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

  1. #31
    Retired Defense Industry Engineer jadnashua's Avatar
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    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.
    Jim DeBruycker
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    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  2. #32

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    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 by Ladiesman271; 12-09-2008 at 09:10 AM.

  3. #33
    Retired Defense Industry Engineer jadnashua's Avatar
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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.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  4. #34

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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.

    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.

  5. #35
    Retired Defense Industry Engineer jadnashua's Avatar
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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.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  6. #36

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    Quote Originally Posted by jadnashua View Post
    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.

    $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!


    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 by Ladiesman271; 12-09-2008 at 02:58 PM.

  7. #37
    Retired Defense Industry Engineer jadnashua's Avatar
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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.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  8. #38
    DIY Senior Member AcidWater's Avatar
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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.

  9. #39
    Geologist sjsmithjr's Avatar
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    Quote Originally Posted by AcidWater View Post
    STILL hoping to discuss my actual question as stated in the title of the thread; hopefully before saturday...
    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 by sjsmithjr; 12-10-2008 at 05:05 AM.
    -Sam Smith
    Licensed Professional Geologist - AL, TN, KY

  10. #40
    Retired Defense Industry Engineer jadnashua's Avatar
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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.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  11. #41
    Plunger/TurdPuncher kingsotall's Avatar
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    acidwater, have you looked into one of these puppies┐

    Price of tea in China... hehe

  12. #42
    DIY Senior Member WV Hillbilly's Avatar
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    Hot or Cold . I don't know if you'd get enough hot water using the cold supply or not but I'd try that first . If it didn't work I'd switch to the hot supply . Cutting the small faucet supply pipes above the shutoff valves , using flexible copper tubing & compression fittings it wouldn't take much to hook it up or switch supplys if necessary . You'd likely need reducers to connect the tubing to the inlet & outlet of the tank .
    Last edited by WV Hillbilly; 12-11-2008 at 01:14 PM.

  13. #43
    Master Plumber Redwood's Avatar
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    Phew! I'd rather boil up a pot o water on the stove and bring it into her than install that POS point of use electric tankless.
    What part of 0.5 GPM don't you understand?

  14. #44
    DIY Member Hillbilly Man's Avatar
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    Quote Originally Posted by Redwood View Post
    Phew! I'd rather boil up a pot o water on the stove and bring it into her.
    Heck, I make my missus carry in her own dang bucket o water n warm it up on the stove.

    Southern Man helped me rig up one o them electric poiter use water heaters on my well hand pump...
    It dint work worth a hillo beens.

    I'd listen ta Redwood thats one smart fella.
    He helped me git my new still runnin real good.
    Hillbilly Eng-in-ear
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