HVAC sizing

There’s always a lot of talk about the energy efficiency of our houses. There’s much less talk about the ‘Building Performance’ of our houses, and there’s very little talk about correctly sizing heating and cooling equipment for our houses.

All of the afore mentioned items are important parts of a well-considered building system, but today my focus is on HVAC equipment sizing.

How is heating and cooling equipment sized? Most of the time, here in Australia at least, a general rule of thumb is used for equipment sizing, such as, floor area multiplied by 120 Watts. By this measure a 200sqm house would have a 24kW system, (200 x 120 = 24,000W).

Other rules of thumb calculate volumes of space, (length x width x height) multiplied by a factor such as 50.

Some HVAC manufacturers even have their own rudimentary sizing software based on volumes and regional multipliers specific to geographic allocations.

The problem with these rules of thumb and basic calculations is that they do not address the building envelope adequately. What about insulation levels? What about thermal bridging? What about window orientation? What about glazing performance? What about shading?  What about skylights? What about air leakage? Without taking these things into consideration, a basic rule of thumb calculation can be prone to over sizing of equipment, especially in the case of a higher performance building envelope.

When I do my own heat load calculations I find that my results are far lower than the results derived from rules of thumb and basic calculations….far lower.

With such a large discrepancy between results, my first reaction is to question my own method. So this is what I did, and as usual I asked my American friends.

As it turns out, this large discrepancy between a calculated heat load and a rule of thumb calculation is not uncommon. In fact, it is the norm.

Rules of thumb have been the same for a long time, and a long time ago houses were different. A modern, insulated house with minimum air leakage is a lot different to an old uninsulated leaky house. This being the case, why are the rules of thumb the same now as they always have been? Should rules of thumb be updated? Shouldn’t we do a proper heat load calculation to suit a new house?

With all of this in mind I recently had a Zoom chat with another American building science professional, and a guru of HVAC design, Dr. Allison A Bailes. Allison recently wrote an article about this very topic. He has come up with his own rule of thumb to gauge if a rule of thumb has been used in HVAC design!

Allison’s rule of thumb is 1000 square feet per ton, which translates to 350W per square, or 37W per square metre. Allison and I discussed the fact that even this rule of thumb is NOT a design option. As much as an old house has less insulation and is leakier, a new house might have far more glass, and disproportionate amounts of glass. Disproportionate amounts of glass in some rooms will result in a greater heat load than in other areas of the house. This is why a room-by-room heat load is important for not just equipment sizing but also for distribution of load.

The thing that piqued my interest in Allison’s article was how his rule of thumb, 37W/m2 compares to our rule of thumb for design of 120W/m2. It is about 70% less! This is roughly the same difference I find when I compare my heat loads to rule of thumb calculations. This highlights the difference between a guess based on the fear of not wanting to under-size a system, and a calculated design based on parameters specific to the building. Would you design an energy efficient car and oversize the engine by 70%?

Designing higher performance into our houses is a systems approach. HVAC design is a critical part of this system, and well worth investing in the thought process from the beginning of the project.

Click here to see Allison’s article about load calculations and rules of thumb.

By the way, just a teaser, heat loads are a component of my new Gold Star building performance system….but more about that later.