What's to blame? Increase in cold water inlet or hot water demand? Both?

Discussion in 'Water Heater Forum, Tanks' started by phillyg, Feb 23, 2010.

  1. phillyg

    phillyg New Member

    Messages:
    4
    Location:
    Philadelphia, PA
    Hi folks,

    I appreciate any advice you can offer.

    I recently had installed a Kohler showering system that has multiple heads (main shower head, handheld, and 3 Water tiles). Each fixture has 2.5GPM, but the valve only allows two accessories at a time. The max requirement would be 10 GPM. The plumber increased the risers from the basement to the bath to 3/4" but also increased the feed from the hot water tank to the risers to 3/4" as well as the cold water input into the hot water to 3/4". This was all previously 1/2".

    You can tell where this story is going... our new shower system is great, but we run out of hot water in about ten minutes. But here's the thing--we run out of water in a little more than ten minutes using just one shower head. When using another bath during construction, we noticed that the shower started to run out of hot water a lot more quickly after the plumbing work described above was done. Coincidence? We never had a problem before the plumbing to the renovated bath was done.

    The water heater is a 50-gallon Bradford White installed in 2006. It looks like it is in good shape, but I am trying to figure out if it is going bad, our demand is too great, or what we should do next to increase the amount of hot water. Do you think that increasing the cold water inlet to 3/4" is dumping too much cold into the tank and limiting recovery, etc.?

    What type of setup would you recommend to satisfy the hot water demand? I estimate that we need about 200-gallon FHR. Replace the 3-year old BW with a larger one? Add a second hot water heater -- what type and size? Go tankless?

    Many thanks for your help -- just a lay person here who needs the professionals!

    Greg
  2. krow

    krow Plumber

    Messages:
    906
    Location:
    Ontario, Canada
    The construction was only a coincidence. Your problem is at the hot water tank. Your dip tube on your cold water feed needs replacing. Dip tube will cost you approx 10-14 bucks
  3. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,824
    Location:
    New England
    You can only get about 80% of the water out of a WH before it cools off so that you would quickly notice (unless the output was a dribble, then the burner coming on would extend it more than it can at full flow). It also helps to start with the water in the tank hotter, but you may want a tempering valve to prevent scalding in that case. At 10gpm, that's only 4-minutes (if you were using all hot, which you aren't, so you'd get longer). Depends on how hot you want it, and how cold your inlet water is.

    The dip tube is likely to be bad, but you are requesting a lot of hot water - at 10gpm, it can go fast!
  4. Dana

    Dana In the trades

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    2,790
    Location:
    01609
    To max out showering capacity you can either buy more burner, more storage, or both, but it takes a lot to keep up with 10gpm with a tankless or keep it going very long with a tank.

    No matter which way you go (more burner, or more storage), you'll get a huge boost in capacity out of a drainwater heat recovery heat exchanger, the output of which will scale somewhat linearly with flow: At 2.5gpm a typical unit might be returning ~30KBTU/hr, at 5gpm the same unit will be returning 55KBTU/h and at 10gpm about 100KBTU/hr (more than some of the smaller whole-house tankless!). Best part about it: It doesn't use any fuel, and requires no maintenance- it's just a dumb heat-exchanger pre-heating the incoming water with the outgoing warm drain flow. At your shower-flow rates it's payback will be less than 5 years on fuel savings. (Less, if it means you can buy less water heater up front. High capacity tanks aren't cheap.) It basically means you can cut your BTU/hr rate by about half, which doubles your showering time at high flow, extends it to over an hour at 2.5gpm with a typical 50 gallon tank heater.

    Natural Resources Canada keeps a list of 3rd-party performance testing on various manufacture's models to separate the marketing hype from reality HERE. In general fatter & longer translates into higher performance (and you need all the performance you get- buy the biggest one that actually fits, if you go that route.)


    [​IMG][​IMG]
  5. Dana

    Dana In the trades

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    I forgot to mention: If you heat the house with a hydronic boiler (forced hot water), it's output is typically much larger than most HW tanks, and in (and in many cases bigger than tankless hot water heaters), which means you'll get a lot more hot water capacity out of an indirect-fired tank running as a priority zone off the boiler than with either a tankless or a standalone tank. If that's your heating system, check the ratings on the nameplate- if it's over 100,000BTUs input a 40 gallon indirect will beat a 75 gallon tank. If it's over 180KBTU in it'll beat all beat all but the biggest tankless units out there in first-hour gallon ratings, and it'll handle much higher instantaneous flows than a tankless.

