Ever since buying a house I've been engrossed in this topic along with indoor air-quality and mechanical HVAC systems. There's so much to know about these topics and I feel it helps when dealing with HVAC companies to make sure I'm getting what I want and not be price-gouged. One expert I follow on these topics is Allison Bailes, he has a blog entry on a similar topic of duct design[0]. I find his way of breaking down certain topics simple to follow and enjoy seeing how he's implemented his teachings in his own home.
The important part of good air shaft design is making them large enough to fit a squad of marines, and yet small enough not to fit biomechanoid xenomorphic invaders.
Isn't there also an important material selection choice to be made - your ducts (getting a bit Brazil about it) have got to be impervious to angry biomechanoid xenomorphs who can smash through thick steel doors but also easy enough to cut and weld with small handheld tools?
Bonus topic near and dear to my heart: Ducting design is a biggie, but a neglected issue, and the biggest sin I see in residential HVAC, is ignoring a design's peak heating or cooling energy demands and installing oversized heat pumps.
The bigger is better mentality in that area results in (sometimes dramatically) shorter run cycles with an unnecessarily large compressor. It's not good for your electric bill or the equipment, and you get a monster motor dimming your lights every 10 minutes.
If I had a dollar for every time an HVAC contractor offered me a quote including an upgrade option for a heat pump three times larger than is needed...
Why not get some indoor sensors and DIY? I’m planning to install an ERV and a whole home dehumidifier to see if I can get that ideal indoor air quality.
I can't help but think HVAC is the perfect industry for the limited AI we have right now... it just need to hit a sweet spot of relative humidity and temperature, with constraints given by the assets you have and its' performance envelope given input from mixture of indoor/outdoor air..
but right now HVAC industry is treating relative humidity like some ultimate luxury tier item..
PID and other basics of control theory have been around for ages, they're just not used in residential HVAC. Perhaps the limiting factor has been the traditional split system with only on and off states. As we move towards variable speed compressors and fans, we may see more intelligent controls. Maybe an AI can help tune the control constants?
Maybe because relative humidity is more expensive to control? in an ideal case it's just moving and mixing different levels of air temperature and humidity, but in reality you need to de-humidify or add humidity
I found it inordinately difficult to get humidity to a stable point in Canadian winters, because practically no homes are designed with relative humidity in mind.
A lot of it is managing the air barriers (not vapor barriers!) and sealing the house to prevent air leaks.
But if you're concerned about air quality there are some major things you can attack first - water heater being electric or forced-vent, furnace being forced vent, etc.
The less "burning" inside the house the better off you'll be to start.
Instrument things and DIY to tune, but start from Passivhaus standards. There's a ton of great developments on ERVs and insulation that could use more attention in the US.
Their youtube channel/resources focus a lot on testing/verification.
If anyone near San Jose / Santa Cruz wants to borrow a blower door and some testing tools for a few days or a week I'll make you a good deal =) -- If you want a non-professional evaluation I'd probably have a day or two of availability in the next few months as well. (I don't have duct testing tools yet, but with notice would probably get them.)
I am in Houston and AC is a necessity. Most residential AC guys here are awful- they can make a killing in the hot seasons for simple calls to replace a $30 run capacitors on compressors (very common failure, often encouraged by power blips). Most of them do not fully understand duct design and many will pass on jobs to re-do ducts if they can since it is more profitable to focus on break-fix type calls. Because of this, I had to learn about AC systems and duct design myself. Here are some things I have learned which are not often mentioned:
- Intake filter flow rates are important, 1" filters have varying pressure drops across them and can account for a significant loss of available static pressure in your system. Most companies do not list this figure on their product, or even provide it.
- Return air is far too often neglected in duct design. I have a section of my house which requires %25 of the available air flow to cool but it has no return air path. So if the interior door to that area is closed, the air is instead forced outdoors and the loss is reclaimed on the other side of the interior door from (you guessed it) outdoors. So if a door to an area is barely open and then closes when your system kicks on, you probably don't have adequate return air.
