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Thread: I need to size a commercial hot water heater for my building.

  1. #1
    DIY Member Erico's Avatar
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    Question I need to size a commercial hot water heater for my building.

    As Condo board president (and head maintanance man, I guess), I am stuck with the task of having the new heater installed.

    History: it's a 10 year old 75 gallon 135,000 BTU gas water heater servicing 6 - 2 bedroom condominiums. I think it's a Bradford-Whitley (or something like that .... or is that the name of the actor that played Josh on West Wing?). We had some service work done last summer and it was reccommended that we replace it soon. The servicer seemed to think the thing is half filled with sediment (whoda thunk you need to flush these thing once a year? now we know). He was supposed to get us a price but never called me back so here we are 6 months later and she sprung a leak.

    The previous servicer seemed to think the unit was TECHNICALLY a little undersized but PROBABLY ok as the building population consists of either single people or couples - no kids or big families (this is near Wrigley Field after all).

    Anywhoo........My local hardware man gave me what seems to be a good price on a Rheem 75 gallon 125,000 BTU unit - a little LESS BTUs than the current unit. His logic is, the new unit will be much more efficient and not filled with gunk like the current unit therefore we can get away with a little less BTUs. BTW, the current unit still works, and always has worked, great. Never any supply/recovery problems. It cranks out the water like nobody's business.

    Here's the question: Do I take the hardware man's advice and go a little less on the BTUs? His logic regarding the gunk doesn't seem out of whack. Btw, this is a real old fashioned hardware store - they sell to local plumbers etc. The hardware man reccommended an installer who, coincidentally, has done work for us and other people I know (he has a good rep. too).

    Then there is ME - who tends to over build and spend "just to be safe." The last thing I need is ticked off neighbors running out of hot water at 6 am. Best I can tell from searching the internets, a size or two up will cost another $500-900 buckaroos. Should I spend it "just to be safe"?

    Any thoughts on sizing? Brands?

    Thanks,

    Eric
    Condo Board President
    Head grounds keeper
    Head maintanace
    Head Locksmith
    Head Bug Killer
    Head mediator
    Snow Desk

  2. #2
    Forum Admin, Expert Plumber Terry's Avatar
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    Here is the Bradford White water heater sizing page

    http://www.bradfordwhite.com/rightspec.asp

  3. #3
    DIY Member Erico's Avatar
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    Default Thanks Terry!

    Hmmmm..........my thinking has been a little off (big surprise there).

    I plugged in the current building configuration of:

    2 - 2 bath w/clothes washer
    4 - 1 bath w/clothes washer


    ...and came up with 75gal./76,000 BTU OR 100gal./85,000 BTU

    Then I changed the configuration for two baths that will be added.


    4 - 2 bath w/clothes washer
    2 - 1 bath w/clothes washer


    .....and came up with 100 gal./85,000 BTU


    This sounds like the way to go. Quite a difference from what the hardware man recommends and what we currently have.

    I'm quite surprised at the lower BTU recommendations. My inner "over build" brain is squirming.


    Terry, do you like any particular brand for this type of application? I'm guessing my hardware man has a connection with Rheem and I doubt the installer will step on the hardware man's toes as he referred us. The hardware man clearly wants the heater sale and I like supporting this mom and pop store when I can - they give GREAT service and are always helpful. I just need to make sure I do the right thing as 5 other owners are involved - not just me.

  4. #4
    In the Trades kordts's Avatar
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    Do you run out of hot water? If not, go with the 100 gal model.

  5. #5
    DIY Member Erico's Avatar
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    Default Never have run out.

    ......using a 75 gallon 135,000 BTU gunked up unit.......but the chart says go lower on the BTUs and higher on the gallons.

    I'm trying to wrap my little pea brain around the gallons versus BTU relationship. My understanding is the BTUs are all about recovery time. I'm just wondering how quick you can go through 100 gallons, say, on a Monday morning with 2 or three showers running.

    Perhaps we would be feeling the cold right now if it wasn't for the fact our gunked up inefficient unit was so over BTUed?

  6. #6
    retired Industrial Arts teacher Drainplug's Avatar
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    Hello,
    My house has a 50 gallon water heater and it is just my wife and myself. For an apartment building with the possibility of 6 people or more taking showers at around the same time and maybe doing some laundry, even 100 gallons sounds small to me. Running out of hot water in the shower in the morning is a bad way to start the day ! Just my opinion.....everybody has one !

    Drainplug

  7. #7

    Default

    Why not a couple or three smaller units ganged up? It might be cheaper and then you would have backup when one unit was down. In summer you could likely shut one off and save a little gas.

  8. #8
    DIY Member Erico's Avatar
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    Default It's funny you should mention that.

    One of the options on the BradforWhite site was two smaller units. That idea seemed foreign to me but I guess it makes sense. I don't think we have the room though. The unit that is currently in use is in a rather small room the builder partitioned off.

    I'm with you though, 75 gallons doesn't sound like a lot but it has been working like a charm for the last ten years........????

    I guess we'll do 100 gallons. The question is - how many BTUs?

  9. #9
    Plumber jimbo's Avatar
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    70% rule says you get 70 per cent of the gallon capacity of the tank as hot water....about 52 gallons on a 75 gallon tank. After that, you are living on the recovery rate. For a rough estimate, figure recovery of a gas WH at 1 gallon per hour, per 1000 BTU.

  10. #10
    DIY Member Erico's Avatar
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    Default Thanks......

    If I do my Jethro Bodine type ciphering I come up with:

    100 gallons = 70 gallons usable - 28 minutes of shower time figuring 2.5 gal/minute shower heads.......is 2.5 about right?

    To figure recovery time, do I use the "usable" or the "total" gallons.

    Usable: 70 gallons divided by 85(1000btus)= .82 or 49 minute recovery.

    Total: 100 gallons divided by 85(1000btus) = 1.18 or 70 minute recovery.

    I think I may need to up the BTUs. I think if we get 3 people all showering on a Saturday or Monday morning we may be a little short if I use the Bradford chart. Their chart is figured for "apartments"...I wonder if there is a chart for condo owners with a sense of entitlement?

    Or am I over building AGAIN?

  11. #11
    Homeowner geniescience's Avatar
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    Condo owners are responsible people who do not have any greater desire to use free (zero incremental cost) resources any more than rental apartment dwellers.

    Tenants have been known to leave the hot water running from a faucet in winter as "it improves the relative humidity." Condo owners deal with co-owners permanently (are answerable to discussions about questionable practices).

    My opinion is to go share this with some co-owners without ever asking as you are doing, whether all this is too small. Cut out that question and let it sit for a few days.

    David

  12. #12
    Plumber jimbo's Avatar
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    We don't look at recovery in minutes, but in gallons per hour. This is because in a commercial application, like a restaurant or laundry, they need this X many gallons of water each hour, ongoing.

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