Welcome back! Every heating and cooling (HVAC) system should be able to do 6 things:
BAD ASS HVAC is designed to be able to deliver all 6 of these best practices. We covered load matching and filtration in part 1, the surprisingly complicated dehumidification piece in part 2, fresh air and coronavirus recommendations in part 3. In this installment we will cover Right Place at the Right Time (aka mixing) and Humidification.
HVAC Function 5: Mixing
This is a photo of a drop of food coloring dropped into a glass of water. This uneven dispersion is a good example of what happens when a fluid isn’t mixed. When a contaminant is added it does not spread evenly.
What happens if you stir this glass? You get a glass of water with a very slight blue tint.
This is relevant because the air in our homes works far more like the water in this glass than you might assume. Air at low pressures acts just like a liquid, so the principles are almost the same. Contaminants can disperse very unevenly.
If your home has a central duct system, you can mix the air in your house by turning on the fan for your HVAC. We’ve found mixing to be helpful with comfort and air quality problems. Here are a few areas we’ve found it helpful.
Evening out surface temperatures
About 60% of human comfort is related to surface temperatures, so if we can control them well we have solved over half of the comfort equation. Mixing can help quite a bit with this.
Our bodies like surfaces to be in the 65-75 degree range, so the better the job we can do at creating that environment, the more comfortable we will be. Surfaces above that temp don’t accept heat we’re giving off fast enough, so we can overheat. Conversely colder surfaces suck heat out of our bodies and make us colder. The goal is to keep all surfaces in that 65-75 degree range, or within 2-3 degrees of what your thermostat says.
This is a classic illustration for recommending radiant floor heating, but it also applies to using forced air HVAC system fans continuously to reduce surface temperature differences.
By stirring the air in a room, you move it across warmer or colder surfaces and even out the differences. The effect is magnified with variable speed HVAC that also provides small amounts of heating or cooling while moving that air. That’s called “load matching” which we covered in Part 1.
The technical term for this is “Mean Radiant Temperature”, or the average of surface temperatures around us. Mean Radiant Temperature (MRT) is a very important concept to understand if you want to be comfortable. If you have home comfort issues, a lot of them can be solved with HVAC using load matching and mixing. There’s a caveat though, if your house is especially leaky, you may need to air seal and insulate those cold surfaces before your house can be cozy. The only way to figure that out is with a bower door test which is part of the HVAC 2.0 Comfort Consult.
We consistently get feedback from our clients about how much more even the temperatures feel in their homes. One told us that she thought we had installed “invisible radiators” in her home. Mixing combined with load matching are really powerful tools we use to achieve this effect.
Evening Out Temperature Differences Between Rooms
This is a graphic from Mitsubishi Heating and Cooling to show how their systems can be used to zone a house (we’re not huge zoning fans, but that’s another topic.)
This graphic nicely shows how different rooms can have different temperatures. Sometimes that’s desired, but often a room is too hot or too cold and people are frustrated because they are uncomfortable.
We have found that if the house doesn’t have serious air sealing and/or insulation deficiencies, very often you can fix these control issues by installing properly sized modulating equipment and having the fan set to continuous circulation. This strategy gently moves the heating or cooling around in the house and blunt the temperature differences. We find continuous fan can change room temps by 1-3 degrees, and as much as 2-5 degrees if you also adjust dampers seasonally.
Mixing and Indoor Air Quality
If you sleep in a room with a door closed, and don’t have your HVAC fan running continuously, the odds are high that the carbon dioxide levels in your bedroom break 1000 parts per million (ppm). That’s double atmospheric levels and where you literally start to get dumber according to an LBNL study. If you sleep with someone else in that room, you may be hitting 2000 ppm which is high enough to impact your sleep quality. We talked about this last time as part of fresh air.
One solution to this would be to bring in more fresh air to that space (an open window or fresh air system), but turning on the HVAC fan is a much simpler, more consistent and controllable approach.
Since the rest of your house is not at those elevated CO2 levels at night, by mixing you not only provide continuous filtration, you’ll average out the CO2 levels and bring them down in the bedrooms.
Mixing and Filtration
If your HVAC fan is on, it’s moving air throughout the entire house, but it’s also running that air through the filter(s). This means you get air cleaning at the same time. To repeat, we recommend a minimum of MERV 11, and if you have people with respiratory issues, preferably MERV 13 or higher. See part 3 for more.
