Economizer, ERV or DOAS?

At cfm we are often asked:

“How much outside air can we bring in thru a standard RTU?”

The short answer is you can bring in as much as you want, but you might run into issues as the percentage increases. 

The biggest issue with a large concentration of outside air is the leaving air temperature starts to sneak in to the 60s.

If you are just trying to “knock down the extreme heat”, then a leaving air temp in the mid 60s might be okay. But for most comfort cooling applications, a 55 (ish) degree leaving air temperature is ideal.

Here is a chart showing how the leaving air temperature is affected as the percentage of outside air increases.

In this example we look at a standard 20 ton rooftop unit at 8,000 cfm supply airflow.

As you can see, the leaving air temperature starts to exceed 55 degrees as the outside air percentage reaches 15-20 percent.

So, as a general rule of thumb, 15-20% is the maximum outside air that should be brought in through a standard RTU (or split system) – especially if the application requires constant fan.

“What should we do when the outside air requirement exceeds 20 percent?”

One of the best ways to combat a high outside air requirement is by adding an energy recovery ventilator (ERV).

In the following chart, we explore the system affect when an ERV is added.

Note, specifically, the leaving air temperatures as the outside air percentage increases.

As you probably noted, adding an ERV reduces the leaving air temperature by as much as 10-12 degrees in some cases.

One of the biggest benefits of ERVs is the load reduction (see chart above for ERV load reduction capacity). This means that in many cases you can REDUCE the size of the rooftop unit which will drastically reduce energy cost.

Although ERVs can be expensive, with the reduction in RTU size (which typically includes a boost in efficiency), pay backs can be as little as 3-5 years. 

Worth mentioning here – an ERV may be required by code for your project. Below is a snapshot from the 2018 International Energy Conservation Code (IECC). Check your local codes to see which IECC your project must follow and then locate this chart to see if an ERV is required. 

Climate zones 4A, 5A, and 6A are highlighted since most everyone on this list is located in these zones. 

“What equipment should I use when the outside air requirement approaches 100%?” 

In this case you will likely need a DOAS (dedicated outside air system) unit. 

I think we’ve covered enough this week though. More on DOAS equipment another time.

For now we’ll leave you with this handy rule of thumb chart.

➡️ Are you working on a job that we can help you with? If so, shoot me an email or reach out to one of our sales engineers (contact info below).

Brad Telker
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