One room the central system can’t keep up with
Happy Valley has the newest houses in the metro, and most of them already have a gas furnace and ducts. The call we get is about one room.
It’s usually the bonus room over the garage, a top-floor office or a daylight basement. Firlands Portland Mini Splits gives that room its own head and thermostat, so it stops dragging the whole system along with it.
Why that room misbehaves
- it’s at the far end of the longest duct run
- it has more outside surface, often over an unheated garage
- the thermostat is downstairs, so the room never gets a vote
Project
What a bonus-room single zone in Happy Valley looks like
The problem
The house has a gas furnace and central air, and every room is comfortable except the bonus room over the garage, which runs 8 to 10 degrees warmer than the rest of the upstairs in summer and cold in January. The family uses it as an office and a playroom.
The house
The bonus room sits at the far end of the longest duct run, over an unheated garage, with three walls and a sloped ceiling exposed. The furnace's thermostat is in the downstairs hall, so the room never gets a say. Happy Valley issues the mechanical permit; Clackamas County issues the electrical.
What we decide, and why
Rebalancing the ducts or adding a booster fan doesn't change the room's own load, so the room gets its own system: a 12,000 BTU head on the gable-end wall, sized from the bonus room's load with its west window and the garage below. We leave the existing supply registers open; the head handles what the ducts can't. The outdoor unit goes on a wall bracket on the garage's side wall, above the side-yard path, because the yard behind slopes away too steeply for a level pad.
The work
One three-inch hole in the gable wall, a 20-foot line set run down the garage side wall in a painted cover, and the bracket bolted into the garage framing with rubber isolators. A 15-amp, 240-volt circuit runs from the garage panel to a disconnect beside the unit. The drain falls outside by gravity alongside the line set. The line set holds a nitrogen pressure test and a vacuum below 500 microns before the refrigerant is released.
Because the room's head runs on its own thermostat, the central system stops overcooling the downstairs while it tries to reach the bonus room, which is where most of the household's complaints came from.
What it changes
The bonus room matches the rest of the house year-round, the furnace and central air run less hard, and both final inspections close the job: the city's mechanical and the county's electrical.
- Location
- Happy Valley, near Scouters Mountain
- House
- 2008 two-story with a 22 by 16 foot bonus room over a three-car garage
- Equipment
- One 12,000 BTU cold-climate wall head, outdoor unit on a wall bracket
- Price
- $7,900, permits included
- Time on site
- 1 day, then two final inspections (city mechanical, county electrical)
Two offices for one permit
Happy Valley is one of the Clackamas County cities that split the permit work. A mini split here has two inspectors.
| Part of the job | Who reviews and inspects |
|---|---|
| Heat pump, heads and line sets (mechanical) | City of Happy Valley |
| New 240-volt circuit and disconnect (electrical) | Clackamas County |
That changes nothing about the install itself, but it means two applications and two final inspections. We file both, include both offices’ fees in the price, and keep the inspections in order so the job closes cleanly.
Addresses that used to be Damascus
Happy Valley annexed more than 1,000 acres of the former City of Damascus. If your address used to be Damascus, it may now be Happy Valley for permits, or it may be unincorporated county land. We check before we apply.
Fix the ducts, or add a head?
We look at the ducts before we recommend a head. Sometimes a damper or a sealed run is enough.
If the supply register barely moves air, the duct may be crushed in the attic or disconnected at a joint. That’s worth fixing either way.
When a head is the answer
If the duct is sound and the room is still off by several degrees, its load is simply bigger than the run was sized for. Mini split vs a central heat pump explains where each one fits.
Sizing one room properly
A bonus room’s load depends on its windows, its sloped ceiling and the garage underneath, not just its floor area.
We work it out room by room. A 350-square-foot bonus room with a west window often comes out at 12,000 BTU; the same floor area over living space might need 9,000. Oversizing makes the head short-cycle and leaves the room clammy, so we size to the load. Mini split sizing has the method.
Daylight basements on the slopes
Much of Happy Valley is built on the flanks of Mount Scott and Scouters Mountain, and many houses there have a lower level half below grade.
That level runs cool and damp. A head in it heats, cools and pulls moisture from the air. Where the head sits below the outside grade, the drain can’t fall by gravity and needs a small pump. Basement mini split installation covers the details.
Wiring for one more head
If your panel is a 200-amp service with a couple of spare spaces, a single head rarely needs a panel upgrade.
A single 9,000 or 12,000 BTU head typically needs a 15- or 20-amp circuit and a disconnect within sight of the outdoor unit. We check the panel’s spare spaces and the load on the first visit. Mini split electrical requirements explains the circuit.
What one more head costs to run
A head in one room adds a little to the PGE bill and usually takes a little off the gas bill, because the furnace stops chasing that room.
Why the furnace runs less
When the bonus room has its own head, the main thermostat stops calling for heat or cooling just to push air down a long run. In summer the central air no longer overcools the downstairs to get the bonus room down.
The head itself runs at low speed most of the time, which is where an inverter unit is most efficient. Mini split electric bill shows how to estimate the cost for one room.
When it’s more than one room
Sometimes the visit shows two problem rooms, or a whole upper floor that runs warm because the return is downstairs.
| What we find | What usually fits |
|---|---|
| One room off by several degrees | A single head on its own outdoor unit |
| Two rooms at opposite ends | Two heads on one outdoor unit |
| The whole upstairs warm | A multi-zone system upstairs, or a look at the central system |
| An aging furnace and AC | Comparing a central heat pump with heads before replacing anything |
We say which one your house needs on the visit, and the written estimate lists every head and its capacity, the route, the drains, the circuit, both permits, the price and what’s excluded.
Prices in Happy Valley
Firlands’ typical range for one head is $4,000 to $8,500, and harder single-zone jobs run $8,500 to $12,000 and up.
If two rooms need help, two heads on one outdoor unit typically cost $6,500 to $12,000. A ceiling cassette in a finished basement costs more than a wall head because of the ceiling work and pump. Mini split installation cost in Portland breaks it down.
Winter in the hills
Clackamas County’s heating design temperature is 24°F, the same as Portland’s, and the January 2024 freeze took nearby stations into the teens.
For a room that has a furnace behind it, the head doesn’t need to carry the coldest night alone. We still size it to the room’s load at the design low and use a cold-climate model, so the room stays comfortable without the furnace running harder.
Around Happy Valley
The jobs follow the terrain and the age of the subdivision.
- Scouters Mountain and Rock Creek. Newer two-story houses with bonus rooms and offices, mostly single heads.
- Sunnyside and the King Road area. 1990s and early-2000s houses with an older furnace, where two heads upstairs sometimes beat a new central system.
- Detached garages and shops. A garage mini split keeps a workshop usable year-round.