Gas Water heater for bathroom renovation

Discussion in 'Water Heater Forum, Tanks' started by tomtbone, Feb 11, 2010.

  1. tomtbone

    tomtbone New Member

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    tomtbone in Boston
    I am planning a bath renovation a few months down the road and I am pretty sure our 30 gallon water heater aint gonna cut it. Its the shower that has me concernced and I'd like some advice as to the size/recovery rates I should be looking at.

    The shower will have 3 shower heads with one shower head (approx 2.5) and two Kohler body sprays which give out 2 gpm each. So it looks like 6.5 gpm max. I did a flow test with a 5 gallon bucket in the bathtub and got around 8-9 gpm so it seems like I have enough flow.

    This is the only bathroom in the house, so apart from the rare dishwasher/washing machine going at the same time, I just need to compute this for the shower. Can someone help me out?
    Thanks!
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    You have to ask yourself how long a typical shower will be, and that may include adding back-to-back shower times. Discounting the WH's ability to reheat water, you can use about 75-80% of the capacity. Now, depending on how hot you set it, your shower doesn't use full hot, so figure out how much actual hot you need. Then, look at recovery rates and first hour capacity. As soon as the WH detects the temp dropping, it will start up the burner, and that will extend it some beyond the 75-80% (which is what gives you the first hour capacity). Tank burner output varies quite a bit, so you can get quite a bit out of a smaller tank if you want. You can work out what you need from that, then try to match it up with a model and brand.
  3. tomtbone

    tomtbone New Member

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    tomtbone in Boston
    Thanks!
    So lets say I want to take 2 10 minute showers back to back. That totals 130 gallons. Just for example I am looking at a 50 gallon Kenmore right now that has a recover of 42 gph and a first hour of 90 gallons. Can you help me decipher this a bit to see if this will do the job under normal shower temp and wh temp setting?
  4. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Another factor you need to include is how cold the winter cold water is. Say that is near freezing if you live in a really cold climate (or you have a deep well), verses say souther FL, where it might be closer to 60-degrees or even warmer in the winter. That Kenmore wouldn't cut it. The pros here recommend Bradford White or Rheem from a quality viewpoint.

    Say the incoming cold was 50-degrees (a common spec) and you wanted your shower at 110 (may be a little hot). And, say you set the WH temp to 120 (about the minimum you should consider). You'd be using almost all hot, since the 50-degree water would cool it off with only a little cold. If that incoming water is 33-degrees (I've seen that at my house), then even less cold would be required to temper it down to a desired point. That first hour assumes the use is constant, so you'd need probably 120 gallons of hot in 20-minutes. You don't have the extra 40-minutes of adding heat, so it would run out quicker. You can make a WH look bigger if you raise the temperature, but there's a limit on how high you can go and still keep things safe.

    You're asking the WH to provide hot water for 1/3 of an hour, so you need to be able to provide that hot water faster - that means a bigger tank, or a bigger recovery rate. You'd get about 40-gallons from the tank initially, and only have 1/3 of that it could produce during the 20-minutes of recovery verses a full hours rating.

    That tank could provide hot at a flow rate of 1.5gpm for that hour. More initially, but then you'd have to throttle way back, since it can't recover.
    Last edited: Feb 11, 2010
  5. Redwood

    Redwood Master Plumber

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  6. tomtbone

    tomtbone New Member

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    tomtbone in Boston
    For what its worth, I am getting 42 degree cold water out of the fauces living in Massachusetts.
  7. tomtbone

    tomtbone New Member

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    tomtbone in Boston
    Last edited: Feb 11, 2010
  8. hj

    hj Moderator & Master Plumber Staff Member

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    If you take two 10 minute showers that is 20 minutes so you will only get a third of that "hourly recovery rate", but the recovery rate is based on a specific incoming temperature, usually 90 degrees below the heater's temperature setting. If the water is warmer than that the recovery will increase, and as the tank's temperature increases the recovery rate also increases. Because the parameters are dynamic and constantly changing your decision may just be a "WAG", but in many cases a 50 gallon heater should do the job. If you go bigger than a 50 gallon one, such as 65 or 75 gallons, expect to pay two or three times more for the heater itself, plus higher installation costs.
  9. Dana

    Dana In the trades

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    The raw math is

    gpm x 60minutes x 8.34lbs/gallon x F-rise= BTU/hour requirements.

    In a shower, figure on ~105F. If you assume a min incoming temp of 40F (probably true for most of central/western MA- for me it's a bit above your measured 42, ,but not by much.)

