Can pipe (pex) size affect how long it takes to get hot water at sink?

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by emmakiata, Aug 24, 2010.

  1. emmakiata

    emmakiata New Member

    Aug 31, 2009

    I had an addition built 5 years ago. It has a master bath and it takes approx 1 minute for the water to the sink to get hot. The plumber used 3/4 pex on the 30 ft run to the bathroom area and then ran 1/2 to the sink, shower and tub. I think he must have anticipated our concerns about how long it takes to get hot water because he also included a 1/2 hot return to the water tank. The return has always been shut off at the cold inlet to the tank. I assumed that he did this in case we wanted to run a circulating pump? I don't really like the idea of running a constant loop of hot water all of the time because the bathroom is not used that often.

    My question is: will running a stand alone 1/2 pex to the sink only or changing the long run to all three fixtures to a 1/2 instead of the 3/4 decrease the amount of time it takes to get hot water without affecting water pressure/volume?

    Just as an aside, it takes about the same length of time to get hot water to the shower but the bath gets the water in about half of the time that it takes to get to the sink or shower.

    Thanks in advance for any ideas
  2. Fubar411

    Fubar411 New Member

    Nov 15, 2007
    St Louis, MO
    I know probably the least on this board, but you can put a recirculating pump on that small 1/2" line, then have that on a timer or proximity sensor.
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  4. Gary Swart

    Gary Swart In the Trades

    Sep 1, 2004
    Yakima WA
    The timer or sensor would work just fine. I run my pump 24/7. These pumps are not heavy power users, and I have never really considered a timer. I'm sure there is a cost to running full time, but it can't be very much. I really appreciate not having to waste water and wait to purge the cold water from the hot water lines.
  5. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Sep 2, 2004
    Retired Systems engineer for defense industry.
    New England
    A tub filler can flow MUCH more water than a showerhead or a vanity sink faucet. Showerheads are limited to a max of 2.5gpm, and a vanity faucet to 2.2gpm. A tub can flow 6gpm or more. So, you can move twice as much water through the tub, so it should take less time to purge the cold water.

    I'd make use of that recirculation line. If you didn't want to run it constantly, you could install a switch, and turn that on prior to wanting hot water, or, as noted, maybe hook that switch up to an IR motion sensor. You could hook it up so it would turn on when you turned on the light fixture which might be less likely to be left on. Any recirculation system needs a check valve, so it's not simply just installing a pump.

    I don't know the ID of your particular PEX pipe, but if you discount the wall thickness (it is significant), and use the pi*R^2 for the area of the 1/2" verses the 3/4", you'll see that the 3/4" can hold a lot more water - almost twice as much, so yes, bigger pipes (regardless of what they are made of) will take longer to purge. If you drop the whole line to a smaller one, you'll also slow the time it will take to fill the tub, and may notice a flow reduction if someone flushes a toilet while in the shower - it's bigger for a reason!

    Pex conducts heat less than copper, so you'll have less radiated loss, and the water will probably heat up faster than the equivalent sized copper (partly because copper would hold more water and partly because it will conduct heat away faster).
  6. jeffeverde

    jeffeverde Member

    Sep 21, 2009
    Yes, a new 1/2" line homerunned to the waterheater will supply hot water in about half the time of the existing 3/4" line (because, as previously stated, the 1/2" line has about half the volume of cold water sitting in it as the 3/4" line does). But don't use a single 1/2" line to supply the entire bathroom- you'll almost certainly be unhappy with the reduced volume. As others have suggested, to improve your hot water response, consider completing your recirculating system as a better alternative. You have several options-

    -If this bathroom is higher than the water heater (ideally on a higher floor), simply opening the return line (which should be tee'd into the water heater drain outlet) will induce a degree of gravity induced circulation (hot rises, cold sinks, so a natural ciculation occurs). You won't see "instant hot" with a gravity system, but it will be warmer from the start, and quicker to hot. Cheapest to install (free), water savings depends on the degree of circulation your system achieves, which is directly inverse to the energy you'll lose fighting heat loss in the line.

    -Pump with timer -- the timer is set to run the recirculation pump during prime bathroom hours (morning and evening), providing instant hot water at those times. Mid-day and night, the pump is off and the water in the line cools as in a non recirculating system. This is a good compromise between economy and convenience, while providing the environmental benefit of not dumping water down the drain while you wait for hot water

    -Pump with on-demand switch -- the pump is controlled by a switch located in the bathroom (typically under the edge of the counter, side of vanity, etc). When you want hot water, press the switch and in a few seconds you've got hot water at the tap without running a bunch of water down the drain. The most economical and environmentally friendly solution - but not particularly convenient.

    -Always-on pump -- a low-volume pump runs continuously. Convenient, but not particuarly economical or environmentally friendly (you are avoiding wasted water, but you're wasting energy through the heat-loss in the line 24/7).
    Last edited: Sep 6, 2010
  7. jimbo

    jimbo Plumber

    Aug 31, 2004
    San Diego, CA
    For a few years, IAPMO and PMENGINEER mag. have been reporting that the sizing of water pipes is being studied by "the powers that be". From the standpoint of energy and water costs, smaller pipes make sense, and with sinks even showers now being low to ultra low flow, we could eventually see a change in the wfu specs in the code books. If you are planning a niagara falls shower, you still need the volume, but I think the codes will soon catch up with that loophole also.
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