Mixing and Fresh Air
If you have a fresh air system (aka mechanical ventilation) that is part of or attached to your central ducted HVAC system, turning the HVAC fan on will help mix that air. In an upcoming article we’ll dig into what that looks like in a BAD ASS HVAC system.
Mixing Caveats: This is not for every home or HVAC system!
That’s mixing. Pretend like your house is a vinaigrette you want to keep from separating - stir it all the time!
HVAC Function 6: Humidification
Humidification is the last function because it’s probably the least important in some ways, but as I’m writing this during the COVID19 quarantine, humidity control looks like it may be very important in reducing how long the virus remains viable.
Keeping humidity in the middle of the range helps a number of things, as you can see in this older chart.
The boilerplate recommendation is 40-60% relative humidity, but the temperatures this is recommended at aren’t clear (relative humidity is relative to temperature.)
Our recommendation is to stay in the 30-50% relative humidity range. Aim for between 30-40% in winter (less if in climate zone 6 or above), and between 40-50% in summer. This typically equates to 35-55 F dew point.
That summer RH number may seem hard to hit, we do it by raising the temperature set point to 75-78 degrees. That may feel warm if it’s humid, but most find it really comfortable if it's dry. We achieve low RH most of the summer by installing equipment with reheat dehumidification, or whole home dehumidification. No More Thermostat Wars - we have found 75-78 is comfortable for both men and women if you are able to keep RH in the 40-50% range.
How to Humidify
Dr. Dustin Poppendieck of NIST recommends using purified water for humidification purposes. This could be either distilled water or reverse osmosis and is recommended for either room or whole house humidifiers.
For whole house humidifiers we’ve come to prefer “water saver” models. They use about ⅔ less water and are less prone to leaks. They do require changing the pads more often, so be sure to have some on hand.
We recommend people pay close attention to keeping their homes dry in summer, and being careful not to add too much moisture in the winter. By limiting the amount of water you put into your house, you are reducing the risk of mold or mildew forming in your attic or walls. We’ve seen a lot of mold in attics likely caused by running humidifiers set too high all winter. Humid air is buoyant, so moisture rises through the house, slips through air leaks into the attic, hits cold building surfaces and condenses, creating all kinds of moisture related problems. This is part of why we recommend 30-40% relative humidity in colder weather.
If you need more humidity than a water saver model can deliver, you might need to consider air sealing your home. Higher capacity humidifiers increase the risk of moisture damage to your home.
We also strongly recommend that any whole home humidifier you install has an outdoor temperature sensor that reduces humidity levels as outdoor temperature drops. This helps prevent condensation in unseen places and on windows during cold snaps. Many smart thermostats can serve this function.
If you are chemically or mold sensitive be super careful humidifying. Rather than whole home humidification it might be wise to stick to room humidifiers as needed.
Lastly, you can’t manage something that you don’t measure, so we highly recommend buying an IAQ monitor with at least a temperature and humidity sensor. GoVee has a Bluetooth datalogging model for $20, they are so inexpensive that there is no excuse not to have one.
COVID 19 Notes
If you’ve been following me for long, you know that I’ve dug deep into Indoor Air Quality for years now, heck, it’s how I got this column.
After masks, surface cleaning, and handwashing, I suspect we’ll find that creating a healthy indoor environment and keeping your respiratory system protected and strong is one of the most important factors to reducing the spread of COVID19 and reducing the severity of it at home.
Retrotec Inc, air quality researcher Dr. Shelly Miller and I gave a webinar digging deep into how air quality and viruses interact. Here are the key takeaways:
The first three are low risk interventions that have a fair amount of science behind them, so our recommendation is to stick with humidity control, fresh air, and filtration. Note that they are all part of BAD ASS HVAC.
Next Up: What the heck does BAD ASS HVAC look like?
When we set out to create BAD ASS HVAC, we had 3 goals:
Basically, BAD ASS HVAC is meant to be nearly perfect HVAC. It’s best practices encapsulated in one simple system.
Now that we’ve made it through the theoretical side of the 6 Functions of HVAC, we can finally dig into what it is. Next time we’ll start unpacking that. See you next time!
Nate Adams is fiercely determined to get feedback on every project to learn more about what works and what doesn't. This blog shows that learning process.