Could use some help figuring something out. Boiler

Discussion in 'Boiler Forum' started by JustAHomeOwner, Jun 14, 2014.

  1. JustAHomeOwner

    JustAHomeOwner Member

    Messages:
    41
    Location:
    NY
    G'day All,

    My daughter and hubby have a 2 story home with one zone of hot water heat on each floor. There's no heat in the full basement. DHW is supplied with a indirect as a 3rd zone from a oil fired boiler. Using data from 12/9/13 to 4/21/14 they burned 544 gallons of fuel oil. HDD's for that period based on 65 degrees total 3881. Therm per HDD is .196, 694 BTU's per HD hour, w/ a 85 percent boiler, 99 percent temp of 15 degrees, output was 34,700 BTU's per average hour. If I multiply that by 150 percent it's roughly 52K. The part I'm unsure of is the indirect ( I'm waiting for the size from them but it's not larger then 35 gallons ) recommends a boiler size of no less then 99K. Are the specs for indirect water heaters that far off because they were warm last winter and had plenty of hot water??
  2. Tom Sawyer

    Tom Sawyer In the Trades

    Messages:
    3,048
    Location:
    Maine
    The indirects requirements are generally not added to the btu/hr requirement to heat the home. IOW, if the heat loss of the house is say 50k, you need. 50k boiler. The indirect can be set to override the heat call so what happens is that if the indirect is calling, the control shuts down the heat call for the house while the indirect recovers.
  3. Dana

    Dana In the trades

    Messages:
    2,783
    Location:
    01609
    Sometimes indirect manufacturers recommend a minimum boiler size, but if your zone controller is set up with the indirect as the "priority zone" and suppresses calls for heat from the other zones whenever the indirect is calling for heat, there's no need to follow that recommendation.

    A typical standalone gas-fired tank delivers only about half the heat rate of the smallest oil-fired boilers out there, so the recovery rate of a 40 gallon indirect on an oil boiler will always be faster than that of a standard 40 gallon gas fired HW heater, as long as it is set up as a priority zone.

    The indirect can be sized for the largest tub you need to fill- only if the indirect is undersized would you need to up-size the boiler to meet the domestic hot water loads. A 99K boiler can handle a 24/365 standard shower load with margin for other purposes, but that isn't a typical hot water use profile for a single family home.
  4. BadgerBoilerMN

    BadgerBoilerMN Master Hot Water Mpls,MN

    Messages:
    303
    Location:
    Minneapolis
    There is much confusion in the sizing of indirect-fired water heaters. The manufacturers do not regularly "recommend" any particular size boiler for a given indirect tank rather; specify a minimum output of a boiler to meet the specified output of domestic hot water to a given tank size. The "first hour recovery" in particular is determined by the volume and orientation of the indirect and the square feet of heat exchanger in the tank.

    As Tom says the boiler output for the house heat load will suffice in all but the rarest cases.
  5. Dana

    Dana In the trades

    Messages:
    2,783
    Location:
    01609
    You're right "recommend" isn't the right verb here. The boiler sizing numbers are really the BTU/hr at which it meets the first-hour specifications for output, they just pick a "typical" (or probably "random") boiler size from which to derive the hot water performance numbers.
  6. JustAHomeOwner

    JustAHomeOwner Member

    Messages:
    41
    Location:
    NY
    Gentlemen. Thank you all for taking the time to go over this. Methinks I got it.
  7. BadgerBoilerMN

    BadgerBoilerMN Master Hot Water Mpls,MN

    Messages:
    303
    Location:
    Minneapolis
    Actually, we tested our boiler/water heater models and calculated the "first-hour" rating from empirical data. But the math is not that difficult so the factor "maximums" aka "1st hour ratings" are usually quite accurate. The point is, the maximum is rarely relevant in residential applications.
  8. JustAHomeOwner

    JustAHomeOwner Member

    Messages:
    41
    Location:
    NY
    That's good to know for me going forward. I see what you mean with first hour ratings. It sort of reminds me of how my hot water heater actually works. Granted it's just two old folks, me, the wife and one stray son living here but we like lots of hot water. I installed a 75g. 75,000 BTU NG hot water heater. During the summer when the NG boiler is off, cloths drying with yet another son showing up for that, cooking (not too much when it's hot outside) and that heater that I thought would be too expensive to run totals no more then 14 therms per month. My take away now is, the cost of getting hot for that first hour has little to do with the cost to staying hot. It's a hard to learn lesson for a home owner like myself but it is good to know that people here like yourself willingly share your knowledge and I do thank you all for that.
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