- Most newer homes use flexible duct. It is much cheaper and lighter than metal duct work but how it is installed makes a massive difference in how efficient your system is. Flexible ducts lengths which have slight compression (15%) or sag might double the loss for that section.
Overall the duct work design and installation is one of the most important aspects of your HVAC system which is the LEAST profitable (earnings/time) to your residential AC guy and therefore, is often neglected.
If you want to go deeper I recommend the ASHRAE Fundamentals handbook which covers heat load calculations, fenestration, systems, and duct design. You can pickup a older used copy for cheap. Avoid ACCA guides, they obscure all the underlying math and engineering behind a more simplistic design method based on total 'effective' length calculations which makes duct design difficult to understand intuitively.
All this assumes airflow to every room all the time.
But in real systems, you only want airflow to rooms with people in. And in a residential setting, you can be fairly sure that won't be all rooms at once.
Which means ducts can be sized differently, as long as you have sensor vents on the end of each.
I like to have CO2+humidity+temperature sensors in each room, and then when any go out of an allowable range you open the vent.
I sometimes also install PM2.5 sensors hooked up to the vent because unattended cooking or toasters can make a lot of PM2.5 and not much of anything else.
Also obviously you want the vent to open proportionally to how in-range the measurements are, otherwise you end up with a vent opening and closing every 5 minutes all day long which is very annoying.
I saw a Swedish "dream house" TV series where someone installed something like that, it cost a small fortune and I don't see the big benefit in it compared to "normal" mechanical ventilation, maybe with heat recovery.
The big benefit is that heat recovery units only recover ~80% of the heat.
And if you want really good air quality in a room, you want 10 air changes per hour or so.
So, that means you are losing 20% of the heat in the air every 6 minutes. If you're doing this in the UK with gas heating and outdoor air of 0C, indoor air of 20C, a 2500 square foot average house and 9 foot ceilings, then it costs a whopping £14/day to heat just the heat lost in the heat recovery unit.
Considering that you can buy a CO2+temp+humidity sensor for £15, and then only use as much air as is actually needed for the occupants of the house, you can normally reduce this by 90% typically - saving £4000 per year - well worth it for £100 of sensors.
You also don’t gain anything by only exchanging outside air with one room.
Let’s assume people are willing to let indoor CO2 levels reach X PPM over outside air in a room with N people that takes say 1,000 cubic feet per minute to maintain. The same number of people in a room 50x times as large still takes the same 1,000 cubic feet per minute to maintain. In a larger room it takes longer to reach an equilibrium, but the amount of CO2 you’re removing is a function of the amount of CO2 in the air which reaches the same level. Thus you can setup your air exchanger to vent in response to a single CO2 reading above a fixed level in your HVAC system and automatically adjust for a packed multi hour thanksgiving dinner in your living room or an empty house.
Circulating Air within a house is beneficial independent of how much air your venting to the outside. For one thing you can better regulate temperature with increased airflow for another you can filter the air for dust etc. So you really should treat them as separate systems.
If it makes you feel better imagine that 10’x10’ room with ventilation to the outside is attached to a sealed 100’x100’ chamber by a ventilation system you can turn on and off. The larger room will only have the same or better air than the 10’x10’ room and thus can’t make the small room worse off.
PS: Detecting people in rooms or activity levels does let you adjust temperatures accordingly which may save some money. Also, you may want more ventilation for other reasons thus kitchen and bathroom exhaust fans.
I put it behind a Lutron Caseta Fan Speed controller, it struggles a bit to communicate due to range, but at the low settings I can barely hear it and it keeps CO2 PPM to ~1k if it's just me in here (usually), I turn it up to 3 if I want a gentle wind noise and rarely go to 4 (but would if I wanted to freshen it up). I used the Fantech 4" ducts and it was pretty easy to self-install.
[1] https://airflow.apache.org/