Summary of product warranty activity (Details) (USD $)	Dec. 31, 2013	Dec. 31, 2012
Summary of product warranty activity
Beginning balance - January 1	$ 2,671,409	$ 1,210,909
Additions for current year deliveries	970,775	996,439
Reductions for payments made	(2,034,053)	(2,809,263)
Reserve Adjustment		3,273,324
Ending balance - December 31	$ 1,608,131	$ 2,671,409