This article is Part 4 of our series on Manual J load calculations for new HVAC installations.

When you opt for a load calculation before buying a new AC or furnace, you’re taking the right approach to HVAC. Compared to your neighbor who just bought whatever the least expensive HVAC contractor tried to sell him, you’ll likely enjoy better comfort and more energy savings for years to come.

But none of that means load calculations are always done perfectly.

Sure, you’re far less likely to get an imperfect load calculation when you hire a reputable, experienced contractor. However, you should always discuss the load calculation results with the person or people who did the work to ensure they didn’t overlook anything important or make any glaring mistakes.

What mistakes might they have made? Here are a few of the most common ones.

1. Improper measurement inputs

The contractor performing your load calculation absolutely must input all the right data into all the fields in the Manual J software. Otherwise, the results won’t be accurate.

For example, say your contractor accidentally entered the R-value of your wall insulation as “0.” If you do have insulation in all of your exterior walls (and you probably do!), this error could throw off the load calculation results. You might be advised to buy a bigger system than the one you actually need. Not good.

So, how do you avoid this sort of outcome? By hiring an experienced contractor.

When an experienced contractor makes this sort of mistake – and it can happen, whether from faulty measurements or a “butterfingers” incident – he or she should be able to recognize that something isn’t right. After that, it’s easy to correct the error and move on.

Inexperienced load calculation contractors, unsupervised newbies, or untrained HVAC salespeople probably will not realize when they’ve provided improper inputs. Their mistake could result in your discomfort.

2. Window types

There are all sorts of windows out there! Compared to old single-pane windows, today’s double-pane, argon-filled, low-E varieties provide vastly better thermal performance. These differences can make a difference in your load calculation results, especially if you’ve got a lot of windows.

The thing is you can’t always gauge the thermal performance of a window just by looking at it. Sure, it’s easy to tell if a window has one or two panes. But aside from that, you might have insulated glass… or not. You might have low-E glass… or not.

A competent contractor will be sure to figure all of this out. A less experienced one might rely on assumptions or just “eyeball” it. That’s not good.

To avoid this problem, be sure to talk to your contractor about the windows in your home. If you’re planning to replace your existing windows with more efficient ones, that’s also something your contractor should account for in the calculation.

3. Ceiling heights

In many homes, this metric is very straightforward. You might have eight-foot ceilings in every room. Or maybe you’ve got nine-foot ceilings on the first floor and eight-foot ceilings upstairs. No big deal. That’s easy.

What can get a little out of hand – and it can have a profound impact on the load calculation – is when you have non-standard or inconsistent ceiling heights. Vaulted ceilings fit into this category. So do unusually shaped ceilings with different heights in different parts of the same room. Your load calculation needs to account for the variation.

Case in point: The living room below.

Imagine, for instance, that your living room is 625 square feet. The ceiling is flat, and it’s nine feet high. Your neighbor’s living room is also 625 square feet, but they’ve got a vaulted ceiling that starts at nine feet and climbs to over twenty feet. At the top, they’ve got a loft area, which also needs to be conditioned. Your neighbor will have vastly different HVAC needs, all due to the ceiling height and the resulting volume of conditioned space.

The lesson? Ask your load calculation contractor whether (and how) they account for ceiling height, especially in rooms where the height varies from one side of the space to another.

4. Air infiltration

Even if you don’t have a ventilation system, your home is getting some “fresh” air all the time.

No, the air isn’t filtered. And it probably isn’t entering your home from the places you’d like it to enter, but it’s getting in. That’s why we put “fresh” in quotes. Really, this air is infiltrating your home through gaps and cracks in the envelope. It’s not fresh in the filtered/purified/cleaned/dehumidified sense.

All that aside, air infiltration affects your load calculation. For that reason, your contractor needs to account for it.

The best way to do this? A blower door test. The blower door test depressurizes (or, in some cases, pressurizes) your home so that a contractor can identify all of the areas where the air is entering and leaving.

Just be sure your contractor is performing a blower door test as part of the load calculation. Air infiltration impacts HVAC performance in a very big way.

5. Post-installation hiccups

While this issue isn’t technically a load calculation mistake, it does represent a potential pitfall of some Manual J-guided HVAC installations. Sometimes, a contractor does a great job with the load calculation and either recommends changes to accommodate a more efficient HVAC system or to improve the performance of the system you need to install.

But when that work isn’t done properly, even the “right” AC and furnace don’t perform as intended.

To understand what we’re talking about, consider an existing home that’s inadequately insulated. The homeowner hired a contractor to perform a load calculation, and they told the contractor they were planning to improve the insulation in their attic. The contractor produced an accurate load calculation based on the assumption that the homeowner would increase their attic insulation to R-40 and air seal the ceiling.

That’s all fine and good until the homeowner hires an insulation contractor to improve the attic before installing new HVAC. Instead of air sealing the attic properly and blowing new insulation into the attic, the contractor forgets to seal several attic penetrations and fails to insulate all the way to R-40. They only add enough insulation to boost the existing R-13 batts to about R-20.

These mistakes can render an otherwise accurate load calculation inaccurate.

If the homeowner gets the newer, smaller HVAC system after paying for the shoddy insulation work, she might wonder why the AC isn’t able to properly cool her upstairs rooms during the summer.

This kind of scenario isn’t limited to insulation issues and can also occur with ductwork, window and door installations, and new additions. Here’s how to avoid it:

  1. Find an HVAC contractor who also performs home performance improvements. That way, they’ll be sure the load calculation matches the type of improvements that they know they’ll be making to your home.
  2. If you can’t find an HVAC contractor who does home performance projects, find one that audits and guarantees their work. A reputable contractor who does things this way is unlikely to make mistakes that throw off the load calculation.

Conclusion: A load calculation is only as accurate as the numbers you enter into it.

That’s the bottom line. By avoiding these common errors, you’ll get an accurate load calculation that provides reliable recommendations for HVAC equipment.

And yes, choosing the right contractor is a major part of the equation.

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