    The aforementioned capacity boost from drainwater heat recovery works with an indirect as well. The capacity boost is only for showers though- you get no extra capacity for filling big soaking tubs 'cuz the drain has to be flowing for the heat transfer to occur.
  6. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,293
    Location:
    Cave Creek, Arizona
    The fact that the water heater is a Bradford-White is immaterial. Its size, 50 gallons, and KIND, i.e., gas or electric, are what are germaine to solving your problem. Using the residual heat of the shower drain to preheat your water would be a good way to spend a lot of money, but not get any results.
  7. phillyg

    phillyg New Member

    Messages:
    4
    Location:
    Philadelphia, PA
    I should have mentioned it is gas. The plumber is coming tomorrow to check out things. He said that the larger cold water input combined with the colder water temp could be contributing to the quick run-down of the tank. This morning it was out in 11 minutes using one showerhead.

    Thanks for all the responses so far. I do not think the recovery system is in the budget. We only plan to be in the house for about three years more, so I am looking for economical and efficient solutions. Thanks again!
  8. Dana

    Dana In the trades

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    2,790
    Location:
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    The "economical & efficient solutions" with a sub-3-year payback is to lower the gpm on your shower heads (or disable the side-sprays) and bump up the temp on the tank to the max after you fix any dip-tube issues, insulate the tank with a retrofit blanket, and put R4 (5/8"+ wall thickness) pipe insulation on all near-tank plumbing (including the cold feed, and the temperature/pressure saftey valve outlet.) Be sure not to impede either combustion air or exhuast venting with the tank-blanket and it'll be fine.

    As more of a code/safety issue, a tempering valve on the output limiting the temp delivered to the distribution to ~120F to limit the scald hazard is also a good idea for (but has no payback). 140F+_water is dangerous, but possible with many gas tank heaters.

    It'll increase your showering time, but probably not as much as you'd like, but that's about as much as you can get without dropping more coin into the system. Showers like typically need far more first-hour capacity than a run-of-the-mill standalone 50 gallon tank. Boosting the temp might buy you 20% more showering capacity, but it'll double the standby loss of the tank- it's nothing like the "right" solution.

    If the heavy-spray shower is seeing 2 or more showers/day with 8+ gpm flows, it's not clear that tdrainwater heat recovery would still be NPV-negative at three years. That's a typical break-even point for far more modest use of electric-heated water. One shower with all side sprays running continuously is the hot water use equivalent of three or four "typical" showers used in the financial analyses showing short term payback on electric hot water. Is gas at 60% efficiency really 1/3 the cost of electricity at 100% efficiency in your market? And if it means you can stick with your 50 gallon tank and deliver longer showers than you'd get out of buying a (much more expensive) 75-80-100 gallon tank the payback is immediate, since the installed cost is comparable or less than going with a monster-tank. You don't get any payback out of a bigger tank ever- you'll burn ~1.5-2x the fuel as you would with drainwater heat recovery, and pay as much or more for the hardware up front.
  9. phillyg

    phillyg New Member

    Messages:
    4
    Location:
    Philadelphia, PA
    But what about the option of adding a second conventional water heater of same or smaller size? Won't the second unit (connected in series? parallel?) only kick on when the first is exhausted? Is it any more efficient than having a single larger capacity water heater? Sorry for what may seem like elementary questions -- just a green-behind-the ears homeowner here.
  10. Dana

    Dana In the trades

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    With two tanks your standby losses roughly double. Larger single tanks have only a slightly higher standby loss, so the 2-tank solution is measurably less efficient. It's a geometry thing- volume grows faster than surface area, and losses are roughly proportional to surface area. But with gas fired tanks flue convection with the burner off is also a huge loss factor, and 2 flues are lossier than one. Bigger tanks also have bigger burners, so the first-hour gallons of a 100 gallon tank can sometimes be more than two 50 gallon tanks.

