suck and blow

March 27, 2013

rough-in work has wrapped up on skidmore passivhaus and it’s time to look at a few of the features under the hood. this new airtight home (or shall we say leak-free) is equipped with mechanical lungs.


we chose the PH certified zehnder comfoair 200 hrv, capable of producing up to 125 cfm of continuous ventilation.  this project requires about 70 cfm (60% setting) to achieve .31 air changes per hour (or roughly 1 air change every 3 hours).


the 3″ flexible ductwork is a double-walled proprietary system called comfotube made by zehnder.  very easy and fast to install.  the goal is to keep the runs as short and straight as possible.


the round ceiling rough-in boxes are used for either supply or exhaust air.  either 1 tube or 2 tubes are connected depending on the cfm requirements.  exhausts are located in the bathrooms (20 cfm continuous) and kitchen (30 cfm continuous).  we have 3 exhaust locations – each with 2 tubes.  there are 4 supply locations.


supply air is typically provided at the ceilings but we have a low supply register in the double height living room.


the tubing terminates in the ceiling of the mechanical room.  6 supply tubes and 6 exhaust tubes.  manifolds gather the supply and exhaust tubes respectively and connect them to the unit.


exhaust air flows through the machine, the heat is captured in the core, and the air is expelled to the outside.


fresh air is brought into the machine from the exterior, and warmed by the heat captured from the outgoing air.  the zehnder is about 92% efficient and uses just 7.1 watts / cfm.


the fresh air supply and exhaust are ducted in well insulated metal ductwork from the bottom of the unit to the exterior.  the fresh air supply is located high on the exterior wall with the exhaust about 10′ below.  the duct penetrations were carefully air sealed to the plywood air-barrier.  this unit also requires a condensate drain that we’ve tied with an air gap into the kitchen waste pipe.


the main controller will be located in the kitchen, providing the ability to boost the ventilation rate temporarily to 35 cfm while cooking.  each bathroom will have a timed switch to boost the rate to 24 cfm if needed after a shower.  we also conveniently have an operable window located in the kitchen and both showers.

thanks to IM Plumbing for help with the installation, and to Matt Grove from Zehnder for help with the design.

sheetrock has wrapped up and painting has begun.  check back soon for more.


2 Responses to “suck and blow”

  1. 1/ how much did it cost? 2/ The one tight house I’ve been in is, frankly, unpleasant. 3/ An alternative: instead of saving energy with this expensive system, ADD energy via solar input, a highly efficient woodstove supplied with outside combustions air, and thermal mass. IMHO.

    • insituarchitecture Says:

      @David – I’m guessing the tight house you’ve been in didn’t have a proper ventilation system if it was unpleasant. PH buildings are notoriously very pleasant with exceptional air quality, and that’s been my experience as well. A leaky building just means you are bringing in your fresh air through undesirable and unintended places (think nasty crawlspace) and dumping warm air (in heating season) to the outside. In a tight building with an HRV, your fresh air supply is clean and filtered, and fully controlled.

      Our strategy is to first aggressively reduce the load (using a highly efficient envelope, solar gain, and internal gains) and only then add energy via active solar input. A tight leak-free building is as much about comfort as it is reducing energy consumption.

      We will be addressing costs for the entire project (including the HRV and our PV system) in future posts.

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