Responded to no heat service call for a residence with dual heating system (geothermal and propane furnace). Upon arrival, found furnace operating but struggling to maintain temperature in poorly insulated home. Diagnosed system was functioning correctly but efficiency was compromised by severely clogged air filter. Replaced air filter and verified normal operation of both heating systems. Determined geothermal unit was running properly but unable to keep up with heating demand in cold conditions, causing automatic switchover to propane furnace as designed.
