First, some background…
Using a ductless mini-split to heat and cool a home (even a passive house) takes some thought. One needs to think about where, exactly, the mini-split head, or cassette, will be placed, and how the warm (and cool, in the summer) air that it creates is likely to flow. Take my home, for example. We opted to place the “winter” mini-split head in the “living” area on the first floor (we also placed a second, separate, mini-split head at the top of the stairway to the second floor, which is essentially the second floor “hallway,” to cool the entire house in the summer).
That single mini-split head (on the first floor) heats the entire 2,000 square feet of living space (1,000 sqft on each floor) on all but the coldest days, and does the job amazingly well. When the outdoor temperature is above 20 degrees, the 2nd floor stays within two degrees of the first floor (which, again, is at 70 degrees). Frankly, that’s warmer than I prefer, since all we do is sleep on the 2nd floor). But it makes my wife happy. When it gets below 20 (and the sky is overcast during the day), the 2nd floor can drop to 66 degrees, and if the temperature drops to the single digits for prolonged periods it can eventually go a bit lower. In those instances, I’ll turn on the 2nd floor min-split, and it will periodically kick on, when necessary.
And now, the problem…
When the house plans were drawn, the architect anticipated that the entry to the first-floor office would be directly off of the living area. He also anticipated that the first-floor mini-split would be mounted on the narrow wall between the living area and the kitchen area. Both are shown in the drawing below:
(Ther drawing also shows an exterior door on the south wall that I decided against because it just wasn’t necessary.)
When I framed the interior of the house, I decided to move the office entry to the hallway because it unnecessarily ate up wall space in the living area, leaving no room for a T.V., for example.
I also had to find another place for the mini-split because the wall that the architect had intended to place it on was too narrow for the mini-split head; it wouldn’t fit.
Given those issues, I changed the first floor as follows:
Moving the office door to the hallway (and eliminating the unnecessary hall closet) freed up valuable wall space in the living area and widened the hall.
And as the drawing also shows, I moved the the mini-split to a wall that had suffficient room.
Here’s how it looks:
In this photo, you can see the location of the mini-split relative to the back hall:
So what’s the problem? Somewhat ironically (at least in my view) the only problem that we’ve had is that, during the winter, the office, which is the room closest to the mini-split, can be two to six degrees colder than the rest of the house, depending on the outdoor temperature. The only exception is during sunny days, when the office warms up perfectly. But that leaves many winter mornings, days, and evenings when my wife isn’t happy (because the office is her place of work). And it should be noted that she almost always works with the door fully open.
I pondered over this problem for two years. Initially, I thought that the problem was the ERV. More specifically, there is only one first-floor ERV supply, and it is in the office. Why? Because that’s where Zehnder said it should go. And it’s consistent with the general rule; pull stale air from the kitchen/baths/laundry room and push fresh air into the bedrooms (the office could also be considered a bedroom).
But I eventually found out that the ERV wasn’t causing the problem, and had no detectable (at least by me) effect on the room temperature. So while the ERV is constantly pushing cooler air into the room, the temperature (above 60 degrees) and flow ( about 24 cfm) of that air just isn’t enough to make a difference in the room temperature. This should have been no surprise, as it is consistent with what the experts suggest.
So my current theory is a bit different. First, after framing the office walls, I insulated them Roxul so my wife could work in relative peace if I chose to watch TV; admittedly a bad idea with regard to heating the room. But I also suspect that any warm air that makes it into the rear hall is probably more likely to rise up the stairs (which are to the right) than into the office (which is to the left).
The easy thing to do would have been to install some baseboard electric resistance heat in the office, which my wife (or a thermostat) could turn on as necessary. But that clearly seemed like a wasteful overkill to me. So what I decided to do instead was install a Tjernlund ASI AireShare Room-to-Room Ventilator fan. The idea came to me as I was painting the wall in the family area that separates the family area from the office. Unsurprisingly, the air near the ceiling (which is almost 9′ high) was several degrees warmer than the general temperature of the house down among the living. It therefore seemed reasonable to believe that, if I could push some of that air into the office, the problem might be solved and, since the fan only draws 25 watts at full speed and pushes the air at 75 cfm, I figured that its use would prove to be much more user-friendly and cost effective than baseboard heat.
If the intent is to move warm air, ideally, these ventilator fans would be installed with the intake high on one side of a wall and the diffuser low on the opposite side. In doing that, the bay in which the fan and diffuser are installed acts as the vent; air enters from the top and is pushed out through the bottom. However, I decided not to do that because, as I stated earlier, I had insulated the office walls with Roxul, and I didn’t want to completely remove the insulation from the bay in which the fan was to be installed. Of course, this meant that the warm air would be brought into the room closer to the ceiling than the floor, which is less than ideal. But the room isn’t that big (it’s about 120 square feet), so I was hoping that it wouldn’t matter.
With minimal effort, I was able to cut the hole on the intake side of the wall about a foot from the ceiling, clear out the insulation in a space of about 18 inches, and seal off the bottom of that space with a piece of 1/2″ foam board and caulk. Prior to sealing off the bottom of the space, I fished Romex up from the basement and installed a variable speed wall switch. The intake vent and fan are located at the top of the space, and the diffuser is located at the bottom.
You can see the intake for the fan in the upper left-hand corner of this photo…
…and the exhaust above the chalk/cork boards in this photo (it’s a bit difficult to see because it is a narrow diffuser rather than a traditional vent)…
Here’s a better photo of the diffuser (my apologies for the temporary “flow-detector”) and the variable speed switch…
The fan has now been in service for about a month, and I am happy to say that it has performed better than I had anticipated. So far, it has resolved the issue completely. For example, last night the low was 23 degrees, and the entire house, including the office, maintained 70 degrees +/- one degree. While not annoying, the fan is detectable at full speed. However, we run it at what I would estimate to be one-third to one-half speed, and it is essentially silent.