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Posted by HeatPro on November 06, 2009 at 12:42:53:
In Reply to: oil boiler sizing posted by johnfnwayne on November 06, 2009 at 09:52:00:
One of the handicaps of heating with air.
The coils must be fed with the water temperature specified for the coil to provide its output, somewhere around 180F.
For instance, a Spacepak AC-WPAK-90
duct coil can provide about 90,000 btuh at 180F
when 6 GPM is pumped through it.
http://www.spacepak.com/modules/lit_lib/download.asp?litFileID=1358
If you pump less water or have lower water temperatures, you don't get that much heat.
That's alright when you run the boiler on an outdoor reset to match the heat needed from the coil as the weather changes. However, it isn't good when you don't have that heat supplied when you need it, so you plan for the worst case weather.
There is no rule of thumb unless you make the rule to supply all the heat and gpm necessary when you need it by selecting the right coil and the right pump to supply the necessary water at the right temperature.
You don't need two boilers if one can supply all 190,000 btuh for all three coils. With that much heat, you don't need to add more beyond 190,000 btuh to heat an indirect, as you aren't heating 40 gallons of hot water every hour every day, just two hours total time broken up in minutes out of the day.
http://www.heatpro.us/indirectree/