An office building in Pontiac spent six hours blowing hot air into a 78-degree afternoon in late April. The heat pump was still locked in heating mode from the night before, and the reversing valve wouldn’t shift when the thermostat called for cooling. Tenants noticed before anyone in the mechanical room did.

Switching a building between heating and cooling mode is a mechanical event, not a thermostat setting. Equipment that ran in one mode all winter gets tested in a way it hasn’t been tested in months the moment the season flips. A reversing valve that sat idle since October doesn’t always shift cleanly the first time it’s asked to.

Why the Changeover Is Riskier Than It Looks

Michigan shoulder seasons swing hard within a single day. A cold morning can call for heat. By afternoon, the same day calls for cooling. That back-and-forth puts more mechanical stress on components than a steady summer or a steady winter ever does.

A stuck reversing valve doesn’t announce itself in January or in July. It shows up exactly when the season turns, which is the worst possible moment to discover a problem that’s been sitting there since the last changeover.

What Actually Fails During Changeover

Reversing valves on heat pump systems are a common failure point. They sit unused in one position for months, and the first shift after a long idle stretch is when they’re most likely to stick. Economizer dampers cause even more trouble. According to a New Buildings Institute analysis of more than 500 rooftop units, economizer faults were the single most common problem area found, with roughly two out of three units showing a malfunctioning economizer damper. Months of disuse let these dampers seize. A damper that won’t open wastes free cooling. One that won’t close afterward lets unconditioned outdoor air into the building at the wrong time.

Thermostat and control programming is the other common issue. Systems built for a simple summer or winter schedule often can’t handle the shoulder season’s day-to-night swing. One built for a warm afternoon calls for cooling correctly. That same system can fail to hand back to heating mode when the temperature drops that same night.

What a Real Changeover Inspection Actually Checks

A changeover inspection tests the system in the mode it’s about to enter, running each component rather than glancing at it. Running the reversing valve through a full cycle is the first step, confirming it shifts cleanly in both directions. The economizer damper gets the same treatment: cycled through its full range, with the actuator checked at both ends. A damper can sit in one position looking perfectly normal and still fail to move the moment it’s commanded. Refrigerant charge and electrical connections get checked too, since anything marginal that never affected heating can absolutely affect cooling once the system switches over. Control programming gets verified against the weather the building is actually walking into, matched to the current forecast rather than a schedule someone set up last year.

The Simple Fix Most Buildings Skip

Schedule the changeover inspection before the season actually forces the mode switch. A stuck reversing valve caught during a scheduled spring startup is a maintenance item, usually a $300 to $400 repair. The same valve discovered mid-heat-wave is an emergency call. The Pontiac building’s invoice came to $1,100, roughly three times the cost, for the identical part.

Michigan’s shoulder seasons don’t follow the calendar. The right time to schedule this inspection is a few weeks before the historical average shift, not a fixed date picked at random. That’s the gap between a $300 to $400 repair on a Tuesday morning and six hours of a hot building blowing heat at tenants who are already calling to complain.

Sources

New Buildings Institute: RTU Savings Primer (Economizer Fault Rate Data)