Cold water tank to increase incomming temperature to tankless heater?

Discussion in 'Tankless Water Heater Forum' started by Jeff Phillips, May 12, 2021.

  1. Jeff Phillips

    Jeff Phillips New Member

    Joined:
    May 11, 2021
    Location:
    Upper Peninsula, Michigan
    Being in a northern climate with cold winters, I am interested in exploring creative approaches to raising the incomming cold water temperature before the plumbing supplies it to the tankless water heaters.

    One idea I have is to run it into a non-insulated tank placed in the boiler room where it is excessively warm all the time. This way water coming in on those coldest winter days will stand in a tank indoors in an above room temperature space for a good while to let it take the chill off before heading into the tankless heater.

    Earlier today I was reading about people using thermostatic mixing valves to enable increasing the temperature of their tank water heater while delivering a lower temperature to the faucets by mixing in cold water as needed to regulate the temperature. It was mentioned that the aim would be to prevent bacterial growth by increasing the temperature of the water in the tank. Hmm...

    That got me to thinking... Would having the incoming cold water sit indoors in a non insulated tank for a while before flowing onward to tankless heaters lead to a problem of bacterial growth in that cold water storage tank?

    We have a large filtration, softener, and clorination injection system on our cold water supply plumbing currently, with the clorination system curiously set to its maximum level. So we would be putting highly clorinated water into the tank, unless for some reason that were undesired.

    What methods would one take to combat bacterial growth in a cold water storage tank, or what alternate ideas might anyone have for taking the chill off the incomming cold water before feeding it into a tankless water heater?


    Thanks,
    Jeff
     
  2. Bannerman

    Bannerman Well-Known Member

    Joined:
    Mar 19, 2014
    Location:
    Ontario, Canada
    Is your water source a private well or municipal. Municipal water will the chlorinated to prevent bacteria growth.

    Why was the chlorination system installed? We won't know what 'maximum level' represents. Do you test to determine the amount of chlorine in the water after the chlorinator? Does the chlorinator use a pump and if so, does it use a flow meter to activate the pump only while water is flowing?

    You mentioned a filtration system, but listed it first and the chlorinator last. Is that the order of treatment?

    Consumption of Chlorine and disinfection byproducts is not exactly healthy, and chlorine will be adsorbed through skin and breathed in while bathing.

    Does the filtration system contain carbon and does it utilize a large tank and backwashing valve, similar to a water softener?
     
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  4. Dana

    Dana In the trades

    Joined:
    Jan 14, 2009
    Location:
    01609
    That sounds like an oversized boiler in a (possibly un-insulated) basement room problem, or too high a boiler temp problem. Assuming it's in a basement, are the basement walls insulated?

    A right sized boiler wouldn't be throwing off excess jacket heat. That's an efficiency problem bigger than the water heating efficiency problem. To get a handle on your actual oversize factor, run this math. Depending on the boiler (and some other factors, such as your zone radiation) there are several ways to improve that short of outright replacement.

    Rather than an uninsulated tempering tank ahead of the tankless, if you really wanted to harvest significant heat from the a hot boiler room a heat pump water heater would be a better bet. That knocks the temp in the boiler room down degree or two (more, if it's a small-ish room), lowering the amount of heat lost to the outdoors, putting a fraction of that excess heat into the tank instead.

    But a heat pump water heater also helps dehumidify the basement. In the July/August time frame even in da Yoop outdoor dew points are above 60F about 25% of the time or more, well above normal basement temps in that area and WAY above your incoming water temps. An uninsulated tempering tank would need something for dealing with summertime condensation on the outside of the tank, since the incoming water temps are well below the summertime outdoor dew point temp. If your basement ever experiences "musty basement" odors or needs a dehumidifier to keep the mold under control, a heat pump water heater does double-duty, handling some of that dehumidification load. A room dehumidifier turns that latent heat of vaporization of the water condensed out of the air into warmer air in the basement, something you don't really want in summer. A heat pump water heater stores that latent heat inside the insulated tank as hot water.

    The smallest 40 gallon Rheem ProTerra runs about a grand at box store pricing, and can probably heat water for less money than your tankless. If you want to use it to pre-heat water into a gas fired tankless , setting it to 120F (to limit legionella risk) then mixing it down to something closer to room temp at the input of the tankless might be needed to keep the tankless from flaming out at low flow, since it can only turn down by so much. Or you could just bypass the tankless altogether, and size the heat pump water heater for the biggest tub you need to fill. A 50 gallon heat pump water heater runs about $1500, and is plenty for most 4-5 person families. If it's running short in the dead of winter it can be run in "hybrid" mode for faster recovery (comparable to a standard electric tank water heater.)
     
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  5. fitter30

    fitter30 Well-Known Member

    Joined:
    Feb 2, 2020
    Occupation:
    Retired service tech
    Location:
    Peace valley missouri
    Any heat that the water would pick up in a tank you paid for it. Remember hot goes to cold. Boiler room in a basement either radiates out or up into the house. Gas or oil boilers and or water heaters that have no outside air for combustion is getting by infiltration.
     
  6. Jeff Phillips

    Jeff Phillips New Member

    Joined:
    May 11, 2021
    Location:
    Upper Peninsula, Michigan
     
  7. Jeff Phillips

    Jeff Phillips New Member

    Joined:
    May 11, 2021
    Location:
    Upper Peninsula, Michigan
    Yes I know, but the tenants upstairs have seperate furnaces on their own utility bills so any heat that escapes from the basement to the upper floors is seen as a loss to us and an unintended windfall blessing to the people who live upstairs.

