Starting an HVAC Inventory for Your Buildings

I’ll let you in on a secret. Most people have no idea what heating, ventilation, and air conditioning (HVAC) systems are keeping them comfy on a day-to-day basis. Sure, you notice them when they don’t work or they are too loud or they’re in the way or blowing on you. (If you work in facilities or maintenance, this is when the grumbling and service calls come.) But even then you are usually only noticing one little piece of a much bigger set of thermostats, heating elements, cooling elements, and fans or other distribution systems.

So, let’s talk about how you get a handle on what all these pieces are, what powers them, and how much energy they use. An important related question is how are they metered and whether you can actually separate your HVAC costs from the rest of your energy bill. Our goal is to map the entire HVAC system and how much energy it uses, so that we can figure out how to modify the system to save energy and money. We will start with a process overview and use a typical home as an example. But our real objective is a process for mapping HVAC systems at an institutional level, like a college campus, an office park, or collection of municipal buildings. That’s more complex, but still quite possible. And that is where the big energy (and money) savings can be found.

A Simple Process

Step 1: What’s Inside Your Boundaries?

What spaces are you interested in? Thinking about your home, does that include the attic, the garage, the garden shed? On a campus, what buildings are you heating and cooling? Are there off-site buildings you should include or buildings on campus that should be excluded for some reason?

Step 2: What Powers Your HVAC?

What different energy sources are you using for HVAC in the spaces you identified in Step 1? If you have more than a fireplace, there is probably electricity involved. But most places use more than electricity, especially in colder climates. Heating oil, propane, kerosene, natural gas, wood, coal, geothermal, solar, and other energy sources are all possible. And it’s not uncommon to have more than one type of fuel.

Step 3: How Much Does It Cost?

Fuel costs money. Even renewables like geothermal and solar may use electricity for pumps, controls, and other subsystems. You want to understand your fuel costs. You want at least a year, but preferably several years of data. For each energy type you are using, you probably have a bill somewhere for what it costs. You need these numbers to really understand you HVAC systems and how they account for 40% or more of building energy costs.

Now, if you only make it this far, you will learn a lot. You can organize your costs against your fuel types and the spaces you are heating and cooling. This is where a lot of energy audits stop. It gives you a sense of the aggregate impact of your HVAC systems and may help drive certain decisions, like adding new insulation or windows, or perhaps replacing an entire system with a less expensive fuel or newer, more efficient furnace. But the real power of an HVAC inventory is about diving into the weeds of how these systems actually work together (or don’t). That’s how you can find major savings from small improvements (and make smarter choices for big improvements).

A More Complex But Worthwhile Process

Step 4: Break Out Your Spaces and Zones

You want to know how much total space you are heating or cooling within any given building. Then, you want to define the different spaces and uses within those spaces. In HVAC, we generally think in terms of zones. You will have to decide how detailed you want to be, but it is often handy to use thermostats or other controls to help define your zones. Your house might be zoned by room or floor or not at all. It’s also possible to have different systems with overlapping zones, but you can often simplify by focusing on the primary source or treating heating and cooling separately.

Step 5: Find the Systems

Each of your spaces or zones will have one or more systems that contribute to their HVAC. Generally, this will include thermostats (and other control systems), heating sources, cooling sources, distribution systems (e.g., pipes & valves and ducts & distribution fans), and terminal fans. Obviously, not all systems have all components (a fireplace may be a heat source and that’s it) and some systems put all the components in one box (PTACs). Again, some judgement may be involved in defining the systems, but a good guiding principle is to look for model numbers.

Step 6: Map and Classify

It’s time to map your systems, their functions, and your spaces and zones. While you can imagine some pretty complex engineering schematics for this sort of thing, for an HVAC inventory we are going to boil it down to a single spreadsheet. Each HVAC component gets a row and columns are used for classifying system types, functional groups, zones/spaces/buildings, and fuel sources. For institutional HVAC inventories, it may also be helpful to aggregate identical components into single rows and introduce hierarchies for aggregating useful categories (such as looking at things at a building level or floor level, rather than a zone level).

Step 7: Allocate Costs

Ideally, you would like to be able to figure out what each piece of the system costs to operate in terms of primary fuel and electricity. Note that some systems, PTACs, electric heaters, window air conditioners, etc. use electricity as their primary fuel and are that much easier to keep track of. Unfortunately, very few people have invested in smart metering to the level of individual HVAC components (with today’s inexpensive sensors, you could if you wanted to). For everyone else, you will need to start from whatever level you can break things down based on how you are metered and billed, such as by building, unit, tank, etc. Beyond that, you can use calculations to approximate how different systems most likely contribute to your overall costs. You might start simply by dividing costs by the number of units or systems, but there are more nuanced calculations that can be made based on square footage, unit size, and other factors. The nice part of building this in a spreadsheet is that you can use a variety of models for breaking down costs and look at them side-by-side.

Now, you have your very own HVAC inventory and cost model. It is a great first step in evaluating projects to reduce your building energy costs. As a starting point, it helps you compare your HVAC costs against other energy consumption, like lighting, office equipment, refrigeration, hot water, etc. It can show you the magnitude of what you spend and highlight areas where investments in upgrades or replacement might make sense. Perhaps most importantly, it provides you with detailed baseline data to see whether improvements you invest in actually lead to their intended energy and cost savings.

If you take it step-by-step, this is something anyone can do. But if you would like help, especially with a large building, complex, or campus, we are available. Please Contact Us for more details about HVAC Inventory projects.

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