Solving a (self-created) Problem with Heat Distribution

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:

And here is a similar view in which you can see the door to the office: 

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.

Transom Window Over Powder Room Door

As previously discussed, our house is heated with two ductless mini-split heat pumps; one at the top of the stairwell to the second floor and the other in the main living area on the first floor.  To ensure that pressure is equalized between the rooms and conditioned air can freely moved into those rooms when the doors are closed, I installed transoms above the doors to the bedrooms and bathrooms.

At least for now, I don’t plan to install windows in the transoms over the bedrooms and 2nd floor bathrooms.  As I see it, this type of detail would be largely decorative, and would have almost no functional purpose.  Yes, there’s the issue of privacy (sound).  But in a 2000 square foot house with only two or three occupants, that just hasn’t proven to be an issue.

I did, however, recently finished installing the transom window over the powder room door on the first floor.  This is a necessity due to the proximity of the room to the main living area.  The room is actually off the front foyer.  But it’s still close enough to provide some potentially embarrassing or uncomfortable moments for guests.

It didn’t occur to me (until it came time to install it) that a properly installed transom window is in the same plane as the door below it.  Initially, I thought that I’d have the window open to the inside of the powder room.  But that wouldn’t have looked quite right because the door opens to the foyer, and therefore the window should do the same. At first I thought it might look odd with the window mechanism on the outside of the bathroom.  But looking at the final installation, I think the window and the mechanism add something interesting and unique to the foyer.

Also, as I understand it, ideally the window stiles should be the same width as the stiles on the door.  I cheated on this a bit because it seemed a bit “heavy” to me.

I laminated up two pieces of 3/4″ clear pine to make the frame.  That allowed me to end up with a thickness that matched the door (one and three-eights inch).

Both the construction and installation of the window are pretty straight-forward.  But the mechanism isn’t cheap, at just over $100.

IMG_1102 IMG_1101

Raw Data on Electric Usage and Temperature/Humidity

In this post, I’m including links to files that contain the historical electrical and temperature/humidity data for our house from February 1, 2015 through the date of this post, October 23, 2015.  The data begins on February 1st because although I started recording in early December 2014, the heat pumps were not working correctly until late January.  My intent is to offer this data to anyone who desires to use it for analytical purposes.

The electrical data, which was recorded by an eGuage energy monitor, contains daily numbers for total electrical use, the first floor heat pump, the second floor heat pump, the combined total of the two heat pumps, the dryer, the water heater, the ERV, the well pump, and the family room, which basically includes the lights and TV in that room.

The temperature and humidity data was recorded by Hobo data loggers that were placed on each floor (basement, first floor and second floor), and outside on the north side of the house under the porch roof.

eGauge Data 2-1-15 thru 7-31-15

I will update this data on February 1st of next year.

In early December (once I have a complete year of eGuage data), I will post the yearly total electrical usage for the appliances and mechanicals that are listed above.  As I indicated, the heat pump data won’t be totally “accurate” (because they were “short cycling” until the end of January).  But the information should still provide a reasonable indication of how efficiently the house is operating.

Energy Usage Update #2

Now that our first heating season is essentially over, I thought I’d provide an update on the cost of heating the house.  As mentioned in a prior post, I wasn’t able to track the energy used by the heat pumps until December 7th, when I installed the eGauge energy monitor. For that reason, I had to estimate the heat pump usage prior to that date.  To do that, I first determined our average non-heating electrical usage during December through April 22nd (the end date of our most recent electric bill) and compared that with the total electric usage in October and November, as reported to us by the electric company.  The difference is approximately the amount of electricity that was used for heat.  It’s not exact, but I’m confident that it is reasonably close to actual.  Here’s the actual summary from eGauge:

Screen Shot 2015-04-23 at 10.55.26 AM

So using the above data, and estimating the usage for the period before December 7th as described above, at our current rate of 15 cents per kilowatt hour this heating season cost us approximately $390.

As previously discussed, I put that in perspective by viewing it relative to our prior house, which is located about four miles away.  That house was larger (2800 square feet vs. 2000 square feet), had two-by-six walls with three-quarters of an inch of exterior foam and was heated with propane.  It took and average of 1,080 gallons of propane to heat that house each winter, at a cost of $2,430 per heating season (based on our most recent cost of $2.25 per gallon for propane).

So from a comparative perspective, given the facts discussed above, when adjusted for the difference in the size of the two houses, our current house costs us about 22% of the amount it cost to heat our prior house.

One other factor can’t be easily be quantified in dollars and cents.  Specifically, we had a programmed thermostat in our prior house, and kept the temperature at 69 in the evenings when we were home, and 61 when we were at work and through the night.  In our current house, we kept the temperature at 69 to 70 degrees the entire winter.

