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

  1. #16

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    Quote Originally Posted by Gary Swart View Post
    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.

    Water is cheap. What do you waste, 1/2 gallon to get hot water. How many times a day and night are you using hot water anyhow?

    Figure out the cost of electric of a continuous pump run over 10 years.

    You ignored the biggest cost of heating the hot water over and over again to compensate for heat loss in pipes during recirculation.

    What do you guys use for a hot water temperature setting?

  2. #17

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    Quote Originally Posted by Redwood View Post
    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.

    I don't think that I would like water from a hot water tank to be mixed with my cold tap water. Those tanks are filled with crud. I don't want to drink that!

  3. #18
    Retired Defense Industry Engineer jadnashua's Avatar
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    If you ever had the opportunity to see the insides of the supply lines going into your house, the inside of the tank is not bad at all.

    The smaller the WH tank, the more surface area per volume, so unless it has superior insulation, you're only going to be heating up the cabinet and wasting a fair amount of energy right there. It might also limit what you want to store under there, partly because it will take up a fair amount of room, and some things don't last as long when they are warm. Using a recirculation system on a timer so it only runs when you are home, or setting it up with a motion sensor so it turns on when you are in the room, or a timed action so you push a button and it runs for say 2-minutes, and it won't use much electricity at all. The pumps used on those things draw probably in the 24-30W range...not much. Plus, depending on the house plumbing, you may end up with ALL of the sinks, or hot water taps being hot nearly instantly, rather than just one. Want hot water in the kitchen? It's there. Instead of running the DW with cold water and having the DW heat it for you, or wasting a bunch to get it warm before you start it, and you'd find a recirculation system would end up cheaper and more convenient. I really like only waiting a few second to have hot water in the shower in the morning. Some people pay a lot for sewer charges, and you'd be surprised how much you waste waiting for hot to arrive.
    Jim DeBruycker
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    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  4. #19
    DIY Senior Member gardner's Avatar
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    Quote Originally Posted by Ladiesman271 View Post
    heating the hot water over and over again to compensate for heat loss in pipes during recirculation.
    In the winter this is no big deal as the lost heat goes to heat the building and just offsets the BTUs that go into the furnace.

    In the summer though, you have heat coming out of the HW system and then have to put even more energy into the AC to pump the wasted heat out.

    I'm curious about the economics of recirculation setups. What is the savings that people are perceiving as most important? Is it the water or the energy? If one's individual circumstances are cheap water, expensive energy, or vise versa, does the equation change?

  5. #20
    Retired Defense Industry Engineer jadnashua's Avatar
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    Somebody ran some numbers, and calculated the break even point for an average home is to run the pump about 2.5 hours during a day. Use it less than that, and you save money, run it longer than that and you spend more (but does not take into account wasting water as part of an environmental event). So, running it on a thermostat, or a proximity sensor, demand switch, or limiting the run time, and you'll save money on both heating costs, electrical, water (remember, if you don't have a point of use or recirculation, you may waste 2-gallons or more each time you want hot water - even though you didn't actually get to use those two gallons, you paid to heat them), and sewer costs. Also, keep in mind, it costs energy to pump those wasted gallons to your house, then it adds to the sewer treatment costs, which uses energy to move it back to the treatment plant and actually treat it. So, to wash your hands with what may only take 1/2-gallon, if you wait for warm, you are using 2.5-gallons. Multiply that for everyone who washes their hands after using the bathroom, washing dishes, running the DW, taking a shower, and it really adds up to wasted water AND energy. Recirculation, done right is not only cheaper, but more convenient and saves significant amounts of water overall. Some locales are mandating it in new construction as part of the green initiative.

    The unit I have is on a timer so it runs for a few hours in the morning, again in the evening, then most of the day during the weekends. It differs from most of them out there in that it has a user adjustable thermostat and only runs the pump when the temp drops below your set point. It probably runs maybe 60-seconds, every 10-20-minutes depending on the temp in the room. I have it set where the water is warm at the vanity, but it's essentially hot everywhere else in the house. Cycling will wear the pump and control out quicker, but so far, it's lasted 6-years, and still works as well as when installed.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  6. #21

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    Quote Originally Posted by jadnashua View Post

    Want hot water in the kitchen? It's there. Instead of running the DW with cold water and having the DW heat it for you, or wasting a bunch to get it warm before you start it, and you'd find a recirculation system would end up cheaper and more convenient.


