Indeed, our topic of HVAC manuals was inspired by a thread on the GreenBuildingAdvisor blog, “ Not all duct design manuals are created green.” It has become increasingly apparent to me that the ability to design proper HVAC solutions is one of the biggest hurdles to achieving widespread adoption of super energy efficiency homes. One of the readers, Whetstone Green, posted the recommendation of having a 3rd party specialist do the Manual J and Manual S (equipment selection). I wholeheartedly agree, as this would disaggregate the design and building science aspects from the sale of equipment. The poignant question is - where are these guys? See AirBalancing for some ideas.
In my need to know, I posed the questions posted on-line to my contacts over at ACCA, in their Technology and Research Division, and a few of my successful builder friends. What follows is a combination of my newly augmented information, with some of my thoughts – in hopes to shedding some light on the matter. Caveat emptor - I have not run these scenarios through the manuals – yet.
The key consensus was that the ACCA manuals are capable of designing for energy efficient houses, not just the average leaky house. The key is to avoid assumptions, and model the details. For example, Manual D will default to designing for the worst-case scenario - a good solution for the average leaky house client. For an energy house, reject the defaults and design for actual conditions. The manuals, and their aftermarket software, are just tools – the quality of the output is reliant on the operator input.
Step 1. Load – Base and Room by Room
|Skylights change room design load|
Step 2 & 3 – System Choice and CFM Delivery
Once the load is determined, Manual S can help determine the best system. It can identify tolerances for equipment, as well as determine which equipment is best suited. This may be a choice between radiant or forced air, or even between different equipment components of the same manufacturer.
Manual T, and its new side-kick, Manual B are the up-and-coming power players. Manual T identifies how to select, size, and locate the supply air diffusers, grilles and registers for the optimal air distribution needs for a space. In a superinsulated home, for example with ICF walls, the exterior thermal envelope holds a relatively steady temperature and has a minimal convective heat loss. There is no need for a supply duct at the point of the wall, but the air does still need to be exchanged. This could be achieved with an air supply high on an interior wall, with the correct diffusor, saving on long duct runs.
The placement of ducts, and related total CFM needed informs the type of system to choose – thus the two steps are iterative.
Manual B, is all about Air Balancing for both air and hydronic systems. And here is the crux of the matter. In homes reaching Passivhaus standards, there is little need for supplementary heating and cooling, and with no convective losses or air currents caused by envelope leakage, supplying air is pretty easy as well. But now it is like playing pianissimo in a concert hall. Every little bit counts. The balance is very fine. With small amounts of air being moved about, the act of balancing the pressures becomes the new testing ground. Jumper ducts join the fray.
Step 4. Finally –the ductwork.
This happens AFTER you have figured out how much air to deliver and where. The Manual D can help connect the dots. In an ideal (future) world, this will happen at a draft phase of design, to help transfer the flat drawings of Manual D to the framing plan, and adjust it to minimize loss of headroom, choose the best location for the equipment, and further fine tune Step 1, 2, 3, and 4. As mentioned above, use the tool to design all the loads, and there need not be any oversized ducts.
I’ve said it before – this is the field with the greatest potential for growth in residential construction. Who are these third party people? Will we evolve, as has the commercial construction, to defining a position of a “project integrator” who takes on this task, as well as the other complexities of interfacing green, structural, durability, smart home...? Not such a bad idea - time to start studying those Manuals!