Tuesday, September 11, 2012

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.