I just completed my first house using the "Perfect Wall" method invented by Building Science Corporation. Perfect Wall is a method of building that results in a super-tight envelope with thick exterior insulation. We didn’t specifically design this house to meet Passive House standards, but the results are very close. (See these blog posts for more on the Perfect Wall system.)
One big benefit of a super-tight and superinsulated house is that you can severely downsize the HVAC system and generally use less expensive systems. Here are my four keys to designing an HVAC system for an ultra-efficient house.
1. Get a good HVAC designer involved. I use Positive Energy, here in Austin, for my HVAC designs. They start with an energy model of the house, then do a thorough Manual J (HVAC Load Calculations) to determine the size of the system. I usually meet with them after those two models are produced, to discuss the system options. In this case, we had very low loads, so a simple system was the way to go.
2. Consider ductless. Traditional houses ran ducts near windows and doors to counteract the heavy loads coming through those openings. This Perfect Wall house is super-airtight, and we were judicious with regard to how much glass we used. A ductless minisplit system was the perfect system. The house was only 1,700 square feet on two levels. We used a three-head Mitsubishi system tied to one variable-capacity outdoor unit. Because we are in a hot/humid climate zone, we added a supplemental dehumidifier. We used a very efficient Santa Fe Compact 2 unit, which removes 70 pints of water a day and is Energy Star rated. This stand-alone dehumidifier is intended to be used for a basement or crawlspace, but works great for this application and takes up very little space.
3. Supply fresh air. Built Tight, Ventilate Right! We want to build the tightest envelope we can, then bring in fresh air on our terms. This house had a tight budget, so the budget-friendly ERV from Panasonic was a perfect fit for fresh air. This little Energy Recovery Ventilator has three setting for fresh air depending on your needs or the size of the house. It runs at 10, 20, and 40CFM of supply/exhaust. As the two air streams cross in the core, this unit has the ability to exchange heat and moisture. In theory, the humid air coming in will shed some of its moisture to the exhaust air. I’ve used this unit in a dozen houses, and for the money, it’s hard to beat.
4. Counter stack with a fan. Two-story houses are notorious for temperature stratification (meaning it’s typically hotter upstairs). To counter this effect, we used a dedicated in-line Panasonic 120CFM exhaust fan that moves air from the tallest part of the house back downstairs to the laundry room where the ERV and dehumidifier are located. Granted, there is a hit on the comfort of that room, but we felt like that was the right place to do it. Louver doors into the laundry space allow that air to return to the main spaces where the minisplit heads can condition them.
To read this original article click here.