The Manual Zr from ACCA is helpful for offering general guidelines for zoning rooms in a house, but I am wondering if there are any more concrete methods to figure whether a house needs zoned systems or not.
I understand that every house is a unique scenario and it depends on climate, orientation, amount and placement of windows, and many other things, but does anyone follow any quantitative guidelines to decide if you zone a system or not?
Hi Brett, great question! Unfortunately, beyond Manual Zr, I'm only aware of one 'quantitative' guideline: multi-level homes should either be air-zoned or equipment-zoned. Some states require multi-level homes to be zoned.
Automatic zone control ('air zoning'), especially zoning within a single level, is often applied for the wrong reason: to shut off unused rooms to save energy. But here's the thing... the more efficient we make the envelope, the less benefit this strategies offers. Also, in cold climates, tight homes generally have higher humidity levels in winter. This improves comfort but increases mildew and mold risk in nooks and crannies that are cooler than the rest of the house. In super tight homes in a cold climate, shutting off supply air to any area is risky business!
I advise clients that the purpose of zoning is to account for changes in the load balance -- from day to night, from morning to afternoon and from winter to summer. Secondarily, zoning allows for different comfort temperature preferences by room. The latter is subjective (qualitative) and thus is based on owner preference. However, the former can be quantified...
First, look at the ratio between the room-by-room design cooling and heating airflows. And second, analyze the room-by-room AED load curves generated by MJ8 compatible software (see note below). Through experience, a designer can establish criteria (thresholds) for when the imbalances revealed therein are sufficiently large to warrant zoning.
NOTE: Adequate Exposure Diversity (AED) is a concept introduced in the 8th edition of Manual J whereby the software calculates the hourly load (by room) imposed by solar gains through glazing (i.e., morning vs afternoon). The Manual J procedure adds an "AED" load to the base room load to the extent that solar gains through glazing exceed the average glazing load by 30%.
Based on my analysis of AED curves and heating/cooling airflow ratios, I may recommend zoning to the client, caveatted by the limitations of zoning. When designing a zoned system, I try to group rooms with similar winter-summer ratios and AED excursions. I find that most mechanical contractors ignore the quantitative approach when designing zoned systems, which is one reason why most residential zone systems perform so poorly. (Another is (mis)use of bypass, but that's a whole other topic!)