One room always hotter than the rest of the house? Run a thermometer week
One chronically hot room is almost always an airflow problem: a blocked or blown-shut vent, a dirty filter, or a duct damper somebody set wrong years ago. Put a cheap thermometer in the hot room and two normal rooms, log them for a week, then fix what the numbers point at. Mine was a half-closed damper. Total cost, about $41.
- A flashlight the crawlspace has opinions about light
- A marker to mark the damper positions
The back bedroom runs hot every summer. Not cozy hot. Seven degrees hot. Deb calls it the terrarium, and for years my scientific method was standing in the doorway in July saying "yeah, it's hot in here" and closing the door like that solved something.
This year I did what I should have done in 2015. I grabbed three little AcuRite monitors off the shelf at Home Depot, about thirteen bucks each, stuck one in the back bedroom, one in the hallway, and one in our bedroom, and wrote the numbers down twice a day for a week. Same July that swells every door in this house, for the record.
The spreadsheet does not guess. Hallway, 74. Our room, 75. Back bedroom, 81 at 4 in the afternoon, every single day.
What I checked, in order
- The vent. Fully open, but the dresser sat half in front of it. Slid it over. Worth a degree, maybe.
- The filter. Three months old. Swapped it anyway, because a choked filter starves the room at the end of the duct run first, and the back bedroom is the end of the line.
- The damper. This is the one. Down in the crawlspace, where the ducts branch off the trunk, there are little metal levers most people have never looked at. Mine were set by whoever serviced the furnace years ago. The branch feeding the back bedroom was half closed.
I opened the back bedroom branch all the way, nudged the front bedroom branch down a touch since that room runs cool, and let the thermometer week keep going. Back bedroom, 76 by Thursday. Deb moved her reading chair back in there, which is the closest thing to a certificate of completion this house issues.
I marked both damper positions on the duct with a sharpie, because come winter the balance flips and I will be back down there. Ask me about that in January.
If your hot room is upstairs
Same physics, taller house. Heat rises and duct runs get long, so the upstairs bedroom at the end of the line loses twice. Run the same thermometer week before you touch anything. And if it is a bonus room over a garage, like the one in Casey's townhouse, you are also fighting a hot garage under the floor and skinny insulation. The thermometers will tell you how bad. The fix list starts the same: vents, filter, dampers.
When it is not airflow
If the numbers stay ugly with the vents open, the filter fresh, and the dampers balanced, you are probably looking at afternoon sun through a west window or thin attic insulation over that room. Curtains are cheap and insulation is a fair weekend job. But if a duct has come apart somewhere you cannot reach, or the room needs more supply than the duct can ever deliver, that is a duct crew conversation, and an honest one will give you a number before anyone cuts sheet metal.
For the record, the whole investigation took 130 minutes of actual work spread across a week, most of it in a crawlspace that had opinions about my being there. The jar went hungry the entire time. A first.
The Damage
| Item | Where | Cost |
|---|---|---|
| AcuRite 00613 humidity + temp monitor, 3 at $12.99 | Home Depot | $38.97 |
| Sales tax | $2.34 | |
| Total | $41.31 |
Deb's verdict. "You put three thermometers in my house and I got my chair back. Fine. Good week."