    If you're assuming 6.5gpm you have a net requirement of 211,419BTU/hour, which you AIN'T gonna get out of any tankless. The NR98s output will be ~45KBTU/hr shy of filling the bill. A condensing tankless with a 199K burner would still be 15-20K short.

    A 75gallon tank with a 75K/80% burner would probably make it.

    The other solution would be to install a drainwater heat recovery heat exchanger (PowerPipe or similar) big enough to return 50%+ of the heat going down the drain measured at the 2.5gpm standard. At 6gpm it'll still be delivering 40% back. This would use substantially less fuel too. A list of models tested by 3rd parties is maintained by Natural Resources Canada here: http://oee.nrcan.gc.ca/residential/personal/retrofit-homes/drain.cfm?attr=4

    You'll need at least 5' of vertical drain to fit one in, and both length & diameter increase the heat-transfer efficiency. The output of the HX need to feed both the water heater & shower's cold feed (but if it has to be one or the other, you'll still get ~75-80% of the performance feeding just the hot water heater.)

    [​IMG]

    At 2.5gpm it's like having another 30K of burner behind it, but at 6.5gpm it'll be more like an additional 80-90KBTU/h. With that a tankless solution would have some margin, and you might even squeak by with a high-recovery rate 50 gallon tank like the Bradford-White M-2-XR504T6FBN. Figure on a ~$1-1.5K installed cost for drainwater heat recovery. EFI in Westboro MA reps PowerPipe in the US, and have been known to sell at the wholesale price in onesie-twosies (it doesn't take much to set up an account with them either.) It's not necessarily about payback on fuel savings- think about the price/performance of alternate hot water heating systems that would actually support the load.

    Last, and definitely not least, if you're heating your house with a boiler that has more than 150KBTU/hour of output, and indirect-fired 50 gallon tank running as a priority zone off the boiler would fill the bill for less cash outlay. Even if the boiler's output is ~75KBTU output and indirect tank + drainwater heat recovery or just an 80gallon indirect would still come in at about the same or lower installed cost (and lower operational cost) than a tankless that's still falling short, or a 75gallon/75KBTU standalone that's just squeaking it.
  10. Dana

    Dana In the trades

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    BTW: If you're sparing with the side sprays, using them for maybe only a minute or so at the end of a shower rather than 5+ minutes at full flow, a high recovery 50 gallon tank should do you just fine.
  11. tomtbone

    tomtbone New Member

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    tomtbone in Boston
    I have a Wells Mclain cga-5 furnace with a btu input of 140k and output of 117k. Our house is operating on a single zone currently.

    It looks like any indirect system would cost me about 1k to 1.2k like the turbomax 23. How would this be any cheaper, especially considering the plumbing that has to be done to convert to a dual zone.
  12. Dana

    Dana In the trades

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    A TurboMax 23 is a "reverse indirect" with a bigger heat exchanger, and more expensive than a typical indirect. But if it's plumbed as a buffer tank/hydraulic seperator it'll improve the operating efficincy of the heating system, and allow you to micro-zone the house later without loss of efficiency. A priority zoned standard indirect will be hundreds cheaper. Either way you'll be into it for another pump and some plumbing, but if you went with the TurboMax you could skip the smart controller. It would have to be plumbed roughly like this:

    [​IMG]

    But since the TurboMax doesn't com with separate port for the boiler & radiation loops (the way ErgoMax does), it'l have to be Tees on each. Set the temp on the TurboMax to whatever is required for the radiation (not lower than 130F to protect the boiler) slave the boiler's controls to the aquastat on the tank, and the radiation's circulation pump's relay to the room thermostat. The mass & hysteresis of the tank will prevent the boiler from ever short-cyling, and as the DHW draw brings the tank temp lower, the radiation will put out less heat automatically. (It's the right way to go, IMHO.) An alternate system schematic from TurboMax:

    [​IMG]


    To put in a 199k tankless the unit itself is gonna run about a grand, then you'll be into it for several hundred in stainless venting and gas line upgrading, etc, it'll be $2k or more when all is said & done. A 75gallon standalone will be ~$1500-2000, installed price, similat to the TurboMax/ErgoMax solutions, without the efficiency boost on the heating system if done right.
  13. jimbo

    jimbo Plumber

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    You might get 90 gallons in the first HOUR but in 20 minutes, you get 35 gallons. Not gonna work!
  14. tomtbone

    tomtbone New Member

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    tomtbone in Boston
    I found a great deal on a new never installed bradford M-I-75S6BN for $200. Can't beat that.
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