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

Discussion in 'Water Heater Forum, Tanks' started by AcidWater, Dec 3, 2008.

  1. AcidWater

    AcidWater New Member

    Messages:
    123
    Location:
    .
    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. gardner

    gardner DIY Senior Member

    Messages:
    217
    Location:
    Ontario
    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.
  3. Redwood

    Redwood Master Plumber

    Messages:
    7,452
    Location:
    Connecticut
    I would consider a recirculation system.
  4. Gary Swart

    Gary Swart In the Trades

    Messages:
    7,334
    Location:
    Yakima WA
    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. AcidWater

    AcidWater New Member

    Messages:
    123
    Location:
    .
    The tank is under the sink. Hot or cold???
    ***

    Recirc constantly wastes money.
  6. Redwood

    Redwood Master Plumber

    Messages:
    7,452
    Location:
    Connecticut
    Compared to buying & running a second water heater its nothing!
  7. Ladiesman271

    Ladiesman271 Homeowner

    Messages:
    221

    Since when is a recirc retrofit inexepensive to install and operate?
  8. Ladiesman271

    Ladiesman271 Homeowner

    Messages:
    221

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

    Redwood Master Plumber

    Messages:
    7,452
    Location:
    Connecticut
    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!:cool:
  10. Gary Swart

    Gary Swart In the Trades

    Messages:
    7,334
    Location:
    Yakima WA
    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. gardner

    gardner DIY Senior Member

    Messages:
    217
    Location:
    Ontario
    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. Gary Swart

    Gary Swart In the Trades

    Messages:
    7,334
    Location:
    Yakima WA
    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. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,917
    Location:
    New England
    Feeding hot into the inlet of a WH won't hurt it...
  14. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,510
    Location:
    Cave Creek, Arizona
    recic

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

    Redwood Master Plumber

    Messages:
    7,452
    Location:
    Connecticut
    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.
  16. Ladiesman271

    Ladiesman271 Homeowner

    Messages:
    221

    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?
  17. Ladiesman271

    Ladiesman271 Homeowner

    Messages:
    221

    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!
  18. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,917
    Location:
    New England
    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.
  19. gardner

    gardner DIY Senior Member

    Messages:
    217
    Location:
    Ontario
    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?
  20. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,917
    Location:
    New England
    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.
Similar Threads: undersink heater
Forum Title Date
Water Heater Forum, Tanks Tankless or 2.5 gal for Undersink Supply of HW for Diskwasher and Sink Mar 1, 2013
Water Heater Forum, Tanks replacing a gas heater, braided cold water inlet hose Sunday at 8:26 AM
Water Heater Forum, Tanks Water heater and high gas usage question Aug 11, 2014
Water Heater Forum, Tanks Hybrid water heaters and types of issues. Aug 6, 2014
Water Heater Forum, Tanks water leaking from the top of gas water heater. Jul 30, 2014

Share This Page