Good point, Andy. That table in the code (M1504.2 in the 2018 IRC) uses a set 15 feet deduction for each elbow. That 180 degree turn would take two elbows, as you say, but I’d guess the actual effect probably would be worth more than a 15 foot deduction because they didn’t make the turn gradually enough. The other problem here is that the flex isn’t pulled tight, which the IRC doesn’t address.

They’re 1/2 way with the 4 inch duct, but need to read note ‘c.’ I see a deduction of 45 + feet – 30 for the 180 and 15 for the little jog – that duct is “too long.”

Eric, David lays it out for us about CFIS. And I know little to nothing about multi-family fresh air strategies, but I’d think it’s more difficult to do than single-family. Smaller volumes of living spaces, just as many people breathing, cooking, washing etc. Would ERV’s be feasible at this point? Are they even allowed to be installed on a one-per-many units basis? Commissioning those would also be crucial in order to get the correct flow rates, and I am sure the right section in ASHRAE deals with those. Curt also had a point with new construction materials drying out.

Would CO2 sensors (to control FAS) in each unit be feasible?

At this point in the conversation about your high RH readings, I’d arrange the contributing factors discussed here in the following order of influence:
1. FAS
2. Backdrafting through dryer ducts
3. Others, including new-construction materials drying out.

MattM, I understand your point, but stubbornly remain in the position that today’s dryers are (most of them) yesteryear’s technology, like internal combustion engines.
And so we keep trying to find excuses for their existence, such as venting. And the hickups with venting, and lint, etc. etc.

OK, coming back to earth now, from the building science perspective I understandably agree with you. However, my builder’s instincts tell me that lint will find a way to accumulate given an opportunity such as an inline damper.

Paul,

I’m replying here because it wouldn’t show reply to your post.

Inline backdraft dampers are available in several flavors. I most certainly would use them in the positive airflow environment of a dryer vent. They close themselves when you have negative gradients. No different than damper vents on the exterior walls. The commercial dryer vents shouldn’t have all the fibers going through them like on residential, so less prone to clogging. Besides that, you always put them in a place where you can service them if they do stop functioning.

Matt, backdraft blocking is a good idea, except I would not put any on a dryer vent. This whole thread should remind us how we are still hanging on to a relatively primitive method of drying cloths.

@Eric, that’s a timer-based ‘central fan integrated supply’ ventilation (CFIS). Those are notorious for not being properly commissioned, and therefore often improperly sized for the attached blower. And even if installed perfectly, can lead to excess indoor RH, especially in Houston. On top of that, off-cycle operation of the HVAC blower can consume copious amounts of energy if the blower is not ECM and properly configured to operate on lowest speed during off-cycle calls. Here’s Allison’s article on CFIS.

Paul, sorry for the delay.

As explained to me, the fresh air system is ducted directly outside. There’s a controller that turns on the blower- whether havc is on or not- and draws-in fresh air based on the controller settings, and controls one damper. It seems to be strictly time-activated, unless it ties into the t-stat humidity readings; I haven’t seen any additional sensors. There’s a manual damper and an activated damper.

Dealing with the high humidity issue, I’ve experimented with the timer, dampers, turning it off entirely. My argument is we’re drawing HOUSTON AIR- you know, the super humid stuff?! Minimizing the FAS did help lower the humidity levels a few points. I don’t know what the long-term effects of this will be… I guess it’s there for a reason : /