That's a LOT of wood!
The firebox on the 5000 holds just over 20 cu.ft. of wood.
If you are filling it 3x daily with only 15 cu.ft., that comes out to roughly a cord every 3 days.
Even if you only put 10 cu.ft. in 3x daily, that would still be a cord in about 4 1/2 days.
That would make it cost prohibitive to heat with wood if you had to buy it...even if you could buy it for $100/cord.
I'd listen to good 'ol Slim on this one and check to see how much heat loss you have from your underground lines.
I have no way of knowing, but I'll bet your heat load isn't that far off from mine(1800 sq.ft. not well insulated-heats like a corn crib).
My boiler holds just over 11 cu.ft., and if I stuff it full of Ash I can easily go 24 hours in the very coldest, windy weather that our mitten state can throw at us. I usually fill it roughly 1/3 - 1/2 full at 6 am and 6 pm.
I use the very best 5 wrap on the market (from Zsupply in Muskegon, Mi), so heat loss is minimal in my 95' run.