HVAC by the Manuals

First – a shout out to Martin Holladay,of the highly acclaimed GreenBuildingAdvisor, who gave us a “thumbs up” review. Since this may have resulted in some new readers, today I will put on my Sunday best and make sure to give you a blog worthy of this honor.

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

Determine the load for the building as designed. Energy modeling software used in conjunction with a drafting tool might help determine the point of parity for energy savings return relative to cost of additional thermal efficiency. Of course, the installation must live up to the modeling, or the rest of the equations are subject to change. Manual J can also handle this identification of load, and it should be done room by room, taking into account potential heat transfer of interior walls, windows, and skylights. For example, a partition wall between a mud room and a bathroom can be a source of cold transfer. Window exposures can also influence heat gain to a room, and relevant SHGC entered into the equation. Skylights may influence heat gain, but are also prone to leakage, altering the overall “tight” envelope.

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.

Air Balancing

The newest guy on the block, the 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!