Thermostats & Airflow

Duct leakage. California's energy standards treat this as a measured quantity for a reason: leakage in existing systems is commonly held to a 15% threshold when a changeout is permitted, and plenty of untouched systems are far worse than that. Air lost into an attic is air you paid to cool being delivered to the roof.

Crushed or disconnected runs. Flexible duct dropped across a joist, kinked around a corner, or pulled off its boot by somebody storing boxes. This is the classic "one room is always hot", and it is visible in a photograph if you can reach the attic safely.

Return air. Supply gets all the attention, but a system that cannot get air back is a system that cannot deliver it. Undersized returns and closed interior doors both show up as rooms that never come to temperature.

Registers and balancing. Dampers that were set once during construction and never touched, or registers closed by whoever had the room last. Free to fix and worth checking before anyone quotes anything.

Modern thermostats need a common wire for continuous power. A house wired for a mercury-bulb thermostat often does not have one, which is why a new smart thermostat can behave strangely, drop offline or short-cycle the equipment: it is stealing power through the control circuit. The fixes are a C-wire, an adapter at the air handler, or a model that does not need one — and which of the three applies is visible from a photo of the wires at the wall plate.

A blank display is usually not the thermostat at all: it is the float switch under the indoor unit, the door switch on the furnace, or the breaker.

A filter too restrictive for the system chokes it as effectively as a dirty one. Very high-MERV filters in a system never designed for them cause exactly the symptoms people install them to avoid. Match the filter to what the equipment can move, and change it more often instead.

Duct repair spans everything from re-attaching a boot to replacing a system: Bay Area published ranges for duct replacement run $3,000–$8,000, while a single register run is quoted from around $720 at one San Jose shop. Thermostat installation runs $199–$299 locally. Testing is its own item — when a changeout is permitted, HERS duct verification is performed by an independent rater, not by the installing contractor.

The thermostat and, if you can remove the faceplate safely, its wires. The register in the problem room. The attic run if it is reachable from a walkway — do not crawl across a ceiling to take a photograph. The filter, with its size legible. And a note on which rooms are wrong and at what time of day.