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“Hey Rick, I need to set a fresh air vent to 10% outdoor air. How do I measure that? More importantly,
how do I set that?”
“Calculating the percentage of fresh air requires that you know just a few things.” Said Rick
With the unit running in the Fan On position only, measure three different temperatures.
1. RAT Return air temperature.
This can be measured at the R/A grille in the building or somewhere in the Return air trunk
2. OAT Outside Air Temperature
This is measured at the fresh air opening where outdoor air is being brought into the building.
3. MAT Mixed Air Temperature
This is measured at the opening to the blower where the RAT and the OAT are mixed and being
delivered into the building.
“Let’s work with some numbers you would expect to see on a day like today”, said Rick.
The temperature outside is 45 degrees. That will be our OAT
The temperature here in the room is 70 degrees. That will be our RAT
The temperature measurement with just the Fan On, at the blower, is 65 degrees. That will be our MAT.
Click here to download the .pdf version of this blog to see the diagram with formula.
“OK Rick, that’s great! This tells me what percentage of OA that I have, but do I have to keep measuring
and adjusting to get it to the proportion that I need?”
“Well… it’s not as complicated as that. Using our same temperature readings and our trusty calculator,
let’s see how we can do it with a minimal amount of effort.”
“You said that you want .10 outdoor air and .90 return air is that right?” Asked Rick
“Yes, that would be minimum position for most of my customers. “ Replied the young technician
OK let’s use the formula below to see what we come up with, said Rick
MAT = (Proportion of OA x OAT) + (Proportion of RA x RAT)
MAT = (.10 x 45) + (.90 x 70)
MAT = (4.5) + (63)
MAT = 67.5
“So there it is”, said Rick, “Close the fresh air damper slowly until the MAT (Mixed Air Temperature)
measures 67.5 degrees and you now are set at 10% Outdoor Air.”
As you might have guessed, the greater the difference between the OAT and the RAT, the more accurate
these formulas will be for you to use. Take several readings before adjusting anything. Remember
garbage in = garbage out.
“Wow, said the young tech, that’s pretty easy. Thanks Rick”
“No problem, all you have to do is ask.”
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