Replace 1 gas WH with 2 gas WHs ?

Discussion in 'Water Heater Forum, Tanks' started by akm, Dec 8, 2009.

  1. akm

    akm New Member

    Messages:
    12
    Location:
    Seattle, WA
    Existing 50gal nat gas WH in home (kitchen/DW + tub/shower bath + laundry), with basement 'mother-in-law' rental (kitchen + shower bath + laundry), needs to be replaced.

    Seems like need more than 50 gal, but next step is 80gal.

    2-40gal is much less initial fixture cost than 1-80gal.

    Make sense/advantage to go with 2-40gal in series (or whatever piping setup is best, not a plumber) ?
  2. Doherty Plumbing

    Doherty Plumbing Journeyman & Gas Fitter

    Messages:
    810
    Location:
    Penticton, BC
    You could pipe them in series with the 1st tank heating the water up to say 90* and then have the second tank heat from 90*->120*. You could even go like 80*-120* for a 40* delta T.

    You could also pipe them in parallel and have each tank store 120* water but this is the more costly system to run.

    You could also go with an 80 gal tank and run high temperatures and install a mixing valve.
  3. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,263
    Location:
    Cave Creek, Arizona
    heaters

    Two 50 gallon heaters would cost only slightly more than two 40 gallon ones. If you have the gas line and chimney/flue capacity for two heaters, (and it would be the same whether you used 40 or 50 gallon heaters or a combination of both), I would go that way and install them in series with both at the same temperature.
  4. Doherty Plumbing

    Doherty Plumbing Journeyman & Gas Fitter

    Messages:
    810
    Location:
    Penticton, BC
    Why would you install both tanks in series at the same temperature? This would be pointless... You might as well pipe them in parallel if you're gonna have them at the same temp.....
  5. Thatguy

    Thatguy Homeowner

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    Location:
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    How old?
    ***************x
  6. akm

    akm New Member

    Messages:
    12
    Location:
    Seattle, WA
    Thanks to all for comments.

    1. Existing 50gal WH was in house when we moved in, but WH dated 1994, and according to bldg permit records, looks like WH would have been installed around 1996.
    Currently we are starting to notice limited hot water supply.

    2. Seems like 50gal WHs in series with lower temp at 1st 50gal and then higher temp at 2nd 50gal makes sense.
    Not sure why would set same temp for both, unless greater capacity needed and then put in parallel ?
    Could we change piping and/or temps if more HW was needed ?
    Also, may it be possible (wild shot) that would not need to replace 1st lower temp WH as early as 2nd higher temp WH... at least some redundancy in case of 1 WH failure ?
  7. Doherty Plumbing

    Doherty Plumbing Journeyman & Gas Fitter

    Messages:
    810
    Location:
    Penticton, BC
    1. Replace it.

    2. With the 1st tank doing so much preheating the 2nd tank is gonna have a QUICK recovery rate. With the setup I suggested you're going to have plenty of hotwater at your disposal with a good ability to recover that water once you've used it.

    I don't think you're gonna have to worry about increasing capacity. If you do then tell people to take shorter showers :D Not but in all honesty yeah you could crank up the temp on the 1st tank so you had a full 70 gal on of useable water. But the idea of having the 1st tank preheat the water is to save $ because I think you're gonna find you're not running out of hot water

    And sorry but I don't understand your last question? If you have 2 tanks either in series or parallel you have some redundancy.
  8. akm

    akm New Member

    Messages:
    12
    Location:
    Seattle, WA
    Thanks much.

    Last question was pretty much what you answered.
    Other part was a bit of thinking/mumbling about how may be able, in future, to stagger replacement of 2 WHs next time if lower temp WH lasted a bit longer.

    Also need to look at space for 2 WHs, and venting, etc.
    May post sketch of piping and venting of both, after get specs and b4 purchase, just to check if thats ok.
  9. Thatguy

    Thatguy Homeowner

    Messages:
    1,460
    Location:
    MD
    Tanks in parallel increase reliability since you can bypass the failed one.

    I get an 18 yr avg. replacement age for NG WHs, with 2/3rds being replaced between 7 and 27 yrs.

    If one heater is 99% likely to last some period of years, two in parallel give you 99.99% reliability.
  10. nukeman

    nukeman Nuclear Engineer

    Messages:
    706
    Location:
    VA
    A negative on doing two tanks is increased standby losses. Two 40 gal tanks have more surface area per volume than a single 80 gallon tank. This means more heat loss assuming both sizes have the same insulation and operating temperature. If you ran in series with the 1st tank at a lower temperature, these losses would be reduced.

