The 3720 is Le Petit Godin, the 3721 is Le Grand Godin.
The 3720 is a 5kw (about 17,000 btu/hr) stove. With wood you can probably get 2 or 3 hour burns, with coal it is capable of a 12 hour burn. Good coal is about 12,500 btu/lb so to get the 17,000 btu/hr would need about 1.5 lbs of coal per hour.
It will never heat 5,000 sq ft, maybe 800 - 1,000. The Godin manual says it will heat 70 to 150 cubic meters. 70 cubic meters is 2400 cubic feet. With 8 foot ceilings that is about 300 sq ft. 150 cubic meters comes out to about 700 sq ft.
The manual, in french, can be found here:
http://www.godin.fr/notices-siteweb/3720A_petit-godin_4238B-05.pdf
The paint on the stove in the picture is not right, probably repainted. It is missing it's ash tray. Unless the insides are in mint condition and all the parts are there it is not worth $300.
Here is a picture of my 3721. It is a 10kw stove (about 35,000 btus/hr). I easily get 12-15 hour burns on coal and 4 to 6 hours on wood. It has been installed since the early 70's and is still used when the temperature is forecast to top out in the 30s or less for more than a couple of days in a row.
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