Is everyone badly oversizing me?

Discussion in 'HVAC Heating & Cooling' started by dobalina, Jun 26, 2014.

  1. dobalina

    dobalina New Member

    Jun 26, 2014
    Hi. We have an old, window-heavy, poorly-insulated 5500 square foot house in Danbury, CT. One hydro-air zone (40% of the house), one radiator zone (50% of the house), and one kick heater zone (10% of the house).

    I've had two (reputable) companies perform a Manual-J on our home and they have come back with proposals for a 350 MBH boiler and a 400 MBH boiler (IBR).

    Our oil company looked at the oil consumption history of our (old, inefficient) setup and concluded that we need a 300MBH boiler (IBR).

    As I am reading this forum I am coming to the conclusion that all three proposals seem to be insanely oversized.

    Does it seem like I am missing something?
  2. Dana

    Dana In the trades

    Jan 14, 2009
    With hydro-air you need to size the boiler at least high enough for the coil, or you'll be delivering tepid air at the registers, and possibly compromising the boiler with too cool condensing temperature return water. What's the model/make or BTU numbers on the air handler? How old is the ancient beast, and what are it's BTU in/out ratings?

    Even 300MBH is a heluva lot of boiler. What does YOUR fuel-use history math look like? (Do you have a "K-factor" stamped on a mid or late-winter oil bill?) Are you going through 2500-3000 gallons of oil per year or something? Did the contractors give you copies of the Manual-Js?

    For 300MBH for a 5500' house is 55BTU per square foot of living space. Even if the place has NO insulation you'd probably have to leave some windows open to hit that for a heat load. A typical so-so insulated house in Danbury at 0-5F (or whatever the 99% outside design temp is in Danbury) would be under half that at about 20 BTU/ft (less if it's pretty tight). A "window heavy" house would still probably come in under 35 BTU/ft.

    If the true heat load is truly over 150 MBH (or even over 100MBH) it's worth doing something about that at the current and projected future price of oil. Antique single pane windows can be tightened up, clear glass storms can be replaced with very tight low-E storms (Harvey Tru-Channel is the tightest in the industry, and they have a low-E hard coat glazing option). The biggest bang/buck is usually in air sealing, followed by insulating whatever can be done without gutting the place, and that includes the foundation.
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  4. BadgerBoilerMN

    BadgerBoilerMN Hydronic Heating Designer

    Mar 4, 2011
    hydronic heating designer/contractor
    I an heating a greenhouse with less. We rarely use IBR but for a fan-coil, as Dana correctly illustrates, the delivered air temperature matters. You could benefit from a two temperature system with the radiator side operating on outdoor reset.

    Finding someone who is willing and able to perform a relevant Manual 'J' can be challenging. The designer needn't be local, as all of the information for Manual 'J' is specific to your location and the input to your building.

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