    But we are going to keep the common areas and basement heated with the boiler anyway regardless of its issues. I am interested in minimizing the excessive heat build up it creates in the boiler room but as long as it is warm in there i see little reason to not make effective use of the heat that is in there.
     
  8. Jeff Phillips

    Jeff Phillips New Member

    Joined:
    May 11, 2021
    Location:
    Upper Peninsula, Michigan

    I thought the same thing about the excessive heat around the boiler room suggesting its oversized, but i also have a 4 places I want to add new zone loops to heat from that boiler so if it is over sized, it won't be once I'm done giving it more work to do.

    I am curious, for people who have their own well and pump to get cold water, they pump it into a pressure tank. What prevents bacterial growth like legionella in cold water tanks? Obviously those don't get heated to 120 degrees yet millions of people use a well with a pump and tank for cold water. Hmm.

    I am definatley intrigued by the idea of the hybrid heat pump water heater acting also as a dehumidifier. There is one corner of the basement that i do have a dampness and mold issue. We plan to do some serious excavating work outside the basement wall there and make some changes to improve drainage but i do occasionally run a dehumidifier in that area currently as needed. It would make lots of sense to swap out the conventional electric tank water heater for the apartment directly above that corner with one of of those hybrids and install it in the basement right underneath the one they were using. Even if we go tankless for the other 9 units, that one in that corner I think your idea would be perfect for instead.
     
  9. Dana

    Dana In the trades

    Joined:
    Jan 14, 2009
    Location:
    01609
    In my area the average oversize factor I see for cast iron boilers is ~3x, some have been north of 5x.

    If your boiler is currently 3x oversized for the load you'll only have a problem if the 4 new zone loops will more than triple the heat load (roughly proportional to the square feet of conditioned space, with large margins of error.)

    About how many square feet of conditioned space is currently served by the boiler, and how much would the 4 new loops add?

    Seriously, run the fuel use load calculation math (which also includes the distribution & standby losses that won't change much with added loops), and figure out the oversize factor. Run a room-by-room/zone-by-zone Manual-J(ish) load calculation on the new space that would be added using LoadCalc or CoolCalc (with aggressive rather than conservative assumptions on air tightness, etc.). This would be a necessary first step in figuring out what radiation to hang on those loops anyway. Then run a block load (the whole space, not room by room) Manual-J on the space currently heated by the boiler, and compare that to the fuel-use based load number. Nine times out of ten the load number will be ~25% lower than a cheap freebie Manual-J, in which case you should scale back the Manual-J proportionally, eg: The fuel use calculation says 47,000 BTU/hr the block load Manual-J comes in at 59,000 BTU/hr, so whatever the calculated room loads were for the added new zones would be whatever the tool came up with times 47/59. Since the standby losses weren't separable from the fuel use calculated load, even the scaled numbers are going to be bigger than reality.

    Only with a reasonably accurate set of load numbers and the boiler's DOE output numbers would it be possible to make rational decisions on how to optimize system efficiency. (A tempering tank isn't going to do very much.)

    The sweet spot for legionella growth is between 85F and 115F, with the most active zone between 100-105F. (Hot tubs are terrible for this.) Most basements in your area would be under 70F in summer, and in most cases it would not much more than that in winter even with the jacket losses of an oversized cast iron boiler heating the boiler room.

    If it's north of 85F in the boiler room in winter the efficiency problem has to be REALLY bad! Most cast iron boilers are cranked much hotter than they need to be in the first place, and if the distribution plumbing isn't insulated the pipes behave like a weirdly shaped convecting radiator. Cranking back the boiler's high temp to something more reasonable and insulating the pipes are cheap mitigation. Retrofitting a heat purging economizer control is pretty cheap too, and quite cost effective at 3x+ oversize factors. (Many/most new cast iron boilers come fitted with smarter heat purging controls as standard equipment, which reduces the efficiency hit from ridiculous oversizing.)

    If the foundation walls in the basement are not air sealed and insulated the jacket losses are truly lost- more than 10x what they would be if the basement insulation was brought up to current IRC code minimums (= R15 continuous insulation in your climate zone.)

    If it's just one corner of the basement it's clearly a ground water or bulk water leakage issue. Surface grading to slope away from the building and exterior "French drain" surface drains are the first steps, followed by water sealing the foundation on the interior side (a whole other subject). Once the foundation walls are known-dry, insulating it with 2" (~R14) of HFO blown closed cell spray polyurethane) provides further waterproofing as well as air sealing and insulation.

    How big is the basement? The absolute minimum amount of available volume to support a 50 gallon heat pump water heater (HPWH) is ~700 cubic feet (say a 9x9 room with 9' to the subfloor above with open joist ceiling). Twice or even three times that is better to avoid over-cooling the space during the shoulder seasons. If the basement is larger than 1000 square feet (probably is) multiple HPWHs may be be needed to have a significant dehumidification effect. Taking a WAG at it (without doing the math) with an uninsulated basement figure 500-700 square feet per HPWH to avoid over-cooling the basement in winter (assuming the boiler's losses get fixed). If the foundation gets air sealed and insulated that can probably drop to 400 square feet, 300 of you don't mind it occasionally hitting 45F or lower down there.

    Nine gas tankless water heaters suitable for a cold-water location would be north of 1.5 million BTU/hr, which requires a HUGE gas service into the building- that needs to be checked!
     
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