Yes, volume matters in the sense that your goal is to change the entire volume of stale air in a building with fresh outdoor air some number of times per hour to maintain a healthy indoor environment for people. The volume of a building can be calculated as the square footage times the average ceiling height. Ventilation units are either in Cubic Feet per Minute (CFM) or Air Changes per Hour (ACH). Buildings are sealed to different degrees which allows natural air infiltration to occur to some extent. 1.0 ACH or more is typical of older, leaky building construction. 0.5 ACH is typical of new construction. 0.35 ACH is typically the minimum allowed for health reasons and is often accomplished in airtight, energy efficient building construction by using a basic air ventilation system, Heat Recovery Ventilation (HRV) system, or Energy Recovery Ventilation (ERV) system.

ASHRAE 62.2 calls out 7.5 CFM per occupant plus 0.03 CFM per square foot of conditioned space. ASHRAE counts "occupant” as the number of bedrooms + 1. ASHRAE 62.2 assumes an average ceiling height of 8 feet, so you can relate this to volume if desired.

Building Science suggests that this ASHRAE number is too high, and recommends 7.5 CFM per occupant + 0.01 CFM per square foot of conditioned space.

If allowed, we recommend using the Building Science number and proportionately increase it if your average ceiling height significantly exceeds 8 feet (e.g., if your average ceiling height is 10 feet, increase it by 25%). Local building codes often specify what is allowed and some inspectors don't allow deviations.

Posted By sailawayrb on 06 May 2020 10:50 PM
Yes, volume matters in the sense that your goal is to change the entire volume of stale air in a building with fresh outdoor air some number of times per hour to maintain a healthy indoor environment for people. The volume of a building can be calculated as the square footage times the average ceiling height. Ventilation units are either in Cubic Feet per Minute (CFM) or Air Changes per Hour (ACH). Buildings are sealed to different degrees which allows natural air infiltration to occur to some extent. 1.0 ACH or more is typical of older, leaky building construction. 0.5 ACH is typical of new construction. 0.35 ACH is typically the minimum allowed for health reasons and is often accomplished in airtight, energy efficient building construction by using a basic air ventilation system, Heat Recovery Ventilation (HRV) system, or Energy Recovery Ventilation (ERV) system.

ASHRAE 62.2 calls out 7.5 CFM per occupant plus 0.03 CFM per square foot of conditioned space. ASHRAE counts "occupant” as the number of bedrooms + 1. ASHRAE 62.2 assumes an average ceiling height of 8 feet, so you can relate this to volume if desired.

Building Science suggests that this ASHRAE number is too high, and recommends 7.5 CFM per occupant + 0.01 CFM per square foot of conditioned space.

If allowed, we recommend using the Building Science number and proportionately increase it if your average ceiling height significantly exceeds 8 feet (e.g., if your average ceiling height is 10 feet, increase it by 25%). Local building codes often specify what is allowed and some inspectors don't allow deviations.

Thanks for your advice. Do these ervs have variable speeds? And what would control the speed? Some of them claim to have a 'max' airflow of some figure, which implies they could be dialed back. Also, why would one pick an HR over an ERV? I'm in Western NY, our house has no AC, and it's a log cabin that maintains fairly higher humidity through the winter. I've gotten mixed reviews from people. What better for me? Is one more efficient than another?

HRVs and ERVs are similar devices in that both supply fresh air to the home and exhaust stale air outdoors while recovering energy from the exhaust air in the process. The primary difference between the two is that a HRV only transfers heat while a ERV transfers both heat and moisture which translates to energy. Well-sealed homes will greatly benefit from using these devices.

The better option between a HRV or ERV depends on your climate and specific needs. If your house is too humid in winter (above 60% RH) then an HRV is the better choice, as it would help get rid of excess humidity while an ERV would tend to keep it at a high level. If the opposite is true and your house is too dry in winter, then an ERV would be a better choice as it helps retain humidity, eliminating the need (and cost) for you to generate it through other means.

Ventilation speeds are usually MIN, MAX and INTERMITTENT (e.g., 20 minutes ON and then 40 minutes OFF, repeating each hour) and controlled manually via switch interfaces or the device itself. Normal daily operation is typically MIN and occassionaly INTERMITTENT. MAX is typically activated from switches in bathrooms for obvious reasons. Likewise, stale air inlet registers are typically located in bathrooms and fresh air outlet registers are located in living space rooms.

Last winter my indoor humidity was 50%. Would you go ERV??

I'm retrofitting my unit into the home, so I won't have switches in the bathroom controlling the unit, or anything like that. If I install it myself. Should I just let it run on intermitten or something?

30 to 50% RH is generally considered the ideal range. So I think you could go with the HRV and run it on MIN or INTERMEDIATE based on where it takes your RH. Presumably, given that you don't have one now, your air quality isn't too bad. So I would size it properly or even a little less and don't over size it. And by size, that could be the MIN CFM if you will never use MAX...or could be the MAX CFM if you will always run it at MAX. Likely running at the lower CFM setting would result in longer device life. Getting your RH to 30-35% will reduce window condensation issues and moisture/mold issues caused by infiltration. I think all your wood would likely prefer a stable year-round humidity level and resulting moisture content level too.

We have pretty humid summers. So using an HR to drop the humidity in the winter, while likely a good idea, would create more of a swing in humidity levels over the course of a year.