    It is probably not a big factor, but just something to consider.
  11. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,263
    Location:
    Cave Creek, Arizona
    heaters

    If you need 80 gallons of 120, or whatever, degree water, what good is it if you have 40 gallons at 120 and 40 gallons at 105? Parallel depends on too many factors, and the dynamics can charge over time AFTER installation, to depend on you getting ALL the hot water in the tanks before the performance degrades. I have only installed a couple of parallel installations and they were with commercial water heaters using a $2,000.00 engineered manifold system.
  12. akm

    akm New Member

    Messages:
    12
    Location:
    Seattle, WA
    Good thoughts...
    1. Thinking about 2-50gal in series. Usage is unpredictable (with mother-in-law rental apt), and an 80gal WH is another 30% more expensive. Also, height may be a question for (existing fixed height) venting, dont know yet if 80gal would be taller or just larger diameter than 50gal WH.

    1. Good point. Both WHs in 'semi-heated' bsmt util area, and will be wrapped with extra insulation.

    1. Maybe install a bypass line/valve ?

    Ps: Where to get input for WH best buys and brand/model comparisons ?
    Last edited: Dec 14, 2009
  13. Runs with bison

    Runs with bison New Member

    Messages:
    891
    Location:
    Midwest
    If I went two tanks I would likely go with parallel. Two in series actually increases the time before the second tank burner fires. I'm not sure what the controller offset is, but I've noticed some draws (5-7 gallons/hot water) don't always trigger the Robert Shaw valve rapidly on my 50 gallon tank. Therefore, in series, burner #2 won't even fire until a significant lag time has developed--call it 2x the parallel tank lag time when both burner #1 and #2 would be firing. I haven't run this through a process control dynamic simulator so perhaps I've missed the salient point for real world control.

    I would go with a larger single tank rather than even consider 40 and 40 for reasons already mentioned. A high output/larger volume tank should handle it easily. The installation cost should be less with half as many fittings.

    Alternatively, put a 1.6 gpm Roadrunner showerhead in your mother-in-law's shower and be done with it. Full disclosure: I use this same showerhead and like it, so I'm not out to punish your in laws.
  14. Doherty Plumbing

    Doherty Plumbing Journeyman & Gas Fitter

    Messages:
    810
    Location:
    Penticton, BC
    That would be the point... to save $ on heating water that doesn't need to be heated. The problem is he doesn't "know" how much hot water he needs and wants to go with 2 40 gallon tanks because they're both cheaper then 1 80 gallon tank.

    Piping them in series with the 1st tank preheating the water (to any extent) will increase the recovery rate of the 2nd tank allowing it to produce hotwater faster if needed. If he doesn't need all 80 gallons of water to be heated up to temp there is no sense in paying to do that.
  15. akm

    akm New Member

    Messages:
    12
    Location:
    Seattle, WA
    Thanks for the additional input and patience.

    More research and still somewhat undecided, but getting closer...

    1. like the redundancy and capacity of (2) WH, but seems a little over-kill

    2. found (1) 75gal, cost is not that bad considering the simpler piping/venting etc (as previously mentioned in this thread).

    Size (all seem to work for venting height, assuming min 1/4' per ft slope) and cost factors are as follows.

    Cost of (1) Heavy Duty 60gal = $600ish
    Cost of (2) (not Heavy Duty*) 50gal = $550ish
    Cost of (1) Heavy Duty 75gal = $850ish

    *HD and non-HD seem to have the same specs and both 'self-cleaning'

    Guess am leaning toward (1) 75gal unless you all see a problem.
    Last edited: Dec 15, 2009
  16. Thatguy

    Thatguy Homeowner

    Messages:
    1,460
    Location:
    MD
    60 @ 600 = $10/gal
    50 @ 550 = $11/gal
    75 @ 850 = $11/gal

    So much for Economy of Scale. . .:eek:
  17. akm

    akm New Member

    Messages:
    12
    Location:
    Seattle, WA
    Seems like good way to compare !
    Actually could look at (2) 50s as 550/100(or maybe a little less if in series) = $5.50/gal (plus a bit more for piping/venting ?
    Still leaning toward (1) 75gal.
  18. Thatguy

    Thatguy Homeowner

    Messages:
    1,460
    Location:
    MD
    Once you make your choice, your Utility Function

    "While preferences are the conventional foundation of microeconomics, it is often convenient to represent preferences with a utility function and reason indirectly about preferences with utility functions."

    will be known to all of us, including you! In principle, at least. . . :eek:
  19. Runs with bison

    Runs with bison New Member

    Messages:
    891
    Location:
    Midwest
    Actually, it makes sense since the 75 gallon one has a higher capacity burner: 75,000 Btu/hr. If you based this on 1st hour ratings the result is different.

    Based on the Energy Guide the efficiency factor works out to 0.53, which is pretty low but still will work out to slightly less in the way of losses than two tanks would.
  20. akm

    akm New Member

    Messages:
    12
    Location:
    Seattle, WA
    Think we are ready to take the plung, just need to click the 'purchase' key.

    Drawing of install is attached, does it look ok ?

    Attached Files:

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