(some of you will recognize me from a recent post, but now that we're starting over, let's start from the beginning.) I'm planning to install a new boiler, and I'm getting conflicting information on the "right" size. I believe the old boiler is too big. Here's what I know:
Old boiler: Bryant 185 MBTUH input, 65% eff., 50yrs old.
Zone1, 60 of bb (x600BTUH max radiated capacity = 36000), 120 of pipe excluding bb, serving 850sqft to be heated
Zone2, 60 bb (36000 max radiated), 90 pipe, 600sqft
Zone3, 70 bb (42000 max radiated), 50 pipe, 400sqft
Zone4, 30 bb (18000 max radiated), 90 pipe, 330sqft
Zone5, 30 bb (18000 max radiated), 50 pipe, 220sqft
House Totals: 250bb (150000 max radiated), 400 pipe, 2400sqft, approx 23 gal in pipe & radiators plus 6 gal in boiler.
2 heat-loss models calculate loss to be about 60MBTUH. Rule-of-Thumb says about 96MBTUH.
Some contractors say, "size the boiler to match the installed radiation, other wise the bb at the end of the zone might be cold. Also, it's nice to know the capacity is there if you need it on the really cold days."
Dept of Energy says, "slightly undersizing a boiler ensures that it won't short cycle, and will always reach peak efficiency, it won't condense, it will last longer."
I live in SE PA. My gas bill during heating season averages $350/month. What size boiler do I need?
Anxiously waiting your replies.