We have a wood furnace that blows heat to gas furnace. When fan/limit switches reaches temp blower kicks on blowing heat in.Worked fine last winter now blower will not kick in when limit is reached.Any idea why?

The limit switch could be bad, but more likely it is the relay. If you have power to the coil and the line contact, but not the load contact, the relay is bad. Unfortunately the newer furnaces have the relay on a board making it hard to do much except cough up big bucks for a new board and hope it fixes the problem. Check the limit switch before buying a new board. It is at the heat exchanger and should at least have leads running to the board. When it heats up, the contacts should close allowing you to check it with an ohm meter or test light.

Thanks Checked limit switch and found loose wire. Fan kicked in. Thanks for answering so fast.