Tankless booster versus recirculation

Discussion in 'Tankless Water Heater Forum' started by PaoliBulldog, Jan 4, 2013.

  1. PaoliBulldog

    PaoliBulldog New Member

    Jan 4, 2013
    Hello all,

    My kitchen sink and dishwasher are located about 35 pipe feet from the water heater, so it takes probably 45 seconds to deliver hot water at the tap and dishwasher. To reduce the lag time, I am considering installing, under the sink, either (1) a small tankless booster heater or (2) an on-demand recirculation system such as D'Mand.

    Can anyone tell me the pros and cons of these options?

  2. Dana

    Dana In the trades

    Jan 14, 2009
    Since the volume draws are small, a tiny electric tank is sometimes the best solution. Tankless heaters have HUGE instantaneous power draws, requiring dedicated (and heavy gauge) wiring & breakers. A tiny tank can be a plug-in.

    Rather than feeding the tank with the output of the main HW heater it's usually better to just size the local heater for kitchen draws. At 35' with 3/4" plumbing you have nearly a gallon of water in the distribution plumbing, and simple rinses or hand washing draws are less than that at a time (as are some Energy Star dishwashers). If you're drawing hot water from the main heater to a local heater, a large fraction of that draw from the main heater gets abandoned in the plumbing and cooling without reaching the local HW heater.

    Recirculation schemes get you nearly-instant hot water, but abandons even more heat in the plumbing on short draws, since it's sending 35' of tepid (but not usually cold) water back in the return line whenever you need the 100F water rather than 90F (or whatever) water at the tap.

    Any long distribution plumbing runs are cost-effective to insulate with 5/8" closed cell foam pipe insulation, since that extends by a few 10s of minute how long the water remains at a useful temperature after the first draw. See: http://www.leaningpinesoftware.com/hot_water_pipes_pipe_cooling.shtml



    Most of the stuff sold in box stores is ~R2 3/8" wall goods- you may have to buy the better stuff at a plumbing distributor (or online).

    You may want to start there first- even if insulation alone doesn't make it a fully acceptable solution it's not a wasted effort if you're going continue sending water from the main hot water heater to whatever bigger deal solution you decide upon.
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  4. lifespeed

    lifespeed Member

    Apr 27, 2007
    recirc is my preference

    An undersink electric tankless is impractical unless you can easily install a 100 amp circuit. An electric undersink tank is reasonably practical, but you'll pay to heat both the electric tankless, and the water drawn through the gas tankless both. Not as energy efficient as the gas tankless alone.

    I chose the gas tankless with recirc solution, and am paying only 0.5 therms per day (about $13/month) for hot water with a family of four. My heater is 65 feet from the kitchen! I use a Redytemp recirc pump which includes a very flexible timer configuration. This way, no button push is required for the common use times of morning and evening, but hot water is still available at any time via the Leviton rocker switches in the kitchen and each bathroom. I think with a tankless one should not use a full-time circulator that would cycle the heater every 15 - 30 minutes 24/7, even though modern tankless are warranted for use with recirc pumps. Obviously, efficiency would suffer without the use of a timer.

    My only regret is I wish I had got the larger version of the recirc pump and used a 3/4" (instead of 1/2") dedicated return to speed the purging of cold water from 20' of 1" pipe, then 45' of 3/4" pipe. That's a lot of water, and it takes 1-1/2 minutes if it is stone cold.

    If you consider recirc, important features are a combination of timer, adjustable thermostatic, and pushbutton control. A dedicated return line is ideal. If completely impractical, the best use of the existing cold line as a return requires a positive action solenoid valve to isolate it when not in use, and prevent thermosiphoning and always having warm or hot water in the cold line. Not all recirc systems use this type of valve, but use a cheap check valve. Lastly, you want a standard, easily replaceable pump.

    Hope this helps.
  5. PaoliBulldog

    PaoliBulldog New Member

    Jan 4, 2013
    Thanks, sorry it took so long to thank you!
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