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Thread: How big a water heater?

  1. #1

    Default How big a water heater?

    Hi everyone,
    can anyone tell me how to figure out what size water heater I need?
    I am in Rhode Island. in a converted victorian, the heater would need to supply 3 units, with 2 full baths and kithcen on first floor. 2 full bath & 2 kitchens on second floor.
    I could separete the first floor hot water from the second if i had to, but the replumb would be alot of work. I know I can't separate the second floor units without tearing out walls. Which scares me to even think of the work involved and $. thanks in advance for the input. Ileana

  2. #2
    In the Trades Bob NH's Avatar
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    There are several things to consider:
    1. How many people are going to use each unit?
    2. Will there be laundry equipment in each unit?
    3. Will there be inconsiderate tenants using hot water that someone else is paying to heat, who therefore have less incentive to take reasonable showers?
    4. What is the cold water supply temperature in summer and winter?
    5. Is heating by gas, electric, or oil. Gas and oil have faster recovery so you can use a smaller heater.
    6. Are there low-flow (2.5 GPM or less) shower heads in the showers of each unit?

    A 50 gallon gas unit will usually serve a modest family if they are reasonable users. Two eighty-gallon units in series for three units would correspond to that.

    Heaters larger than 120 gallons usually cost more because they must be made to a Pressure Vessel Code, so if you need that much, you may want to put two smaller heaters in series.

    If you give us some more information, you will get a range of suggestions with rationale.

  3. #3

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    Hi Bob,
    Thanks for all the input. Not sure I have all the Answers for you but I will try.
    1.On average 2 people per unit.
    2.There is one set in basement, and I would like to add a second set later on for the tenants.
    3.Right now they are considerate tenants, but life could change.
    4.I have no clue. Can I just stick a thermometer under the tap?
    5.I have a oil furnace which supplys the hot water for both 1st. & 2nd flr, and heat for both units on 2nd flr. But I could run a gas line fairly easily.
    6.Again have no clue. I suppose I could replace the heads just in case.Now that ya mention it I do feel a bit silly for not thinking of that eariler. But thanks for the weekend project
    Last edited by IleanasColors; 03-16-2007 at 02:45 PM.

  4. #4
    In the Trades Bob NH's Avatar
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    Since you now use an oil furnace for water heating, are you going to replace that source or supplement it? If you are going to supplement the oil, that is equivalent to at least a 50 gallon gas heater.

    One problem with using the oil furnace as a supplement is that it might be hard to get enough flow through it to serve 3 units. That is the kind of setup where you could easily have 3 showers running at the same time and a tankless coil may give you too little pressure.

    How is the pressure now for the three units? I assume there is too little water for all with multiple showers.

    Here is a link that will give you some information.



    You can probably figure 50 gallons per unit x 3 = 150 gallons in a peak hour when everyone is taking a shower in the morning.

    I would consider something along the following lines for gas heaters. Maybe some others will give their opinions. Get prices for installed units.
    Minimum/Marginal - Two standard 50 gallon heaters
    Adequate - One 120 gallon standard heater
    Adequate - Two high-recovery 50 gallon or two standard 80 gallon heaters
    Super adequate - One 120 gallon high recovery heater

    You could test your cold water temperature by just running water for a while until the thermometer reaches a steady temperature. The winter temperature will affect the water heater requirement in the winter.

    For shower heads, I recently replaced on with a moderate cost HydroSpin 5 Spray Settings model from WalMart that was far better than a fancy Grohe or something that I got at HD and took back. Everyone is satisfied with the HydroSpin model.
    Last edited by Terry; 12-14-2012 at 03:15 PM.

  5. #5

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    Thanks Bob!,
    You are the first to give me some real info to work with! I could kiss ya!
    You are correct there is a big hot water pressure problem!Thats what got me started. I priced out replacing the existing system.Sears gave me a price of $10,205.00. Which I can't afford. So thought maybe if i broke out the hot water into a separate system it maybe affordable.And then I won't have to run the boiler all summer long.

  6. #6
    In the Trades Bob NH's Avatar
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    Since you have a pressure problem with the tankless coil, I would go with a large gas heater. I just looked in my Grainger catalog and they have a 100 gallon heavy duty high-recovery commercial heater by Rheem (catalog 5AU63) listed at $2658. It is listed at 199,900 BTU per hour (must be some code breakpoint at 200,000 BTU per hour) with 215 Gallon per hour recovery at 90 degree F temperature rise. That should be enough for six showers in an hour.

    Installing one big heater will be less than two smaller ones, and you need only one flue. You might need a power-vent to minimize the flue costs if you don't have a convenient chimney. You might also have to upgrade your gas supply.

    You should try to avoid getting an ASME code compliant heater (120 gallons or more, and more than 200,000 BTU) because it will increase the cost to about $5000.

    Someone else here has better info than I do on commercial water heaters. Maybe they will chime in.

  7. #7

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    Once again Bob your input has been wonderful
    Now all I have to do is find a affordable,competent plumber! Not easy in Rhode Island given how loose the licensing is here.

  8. #8
    Master Plumber master plumber mark's Avatar
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    Talking Rheem commercial heaters

    Bob , most of your advice sounds very good.
    and I know very little about oil heat and tankless coils....
    ( I dont want to know)

    that expensive $2800 --120 gal Rheem heater from grange
    sounds ok but they only come with a 3 year warranty
    and they can be an absoulte pain in the ass to service..

    and you need a fork lift and a wench, and about 5 dumbasses
    just to get one down into the basement......




    If you have the room in the mechanical area,
    Might I suggest you simply install a either two 75 gallons
    or a 75 and a 50 gallon cheap---easy ---standard Rheem home units
    and make them in series....


    they cost on average 500 bucks for the 75 and 350 for the 50......
    and have a 6 year tank warranty........you cant go wrong.


    you can burn through quite a few of them before you ever hit that $2800 k mark.

    and any moron can service them

    thats all I got to say about that...
    Last edited by master plumber mark; 03-24-2007 at 05:10 AM.

  9. #9

    Question

    Thanks for the input Mark. I'll let you all know what i decide to do. Is a tough choice. I do have a 1000sq ft basement, but with two old boilers and oil tanks to match, and water heater for 3rd flr. not sure how much more space i want to loss. but the idea of lugging in a big heavy water heater, that is finicky when it comes to maintenance. and may not last me long is not appealing either. maybe Uncle Sam will be generous this year and i can go expensive with a new efficiency boiler from sears.

  10. #10
    Plumber Cass's Avatar
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    I don't know what the water quality is in your area but if the water is soft you may want to consider 2 in line tankless heaters.

    Personally I would go with Marks suggestion of a 50 and 75 gal. heater with the 50 feeding the 75.

    The draw back to the tankless is service should they need it. If you can locate a company that installs and services them then that would be a viable alternative for you to look at.

  11. #11

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    The in-line sounds like a nice idea, Like the new and over priced boiler from Sears. But we are first time owners and struggling. so budget is tight think the water heater in series is the way we will end up going. hubby will just have to deal with less space for band and recording equipment, poor boy

  12. #12
    Plumber jimbo's Avatar
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    Regarding the ASME rating.....yes I think > 100 gals and > 199,900 BTU is a mandatory set point, but check with your local authorities. There are places now which require ASME on any large tank in commercial and multi-family. Any local plumber who you contract with should be familiar with your local rules, and guide you to a proper choice.

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