CFIS Ventilation: Supplemental Fan Option
First things first, I'm not sure we can fully discuss a Central Fan Integrated Supply (CFIS) ventilation strategy without first talking about the industry's elephant in the room - proper HVAC equipment sizing! As we know, sizing of systems plays a big role in the run time of an HVAC air handler fan. A properly sized system tends to run longer than an oversized system (just stating the facts). Sizing plays a huge role in moisture removal during cooling season, when oversized we get short cycling and poor moisture control (which leads to...). This is even more important when installing all these Air Source Heat Pumps for electrification and decarbonization efforts. I'm starting here because this may play a part in the decision to use a supplemental exhaust fan integrated into the CFIS system. So, as always, step one - let's properly size those systems!
I also want to take a moment to note that I'm not pushing a particular brand but the AirCycler? G2 with the supplemental FanConnect? exhaust fan switch, is a pretty cool setup and they also have great documentation, including a homeowner guide which I love seeing. If you're installing CFIS ventilation systems, I'd love for you to mention your favorite units and configurations in the comments.
So, let's talk about the setup. A CFIS system needs to include, at minimum, a fan controller for the air handler that is equipped to call for additional run time of the air handler fan when the HVAC system has not run long enough to satisfy ventilation requirements, and a fresh air duct equipped with a damper to control incoming outdoor air when run times are longer than needed for ventilation (more is not better). We'll get into CAR dampers at a later date. With that, particularly when we're dealing with short cycle times, we're using a pretty big fan (hopefully with an ECM motor) to make up for the ventilation needs, the opposite of what we're looking for in energy conservation. We're seeing this play out in the results of our energy simulation models.
Note: A duct supplying fresh air to the return of the HVAC system without a damper and controls is not a CFIS system and might I add, not a good idea. (in my humble opinion)
In the cases where we're not getting enough run time to satisfy building ventilation needs, I am a fan (maybe pun intended) of using a smaller supplemental exhaust fan integrated into the CFIS system. It's important to use an integrated fan controller and not just running separately controlled exhaust fan. Technology has come a long way with microprocessors, allowing all the system components to work in tandem to provide a more balanced and economical CFIS strategy.
To third party verifiers - you need to do your homework here! Please read the documentation and understand how the system should be set up and how to identify if it in fact is. There are also testing modes you may need to be aware of. When performing HERS Ratings, I always loved to be on site with the HVAC contractor so I could understand the strategy being used. This has led to some really great conversations and learning opportunities for both of us. WE NEED TO TALK TO EACH OTHER! (yes, I'm yelling)
Just because I'm writing about this topic and my preferences when CFIS systems are used, does not mean I prefer this ventilation strategy. The main reason, it has to be done right or the homeowner may suffer significant penalties both with energy consumption and health & safety. Improper setup and unfavorable conditions, may lead to premature failure of HVAC system components, another hit to the wallet for the homeowner not to mention additional issues that could be caused during system down time.
Having said all that - I would really love to hear from our industry professionals (many much smarter than me) to get your take on CFIS ventilation strategies, if you're seeing/using it in the field, and what issues you're finding. Speaking for myself, I've seen my share of improperly installed and/or misunderstood systems. I'm guessing you have too!
领英推荐
#CFIS #Ventilation #LetsGetThisRight #HERSRater #HVAC
Unique zone control boards designed for ENERGY STAR, net zero, and low load homes available at FAZSControl.com
3 个月Cindy, I 'm glad you brought this topic up. I have been struggling with outside air equipment designs and control for over 40 years. Trying to bring outside air into equipment that runs intermittently is always going to cause control issues most of the time due to equipment cycle times vs the air requirements. The only solution I have seen that works well is a dedicated 100% outside air handler with temperature control and humidification and dehumidification capabilities working with exhaust fans to maintain a very low positive or neutral pressure relative to the outside. Unfortunately, scaling down that type of equipment for residential applications to my knowledgeable has never been done. Filtration for that type of system could be tailored to the contaminants in the local outside air.
Business Development Specialist Northern NJ at Aprilaire, a Division of Research Products Corporation
3 个月One thing we can all agree on is that ventilation is an important part of good IAQ. There are many ways to accomplish this and with all the different systems each will bring its own set of challenges from upfront cost, operating costs to increasing or decreasing RH. In my opinion ventilation is a multi pronged approach and you will always need to add supplemental humidity control (humidifiers & dehumidifiers) as well as good filtration with what ever system you use.
Residential HVAC Design and Consulting for high performance homes. Founder of Comfort Science Solutions. Youtube content creator @timdestasiohvac .
3 个月Soery, I cannot disagree stringly enough. CFIS is a horrible strategy. With multi stage and variable speed units, CFM will vary. And every green grass climate has low sensible/ high latent weather that loads the building with moisture. This strategy has been adopted by cheap builders who dont want to pay for a erv or ventilating dehu and inept code creators who don't understand psychrometrics.
Full-time Technical Director | Part-time Welder | Aspiring Retiree
3 个月Great conversations happening here - I love it!
Business Development Specialist Northern NJ at Aprilaire, a Division of Research Products Corporation
3 个月CFIS fresh air system are an inexpensive way to ventilate, but they bring some challenges. CFIS systems can increase or decrease humidity in a home and if you don’t use a good control that has the ability to lock out the ventilation if the RH and temperature is adversely affecting the indoor environment you will have an unhappy homeowner. AprilAire offers the 8120X ventilation control that has two control options. You can use the 8120X in the code set up that will ventlate so many minutes out of every hour base on the size of the home and number of bedrooms. It also offers a comfort option that will give you RH and temperature lockouts.