Why Every HVAC System Is Oversized (And How To Rectify This)

The revelation

At university, I learned something so groundbreaking, so vitally important, that in retrospect I should have known it all along…

From first grade, we learn incrementally complexer math. From addition and multiplication, to calculus and imaginary numbers (such as square-root of -1).


Wait, you mean by hand?

The assumption that one develops in such an environment is that everything must require complex math… which isn’t the case. The revelation I had was the following: What is hard isn’t the math, not at all. What is hard is to get the data!

This is especially true in the HVAC world. My god are those equations easy! You can easily fit them in a spreadsheet and you are good to go. But how do you know exactly what heating power will be needed? Unless you have the data, you won’t be able to do more than an educated guess.

The Car Analogy

Let’s imagine a map with a Start location, and a Destination location. You know where you are, you know where you are going, but you have some important data missing: you don’t know how you will get there. Will the road be a straight line, or will there many detours?


You must guess how much gas you need. There’s no way to know which road is available.

Furthermore, you have to fill your gas tank before you start your journey and you won’t be able to fill it en route.

What’s the value of the dashboard’s fuel consumption under these circumstances? What is this ‘equation’ worth? Without the distance, it’s worthless.

You have to guess how much gas you’ll need. You can’t underestimate, because you will then be stranded in the middle of nowhere. So you have to make an educated guess, based on your past experience on the road. You will also add a little incertitude bias, just to be sure you will make it.

Thus, under normal circumstances, you will always have some gas remaining in your tank when you reach your destination. In addition, this gas will cost you more money upfront, and will add to the total vehicle weight, consequently making it burn more fuel. Physics’s a bitch.

Back to HVAC

It’s the same thing with any HVAC component. Suppose you need a boiler for a new school. What size should it be? Engineering firms can guesstimate, but they will also add a little extra, just to be sure. To a certain extend, you can control an oversized boiler to keep the temperature comfortable, but if the boiler is too small, no amount of control will allow you to reach your temperature setpoint.


When is ‘big’ too big?

As with the car, you need to overestimate. Unfortunately, a bigger boiler means an additional upfront cost. Bigger components also means higher operational costs.

You have BACnet? Use it!

All of this, of course, is the consequence of not having enough data! Without data, math equations are worthless.

It’s ALL about data. Let me make this clear: data data data data data.

Hopefully, building owners and engineering firms will tap into the torrent of data the BACnet protocol can offer them.

Building a new installation similar to another one? (Engineering firms love to copy-paste) Check the original installation! Boiler is cycling? Too big!

Same thing for renovation: You want to install a shiny new heat pump in an existing building? Well now you have data on a silver plate! Instead of guessing what is required, you can actually know it.

There you have it. Until the industry starts to seriously use the available data, HVAC systems will be oversized. Not because someone is lazy, but because without data, oversizing is a necessity!

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