What do you think the odds are of a new roof top unit going through 3 control boards in just 3 months since the unit was installed? And, if this truly was the case, don’t you think some one would try to find out why this was happening? Well, it was happening and no one tried to find the reason and as a result, the unit now needed a 4th board. Time to get to the bottom of this because, as you and I know, boards don’t just fail ( I know I preach that a lot). There has to be a reason, a cause, something unusual at the site.
Well, when this contractor ordered the 4th board, I told him I needed to meet him at the site so we could see what was causing this astronomic amount of board failures — now up to 4 boards in 4 months! He agreed, stating to me that: “this unit has had a plethora of problems”. Beside the boards, he also has replaced the contactor for the blower motor.
Any speculation up to this point? The board failures were all “fan related”. The on-board pilot relay for the main blower contactor were all “failing” and then there was also the blower contactor itself. Any guesses? Power problem? Low voltage? Line Voltage? Actual “run of bad boards” from the factory :>) ?
Well, I meet him on the roof and I start looking things over. I check the existing board in the unit, and, the pilot relay for the blower will not pull in and send power out to the contactor. We replace the board! In taking out the old board, the first thing I notice is ALL the bare thermostat wires that were attached to the board are “green” and chalky. Well, when copper conductors turn green, there is a “corrosion issue”. This unit was a replacement for an older unit and they never changed the thermostat or wire — normal practice. But they never “cut back” the wires to get good clean copper connections, so, right from the start, they had voltage drop problems through the thermostat wire connections to the control board. So we clean all of these up, cut off the “green” and strip the insulation back to get good connections.
Do we stop here because we found a potential problem that caused all the board failures? I don’t think so. We need to make sure there are no other issues causing problems.
Now I get out my analog meter (yes analog) so I can check the incoming power to the unit. I use an analog meter because I’m checking for voltage imbalance at the unit. (If you read my post on “voltage imbalance” and the comment from Bruce Porter at the end, You’ll understand why I use the Analog meter)(You can use a digital meter as long as it is a TRUE RMS meter). With the meter we read L1 – L2 =242 VAC, L1 – L3 = 241 VAC (looking good so far), L2 – L3 = 229 VAC RED FLAG!!!!! We have a balanced voltage problem. But this usually would only affect a 3 phase component like the motor or the compressor. It shouldn’t affect the board?
In the unit is a transformer. The transformer has multi-taps for 240 VAC and for 208 VAC so we get proper secondary voltage out of it. The trans former was set on the 240 tap because the installer saw the 242 VAC and left it in the factory position. When we checked the line side of the transformer, we only had 231 VAC. WE had 24. 1 VAC secondary. This powered the thermostat, board, contactors and the field installed economizer on the job. Under load, voltage would drop to 22.3 volts. Given the right set of circumstances, this could cause the contactor and pilot relay on the board to “chatter” which would eventually burn out the board. WE moved the one primary leg of the transformer to one of the other power legs and got it off the 229 VAC leg. We now had 241 VAC on the primary side of the transformer and, even under load, the secondary voltage to the contactor never dropped below 26.4 VAC.
After cycling things, firing the heat, running the fan, and even turning on the A/C with its contactor, the blower contactor and economizer, our voltage never dropped below 25.7 VAC. So between the line side power issue to the transformer and the corroded thermostat connections, and the fact that we see steady voltage under load, I felt confident that we should not have any other board failures under normal operating conditions.
Now, I did warn the contractor that they needed to get the voltage corrected or they would start losing the blower motor and/or compressors. Since there was 3.5% voltage imbalance, that could add 20- 25% increase in the motor winding temperatures and now we’ll be losing motors and compressors down the road.
As always, the point of blogs like this is to help educate all of us. When you see something “strange” happening (like 3 boards in 3 months), you need to find the cause. If you are at a loss, don’t be afraid to reach out to others, to the manufacturer’s tech support reps, to other dealers or workers. None of us know everything but if we use our tools and the knowledge that we do have, sooner or later, we find the problem and and correct it. Sometimes we just have to be tenacious in our pursuit of the answer and not take the “easy way out” like keep replacing boards.