Heating and cooling the Passive House

One of the main qualities of a Passive House building is that the mechanical loads are so reduced that you only need a tiny system for heating and cooling.  Even for this relatively big house, there is no conventional furnace/AC small enough that it wouldn't be way oversized, and therefore short-cycle.

Our heating and cooling is handled with the new stars on the efficiency stage, mini-split (or ductless) heat pumps.  This is a rapidly expanding field that bears little resemblance to the heat pumps of the 1970's and '80's.  Heat pumps exchange heat similarly to the way air conditioners or refrigerators do, but can work in both directions to provide both heating and cooling.  The early units got a bad rap in the 80's for being inefficient at supplying heat below 30F or so, but the new Mitsubishi units we're using supply their full nominal heating output at 5F and continue to operate efficiently down to -13F.  With a SEER rating of 26  and heating Coefficient of Performance over 3, the ductless heat pump is a terrific unit. But they gain even more efficiency from the fact that they can modulate down to run at partial load--so they will run on a very low setting if you just need a little heating or cooling, which is way more efficient than coming on, blowing hard, and shutting off. Another benefit in cooling mode is that the unit can dehumidify continuously, even at low levels of output.  Aside from being a more comfortable way to get your space conditioning, it is also nearly silent.  You literally can't hear these units unless you get your ear right up on top of them.

They have an outdoor unit that looks like a small, upright A/C condenser, and a wall unit with a fan that provides conditioning.  Here's the rough installation of the units:

Master Bedroom

Living Room

Mitsubishi and others also have units that can be ducted, but we didn't want (or need) to get into that.  One of the other qualities of the superinsulated Passive House is that, since the walls lose heat so slowly, you just don't need to duct heat around to provide even temperature in the house.  The ventilation system helps the air circulate, and convection and vapor pressure migration take care of the rest.  Super comfort, Passive style.

Ventilation for the Passive House

One of the qualities of a Passive House is that the construction is nearly airtight., so to allow the house to "breathe" we use an energy-recovery-ventilator (or heat-recovery ventilator).  Yes, houses need to "breathe", or more accurately, people inside houses need fresh air, but the problems with leaky construction are at least threefold: 1.) you don't know if the leaks are getting fresh air where you need it (like in the bedrooms), 2.) as it leaks in, the air picks up contaminants and can cause condensation, mold, and deterioration of the structure; and 3.) you don't know the rate of "leaking", so combustion gases, moisture, etc. may not be getting cleared out of your house.  My analogy is that we want the beast to breathe through its nostrils, not through gaps in its skin.

Like all Passive Houses, ours has a balanced ventilation system (same amount of supply and exhaust) that runs continuously. It has an extremely efficient fan, and recaptures about 85% of the outgoing heat, so it is a low-energy way of providing great indoor quality.  The system extracts air from the kitchen, bathrooms, laundry, basically any place that makes moisture and odors, and supplies fresh air to bedrooms and other living spaces.  Imagine getting fresh air continuously as you sleep...you wake more refreshed, having been breathing air that was cleaned of pollen and molds by the (MERV 7/8) filter. 

Zehnder, beginning of installation

Our system is a Zehnder ComfoAir 550. Zehnder not only has great heat recovery technology, but they have also developed a suite of products that make sizing, installation, and commissioning (getting air flow right) a breeze.  The small flexible ducts clip into supply and exhaust manifolds, and into supply and return diffusers, which have fittings to adjust flow rates.

Zehnder with ductwork connected to supply and exhaust manifolds

 These ducts are flexible and fit in a standard 2x4 cavity, so are quite easy to work into the construction site (though I recommend a riser for ducts running from the manifolds to another floor).  One reason each diffuser is individually ducted is that precise airflow can be guaranteed at each location. Commissioning consists of testing each diffuser at the end of installation and adjusting the unit's fans and the diffusers themselves to ensure design flow rates.  Once set, it typically doesn't take maintenance to maintain the flow rates; all that needs to be done is occasional cleaning of the filters.  Clean, fresh, healthy air, 24/7.