Short-Cycling Mini-Splits

On December 7th, I installed an egauge energy monitor on twelve circuits; the two lines supplying power to the house (2), the two heat pumps (4), the water heater (2), the ERV (1), the well pump (1), the family room circuits (1), and the dryer (1).

At about the same time, I set the first floor mini-split at 70 degrees and the second floor unit at 66 degrees.

One of the first things that I noticed when reviewing the egauge data, was that the heat pumps were both “short-cycling;” that is, they were turning on and off at intervals as short as three to five minutes.  When “on,” the unit would draw about 1kW.  When “off,” the unit would draw about 20 watts; just enough to power the internal fan on the wall unit (which runs continuously).  Here are a couple of examples:

Screen Shot 2014-12-08 at 1.12.28 PM


Screen Shot 2014-12-09 at 1.57.59 PM

This didn’t always happen.  Occasionally, one heat pump or the other would “flat line” at 300-400 watts.  This could go on for hours, and in a couple of instances, days.  But then the unit would revert back to the short-cycling.

Screen Shot 2014-12-13 at 3.51.27 PM

The short cycling didn’t seem to have a strong correlation to outdoor temperature.  For instance, sometimes the first floor unit would short-cycle while the outdoor temperature was in the 30s and flatline when it was in the 40s.  At other times, just the opposite would occur.

Having said that, the first floor unit stopped short-cycling when the temperatures dropped below 20 degrees or so.  But the 2nd floor unit continued to short-cycle even at those low temperatures.

I contacted both the local HVAC company that installed the units and Mitsubishi’s National Sales Support Manager John Bart, and presented both with a summary of the data.  I had learned about John when we were beginning construction of the house.  He had participated in a Passive House webinar where he discussed the Mitsubishi mini-splits. Shortly after that webinar, I contacted him with questions about the type of mini-split system that might be best for my house, and he was very responsive.  In fact, it was his responsiveness that caused me to go with Mitsubishi over the less expensive Fujitsu units.

It appeared to me that the local HVAC guys (who installed the system) didn’t know quite what to make of the data.  However, John, who was clearly more familiar with the Passive House concept, asked for additional information; a floor plan, photos of the wall units, and the Passive House heat load computation, which was (is) about 8,000 btu/hr.  Given that information, he surmised that the units were oversized, and sent the information over to his Northeast Tech office.

To back up, both mini-splits are 12,000 btu units, and can power down to somewhere around 3,500 btus.  John suggested that, at the more moderate temperatures, even 3,500 btus was probably too much for the house.  However, he acknowledged that it was perplexing that the short-cycling stopped for sometimes lengthy periods of time, even at those more moderate temperatures.  He added that even the smallest (9,000 btu) mini-splits would probably have had the same issue because their low end was very close to the low end on the 12,000 btu units.  He suggested that a Mitsubishi regional tech visit my house to assess the situation and to install remote thermostats for both units.

Mini-splits (at least Mitsubishi mini-splits) come with a remote control.  However, contrary to what some may believe (in fact, even the local HVAC contractor believed it), the remote does not have a built-in thermostat.  In fact, the thermostat is located in the “cassette” that hangs on the wall and dispenses the heat.  As I understand it, the sensor measures the incoming air, and in doing so, provides the data that helps the unit decide when to power up or down.  But apparently it isn’t like the old days, when a single thermostat turned the furnace on or off.  Rather, as I understand it, with these units there are five sensors; three on the outdoor condenser and two on the indoor wall unit, that collectively tell the unit what to do.  In other words, things are much more complex than they used to be.

At any rate, last Friday the Mitsubishi tech showed up and installed the two remote thermostats.  These thermostats (product MHK1) are manufactured by Honeywell specifically for the Mitsubishi mini-splits, and it appears that they resolved the issue.  In my opinion, Mitsubishi provided outstanding customer service in dealing with this issue; something that I find sorely lacking with many companies these days.

Since last Friday, the 2nd floor unit has been idling; meaning that the compressor has not turned on.  In other words, even during the evening lows, which have gone down to the low 20s, the first-floor unit has handled the entire job of heating the house, and the second floor has not gone below 68 degrees.  So the picture, from the egauge perspective now looks like this for both heat pumps:

Screen Shot 2015-01-20 at 4.10.23 PM

A couple of notes…The second floor heat pump is the horizontal red line at the bottom of the graph.  It’s showing about 30 watts.  As I said earlier, this heat pump has been in idle mode since the remote thermostat was installed.  The more prominent red line is the first floor heat pump.  The outdoor temperature early this morning was in the high 20’s, so my guess is that the three vertical jagged lines are defrost modes.  However, they’re the only three such lines that occurred during that night.  So I could be wrong.  They may instead be cycling blips.  But at one hour intervals, they’re a far cry from the three to five minute intervals that were occurring.

