Key Points:
- The CERV isn’t just for fancy new homes. Old homes have poor air quality and need fresh air too!
- In this home, a CERV2 was integrated into an existing central furnace/ac system. The existing supply ducts for the kitchen and bathroom were converted to return/exhaust ducts, and a new exhaust duct was added for the previously unducted half-bath. Zone dampers were also installed for smart spot-ventilation.
- The 80/20 rule applies – With some smart planning and a little bit of effort, huge results can be seen. This project was easily performed by the homeowner.
- Immediate results were experienced – better sleep quality, fewer lingering odors!
- Next step: removal of Natural Gas, switching to Mitsubishi Hyper-heat mini-splits and Heat Pump Water Heater.
CERV Retrofit - New Lungs for a 101-Year-Old Home
Written by Alex Long, co-founder of Build Equinox
More often than not, we get the question from people: “The CERV is just for new homes, right? Wouldn’t it be too much work to put one in an older home?” If that was true, this would be a very short article!
The truth is your old home needs fresh air ventilation because it wasn’t designed to have it. In a house like mine, homeowners over the last century have added insulation, sealed up cracks (either purposefully or by slathering layer upon layer of paint), replaced leaky windows, etc. In a home with two adults and four small pets, odors stuck around, bedrooms were musty at night, and allergies were prevalent.
This article will take you through our efforts to bring smart ventilation into a 1919 home in Central Illinois. This project was completed entirely by my dad and me (although, once the COVID-19 lockdowns started he had an excuse not to help).
And as a quick summary of how things turned out: wow. We started Build Equinox 10 years ago, and I’ve worked on development of the CERV since the beginning. I was no stranger to what it does and how it operates. But somehow it still surprised me how big of a difference it made. Sleep quality is drastically improved. Even with multiple pets, the house doesn’t smell. Allergies have been basically non-existent. I know my opinion is absolutely biased as someone who’s employment status is directly tied to the CERV, but I'm now a true believer!
House was purchased in 2012 and I haven’t stopped renovating.Planning - House Layout and Airflow - Before
The home is a two story, 3-bedroom, 1.5-bathroom, 1650 square feet (along with around 700sqft of unfinished basement) single family residence. It has a central furnace (gas) and AC, and as you can see from the above graphic, there are 8 supplies and one central return (in the dining room).
Air from the second floor had to travel all the way down the stairs to make it back to the return, so during the summer time, the second floor got hot, hot, hot. I was having to crank the AC at night to keep it remotely comfortable in the bedroom. The half bath on the first floor has nothing at all, so in the winter a space heater had to be used to keep from freezing to the toilet seat.
Using Sketchup, I mapped out how the ductwork was run throughout the house. The central furnace/ac is located in the basement. All the first-floor registers are easily accessible through the floor rafters and use 6” round rigid steel duct, while the second-floor registers all ran vertically through the walls with rectangular duct stacks (12” x 3”).
After
Now let’s look at how it should be. The rule of thumb is to supply fresh air to bedrooms and living spaces, and exhaust stale air from bathrooms and kitchens. The goal, then, was to convert the supplies in the kitchen and 2nd floor bathroom into returns, and then add a brand-new return for the half bath on the 1st floor. We’ll return to this in a little bit; but first the CERV is installed.
The Installation
The CERV Installation guide has suggestions for ducting configurations with various systems.
The central system for our home is in the basement. We lucked out and had a CERV-shaped space right next to it. This is a non-load bearing wall (mostly used to hold our home gym equipment), so we could take out some boards to get the CERV in.
Voila, a perfect fit! We spaced the CERV off the basement floor using two composite deck boards leftover from an earlier renovation, just in case of flooding.
Installation – Ducting Fresh Air Intake
I’m very happy with the wall vents we used for the fresh air intake and stale air exhaust. They come with a removable flapper (which should be removed), and has about a 1ft section of 6” duct on the back to go through the wall. You can find these at your local hardware store for around $20: Imperial part # 6398901
We hung flexible duct straps from the rafters, then ran 6” flexible insulated ductwork from the CERV out to the previously shown wall vent hood. The fresh air intake duct ended up being around 20ft long.
A little easier to see what’s going on. Ideally, we would use rigid duct, but with such a short run the pressure drop from the flex duct is pretty minimal.Installation – Ducting Stale Air Exhaust (To Outside)
It is always important to survey the impact on local fauna. These two wild beasts enjoy napping next to the warm furnace ducts during chilly Illinois winters.
Once again, flex duct straps were attached to the rafters and flex duct run to the outside wall vent.
