I think we're heading down a road that has very few footprints with this baro system. That's what I love, challenge accepted.
I was thinking that I could take your ideas for a passive system and add a little automation but still be passive with tighter controls. The more I think about it, I think your idea development is much more accurate. Logic dictates that to depressurize an area you must be pumping more air out than in. I also realize that with pressures that low, it likely would not even lift the backdraft damper on the roof cap. Let's develop your idea of putting an in line fan and going with 6" duct. This would also (in my thinking) require enabling the fresh air intake while the baro relief is active needed or not, have the baro fan set to move a little more air than the 50-55 CFM of the fresh air. There are also acoustical considerations to take regarding the cycle time. With that subwoofer system, you might be playing bass for Ft. Worth!
Rather than worry about a fan assist to overcome the resistance of a backdraft damper built into a roof vent cap, I would rather use an open vent cap without the backdraft damper. Given that the ducting is already protected by a motorized damper, I think the integral backdraft damper in the vent cap is superfluous and as you point out, probably detrimental to the function of the overall system. Furthermore, the inline fan option was only a contingency plan in the event the system could not maintain comfortable CO2 levels. For 98-99% of the time, I will be the only occupant. So for the majority of the use, the current system as designed should be more than adequate to maintain acceptable CO2 levels.
As far as the roof cap, I have learned painful and expensive lessons over the years do NOT use plastic or powder coat sheet metal they don't hold up in my area which is a milder climate than yours. Aluminum is the only choice, it has a very reflective surface that will reflect more solar gain than it absorbs. I used aluminum roof caps for bath fans on this house when we built it 22 years ago. When I had the roof replaced a couple of years ago, I reused them, they were still in pristine condition. I'm sure there are many on the market, here's one similar to what I'm talking about
they do tend to look commercial, but maybe a 6 inch won't be too obtrusive.
I had just finished specifying polypropylene vent caps after spending hours scouring the net for a good option when you wrote this. Argghhhh!
Another obstacle to overcome will be the amount of time it takes the damper to open, you don't want to deadhead or restrict the fan waiting on the damper to open fully. For my fresh air project, I chose Famco dampers, I wanted something with a switch to tell me when the damper was fully open. Remember my comment about assumptions and anticipations? There are add on end switches available for a lot of dampers that cost nearly as much as the damper itself. The Famco could be ordered with an intergal end switch. They seem to be made of a slightly heavier gauge sheet metal and have a really nice rubber air seal. https://www.famcomfg.com/product/mot...rmally-closed/
These cost about twice as much as the dampers I had planned, but again, it is not a huge amount in the pig picture, especially compared to the cost of Green Glue!
I could purchase the type with the end switch for the baro system and leave the end switch disconnected. It would be much easier and less hassle to have it available if I needed it later than to have to change out the damper after the attic is closed out.
Adding the fan would require adding a relay to start it, needs to have a 24vac coil and contacts rated for the fairly low current draw of the fan. You would also need a manual speed control to slow it down, a 6" inline fan would likely move far too much air at full speed. This would not be something that requires frequent adjustment, once it's dialed in just leave it.
I have one of these fans: https://www.solerpalau-usa.com/produ...td-silent.html
brand new in the box that I bought for a previously planned project that I ended up not doing. I am so vested in your project, I'll be glad to donate it to you. If you are interested you can contact me privately and I only ask send me a few bucks for shipping. I want to see it go to a good home. I believe I have a speed control and probably have a relay here that will work for this application with the same offer.
That is a very generous offer! I'm not convinced yet there is a need for an auxiliary boost fan. For less than the cost of the fan, I could just increase the size of the baro damper and ducting to 8" and the flow problem is solved. I added a little table (below) to help with duct sizing.
The wiring is starting to get a little more complex, but nothing we can't work out. I very highly recommend buying a junction box and putting a terminal strip in it. I'll try to find something suitable and send you a link. Devise a good wire labeling scheme and make all connections on the terminal strip, it can also house the fan relay. You can probably find one at a big box or maybe cheaper at an electrical wholesale house. By doing this, all of your wiring will be in one location and if you have to make changes during debugging it's all in front of you, as well as it looking professional. I'm going to have a label made for my control panel "FRESH AIR CO2MMANDER".' I probably have some modular terminal strips and some DIN rail that I'll also donate.
I've been thinking about how to simplify installation and have toyed with the idea of a simple PCB layout, much like the early DIY Soundgroup crossover boards. I would then mount this in a medium sized project box with all the wiring protected by grommets where they enter the box from the sides. I have some leftover DIN rail and terminal blocks from another project as well (a PLC-based water well controller that supplies water for 3 separate homes), if I should choose to go down that route.
I want to hear Dave's thoughts on this before making final decisions. I contacted him via his work email and he might not even look at it until Monday. I do believe you have pulled me back from the brink and gotten this back on track.
Glad I can be the one to help, for a change!