First Full Month Heating and Energy Usage

We’ve now been in the house for about two months. But our electric bill runs (roughly) from the 21st to the 21st.  So last week we received our first “full month” electric bill.  Given that it’s the first full month, the information gleaned from the bill is, of course, very preliminary.  But I thought I’d share it anyway.  The important numbers were as follows:

Average Outdoor Temperature During the Period: 53 degrees

Total Electric Usage: 551 KWH

Total Cost for Electricity $90.94

Cost per KWH: 17 cents (all fees and charges included)

The last week of this billing cycle, it got fairly cold.  Nightly lows were as low as 19 degrees.  Daily highs were in the high 20s to mid 30s.  But again, the average temp for the month was a relatively mild 53 degrees

The house is two floors plus a basement.  Square footage is 1,000 per floor.

All lights are LED; 40 and 60 watt equivilants.  Most are 60s.  Only a few are 40s.

The house is completely electric.  Oven is convection.  Cook-top is induction.  Water heater is heat-pump.

We have two Mitsubishi mini-split heat pumps.  Both are 12,000 BTUs.  Each has one cassette.  One cassette is on the first floor.  The other is on the second floor at the top of the stairs.

For all but the last two days of the month, we used only the first floor heat pump.  Some days I didn’t use it at all.  Other days I left it on.  Mostly I was trying to get a feel for how it operated.  I was surprised to find out that the fan on the indoor cassette unit always runs (albeit at a very low rate), even when it is putting out no heat.

The single heat pump, kept the house “comfortable” down to the 19 degree low.  However, when the outside temp got below freezing, it appeared to put out heat all the time.  This kept the first floor at a pretty consistent 67 or 68 degrees at night and 68 to 70 degrees during the day.  The second floor temperature (which is where the bedrooms are) varied; at night it got down as low at 64 degrees, and during the day, with a little help from the sun, it rose to 68 or 69 degrees.

That’s why I put the word comfortable in quotes.  I found this to be quite comfortable.  After all, for us, almost all of the time spent on the second floor is spent under covers.  But my wife was not quite as content.  So for the last couple of days, I turned on the second floor heat pump.  This quickly brought the second floor up to 68 and significantly reduced the working time for the first floor heat pump.  Both heat pumps were set to 65 degrees. I’m not sure why a 65 degree set point on the heat pumps results in a 68 degree environment.  But it does.

A couple of things seem clear to me.  First, at 68 degrees the house is exceptionally comfortable.  Not a hint of a draft, even when the wind is howling. And second, a clear, sunny day makes a measurable difference in the form of a reduced heat load.

Next week, I will install an eGuage energy monitor on the main feed and 12 circuits.  So beginning then, I will have a much better read on the energy consumption for the heat pumps, water heater, and ERV.  I will also install Hobo temperature/humidity data loggers on all three floors and outside. This will give me the ability to monitor and report on the performance of the house over the long term.

Based on this very preliminary information, I have a few initial thoughts.  First, I was a bit disappointed in the first month’s electric bill.  Both the KWH used and the overall cost were higher than I anticipated.  But I’m thinking that this is due more to unrealistic expectations (and an unexpected 17 cent/KWH overall rate) than house performance.

One way to try to put this data in context is to compare this first full-month electric bill to my energy cost from one year ago, when we lived in our prior 2,800 square foot house, built in 1987, which used propane heat. The average temperature last year was 51 degrees; a couple of degrees cooler.  At that house, I followed our energy usage pretty closely.  Electric usage was easy (just read the bill).  But I had to gauge propane usage by my readings at the end of each billing period.  Each night, we set the thermostat back to 60 degrees and at least three days per week we left it at 60 degrees during the day because we were both at work.  We never raised the thermostat past 69 degrees.  But frankly even I thought that was uncomfortable.  It pretty much always felt colder than either of us would like.  At any rate, my total energy cost for that house during the same period last year was $428.43, and my electric usage was 881 KWH.  But remember, we were heating an additional 800 square feet.

It will be interesting to see how things play out in the long run.  Taking a SWAG based on this admittedly limited data, I’m thinking that when all is said and done, we have a good shot at being under 6,000 KWH total energy usage for the entire year.  That would result in a total energy bill of about $1,000.  And if it looks like that prediction is proving to be fairly accurate, I may start looking for a PV system next spring.

Energy updates will follow, once I get the monitoring systems installed.