Achieving and Maintaining Optimum Distributed Interior Temperatures in our Passive House (Passivhaus)

Note: There is an update to this article.

This is the second Winter season living in our passive house and although I have not been posting for a while, I have been measuring the real-world performance of the house during the course of the varying seasons and will be sharing our results in the coming months.

This past summer we got away with needing very little active air conditioning.  We used the two Fujitsu mini-splits primarily for dehumidification purposes when interior RHs approached 60% and our Zehnder ERVs attained equilibrium with the intake air provided by our earth air tubes during certain periods of very high outdoor humidity levels.  Having windows which only have SHGC values ranging in the 0.40s seems to have reduced the propensity of over heating the interior that may otherwise occur with those having SHGCs of 0.60 or greater.

The first two months of Winter have proved to be quite a bit colder and have been accompanied by much higher levels of precipitation than previous years.  We have also been frequently experiencing outside temperatures below our heat-load design temperatures.  These Artic air-blasts (vortexes) are continuing in the coming days.  This morning it was -1°F with a windchill factor of -14°F.

Last December's electric bill was higher than we had expected and I believe we are paying a price, in part, for the increased amount of glazings throughout (particularly on the North side).  During the design phase, we decided to make a little bit of a trade off during the coldest three months of the year for the benefit of having spectacular views and a bright house that is naturally lit.

It is not uncommon in this region of the country to have many days during the season where the sun doesn't shine which further hurts our performance since we do not benefit from the free solar heat gains that we would otherwise receive.  During these periods of heavy overcast and shorter periods of daylight, our windows become net heat losers.  Loewen windows are not a Passivhaus certified (or certifiable) in their present form.

In the three years since we built this home, Passivhaus certified window availability has improved considerably in the U.S and with any new construction project we would likely be selecting another manufacturer who could provide triple-panes with U-values in the realm of 0.12-0.14.

The extremely cold days and nights have pushed our design and have exposed areas that could be improved but have also underscored the importance of regular maintenance of our heating and ventilation equipment (more on this in the next article).

When we originally designed our heating, air conditioning, and ventilation layout with Zehnder we opted for installing one Fujitsu 9RLS2 ductless mini-split in our family room and another one in our basement.  The nominal heating output of these units are rated at 12,000 Btus/hr (at 43°F) and can vary their output between 3000-22000 Btu/hr each.  Given our heat load requirements of our climate we figured two units should be sufficient for 6600TFA.

We had figured that the ERVs would essentially distribute the heating (or cooling) around the home from both of these point sources. We decided to go ductless as the SEER /EER ratings of the 9RLS2 were so impressive--ducted systems are not as efficient.   It was suggested by Zehnder that these configurations of point sources coupled with MHVR ventilation to distribute the conditioned air were used successfully abroad.  But since we are finding our supply air to be routinely 4°F-4.5°F cooler than the return air temperature in the winter this is not bearing out to be entirely true in our case.

Perhaps it is a function of the sheer size of the house or its layout but when the doors are closed in the exterior rooms, the temperature deltas can range from 4°F to nearly 8°F (hotter or cooler) than the family room.  When the doors are open to all of the rooms the temperature differences range a more amenable 0°F-3°F.  We are also finding that we have to do is to essentially overdrive the unit in the family room to raise the temperatures a bit in the other areas because of these temperature imbalances when the doors are closed to the rooms.  We are now considering some corrective action.

One thought was we could have or should have created another ducting system for moving air around the house without going through the enthalpy exchange of the ERVs.  This is something we can always add if it turns out that we can not devise a more simplified less invasive approach.

Another thought was to install small DC fans within our transfers from room to room to assist in moving the air around the house.  Yet another thought was to increase the supply rates (at any given setting of the ERVs) in the rooms farthest away from the center of the house.

Our current thinking is to incorporate a small hydronic post heating system in the supplies of both of our ERVs effectively using our existing ducting system to provide increased levels of heating (or even reduced levels of cooling) throughout the home--depending upon the season.

We are currently evaluating the feasibility of designing such a system around UltimateAir hydronic post heaters/coolers tied into our Stiebel-Eltron DHW system (robbing Peter to pay Paul, so to speak).  We have calculated that we don't need much Btu capacity in either heating or cooling modes.  The cost of such a system we expect to run us about $1000-$1500 installed.  We are expecting to utilize a dual closed-loop system to circulate either heated or cold water through the "radiators" to do the job.

I would like to hear from any others who have encountered similar situations and would like to hear of the success (or lack thereof) of any corrective actions.