38 radiators, average 1.8 gallons each = 68 gallons
0.652 gallons per foot of 4" pipe. 120 feet = 78.2 gallons
0.37 gallons per foot of 3" pipe. 120 feet = 44.4 gallons
0.135 gallons per foot of 1" pipe 400 feet = 54 gallons
0.0229 gallons per foot 3/4" pipe 500 feet = 11.5 gallons
Boiler Content, 2 gallons = 2 gallons

Comes to 258 gallons of water in the visible piping, 8 pairs of risers and the boiler itself. It does not count the expansion tank, piping in the crawl-spaces, or the piping around the boiler and indirect water heater. Just the original iron pipe. It is a 3-zone, 2-pipe system with 3 circulators. So, I think I am pretty safe using 300 gallons as a round number. Our boiler is a WM Ultra 230,000 BTU mod-con that replaced the 400,000 BTU steel oil-burner that was in place when we bought the house in 2008. The math was easy..

Average 6 hours of operation per day for 120 days as the base-line. And based on use-figures supplied by the realtor.
Oil at $2.50 gallon (130,000 BTU per gallon).
Natural gas at $0.77 per therm (100,000 BTU).
Oil Boiler at 55% efficiency (typical of a 1960s steel boiler measuring 6 feet cubic - really).
Gas boiler at 95% efficiency (published specifications)

(6 x 400,000)/130,000 = gallons per day. 18.4 gallons per day.
120 x 18.4 = 2,215 gallons per season - this is a backwards calculation from the supplied use figures.
288,000,000 BTUs expended
158,400,000 BTUs delivered
$5,537.50 expended on fuel.

For gas (NOTE: Work with the DELIVERED BTUs, not the hours of operation!):
158,400,000/0.95 = 166,736,842 BTUs to be expended.
Comes to 1,668 therms
Comes to $1,283.80 expended on fuel.

Comes to a seasonal savings of $4,254

As I did 60% of the work myself, using a registered master plumber and contractor to do the demolition and final inspections, the entire installation paid for itself in three (3) seasons. That includes the cost of replacing 17 radiators and three pairs of risers - the house sat empty for two years (REO) and the system was left full.

I did enjoy the reference to the Simpsons - so the joke was not lost on me.

"The coldest winter I ever saw was the summer I spent in San Francisco."
- Mark Twain

Peter Wieck
Melrose Park, PA