Record a pattern that helps someone investigate
Choose a few ordinary heating periods. Note the bedroom temperature, the thermostat's displayed temperature, the time, and whether the bedroom door was open or closed. Use a safe freestanding room thermometer, away from direct sun and a vent. Note which rooms feel normal and whether the cold rooms are also the hottest rooms in summer.
This record is for comparison, not a laboratory test. It helps answer whether the problem follows a room, a time of day, or a control setting. Do not remove grilles, climb into an attic, or cut openings to investigate.
Keep accessible supply registers and return openings clear. Check the ordinary filter only as the owner manual allows. Avoid closing many other vents to force heat upstairs. Ask for a qualified airflow review rather than making the system work against new restrictions.
Check the rooms before changing the furnace size
We recommend asking whether a room-by-room heating load has been considered. A heating load estimates how much heat each space needs under the design conditions. ACCA distinguishes whole-home and room-by-room load calculations; the latter helps keep a room concern visible. ACCA Manual J resources.
A technician can then compare the room concern with equipment operation, controls, and airflow findings. Ask whether the furnace is producing and moving heat as intended, whether the thermostat location represents the rooms you use, and whether the home's air paths need a separate investigation.
If the bedrooms need envelope work, such as insulation or air sealing, identify the qualified professional responsible. If duct changes are recommended, that is another scope and provider to arrange. MGI's HVAC explanation should make those dependencies clear without offering general duct work.
Decide which change would address the cold room
If equipment operation is faulty, address the verified fault. If the furnace runs correctly but the bedrooms lose heat or receive inadequate air, a larger furnace does not establish a fix for those rooms. Require the recommendation to say what changes in the bedrooms and what remains unresolved.
Your next step is to share the temperature pattern and ask for a room-focused review. Make “usable upstairs bedrooms in winter” the goal of the investigation, rather than starting with a new furnace size.