    The EERE has a different opinion on that matter.

    http://apps1.eere.energy.gov/consume.../mytopic=13050


    "Dishwashers with booster heaters typically cost more, but they pay for themselves with energy savings in about 1 year if you also lower the water temperature on your water heater."





    Quote Originally Posted by jadnashua View Post

    I really like only waiting a few second to have hot water in the shower in the morning. Some people pay a lot for sewer charges, and you'd be surprised how much you waste waiting for hot to arrive.

    Under worst conditions 30 seconds and 1/2 gallon of water for me. If I brush my teeth first there is no waiting time at all.

    You waste more water watering the lawn once a year (which also includes sewer charges)!

  7. #22

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    Quote Originally Posted by gardner View Post

    In the winter this is no big deal as the lost heat goes to heat the building and just offsets the BTUs that go into the furnace.


    I use a tankless water heater, so the entire matter is a big deal even in winter.

  8. #23
    DIY Senior Member AcidWater's Avatar
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    I'm still waiting for an answer to my actual question. I'm not installing a recirc. The tank is already purchased.

    >Water is cheap. What do you waste, 1/2 gallon to get hot water

    No, water is expensive. I pay for electricity to PUMP IT OUT OF THE GROUND. It takes 3 gallons to get hot. That's 3 gallons of water that was heated & got cold.

    ***

    If 2 1/2 gallons is sufficient for a woman to wash her face & get her makeup off then I should plumb it to the cold supply. If she needs more then I have to plumb it to the hot so she doesn't run out.

    What's the GPM thru a lav tap at a moderate flow rate?

  9. #24
    Retired Defense Industry Engineer jadnashua's Avatar
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    First, at best, you'll get 70% of the volume of a WH before the incoming supply overwhelms the hot that is there and makes it at most tepid. On a small one, there probably is only one element, and it probably won't give you that much. I think most faucets are limited to 2gpm, but some may be less.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  10. #25

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    Quote Originally Posted by AcidWater View Post

    No, water is expensive. I pay for electricity to PUMP IT OUT OF THE GROUND. It takes 3 gallons to get hot. That's 3 gallons of water that was heated & got cold.


    What's the GPM thru a lav tap at a moderate flow rate?



    You need to use 3 gallons of water in order to get hot water?

    I use a tankless heater and have a 70 foot run of 1/2" pipe, and that comes out to be about 1/2 gallon before the water starts to get hot. Warm in 30 seconds, hot in a minute.

    If you have a tank type water heater, your water pipes must be either very large or very long if you need to waste 3 gallons to get hot water.

    By the way, compared to the cost of heating water pumping it cost next to nothing. Water is not expensive even if you include the cost of pumping it.

  11. #26

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    Quote Originally Posted by Terry View Post
    If you intend to drink the water, use the cold water.

    You shouldn't drink water that has passed through a standard water heater.


    Does the same thing hold for using the cold water line as part of the recirc system? You are pumping cooled off hot water back into the cold water line until the hot water line gets back to temp.

  12. #27
    DIY Senior Member gardner's Avatar
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    Default what am I missing?

    Quote Originally Posted by Ladiesman271 View Post
    I use a tankless water heater, so the entire matter is a big deal even in winter.
    I don't see that. The recirculation system doesn't waste any water. Yes, your tankless system will run more often to keep the recirculated water hot. But all the heat that leaks out will still go into your building where, in winter, it will offset your main heating energy.

    Not sure -- what am I missing?

  13. #28
    Master Plumber Redwood's Avatar
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    Quote Originally Posted by gardner View Post
    I don't see that. The recirculation system doesn't waste any water. Yes, your tankless system will run more often to keep the recirculated water hot. But all the heat that leaks out will still go into your building where, in winter, it will offset your main heating energy.

    Not sure -- what am I missing?
    Your not missing a thing in my book!

  14. #29
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    Quote Originally Posted by Ladiesman271 View Post
    Since when is a recirc retrofit inexepensive to install and operate?
    you don't need to run a new line. The make a retrofit model where the pump goes on the heater and a bypass goes on the fixture at the end of the line or the fixture ou want instant hot water at.

  15. #30

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    The only advantage to use a recirc line seems to be water use savings and the "convenience" of instant hot water.



    However, on the downside you have to pay to keep reheating that water plus pay for electricity to run that pump. In the summer you have to pay double to pump that hot water heat loss out of the building.



    The cost of running a hot water recirc system for 24 / 7 is a high cost for instant hot water. Some type of demand control system is required to make a recirc system cost effective.

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