HORSESHOES, HAND GRENADES, AND HEAT LOADS: THE ART OF GETTING CLOSE ENOUGH
Heat pump sizing comes in intervals of 6,000 BTU/hr (half-ton) so close enough is the only reasonable goal for heat load calculations. Calculating heat loads down to a single BTU/hr won’t change equipment selection.
Heat loss calculations like Manual J, F280, and HOT2000 (H2K) have a long list of inputs that can be adjusted and manipulated in minute detail. This level of control gives the illusion of accuracy but you’re actually getting its cousin: precision.
NOTE: H2K is the energy modelling software developed by National Resources Canada and used by energy advisors (the Canadian equivalent of HERS Raters). For simplicity, I’ll refer to H2K, but the concepts and criticisms apply to other modelling software and methodologies like Manual J and F280.
Accuracy means getting close to the right answer. It’s about being correct. But precision is about being exact, whether it’s correct or not.
A Real-World Example
Let’s look at an example from 2023. I was helping a homeowner in Toronto (as a third-party consultant, not as an HVAC contractor). It was a hundred-year-old double-brick row house connected to neighbouring houses on both sides. It was leaky because of an issue in the converted attic. An energy advisor assessed the house, completed an energy model, and created a full report with recommendations.
The report included a heating requirement of 83,052 BTU/hr (6.92 tons) and estimated the house would use 3,971 m³ of gas (1,400 therms) per year for heating. Because of the report, the contractor recommended a 7-ton gas furnace.
Such precision.
Here’s the problem: over the previous twelve months, the house only used 1,300 m³ (460 therms) of gas for heating—67% less than the modelled amount. I confirmed that the homeowner hadn’t taken any winter vacations that would’ve skewed the data.
I did a performance-based heat load calculation based on actual gas consumption, and the heat load was 26,000 BTU/hr.
One of the best ways to improve the accuracy of models like H2K is to calibrate the results based on real-world performance data like thermostat runtime or energy consumption. H2K has a very strong correlation between modelled gas consumption and heat loss (see figure 1).
For this house, you can use the actual gas consumption and prorate the heat load. The house used 33% of the modelled gas consumption, so the heat load is closer to 33% of 83,052 BTU/hr (27,000 BTU/hr).
It’s not perfect, but it’s getting closer—and close is the goal.
WHY ACCURATE HEAT LOADS MATTER
You can’t get right-sized HVAC without an accurate heat load calculation.
Sure, but why do we want right-sized HVAC?
Comfort, mostly.
But it also has serious implications for heat pumps. Central ducted heat pumps are often constrained by duct capacity because they need to push more air to move the same amount of heat.
The industry tends to overestimate heating loads, so improving accuracy generally leads to smaller equipment, which reduces the risk of high static pressure.
Smaller equipment will perform better within existing infrastructure, it’ll dehumidify better than oversized equipment, it’ll be quieter and require less maintenance than systems with high duct pressure, and it reduces the odds that the outdoor units will need to be 50% bigger (2 fans instead of 1).
The Comfort Factor
Let’s talk briefly about comfort.
Oversized HVAC is the underlying cause of many comfort problems. Traditional contractors oversize equipment as a way to reduce risk: if it’s too big, it’s not too small. Or so the thinking goes.
We talk about heating loads like they’re a constant, but it’s an ever-changing state. A house needs a different amount of heating or cooling every hour as outdoor conditions change.
The heat load that we calculate using the 99% design temperature is just a tool to size HVAC systems—but it represents a tiny fraction (by definition, 1%) of the year. The rest of the year has heating and cooling needs too.
And when an HVAC system is oversized, it serves the 0.1% at the expense of the 99.9%. During those hours, the system can’t match the needs of the house.
That means short-cycling equipment, which leads to hot and cold rooms on the top floor of the house because the system isn’t running long enough to provide conditioned air to those floors. The thermostat on the main floor tells the furnace to turn off, long before that happens.
Right-sized HVAC is better at load matching, so it can provide the right amount of heating or cooling during more hours of the year. The system can match the needs of the house.
In most cases, right-sized HVAC needs to include a heat pump (either fully electric or installed as a hybrid with a furnace for backup heat—the right option depends on the local climate and the specific house). Even the smallest furnace on its lowest setting is too big for an average house.
This article is Part 1 of a 3-part series on heat load calculations and proper HVAC sizing by Drew Towzer for HVAC Know It All.
For more on heat pump sizing considerations, check out Gary’s article on Important Considerations for Heat Pumps, where he discusses the critical balance between heating load, cooling load, and duct capacity.