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boiler sizing

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Chip_8
Chip_8 Member Posts: 15
I am doing a new install in a 10,000 sqft house. 4 100, 000 btu air handlers with hot water coils . 3 car garage with radiant, and 6 zones of radiant in basement plus a couple tile areas on first floor. The boiler my wholesaler sized this for is a 300, 000 mod con. I feel that this too small. He said loss on entire house is 275 ish. I know we size for loss, but the total load here is roughly 580, 000. Is this boiler going to fall on its face at 100% load?

Thanks, Chip

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  • Zman
    Zman Member Posts: 7,569
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    Two

    I would absolutely do more than one boiler. You will get excellent turn down ratio and redundancy. I single boiler will short cycle like crazy.

    You should be able to size the boiler(s) to the heat loss. The issue you are going to run into is that your rad slabs and your air handlers can suck energy faster than you can make it. When the big slabs come on line they will drag down the system temps and cause the handlers to blow cold air. Check out this http://www.radiantandhydronics.com/Articles/Glitch_and_Fix/BNP_GUID_9-5-2006_A_10000000000001216751

    Even if all 4 handlers come on at the same time the temps will drop.



    I would consider twin 150k to 175k or even 3 110k.  I would use a control strategy  that prevents  the slabs from hogging all the BTU's at startup. The air handlers may need to be rethought. Why 100K? They should be sized for the load as well. Ideally the coils should be sized for the low temps the mod/con loves.

    Carl
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • CMadatMe
    CMadatMe Member Posts: 3,086
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    Multiple Boilers

    Agree with Carl that the ideal install is two boilers but budget may not allow it. You need to do a little homework on your own and pull the rating charts for your hydro's, ie, pressure drops and capable output at different water temps at give CFM's. As long as the boiler can deliver the btu/hr to over come the loss the boiler is fine. Your job is to make sure the capable output of the emitters will do it. Who is sizing the zone or system pump? That may be more important than the boiler. You have enough muscle, can you get it their effectively.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • Chip_8
    Chip_8 Member Posts: 15
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    I want 2 boilers for redundancy

    I will be sizing pumps after air handlers are in place and gpm/head requirements have been figured. General is digging basement this week. I am looking a 2 TT solo 175, 000 boilers. Customer wants Buderus as an option. I guess they have done research online and found Buderus to be a better boiler. No stocking wholesalers here for the blues, so I hope they consider that. Seperate hvac company is installing air handlers, so I don't know why 100k.
  • CMadatMe
    CMadatMe Member Posts: 3,086
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    Because

    The air handlers are sized for cooling. That's the biggest problem with hydro. Big mistake in my opinion having to go with an alum HX vs a stainless steel. I'd go with the Triangle. Size the pumps for the gpm you need to overcome the loss and the pressure drop across the coil. Who's A/h?
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • Chip_8
    Chip_8 Member Posts: 15
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    carrier

    almost positive they are carrier air handlers.... I have done other houses like this with same hvac contractor, just not this big. Chip
  • Ironman
    Ironman Member Posts: 7,376
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    AHU's

    Last time that I checked, Carrier uses First Co. Regardless, get the performance charts for the AHU's and adjust the fan speed down to lower the output of the AHU. First Co. gives performance ratings at 120*, 140* and 180* with related cfm. The numbers you've been given are with max water temp and max cfm. You can trim it back considerably using lower water temp and less cfm. which would make the boiler(s) operate more efficiently.



    If the customer goes with Buderus, you'll also need their MCM10 module to stage two GB142 boilers. And their turn down is only about 3.3 to 1 on each boiler (6.6 to 1 total).
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • Chip_8
    Chip_8 Member Posts: 15
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    TT solo 175

    What is the turndown ratio on the Triangle?
  • Zman
    Zman Member Posts: 7,569
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    4 to 1

    Beware of boilers advertising super low turndown. Many do it at the expense of efficiency.

    Carl
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
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