Joined:
    Sep 2, 2004
    Occupation:
    Retired Systems engineer for defense industry.
    Location:
    New England
    While you have things gutted, insulate the water lines using the best stuff you can. Depending on the use pattern, simply putting the pump on a timer may be easier as you wouldn't need to wait, but in the colder times overnight, it would be off (unless you're going to need instant hot water regularly then). While it might become a habit, remembering to turn the pump on and then having to wait, can be a pain.
     
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  4. Rossn

    Rossn Member

    Joined:
    Dec 12, 2017
    Location:
    Denver, CO
    Thanks -- I think I'm pretty sold on not using a timer, especially for the area where usage could vary between infrequent and regular use. It seems this is a little to complex to ask. I'll break it down.
     
  5. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Joined:
    Sep 2, 2004
    Occupation:
    Retired Systems engineer for defense industry.
    Location:
    New England
    You could use a switch to enable a timer, so while you may have guests using the area, it would work, and when they leave, turn it off. You could use an occupancy switch to enable the system so that you wouldn't have to remember to turn the thing on. So, a typical scenario is you enter the bathroom, do your thing at the toilet, and by the time you're ready to either shower or wash your hands, the pump would have had an opportunity to get the hot water there. This would minimize the actual interaction the person would need to perform. Expecting people to remember to turn it on and then have to wait for the expected result could happen, but it's not particularly convenient.

    100' of pipe at the maximum recommended velocity means at least 20-seconds, and then, you have to warm up that length of pipe, so that would add some more. The system I have has an adjustable shutoff temperature and I have it set to about 100-degrees. My shower is closer to the supply, so it only takes a few seconds to purge the water for hot, and it have warm at the vanity to wash my hands. This also keeps less of the line at full hot temperature so that with insulated lines, the pump doesn't need to run often. With my system, the pump only runs about 40-seconds, maybe every 10-minutes and if someone uses hot water in the interim, the sensor would not turn the pump on since the line is already hot. It's neither a big energy hog for the pump nor a load on the a/c or a factor for the WH. In the winter, that small amount of radiated heat is not lost...it offsets some of the heat that would be required to heat the space (but it's not much). You do not want to use a large pump at high velocity to shorten your wait times...the overall costs will be higher for both the materials and the electricity and likely added maintenance factors.
     
  6. Rossn

    Rossn Member

    Joined:
    Dec 12, 2017
    Location:
    Denver, CO
    Ok, I see what you are saying about the motion sensor ahead of the timer, or maybe you're saying the other way around. I'll have to see what options practically exist. The push button ones can also work directly off a motion sensor, so I'm not sure that is terribly different.

    My general thought on the push button is that it can be used for many more things than the one that is temperature only. For instance, you could always convert the push button to timer, and quite frankly, it would require only a simple circuit to tie into home integration in the future.

    At this point, I'm leaning toward using a pump only for the East end, and looking for options for more efficiently plumbing the west end more efficiently that would lessen wait times. It's really only the shower and adjacent lavatory that are the issue.
     
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