Rear view of the CERV and the new exhaust duct.Installation – Ducting CERV Supply Outlet (To Inside) to the Central System
Back of the CERV connected into the central system return. You can also see at the bottom of the CERV, the condensate outlet drains to the existing central AC’s condensate pump.Installation – Return Ducts
Same graphic as earlier just to refresh your memory of the ducting changes.To summarize changes to the ductwork from before:
- Second Floor - Full Bath SUPPLY – change to RETURN
- First Floor - Half Bath (wasn’t ducted at all) – Add RETURN
- First Floor - Kitchen SUPPLY – change to RETURN
Return Duct – Second Floor Full Bath
Conversion of the full bath’s supply into a return was by far the most challenging part of the process. Since this is a bathroom with a shower, it wouldn’t be a good idea to just try to remove air via the register at the floor. Instead, the existing exhaust fan would need to be connected to the duct to help force air back to the CERV. Doing so, would require a little bit of precision demolition work on the floor and dropping some rectangular duct into the wall cavity through the attic.
Our 101-year-old home has plaster and lath walls. The wall into which I needed to insert the duct into measured approximately 7” x 3”. 5” oval ducting measures around 6.5” x 3”.
The bathroom duct runs through the floorboards, but luckily by removing the baseboard trim on the wall I had enough space to get access. I designed an adapter box to allow the new wall duct to connect into the existing floor duct.
I would like to thank my mother for all of the origami lessons when I was younger. After the box was folded and riveted together, the outside edges were taped, and the inside edges were sealed with duct mastic. 
The adapter box was installed into the floor duct and fully sealed up with mastic.
The 5” oval duct was dropped down through the wall, connected to the adapter box with sheet metal screws, and sealed with mastic. In the process I accidentally dropped a really nice flashlight down the wall cavity, so I guess it lives in the floor duct now.
In the attic the 5” oval duct transitions to 6” round. The bathroom exhaust fan outlet is 4”, then expands to 6”. To connect the two I used 6” insulated flexible duct.
Return Duct – First Floor Half Bath
Before this room was a half bath it was a back porch. A previous homeowner decided to enclose the porch, add insulation and a bathroom. Even though the space was insulated a bit there was never any airflow added to the area (not even an exhaust fan), so it would be freezing in the winter, and boiling in the summer. And also there’s that other reason for needing airflow in a bathroom...
To remedy this I needed to add a brand-new return duct to the half-bath. Once again I used oval duct (this time, 6” oval) and performed a little sheet metal magic to create a register box. The duct would travel through the basement rafters, up through some floor cabinets in the kitchen, and then into the bathroom.
My vibratory saw was put to good use during this project. In this picture, I cut a rectangular hole between the kitchen cabinet and the bathroom for the vent register. You can see on the right there are two water pipes that supply the sink in the bathroom. Unfortunately, this was the only area where I could put the register because of stud spacings in the wall.
More sheet metal bending. The bent sheet metal adapter on the left avoids the water pipes in the wall, and attaches to the 4”x10” to 6” oval duct transition.
Register installed through the wall, taped, and sealed with mastic.
The duct enters the kitchen cabinet (left) then drops down into the basement rafters (right).Return Duct – First Floor Kitchen
No modifications were needed to convert the kitchen supply over to a return, just disconnecting the duct from the central system.
Return Ducting – All Together Now
Now that the return ducts are all hooked up here’s an overview of what they look like together.But wait, there’s more! Let’s add some more intelligence into this system. Wouldn’t it be swell if we could target pollutant removal for the bathrooms and kitchen? We need a system that allows us to prioritize the exhaust from the full bathroom when someone takes a shower, or exhaust air quickly out of the kitchen when Alex sears some amazing Sriracha Honey Garlic Sous Vide Pork Chops on a cast iron skillet. Well, my friends, the future is now!
The CERV2 has the ability to control zone dampers! At the base of each branch to the half bath, kitchen, and full bath, I installed three 6” zone dampers which are wired back to the CERV with low voltage communication wiring. In each of the bathrooms and kitchen Active Circuit Transmitters monitor electrical circuit activity to indicate to the CERV when to ventilate. Note: Want to have a dedicated switch on the wall for triggering the CERV into ventilation, but don't want to connect into electrical wiring? We also have Battery-Free Wireless Switches!
In the full bath the active circuit transmitter is wired into the bathroom exhaust fan (which now helps push air to the CERV instead of exhausting directly out the roof). Once the exhaust fan is on, the ACT sends a wireless signal to the CERV to indicate the bathroom fan is active. The CERV then closes down the two zone dampers for the half bath and kitchen and increases airflow pulled from the full bath.
In the half-bath the active circuit transmitter is wired to the lights. When someone goes in and turns on the lights the CERV starts venting and closes down the kitchen and full bath zone dampers, increasing airflow pulled from the half-bath.
Lastly, the kitchen ACT is wired into the kitchen vent hood. When the vent hood is activated the CERV closes down the zone dampers for the full and half-baths which prioritizes air from the kitchen.
