u/_underthecity_

You know the feeling. It’s 10pm at the cottage. 87° outside and somehow hotter inside. The ceiling fans are just moving hot air around.

The kids can’t sleep. The adults are cranky. The vacation that was supposed to be special is just… sticky and miserable.

We had a cottage on Crooked Lake in mid-Michigan for years. I loved every second of it. Except those nights.

Fast forward to this summer. I’m living in Detroit, tracking every single kWh of my bedroom window AC unit through the worst heat wave of the year with real data.

And somewhere between watching the numbers and thinking about those Crooked Lake nights — I think I figured something out.

The concept:
Most Michigan cottages have a forced air furnace for the shoulder seasons.

No central AC — because historically the lake kept things cool enough.

Climate change has made that a lie.
But that furnace has a blower. And blowers move air through ductwork.

Put a 12,000 BTU inverter window unit in the master bedroom — specifically the room with the cold air return. That’s the large floor or wall vent that pulls air back to the furnace, as opposed to the smaller vents that blow air out. Seal the door. Set it to 70°. Switch the furnace to fan-only.

The blower pulls the cooled air through the return and pushes it through every supply vent in the cottage. Add the ceiling fans you already have and the perceived temp drops another few degrees on top.

The honest promise:
Not turning your cottage into a Marriott. But if it drops the living room from 85° to 78° on a hot July night — that’s everyone sleeping. That’s the kids going down at 9pm.

That’s you and your partner actually enjoying the porch with a drink instead of lying on top of the sheets staring at the ceiling.

A few degrees with moving dry air feels like a completely different cottage.

Under $500. No contractor. No installation. Just a window unit and the furnace you already have.

Anyone Up North right now want to try this tonight and report back?

For everyone sweating through another July at the lake — let’s figure this out together.

Concept: Place a 12,000 BTU inverter window unit in a room with a cold air return. Seal the door. Set to 70°. Switch furnace to fan-only.

The theory: The furnace blower pulls treated air through the cold air return and distributes it via existing ductwork to the rest of the house.

The sealed room forces the return to pull predominantly cooled air rather than mixing with warm air from the rest of the house. Continuous inverter modulation maintains a steady feed temperature rather than cycling on and off like a traditional unit. Ceiling fans in other rooms handle the last mile.

Target application: Cottages and older homes under 1,500 sq ft with forced air furnaces but no central AC. A 12k BTU inverter unit approaches 1-ton equivalent when feeding a return directly. For that square footage that’s meaningful.

I’ve searched Reddit, HVAC forums back to 2009, and X. I found discussions about piping a single unit into a return plenum — consensus was negative, mostly due to BTU mismatch and duct losses.

Nobody has documented this with multiple inverter units running simultaneously or with intentional cold air return placement as the primary feed point.

What I’m looking for: Anyone who has a forced air furnace and window units already running in multiple rooms.

Flip the furnace to fan-only and measure common area temps before and after 60 minutes. Any window AC units qualify — doesn’t have to be inverter.

Happy to be told this doesn’t work. Need actual data either way. Thank you.

I’ve searched Reddit threads back to 2009, HVAC forums, and X. Found discussions about single units piped into plenums — consensus was negative due to BTU mismatch. Nobody has documented multiple inverter units with intentional cold air return placement as the primary feed point.