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

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

  1. Ladiesman271

    Ladiesman271 Homeowner

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    221


    The EERE has a different opinion on that matter.

    http://apps1.eere.energy.gov/consumer/your_home/water_heating/index.cfm/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."






    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)!
  2. Ladiesman271

    Ladiesman271 Homeowner

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    221


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

    AcidWater New Member

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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?
  4. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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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.
  5. Ladiesman271

    Ladiesman271 Homeowner

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    221



    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.
  6. Ladiesman271

    Ladiesman271 Homeowner

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    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.
  7. gardner

    gardner DIY Senior Member

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    Ontario
    what am I missing?

    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?
  8. Redwood

    Redwood Master Plumber

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    Your not missing a thing in my book!:cool:
  9. Winslow

    Winslow Plumber

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    Hawaii
    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.
  10. Ladiesman271

    Ladiesman271 Homeowner

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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.
  11. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Location:
    New England
    There are lots of systems that you can remotely turn on when desired; i.e., a demand system...so, if you wish, you can do it that way with readily available bits.

    Assuming you are on pubilic water and sewer, part of your bill includes pumping the fresh water to you, and then back to the treatment plant. If you have your own well, you pay to pump it out of the ground. Essentially, the water is free, but you are paying for the infrastructure to get it to you then once soiled, away. Pumping the hot water around in your house if your pipes are insulated is a very minor cost. Mine is on a thermostat and a timer...if it runs 1/2-hour a day, I'd be surprised - that's probably in the order of 25W a day for the pump and maybe 200-300BTU in lost heat (good in the winter). Well worth the convenience. Many locales are now requiring them, since they feel they do save resources. It depends a lot on how your pipes are layed out and if they are insulated. From my WH to the furthest fixture is about 40' of pipe, some of it 3/4", and to get warm, not hot water takes close to a minute or more.
  12. Ladiesman271

    Ladiesman271 Homeowner

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    221
    Rough gas cost calculation for me would be:

    1/2 hour recirc time per day.

    100,000 BTU per therm

    Gas cost per therm = $1.80

    Tankless modulated gas water heater uses up to 125,000 BTU per hour. Estimate use at 80,000 BTU per hour. Hot water pipes fully insulated.


    80,000 BTU/hr X 1/2 hr/day X $1.80 per therm X 1/100,000 therm/BTU X 365 days per year = $263 per year.


    Running a recirc pump for 1/2 hour per day would cost me around $263 per year just for gas!
    Last edited: Dec 9, 2008
  13. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Location:
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    Most tankless require a small holding tank be installed if you want to use a recirculation system just for that reason...you don't want the tankless to run. So, recirc with a tankless is much more complicated, and probably not a good idea. For a tank type WH, it's both a convenience and often can save money.

    For you tankless, you get the increased cost of install, and maintenance along with not being able to ecconomically have the benefit of instant hot water at the tap. Finding someone to repair it can be a major pain (as can finding repair parts, especially on a weekend). And, if you ever DO want to run multiple things at the same time, each and every point of use will suffer from lower water temperatures. Yes, we Americans are spoiled - many places use tankless, but they also typically use much less water because not only is the water more precious, but the fuel to heat it is MUCH more expensive than it is here (typically, anyways - there are always exceptions).

    I use an indirect off of a super efficent modulating boiler - mid-90% efficient and I've never run out of hot, and I could sit in the shower all day and not run out, either (the house would get cold because it is a priorty circuit, though!). Filling the 6' tub can readily be done time after time without impact even if the washer is running or the DW is on. You pick your poison, and live or die with it...stand-by losses are one of the major selling points of a tankless...well, my tank loses maybe 1/4-degree per hour and after a major use may not turn on again for a day, so there is no big standby loss. The boiler heating it up is typically much more efficient than a tankless, and I can get any volume I want - no low flow limitations or high volume - as much as the pipes can supply. So, spout the benefits...in most cases they are illusionary.
  14. Ladiesman271

    Ladiesman271 Homeowner

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    You have a boiler. A boiler is used primarily to heat the house rather than to heat hot water. Hot water is just a low cost after thought on that system. Air conditioning is not an option.

    I have a FHA heating system, so that leaves a tank or tankless water heater for me. I can take a shower all day long and I can still heat the house at the same time.:D

    How many therms do you use per month off heating season? Say May, June, July, August, and September? That would include hot water use from your boiler including standby losses. No call for heat during off season, so the cost of gas for hot water is easy to figure out.
  15. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Location:
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    I have a gas stove, dryer, barbeque grill that get used regularly, so isolating heating the water is not easily done. My summer bill for gas is often only a little more than the customer charge. Living in NE, our energy costs are fairly high, but my buget gas bill (equalized payments) is $73/month right now. This last year it came out almost even at the end of the adjustment period.
  16. Ladiesman271

    Ladiesman271 Homeowner

    Messages:
    221

    $73 per month ave is low, but then again you live in the middle of a row of condos. You are stealing a lot of heat from the neighbors!:D


    How many BTU/hr for that boiler?

    I live in MA, and my gas cost is $6.60 monthly customer charge, $.59 per therm delivery charge (first 50 therms), $1.25 per therm for cost of gas. That is about $140 per month for heat and hot water!
    Last edited: Dec 9, 2008
  17. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Location:
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    The home's heat loss calculation only required about 28K BTU. The boiler is an 80K BTU unit that can throttle down to 20%, or around 16K. It gets more efficient at lower burn rates since the heat exchanger can suck more heat out of the exhaust. Run it at max, and you lose more heat out the vent. If I ever get around to springing for new windows, it should drop some...really like those from www.infinitywindows.com.
  18. AcidWater

    AcidWater New Member

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    STILL hoping to discuss my actual question as stated in the title of the thread; hopefully before saturday...

    What temperature is what people consider a "hot shower?" If 120 deg F takes "more than 5 minutes" to scald, then washing temperatures considered pleasant must be something less.

    If I set the tank at 130 deg, which is 30 seconds to a scald, it is safe when accidentally turned on without mixing with cold.

    If the cold is 50 to 60 deg and they mix 50:50 then the temp is 90 to 95 deg and is good for about 4 gallons, which at 2 gpm (I'm guessing that is a pretty high flow rate for hand/face washing) allows 2 minutes, which seems long enough to wash your face.
  19. sjsmithjr

    sjsmithjr Geologist

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    Location:
    Knoxville, Tennessee
    If one were to contact the manufacturer of a small 2.5 gallon tanked water heater, let's say Ariston, and you said "Hey,I want to put a small tanked water heater in-line on the hot side. My hot water heater takes a minute or two to reach the sink. I want to put a small tanked water heater in-line on the hot side." They would say "Sure, that's they way you'd want to do that".

    I'm not a professional plumber (I deal with water at the source) and I've lost track of who has a tanked heater and who has tankless and who heats their water in a kettle over the fire, but while staying at a Holiday Inn yesterday I did look into an answer for you.

    I hate to ask, but do you have your electrical worked out?

    You may now return to debating such relevant topics as the price of tea in China.
    Last edited: Dec 10, 2008
  20. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Location:
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    The anti-scald regulations were put in for several reasons...children and older people have thinner skin, and can scald much quicker than the rest of us, and a radical change in temperature is known to result in a reaction to the change that can cause people to fall as they try to get out of the way. Most people would find higher than about 105-110 degrees to be uncomfortably hot unless it is breezy and showering in a cold room.
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