Nil wrote:
On 12 Feb 2015, benny
wrote in
alt.home.repair:

So I didn't call a service contactor. i did go to home depot and
buy a new thermostat, i open it, follow the instruction inside
and replace it by my self..
turn on the power back with a new thermostat and its working. the
furnace stop when it reach the set temperature...
but i notice again the blower fan still running all the time.it
only stop if i switch OFF the power at the furnace. should be
auto off when it reach the set temperature.. ANY HELPFULL
ADVISE will be apreciated

I went through a similar exercise a few weeks ago. The furnace would
heat and cool normally, but the blower ran constantly. I determined
that the problem was not the thermostat but was unable to troubleshoot
it any further, so I called a repairman. It turned out that the main
computer board in the furnace had gone bad and had to be replaced. Many
furnaces like mine are so computerized that there aren't very many
small replaceable parts inside - much of the system is integrated into
that board.

Hmm,
If relay is stuck, it can be replaced with some soldering skill. If
logic controlling the relay is bad little harder to deal with it,
but still repairable. When run into ASIC, it is almost impossible to
get a replacement chip. They charge few hundred $$ for a replacement
board. It is not really a computer but contains micro processor chip.