I arrived at the customer's house and checked the power supply to the unit. The power supply was 120 V, which was within the acceptable range. I then checked the power supply coming out of the transformer and found that it was 24 V, which was also within the acceptable range. I checked the Preston door switch and found that it was working properly. I then ran the unit and the heat, and the unit was working properly. I watched the unit run for 20 minutes to ensure that it was operating correctly. I also checked the unit's performance and found that it was operating as expected.
