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Very low flow at -50: Use the blower door or the Duct Blaster?

A theoretical question here. I tested a small building that's around 110 CFM50. I have ring C for the blower door, and that's what I used for the entire test. Tec-Tite asked to switch to ring D near the end but I don't have that. During the test the fan speed moved up and down quite a bit as it tried to hold each pressure point. That doesn't hapen during more normal tests with higher flows.

Any chance this test would be more accurate if done with a duct blaster fan?

 

Answer
Good question. If you had Ring D or if you used a Duct Blaster, either would have allowed you get those lower data points. So in that sense, yes the Duct Blaster may have led to a slightly more accurate test. So would Ring D have helped. 

But the issue you were experiencing with the fan speed not stabilizing is a curable one regardless of which fan you are using. In TECTITE you want to decrease the Fan Adjust Rate (one of the Auto Test Parameters found on the Test Settings screen). What is happening is that you have a very tight building (and I would guess from your symptoms it is NOT a very small volume). The combination of very tight and medium to large volume leads to fundamentally slow response times for the building pressure to stabilize. What happens is that the auto fan adjust logic starts making corrections even before the final result of the previous correction has been seen. This leads to overshooting and undershooting of the target (oscillations). By decreasing the fan adjust rate (try 0.5 instead of 1.0) you can likely solve the problem. In extreme cases you may need to resort to using "Semi-Auto" mode which is a mode where YOU are controlling the fan, yet the data are all being electronically captured for you. It is found on the same settings screen under "Method". This allows you to test almost no matter how tight and how large the building is.

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