Part III: Importance of Mini-Split Placement for Whole House Heating and Air Conditioning

Current Configuration Short Comings of our Two Minisplits Requiring Remediation

While Lisa and I made a bunch of progress improving the temperature performance of our home, one final mystery remained.

When the ERVs and mini-splits were properly cleaned the interior temperatures quickly recovered but once we reached an increase in about four to five degrees, we found it became increasingly difficult to raise temperatures further.  For some reason it appeared that we struggled with raising the temperature of the family room in the evening beyond 70-72F and I was at a loss to understand why.   With potentially 16,000Btu on hand (with these outside temperatures) with two units, I couldn't understand how they could so effective at quickly increasing the temperature from 66F to 71F and yet they couldn't seem to raise the temperatures further even after an extended amount of time having the minis set to 76F.

Sure we were experiencing some stratification in our open two story family room and foyer (ceiling fan was not on) and our 2nd floor tended to be warmer as a consequence, but I was still at a loss to explain what was going on.

Speaking with Jason Morosko of UltimateAir he did some quick calculations of how many more BTUs--than we were generating--that we would need to raise the temperature of the house taking into account the heat loss of the ventilation system at a given flow (air exchange rate).  It appeared that all we needed was an additional 3600 BTUs to do the trick during these super cold nights (of -1F).  But 3600 BTUs over what exactly?  How much production were we actually producing?  Between both units I figured we had a total capacity of about 32K BTUs given these outside conditions.  Which would mean we would need a total of about 36K BTUs, but that was assuming these units were always running at maximum capacity (which they were not).

Upon measuring their energy consumption, it appears that their nominal power consumption is about 585 watts each or about 2.7A at 240v.  Assuming a COP of about 1.85 at these low outside temperatures that would mean that each was producing about 3900 or a total of 7800 BTUs combined.  Surely we had another 3600 BTU potential between both of them without then need for yet another unit.

Yet even if we cranked the set temperature to 80F the family room would not get beyond 71.9F.  Something was up. But what was it?  After some serious head scratching and a couple of discussions with our Fujitsu installer, John, the potential answer became clear.  John had suggested that we take a temperature reading on the high side of the unit, the intake side, to measure the temperature.  I knew there was some temperature stratification going on (on the 2nd floor), but I wasn't expecting anything at eight feet off the floor.  Well was I wrong.  As it turned out we measured temperature differences as high as 12F or more above the unit!

I didn't realize how serious this situation could be until I had the mini-split set to AUTO instead of its heating/cooling/dry function.  By happenstance one evening before I was going to bed, I turned the temperature down a couple of degrees C (about 4F).  About 10 minutes later I walked by the unit on the way to the bedroom and was surprised to feel "cool" air blowing out of the unit.  It occurred to me that the unit actually was going into air conditioning mode!  Clearly this was a mistake on my part.  Here I was expecting the unit to go in standby monitor mode, but instead it went to cooling mode to lower the temperature.  Clearly during the Winter season, the minis should be on HEATING mode only.  That was my first lesson.

The second lesson came from the temperature monitoring of the intake side of the minis.  I finally realized that the overhang of the bridge and the nature of its ceiling (recessed as it is) was acting as a trapping mechanism for the heated air that was blowing out of the unit!  This is why the unit peaked and stopped heating even with high set temps.  We measured air with temperatures in excess of 82F that was being trapped.  This could also be accounting for the seesawing or the rapid cycling ramping up and ramping down of the minis.  This hurts power consumption because when the units ramp up, they ramp up with a lot of power before settling down to their nominal operating range.

Fujitsu 9RLS2 mounted in a "pocket" close to bridge overhang

Delving deep into the technical manuals we come across a temperature correction function setting for both cooling and heating modes.  While not documented well the Fujitsu minis have the ability to have their temperature readings altered.  The programming functions to do so are FUNCTIONs 30 and 31.  Each function has four values.

Cooling Mode (FUNCTION 30):

Value 1: 00 - No correction (default)
Value 2: 01 - Slightly cooler correction -4F than indicated
Value 3: 02 - Cooler correction -8F than indicated
Value 4: 03 - Slightly Warmer correction +4F than indicated

Heating Mode (FUNCTION 31):

Value 1: 00 - No correction (default)
Value 2: 01 - Slightly cooler correction -4F than indicated
Value 3: 02 - Slightly warmer correction +4F than indicated
Value 4: 03 - Warmer correction +8F than indicated

At a setting of 02 for FUNCTION 31 this would mean that the unit would allow for a temperature of 76F measured before ceasing heating with a room set temperature of 72F.  In our case the temperature deltas have been as high as 12F.

This really limits are ability to use this feature.  The better option for us is to install a wired thermometer and control panel somewhere in the room away from the overhang.  This is our next step.

In the basement, the opposite situation was going on.  The basement mini split was mounted high to our ceiling on the wall adjacent to the mechanical room.  Once again I was befuddled with another mystery.  The room was consistently warmer than the set points of the mini.  I couldn't understand why the 9RLS2 would continue to heat when the room was quite a bit warmer than its set point.

I didn't figure this out until last night with the help of a Powerhouse Dynamics eMonitor24 whole house energy monitoring system (more on this in a future article).  Just before the Super Bowl, I took a shower.  As I began watching the game, I took a quick look on my iPhone's eMonitor app.  What I noticed was as the Stiebel Eltron heat-pump hot water heater was running, so was the mini split at maximum power.  How could this be?

The room was at its set point temperature and yet the unit is cranking out the BTUs.  Looking at both power charts--those of the DHWH and the basement mini--I saw the correlation.  As the Accelera 300 was running, it was cooling the mechanical room by as much as four degrees.  The problem was that the location of the mini-split put its intake on directly on the other side of the wall and sure enough I discovered the intake temperature  was being artificially lowered as the hot water tank's heat pump was blowing cooled air from its exhaust towards the wall that where the mini split's intake and thermostat was located.  In fact it is appearing, every time the hot water kicks on, so eventually does the mini split.  Once again a correction needs to be made with the installation of a remote thermostat and control panel.

Fujitsu 9RLS2 located high on the wall adjacent to mech room
Stiebel Eltron Accelera 300 Exhausting Cooled Air Toward Min Split

Current room temperature in excess of 69F while set temp is 64F
Remediation Solution

So there you have it.  While I may have made some poor choices in location of our mini-splits (in part for the aesthetic value), I would recommend that any builder/designer of an energy efficient home--be they Passivhaus, LEED, or Builders Challenge--that plans on using ductless mini splits choose their locations carefully and install a remote located controlling thermostat.