    You've not stated whether or not you heat with a boiler, but the cost of an indirect is comparable to the installation of a second 50 gallon tank (no gas plumbing or exhuast routing is involved with an indirect), and it'll be both more efficient and a (likely) higher first-hour capacity than a standalone tank. Indirects typically last for decades, whereas a 2-decade standalone is usually on it's last legs. If you have enough boiler behind it- this would be one of the "right" solutions to boost capacity, but would have a long payback in fuel savings. Drainwater heat recovery would rougly double your showering capacity of your old tank for similar money, but uses less fuel. If this was a place you were going to keep I'd recommmend both, but if you're looking at it from strictly a short-term payback point of view, drainwater heat recovery would have a much higher ROI.
  11. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Location:
    New England
    The total surface area (radiating) on a WH doesn't go up as fast as the amount of gallons it holds (the r^2 factor).
  12. phillyg

    phillyg New Member

    Messages:
    4
    Location:
    Philadelphia, PA
    I heat with a gas boiler. It has an input rating of 150,000 BTU and a "water" rating of 107,800 BTU.
  13. jimbo

    jimbo Plumber

    Messages:
    8,997
    Location:
    San Diego
    I wonder about that heat recovery. There will be delay between using the hot water...with cold going into the WH heater at the same time, and some several seconds later the drained water finally gets to the recovery unit. I guess if the shower runs several minutes, that is just a small %
  14. Dana

    Dana In the trades

    Messages:
    2,790
    Location:
    01609
    Instead of a second 50 gallon hot water heater, go with an indirect controlled as a priority zone, and decommission your aging 50 gallon tank:\

    A typical 50 gallon tank is worth around 75 first-hour gallons, so two might give you to ~150 first hour gallons, but a net efficiency of around 55-60%.

    The heat output of the boiler is ~4x that of a typical 50 gallon standalone tank. That's enough burner to run 3-4gpm forever (x 60 minutes is 120-160 + the storage capacity of the indirect for first-hour gallons), so with the buffering volume of the tank behind it you'll get plenty of showering time, peak flows at other can be pretty high without affecting flow at the shower, and the recovery time from depletion will be less than half that of a pair of tanks. It'll increase the as-used AFUE of the boiler and your annual fuel use will be lower than with the boiler + tanks, and in the summer it'll be only slightly less efficient than a standalone tank due to cyclling losses on the higher-mass boiler. And the indirect tank will outlast the boiler.

    Drainwater heat recovery will be a similar order of magnitude of cash, and roughly double the first-hour showering gallons, and will cut your fuel use even more than an indirect, but if it's difficult/awkward to install, can be more money up front. And although many areas have state & utility subsidy for indirects, very few have subsidies for drainwater heat recovery for installations where the water is heated with natural gas. (Since most homes don't have water-pig showers, the return on investment on DWHR is generally lower for gas-fired customers than for an indirect vs. standalone tank, but in your case the fuel use numbers would favor DWHR over indirect.) Check to see what and how deeply things are subsidized in your area- you may find that with subsidies applied it'll be cheaper than adding a second 50 gallon tank.

    jimbo: The few quarts of cold mixed into the water heater at the beginning of the shower before the feedback of the heat exchanger kicks in has a nearly inconsequential effect on the tank temperature. The pre-heated water of the sustained flow into the tank is also diluted by the tank, but the rate of depletion falls and the recovery rate shortens. With the heat exchanger plumbed for balanced flow to the shower the amount of heat recovered goes up by ~15-20% due to the higher flow through the HX, and cold water to the shower mixer is now considerably warmer, making the flow from the tank slower. From a user point of view all it means is that at the begining of the shower you start with the shower mixer set for a higher hot water flow, but then have to adjust it down 30-60 into it as the pre-heated water finally arrives at the cold side of the shower mixer. (At least that's the way it works at my house.)
  15. kcodyjr

    kcodyjr New Member

    Messages:
    103
    Location:
    Chelmsford, MA
    Wouldn't a thermostatic type shower valve be a better solution there?
  16. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
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    Location:
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    Old thread...depending on the design, many thermostatic valves will run all or nearly all hot until it needs to begin tempering it to achieve the desired set temperature. I have a Grohe thermostatically controlled valve in my tub/shower that I like. My first exposure to them was in a Holiday Inn in London, England where they worked quite well, which led me to consider one for my remodel. They tend to be the most expensive type of shower valve, but they are nice.
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