Designing HVAC System for Humid Climate
I recently watched a Matt Reisinger episode on a beautiful home in Louisiana: https://www.youtube.com/watch?v=WuYvDuOQ-5M.
The house was designed and being built by en engineer (owner) and was simply spectacular. There was one part, however, that really made me wonder – the HVAC system. The house was built tight 1ACH50. The mechanical engineer designed a 24/7 running ducted VRF mitsubishi system, additional 2 Zehnder ERVs and 2 dehumidifiers. This is obviously as complex and expensive as it gets, but it seems that the beauty of simplicity was completely thrown out the window.
Why use an ERV in a hot-humid climate, when the dehumidifier could bring in fresh air and circulate it using the blowers that are already working via a large duct system. What is an ERV adding in this system, with very long runs of small diameter comfo-tubes that may not bring what is needed at the very end of the run? It seems to me these are really designed for a different climate, specifically for houses with ductless mini splits where you need something to bring in fresh air?
If pressurizing the envelope is a concern, why not install something like an air cycler that start a close by bathroom exhaust fan anytime the dehumidifier fresh air vent runs. I doubt this would be a concern in this house as it has a fireplace, which must have been taped off for the blower door test.
I am very impressed by this persons build and have followed a lot of Matt Reisingers advice in my own home which I am building. I guess I am asking this not to criticize but to better understand, as I am wrestling with how to optimally design my own system which already has VRFs with a large, well designed rigid duct system. I think both energy efficiency and IAQ need to play a role here. It seems to me that the HVAC engineer just threw the sink at this house given a clearly unlimited budget, but that should not be the goal. Please help me understand the deficiencies in my thinking.
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You noted..."This is obviously as complex and expensive as it gets, but it seems that the beauty of simplicity was completely thrown out the window." I agree. Maybe I'm just too focused on affordable energy-efficient construction, but I bet just about everyone on this site would agree with you as well. In the engineering world, simplicity is valued to minimize costs, speed time to market, and avoid defects and complex problems that are difficult to troubleshoot and fix. This gigantic single family home is a showcase of what an unlimited budget can do (and overdo).
“It seems to me that the HVAC engineer just threw the sink at this house given a clearly unlimited budget, but that should not be the goal.” Yup.
"Please help me understand the deficiencies in my thinking." Quite the contrary, your thinking isn't deficient IMHO. An understandable reaction to an uber-wealthy engineer's home with duplication of systems with no expense spared, and little attention to sustainability issues.
Risinger is a great educator, and values high performance homebuilding techniques. But he showcases innovative ideas without critiquing them in his videos.
As an engineer, I can tell you that engineers sometimes get a little to focused on "toy value" -- basically putting in an excessively complex system so that they can "play" with it. That might be what you're seeing here: the engineer building the house for himself wants a fancy HVAC system that he can play with and endlessly fiddle with all the myriad settings it probably allows for. This is similar to what is known as "feature creep" in the commercial product development world, which is the tendency for product designers to want to keep adding features to a product. At somet point, you get carried away and the added complexity doesn't really add any real value.
It is entirely possible to go overboard with an HVAC system and have lots of features you can point to that will address all kinds of unusual, or perceived, issues, but won't add anything useful in practice. I once worked with an architect who liked to test designs to meet his "two button press rule". What he meant was that anything that required more than two button presses to access probably wouldn't get used because it was "too complicated". That might be a bit extreme in the other direction, but it is a good idea to try to avoid unnecassary complexity to keep systems user friendly. Most people would probably like to be comfy in their home without needing to be bothered with doing anything to make that happen. That means a properly designed system is important, but so is keeping the operation of that system simple.
Bill