Community Engineering Services, PLLC A product liability claimant retained Dan Robles to investigate the root cause of an air handler failure.  The failure actuated the smoke alarm system and the affected rooms sustained some smoke related damage.  No fire was recorded but extensive damage was evident in the air handler unit indicating a clear danger to occupants was present. Initially, warm air was reported warm air entering the occupied area, but no fan pressure was evident.  The circuit breakers tripped and were reset by the occupant.  Instead of the circuit breakers actuating a second time, the space filled with smoke. Upon interviewing occupant and after removal and inspection of the unit, the following conditions were found: It is unacceptable that such a meltdown should occur allowing the system to become damaged to the extent shown in the following analysis.  The exterior of the unit could have easily ignited any combustible material in the vicinity. 

However, The system did not in fact support flame despite possible repeated resetting of the circuit breakers. It is more likely than not that the sequence of events leading to this condition occurred as a result of a fan mounting failure.  Upon inspection, the squirrel fan was lodged in the housing rendering he motor stalled in place.  One of the three motor mounts was broken in what appears to be a classic fatigue failure in the spring steel bracket.  Since there were only three mounts (instead of a more secure 4 mounts) the fan easily fell off center.  Further, where one would expect to see vibration isolating materials such as rubber pad or felt washers, there was a clear metal to metal transmission of vibration to the air handler housing.  Any number of harmonic conditions could have lead to the pre-mature failure of the motor mount.  As such, it is more likely than not the initial cause for the eventual destruction of the unit. Broken Motor Mounts: One of three motor mounts broken at 45 degrees relative to the mounting surfaces. 

Motor canted inward to housing.  Squirrel cage lodged in housing. Note absence of vibration isolating in left hand photograph. A failed motor mount caused the squirrel fan to fail.  No noise of a rubbing fan was evident at anytime.   At the orientation of the installation, the fan blade fixed to the siding such that no motion was possible. Spring steel bracket likely suffered a premature fatigue failure due to manufacturing or heat treat process failure.  No damping devices were installed between the bracket and the housing.  At 1200 RPM, for example, may experience up to 1.7 million reciprocating cycles per day of operation if not isolated. Three brackets instead of four created a condition of more catastrophic failure.  Four brackets would have provided aural notification of imminent system failure by likely causing the fan to chattering loudly. Wiring Diagram (burned on edges) for heater insert show relationship of high temperature limit switches, fan relay, heating elements, circuit breakers, and thermostat.

Wiring Diagram indicated that three hi-temperature limit switches should turn the unit off in the event of an over-heat condition.  how to replace a fuse on ac unitThe insulation on wires coming off the hi-temp sensors were burned through to exposure. air handling unit noiseSame black wires were fused together and possibly crossed current between them producing a possible electric bridge over-riding the switch.carrier ac unit filter Extensive burned and fused wires.  Leads to hi temp switched are fused.  Note extensive burning on relay switch possibly not allowing thermostat to turn system OFF – contributing to constant ON failure mode.  Extensive burned and fused wires and scorched bi-metal overheat sensors.  Continuity across limit switch suggests run away failure in the ON position.

The damage to the unit was too extensive to test for sequence of cascading failures leading to the severe overheat condition.  Interviews with the operator suggest that the circuit breakers were reset at least once, possibly more times suggesting poor judgement.  The presence of fused wires that may have bypassed safety circuitry suggests an over reliance on breaker circuits by the manufacturer. However, the most obvious failure is the broken fan motor bracket.  Like most lower value appliances that are too bulky to transport over long distances, components are often shipped to a regional manufacturer who assembles packaged systems from a variety of components. The assembly of the fan and housing may have occurred at any number of sources under a variety of quality control conditions.  The combination of an unbalance fan wheel, and fail prone 3-spoke bracket,  and the omission of vibration  damping devices may all have contributed to the failure of the fan assembly.

The unit was approximatly 15 years old and the bracket may have experienced over a billion cycles before failure.  Still, the possibility that a fire danger would be present is not acceptable if further design and quality improvements are made.FAQSeptember 10, 2014I have 4 wires at the thermostat and there is no C wire, how do I install my Smart Si?If you do not have a C wire and are installing a Smart Si, please follow the steps listed here: Please review the steps below and if you are not comfortable performing electric wiring, please request for professional help You require an additional 18 gauge thermostat wire* The assumption is that you have 4 wires available at the thermostat – R, G, Y and W. If this is not true, please contact our support for further assistance Switch off the HVAC equipment from the circuit breaker Go to your utility room to access the furnace control board within the furnace equipment chassis (Note the control board could be in the air handler unit)

On the furnace control board, locate the wiring terminals – R, G, Y, W, C Identify the wires from the thermostat that are connected to the terminals R, G, Y, W, C at the furnace control board Move the wire at the G terminal to the C terminal Connect the additional 18 gauge thermostat wire from Y terminal to G terminal Close the furnace chassis Back at the Smart Si thermostat, connect R, Y, W and the previous G wire to C terminal Connect the thermostat and switch on the power from the circuit breaker (*Your Smart Si box may contain an additional wire) Please refer to the Smart Si install guide wiring diagram “Single Stage Heat/Cool with 4 wires at the thermostat” for further information On the same topicGeneralNeed More AnswersSearch our knowledgebaseOther topicsAmazon EchoBefore you BuyCompatibilityFeaturesGeneralHomeKitIFTTTInstallation, Setup & RegistrationRemote Sensor FeaturesSecurityTips and AdviceTroubleshooting your ecobeeUsing your ecobee Thermostat, Web portal & Mobile AppWhat's new