Cold downstairs home office - mini-split or zoning?
Three years ago we remodeled and added an additional home office downstairs, during winter it is fairly cold, about 8-10 deg cooler than the 70 deg setpoint on the main living area upstairs. Most of the downstairs is somewhat cooler than upstairs, but the office is yet another half-story down and farthest from the furnace. The things aggravating the office temps are:
-shaded and north facing
-supply register is in the ceiling, long 6" flex duct run has less supply airflow than other rooms
-infinite room return, i.e. open to hallway and stairwell exacerbates stratification
-single-pane corner windows (butted glass at corner, about 15 sq ft of R-1)
-outside door primarly glass
-no downstairs return
I've done two separate experiments to simulate mini-split vs zoning modifications:
-mini-split - proxy was a portable dual-hose AC/heat-pump, vented through an access door to an adjacent vented crawl space. Heat pump is only used when office is occupied, between 1-4 hours at a time.
-zoning - proxy is I closed off all the upstairs supply registers, and am heating the entire house only from the downstairs. Carrier Infinity control says static pressures are reasonable. As the runtimes are still short this mild winter, I further have the fan recirculating on low for several additional hours.
Attached is the temp data for anyone to review, the first four days are with the "mini-split", the next four days are with the "zoning". Here's what I see:
-mini-split. The small heat pump brings the room temp up FAST. Its thermostat was inaccurate, so it was overshooting 70 deg. Once off it also cools off fairly quickly to about 68 deg, but then slowly drops overnight to 58-60 before the next morning.
-zoning. The downstairs and upstairs are now both pretty even at 70 deg, though the office is still only reaching 65-66 deg. However, this 4-5 deg difference is about half of the stratification as before, and not entirely uncomfortable. A true zoning system could bring the office to 70, but could also overheat the upstairs (e.g. Downstairs is 70, Up is 85+ )
Which permanent approach do you think is better - mini-split or zoning?
First let me say this, if you have no returns in the basement don't expect to heat/cool it right. And it maybe the whole problem right off the bat. You can't push air or anything else for that matter into a space without a way for air to leave that confined area. There must be an escape route for the exhisting air to leave to let the "new" air enter.
I wouldn't try to zone. Use a mini split or a portable heater!
Yes, just to clarify, the first floor is not a basement and it's the opposite of confined - the office is an open plan with no doorway and a entire 8' high x 4' wide hallway going up the wide stair/loft area to the upstairs return. That however does seem to accelerate the stratification.
Not looking for a perfect solution to an architectural problem, but willing to put some more money to HVAC mitigations. I've had several HVAC professionals come in, all with differing solutions - zoning, low supply, low return. I'm not against a low return, the furnace is roughly the same level as the office, do folks think it would work? It would be about 30' long, with two 90 deg bends, and have to go 3' underground at one point to cross a concrete foundation wall. Would return air follow this path vs the open hallway with 40X larger cross-section (similar distance but goes up a level)?