The blower on my parents Trane XE80 gas furnace stopped working this weekend and a local HVAC company diagnosed the problem as a bad control board. The technician didn't have a board on his truck so he temporarily bypassed the control board and wired the fan to stay on 100% of the time until he returns with a new board this week. I asked the tech about how much the board would cost and he estimated it at between $400 and $700 dollars not including labor.

In meantime I got on line and found I could order the board for $169. I am fairly confident I could replace the board myself. My only concern is not knowing what the tech did to bypass the bad board. Well after examining the control board wiring I am pretty sure I figured out what he did -- looks like he moved the blower's wire from the HEAT-C terminal to the LINE-H terminal -- but in the process I think I may have also found a reason for the board to fail on the first place. I found that the EAC-H terminal (electronic air cleaner) has about a 5 Amp load on it when the schematic clearly puts the limit of the EAC-H terminal at 1 Amp. The 5 Amp load is the combination of a humidifier (.85 A) and a booster fan (4.2 A). Note this HVAC company is the same one that installed the furnace 8 yrs ago so they undoubtedly were the ones who wired the humidifier and booster fan this way. So my questions are (sorry for the long lead in):

(1) Could the apparent overload of the EAC-H terminal have caused the board to fail (even though it worked fine for 8 years)? (If so perhaps the HVAC company should replace the board for free.)

(2) How can I connect the booster fan to the new control board without overloading the board? I've thought about connecting it to the HEAT-H terminal along with the main blower so that they both come on at the same time -- but this would only work for heat. My parents also use the booster fan occasionally with the A/C.

Thanks in advance for some expert opinions/advice!

First go and replace the board if you are capable.

Next add a relay to take care of the supposed board overload.

Since you have a Amprobe to check the unit I guess you should be handy with electric.

Since this will be non stock wiring and I am not there to look it over you just need to follow the wiring instructions/diagram that is on the furnace blower door and wire it up.

Good luck.

The board you buy will probably not carry a warranty since you are not in the HVAC trade but if you are careful you should have no problem.

As far as getting someone to pay for something done 8 years ago good luck with that. Who knows who changed any wiring over a eight year period.

Thanks for the quick ressponse.

Could you be more specific when you say add a relay? Do you mean add a new heavier duty relay to the new board? Or do you mean add a separate relay elsewhere? I'm wondering if a separate pressure switch in the duct for the booster fan would be the way to go?

BTW, I did not use an amprobe to measure the actual load, I just read the specs on the humidifer and booster fan. So the actual load may not be 5A but I've got to believe it's a lot more than 1 amp. The fan is huge... I believe it's an old furnace fan.

Also, any idea why whoever wired the humidifier didn't use the HUM-H terminal instead of the EAC-H terminal?

Thanks.

Or do you mean add a separate relay elsewhere

Yes just add a separate relay to take the load off the board. Use the board terminals to trigger it.A small HVAC relay probably will not take 1/2 amp for the coil.

Or do you mean add a separate relay elsewhere

Yes just add a separate relay to take the load off the board. Use the board terminals to trigger it.A small HVAC relay probably will not take 1/2 amp for the coil.

OK - I understand now. Thanks for you help.