The zone dampers are easy to install and come in various duct sizes. The 6” zone dampers come as a short straight duct section, then three low voltage (24VAC) terminals are wired back to the CERV.
Results
Short answer: It works!
Long answer: I have the data! Let’s look at an example day from Thursday, April 9th. We were working from home (on account of, you know, the whole Coronavirus thing) so the occupancy for this day is two adult humans, one small dog, and three cats. The plot below is actual data from the CERV (users have the ability to view and download historical data if they choose to connect the CERV to their WIFI. Superimposed on the plot is the “CERV Mode” bar which indicates the current mode of the CERV with blocks of color.
Buckle up, because you’re about to witness the white-knuckled excitement of our daily life at home.
12:00AM – ZZZ
Right off the bat, you can see that there are periodic ventilation events occurring, about once an hour for 30 minutes (indicated by the green/orange bars). While sleeping I like to have “scheduled ventilation” enabled on the CERV. This means that even though the indoor air quality sensors might indicate a pollutant level below our setpoint (1000ppm), fresh air is brought in anyway.
You can also see that the CERV mode bar starts out as green and then becomes orange later in the morning. Between measuring the indoor temperature, outdoor temperature, and the user-defined setpoints, the CERV is smart enough to know when to use the integrated heat pump to provide additional heating or cooling to the fresh air brought in from outside.
6:00AM – The Cats…They Hunger…
Breakfast time! Scrambled eggs are a breakfast staple in our household (freshly supplied by the hard-working chickens at the Build Equinox Laboratory in Urbana) so the VOC readings on the CERV start to increase quickly. The CERV catches the increase of cooking VOCs and switches from recirculation mode, which mixes air throughout the house, to Ventilation Heating mode. Why Ventilation Heating? It’s a cold morning outside, so the CERV knows it needs to heat up the fresh air from outside before sending it in.
One thing to note – VOCs increased, but CO2 did not. We cook on an electric cooktop, if we had been cooking with gas, the CO2 would have surely shot up along with the VOCs.
4:00PM – Feel the Burn
Over the years I’ve been accumulating home gym equipment for our basement. I have competed in powerlifting in the past, so we have a great setup that allows us lift weights, ride a stationary bike, and run from the comfort of our own home. This has been a blessing since the Coronavirus pandemic has closed all gyms.
On this day, we decided to bring the pain with some HIIT, or High Intensity Interval Training. Push-ups, Burpees, Mountain-Climbers…self-inflicted cardio torture. With that heavy breathing comes high CO2 production. And the CERV was there too, doing its own high intensity ventilation. One more rep, bro, you got this!
The Rest of the Evening
There are no increased pollutant spikes for dinner time, since we meal prep all of our weeknight dinners. The CERV just does a bit of Recirculation Heating as the outdoor temperature drops, and then when it comes to bedtime, we resume the regularly scheduled ventilation programming.
I could show some more data, but with self-quarantining the days are all pretty much the same and time has no meaning anymore.
Summary
We often like to show off fabulous award-winning new homes using CERV smart ventilation, but sometimes it’s important to show a little love to our old homes! People often think that old homes are leaky and therefore get plenty of fresh air, but study after study has shown that where the leaks are, and where the people are, often doesn’t overlap. The addition of smart ventilation to a home can improve your health, cognition, and sleep quality (check out the studies listed on our Fresh Air page), as well as reduce odors and improve your comfort. In any home.
Overall, this was a project that was easily done mostly by one person, making it a perfect pandemic activity. Every home is unique in its own way, so a little planning goes a long way when deciding how you want to integrate the CERV into your own home (and we can help, just submit our contact form)!
Total cost for the project? If it wasn’t for my five-finger employee discount, the CERV2 typically starts around $5,500. Ductwork, fittings, aluminum tape, sheet metal screws, and duct mastic added up to around $300.
Installation time of the CERV ended up totaling around 30 hours. Getting the CERV in place, cutting holes in the walls for the the wall vent hoods, and connecting the fresh air intake, stale air exhaust, and supply to inside ducts was easily done over one weekend. The return duct branches (including converting supplies to returns and adding the new half-bath return) was accomplished over the next few weekends when there was time.
Next Up – Full Gas to Electric Conversion with Mitsubishi Ducted Minisplits
We will be tearing out our old inefficient gas furnace and central AC and replacing them with new ultra-efficient Hyper-Heat ducted mini-splits from Mitsubishi Electric. The CERV will act as the mastermind of the whole home, not only managing the indoor air quality, but the indoor comfort level (zoned to both floors, of course). We are very excited to start this next phase and can’t wait to show you how it can be done to your home too!
New Lungs for a 101-Year-Old Home Pt.2: Total HVAC Replacement!
New Lungs for a 101-Year-Old Home Pt.3: Heat Pump Water Heater
