One Size Doesn't Fit All

Designing an effective heat pump system requires careful engineering—getting the equipment, sizing, and installation just right for each unique home. But how do contractors actually determine the correct size for a heat pump? Let’s do the math.

Solving for X

In designing a heat pump system, the goal is to install equipment that will heat and cool the home efficiently throughout the year. A few key questions determine how the system should be sized: where is the home located, how is it built, and how cold or hot does it get seasonally? Together, these factors define the ‘load’ - or the heating and cooling demand- needed for a particular home.

But before we dive into how each of these attributes affect the load, let’s discuss some basics.

Starting from First Principles

Every house is unique, so how can we know how much heating or cooling capacity is needed to sustain it? Let’s touch on some basic design fundamentals to give you an idea of how an engineer would think through this problem.

Every engineered system, including a home’s heat load, operates on basic principles. If we understand the core physics, we can model a system on paper and make reliable predictions.

A classic example from every mechanical engineering student’s undergrad career is finding the strength of metals. In this lab experiment, they’ll set up material samples in a tensile testing machine - an instrumented frame that pulls the material until it breaks.

Over the years, materials scientists have conducted thousands of these tests and tabulated all of the data into standard tables which engineers can use to look up the properties for most materials.

Simple building blocks help us model complex systems

We started by pulling a simple metal rod and found its behavior in one direction. But for larger, more complicated systems, we just break them down into a bunch of smaller, simpler shapes and do basic math on each one.

Putting all of this together lets us model all kinds of geometry using the information we gathered from basic tests - and computers let us solve thousands of equations at once to get meaningful predictions.

Models alone are often not enough

However, our computer models can only take us so far. With car crash tests, engineers gather data like stress, strain, and deformation to refine their models, anchoring predictions in real-world behavior.

We can predict outcomes to an extent, but physical testing reveals gaps and confirms accuracy, so the process continues in a cycle of testing, learning, and improving.

This testing and simulation process is expensive and tedious, involving hundreds of engineers and equipment, but it’s critical in high-stakes situations like car safety. And even though we've done this for decades, we still crash-test every new car model because there are always new unknowns. We can’t get everything perfect through simulation alone.

What if we can’t test everything?

For many projects, it’s not practical to spend the upfront cost and time to simulate every single variable and test the system to make sure it works just right.

Instead of trying to perfectly model everything, we build in safety margins. For example, bridge design, you don’t have to worry about weight because a bridge isn’t moving, and you don’t need to optimize for fuel efficiency like you would in a car or a plane. So, you simply overbuild it.

Bridge designers use materials like steel and concrete, which are relatively cheap, and make the beams many times stronger than necessary to cover for unexpected events like a ship crashing into the bridge. Although there is still a lot of engineering work that goes into designing a bridge, it removes the necessity for real world testing of every possible scenario.

I thought this was a newsletter about heat pumps?

So how does all this relate to heat pumps? Homes, unlike cars or bridges, aren’t typically engineered with strict precision. Factors like aging materials, remodels, and varying construction standards impact heating and cooling needs.

The standard method for heat load calculations in homes is ACCA’s Manual J. It takes into account a home’s construction, the weather at it’s specific location, how the heating or cooling is delivered within the home, and how well sealed the home’s envelope is to calculate how much heating and cooling load is necessary.

Similar to how we were able to test the strength of a material in a simple case and use that to model the behavior of more complex systems, we can do that with homes. Using standard values based on different building materials, an engineer uses Manual J to create a representative model of your home.

Testing to get closer

With a lot of unknown unknowns in home construction, a few things can help get a more accurate assessment of the leakiness of a home, making a more accurate load sizing possible.

Blower door tests measure how much air leaks into a home, using a specialized fan installed in the doorway. This data helps calibrate the Manual J model to more accurately reflect real-world conditions. Thermal imaging identifies specific points of heat loss and poor insulation, helping to spot opportunities for improving the building’s envelope.

However, both of these tests require specialized instrumentation that can cost thousands of dollars, as well as skilled practitioners who know how to properly set up the tests and interpret the results. Many times, these inspections can cost hundreds or thousands of dollars and are conducted by independent home energy assessors.

Conservatism strikes again

Since it’s impractical to know every detail of a home’s thermal behavior without spending a fortune on engineering and testing, heat pumps are generally sized with some estimates and assumptions.

Most contractors doing a Manual J assessment will make assumptions based on the home’s age or construction type. Many of the software tools that streamline these calculations offer suggested assumptions for standard building types.

The Manual J calculations themselves have built-in safety margins which prevent undersized systems. Even without thermal imaging or a blower door test, a thoughtful contractor can appropriately size a system if they know enough information about a home.

It’s important to design before installing

However, many contractors cut corners in the design process, simply matching the prior equipment’s capacity or using rough rules of thumb that don’t account for the attributes and location of a particular home.

Because furnaces have historically been oversized, this approach often results in an oversized heat pump system, which can create comfort and energy efficiency issues. If you’re a homeowner, make sure your contractor does the appropriate sizing calculations.

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