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Thread: 2 1/2 gal undersink heater - use hot or cold supply?

  1. #1
    DIY Senior Member AcidWater's Avatar
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    Default 2 1/2 gal undersink heater - use hot or cold supply?

    Since the purpose of the under sink lav water heater is to prevent wasting hot water, it seems to me that I should hook it to the cold supply. Otherwise, hot water will flow into the supply line and cool down, wasting it.

    OTOH, is 2 1/2 gallons enough for a woman to wash her face, get her makeup off, etc? Maybe I'll have to crank up the thermostat setting a bit?

    If she's going to use up more than a tank, then I'll need to hook it to the hot supply.

  2. #2
    DIY Senior Member gardner's Avatar
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    Default comment from the ignorant

    My totally uneducated opinion would be hot.

    In my house the run from the tank to the tap is pretty long, so if you want a cup of hot water you have to run the hot for 60 seconds before you get warm and 90 secs before it is really hot. Getting a cup of water forces the water heater to heat 6 gallons.

    What you are saving is that 90 seconds of running and the time it takes to warm up those pipes so the water is hot. The run time to get full heat drops to maybe 10 seconds and 1/2 gallon and though you heat the water twice, you heat way less than twice as much of it.

    When you're filling the tub or running a lot of hot for dishes or something, you will wind up drawing from the main tank and get lots of hot to work with.

    I have no idea whether it is actually kosher to feed hot into the inlet of a small electric tank, so that's something I'd be interested in.

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    Master Plumber Redwood's Avatar
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    Default

    I would consider a recirculation system.

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    In the Trades Gary Swart's Avatar
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    I concur with Redwood. I plumbed in a recirculating pump 3 or 4 years ago and it's great. Instant hot water anytime and as much as I want anywhere in the house.

  5. #5
    DIY Senior Member AcidWater's Avatar
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    The tank is under the sink. Hot or cold???
    ***

    Recirc constantly wastes money.

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    Master Plumber Redwood's Avatar
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    Compared to buying & running a second water heater its nothing!

  7. #7

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    Quote Originally Posted by Redwood View Post
    Compared to buying & running a second water heater its nothing!

    Since when is a recirc retrofit inexepensive to install and operate?

  8. #8

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    Quote Originally Posted by Gary Swart View Post
    I concur with Redwood. I plumbed in a recirculating pump 3 or 4 years ago and it's great. Instant hot water anytime and as much as I want anywhere in the house.

    How many times in a day do you need "instant" hot water? What a costly and energy wasting feature!

  9. #9
    Master Plumber Redwood's Avatar
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    Grundfos Comfort System - Hot Water Recirculation System

    This unit recirculates cooled off hot water into the cold supply side with a thermostat valve that controls the flow so hot water is not wasted. It may be tied to a timer or, occupancy sensor for additional savings. Insulate the hot supply lines for additional savings.

    It's more economical than you think!

  10. #10
    In the Trades Gary Swart's Avatar
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    I run my pump full time, and I frankly have not been able to determine how much it costs, but the motor is very small and the electricity consumption is minimal. I daresay my dishwasher running once a day costs more to operate that the recirculating pump. I installed the system myself so there was not labor costs. It did help that copper was not nearly as expensive as it has become in recent times, and the fact that all of my plumbing has easy access in the basement helped also. Purging cold water from the hot water pipes in my opinion a greatey waste of resources than that little pump.

  11. #11
    DIY Senior Member gardner's Avatar
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    If the recirulation systems put the feedback into the cold inlet, then I guess it is fair play to feed hot water into the cold inlet of a water heater. Is that right?

    If you wanted to double up your tanks for extra capacity, would you feed them in series like this?

  12. #12
    In the Trades Gary Swart's Avatar
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    I'll bet the farm that I can operate my recirculating system for 100 years for the price of just one of your under sink heaters and have instant hot water everywhere in the house not just in the bathroom where the under sink heater is located.

  13. #13
    Retired Defense Industry Engineer jadnashua's Avatar
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    Feeding hot into the inlet of a WH won't hurt it...
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  14. #14
    Moderator & Master Plumber hj's Avatar
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    Default recic

    Here, the city basically refunds the cost of the recirc pump/control valve system so installation is all you pay for.

  15. #15
    Master Plumber Redwood's Avatar
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    Bear in mind that the hot water it feeds into the cold line has cooled off. The water that comes out of the cold tap will be powerfully close to room temperature.

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