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commercial water heater sizing

superdave Member Posts: 155
I have old building with 2 oil fired boilers each have 2M BTU with coil in side. Client want to go with wall hung boilers gas fired. Building is about 52,000 square feet with 68 units all with 1 bath and kitchen. What would be the needed gals of hot water storage needed. I pane on using 6 wall hung boilers with an output about 2.1M BTU.


  • Ironman
    Ironman Member Posts: 6,684
    edited July 2011
    You Don't Need that Much Horsepower

    That's enough to produce almost 3200 gph at a 70* temp rise at 88% efficiency. Which is about what a mod/con will get when producing 170 - 180* water supply temp for an indirect.  The entire building may use that in a day. That's assuming 50 gal's or so per apt. per day.

    You'll need to know the number of bed rooms to get a better idea of how many occupants there are in the bldg. DHW usage is calculated based on the number of people and peak usage

    I would suggest using reverse indirects like the TurboMax. They have a higher output and require less storage capacity.

    If you contact the manufacturer or their rep, they will size it for you and guarantee it's performance.

    P.S. I'm assuming these boilers are only for DHW. If they are also doing space heating, then that obviously should be factored in the sizing.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • superdave
    superdave Member Posts: 155

    Thanks,  I love the Turbomax I have one in my house it works great. I have 1 standard bathroom and 1 master bath with 92 gal. tub the shower has 4 body sprays, 1 hand shower, 1 fixed head, and 1 over head rain 12" round. With a Baxi 1.31 that has an out put of 105,000 BTU.

    Yes the boiler will heat 52,000 square feet and 68 units of DHW.  The first supply house told me to use 4 109gal. tanks. I thought that did not seem right that is why I am double checking. 

    Thanks for the info.
  • Greg Maxwell
    Greg Maxwell Member Posts: 212
    Commercial DHW

    I have been involved in a few large DHW jobs of this nature, and I agree with Ironman, and would add this. Typically, these types of projects, (apartments), tend to use a majority of their hot water all at once, so you need to size your storage for the peak dump, at the rated GPM of the tub shower valves. Hopefully, the building owner has had their shower heads changed o the lowest gpm that he can get away with. Also, storing that water at a higher temp, typically 140 degrees, then down mixing it to the desired delivery temp, can give you a mixture ratio of approx 25-30% higher output. Ill be interested to hear the outcome. Thats sone serious hot water!
  • tim smith
    tim smith Member Posts: 2,421
    Re: commercial sizing dhw

    I have done several jobs in this size range. Usually I end up with around a 200 gals glass line asme storage tank and a Braze plate heat x. I have been using a GEA heat x for these jobs, quality/price good. Sizing them at 140 or so entering water temp to keep in condensing mode mostly. fairly close approach  so you can still keep min 120 stored.  Works real well and pencils out ok on jobs. Good luck
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,837
    My experience says...

    Size the physical plant to the worst case scenario load, and then prioritize DHW during simultaneous calls and you will get a long just fine. DHW is typically the greater of the two loads here in Denver, CO.

    There are a number of manufacturers DHW sizing programs out there. My fav is RayPak. You can play "what if" games with their software, by decreasing storage volume and increasing fire capacity to come to the same bottom line BTUH capacity.

    The use of reverse indirects as a DHW source, coupled to a mod con boiler is FANTASTIC, if, and this is a big IF, it is properly controlled and set up. Done wrong, it will waste money and energy.

    Us Americans are used to having large storage tanks with small heat exchangers that require an approach temperature that guarantees the boiler will be out of the condensing mode during DHW production. The reverse indirect corrects all of this if it is set up right, and setting the boiler to operate at 180 degrees F during DHW production is NOT the right way to do it.

    These things are basically a HUGE heat exchanger (think Everhot, except bigger) and they only need supply water temperatures maybe 10 degrees F hotter than your target DHW production temperature. So, if you are targeting 140 degree tank temperatures and mixing down from there to a reasonable temperature, then you only need supply the tank with 150 degree F boiler water to hit that target. This keeps the heat source in the condensing mode, even when it is doing DHW production. WIN WIN WIN. (Note that the assumption here is a condensing heat source. DO NOT DO THIS IF THE HEAT SOURCE IS NOT A CONDENSING BOILER, or it will die a thousand deaths...)

    Now, here's the real trick to avoid mega short cycling during non peak load periods if a circulation return is present. Set the differential on the DHW tank as high as you can reasonably set it for. For example, use a 20 degree differential. Tank hits 140, heat source decouples. Storage tank, through radiant/convective losses from distribution piping will lose heat, and when "system" hits 120 degrees F, the heat source is fired back up and can actually modulate before it hits high limit and shuts down.

    What this does is to alleviate the continual start stop short cycling. With the exception of one boiler (Viessmann) that I am aware of, most other boilers ignite and confirm flame at a higher RPM/burn rate, and by the time they release the burner for modulation, the tank is approaching its highest allowable set point, thereby shutting the boiler down before it even gets a chance to stretch its legs.

    With the proper anti scald mixing valve set for say, 120 degrees F, even if the tank hits the bottom of the scale, the water is adequately hot for use, and the "system" will get it caught up quickly anyway.

    As for building heat loss, don't always depend on just an EDR/convector survey to determine real time loss. Look at the utility bills for worst and best case scenario, and run the numbers forwards AND backwards. Perform a "skin loss" calculation on the shell of the building, and compare that to the EDR/convector survey to see if they are reasonably close. If one (usually the convector survey) is out in left field, it is fairly safe to assume that whoever designed it lived in fear of getting the dreaded cold call in the middle of the winter. My experience has shown me that in situations where even the skin loss showed 30 BTU/sq ft /hour, the "real world, real time" loss of the building was half that. Remember, not every square foot of occupiable space is going to lose the 30 btu/sq foot per hour, and the average is probably really closer to 15 btu's/square foot per hour, with some interior hallways approaching ZERO.

    By all means, DO NOT REPLACE LIKE WITH LIKE SIZED EQUIPMENT, even if the new source is modulating. You will create your own new nightmare if you do. Take the time to do it right.

    Ideally, a person could put some data loggers onto the existing equipment for a reasonable cold period of time, and determine the real time energy demands of the building, and size the replacement equipment to those specifications. Unfortunately, very few people have the luxury of time in their favor to do that, so we fall back to alternate means.

    Performing due diligence in advance will pay awards for many years to come.


    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • Greg Maxwell
    Greg Maxwell Member Posts: 212
    Water Quality

    I typically would not recommend either a plate exchanger, or glass lines tanks on this type of job. Although they will lower your bid price consideralby, they will be more prone to problems down the road with scaling, and the glass lined tanks will have a much shorter life span.
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,837
    edited July 2011
    Solution to lime scale...

    Don't let lime scale scare you...

    Look into Fields Clear Wave electro/magnetic water conditioner.

    WIll descale scaled up systems and will avoid lime scale accumulations on new systems.

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
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