Do you recommend any brand.of Hrv or Erv? I've been looking at Renewaire and lifebreath

An ERV won’t reduce your indoor humidity in the winter at all and it won’t reduce it in the summer either when the outdoor air is more humid than the indoor air. If you need to reduce indoor humidity in the summer, you should be looking at AC and perhaps a dehumidifier. Sorry, I don’t recommend brands online. A Google search for reviews and ratings of ERVs or HRVs should get you what you need in that regard.

EVR = magic black box = question its effectiveness..... also newer and not much data on how long they last. HVR are simpler and been along in residential longer.

HVR will remove humidity if the outside air is colder then 60 degrees even if the 60 degree air is at 100 percent humidity.

When heating 60 air that is at 100% humidity to 70 degrees the humidity changes to ~50%
So 50 degree air at 100% humidity changes to around ~30% humidity at 70 degrees.

Thus my run of thumb is if the air outside is under 60 degrees it doesn't matter what the humidity is because when it is heated up it will always help keep humidity at or below the safe level of 50%.

I'd like to resurrect this old thread because I am also struggling to size a ventilator for my house. I'm in zone 6 and still confused if hrv is better for me.or ERV


I have 2400 sf house with 1600 sf finished basement (4000 sf total). 3 bedrooms, 4 occupants. And 46,800 cubic feet of airspace (which includes basement)

There seems to be 3 ways to calculate the size. ASHRAE 62.2 , the International Building Code, and by cubic feet of house.

Problem is I get wildly different numbers.... Which is right?

ASHRAE = 70 cfm
IBC = 150 cfm
Volume = 232 cfm.

And lastly a contractor who told me 130cfm. Not sure how he calculated that

So what size unit should I get, and what speed setting should I set it at?? Is a u it with 40-100 cfm enough?

I don’t know that any methodology is “right”… For your design, I get 150 and 70 CFM for the ASHRAE and Building Science methodologies, respectively. But based on your volume number, your average ceiling height is 11.7 feet (i.e., 46,800 CF / 4,000 SF). So I would suggest getting a unit that when it’s operating continuously at its MIN setting will provide about 102 CFM of ventilation capability (i.e., 70 CFM x 11.7 feet / 8 feet). This is equivalent to 0.13 ACH or Air Changes per Hour (i.e., 102 cubic feet/minute x 60 minutes/hour divided by 46,800 cubic feet). And I would place switches in your bathrooms that can place the unit in its MAX setting as needed which will likely provide 150 CFM or more ventilation capability allowing it to meet code. Plan on running the unit on its MIN setting and put the stale air inlets in your bathroom. You may find that running the unit at its INTERMITTENT setting (e.g., 20 minutes ON and 40 minutes OFF) will be adequate too. I don't think I can explain the differences or motivation for getting an EHV or HRV any better than I already did previously.

Posted By newbostonconst on 23 Jul 2020 03:17 PM
EVR = magic black box = question its effectiveness..... also newer and not much data on how long they last. HVR are simpler and been along in residential longer.

HVR will remove humidity if the outside air is colder then 60 degrees even if the 60 degree air is at 100 percent humidity.

When heating 60 air that is at 100% humidity to 70 degrees the humidity changes to ~50%
So 50 degree air at 100% humidity changes to around ~30% humidity at 70 degrees.

Thus my run of thumb is if the air outside is under 60 degrees it doesn't matter what the humidity is because when it is heated up it will always help keep humidity at or below the safe level of 50%.

Will an HRV lower your indoor humidity in the winter? In NY?

I'm getting really frustrated. For the life of me I have no idea how these units are controlled. The only one that seems to have controls is the panasonic intellibalence, with dials right on it to pick your fan speed and adjusting how often it runs.

How do you control the other ones? Other than just min airspeed or max airspeed?? Like if the unit says 40-250 cfm. How do you set it at 90 cfm? Or 130 cfm? Or 180 cfm?

What if the unit is just marketed as 150 cfm? Is that the min or max cfm? Good grief, don't these companies understand that at the end of the day, these units will be in the homes of people without PHDs in HVAC?!?!?!

Most home owners are not involved with specifying their HVAC so the manufacturers are marketing to builders and architects.

If you want to know how they work, I would start looking for YouTube videos and also download and read the product manuals.

Rocket science isn't required, but spending the time to gain a basic understanding is required. These ERV and HRV units typically contain processors and they need to be booted just like a computer and have considerable installation capability and flexibility . These units come with an integrated control within the unit to set speeds and associate desired CFM with these speeds. You can and likely will need to use separate devices/switches to talk to the unit and control it remotely (i.e., switches in your bathrooms). As Smart Shop suggested, I would also suggest reading the installation and user manuals of a typical HRV to gain a basic understanding:

https://s3.amazonaws.com/s3.supplyhouse.com/product_files/Broan-HRV160TE-Install-Instructions.pdf

https://s3.amazonaws.com/s3.supplyhouse.com/product_files/Broan-HRV160TE-User-Guide.pdf

Yes, a HRV will lower indoor humidity in Winter whenever the outdoor temp is less than the indoor temp. An ERV will not.

So where could I find a "thermostat" type control for these things? All I'm seeing is a button that runs it on Max for 20/40/60 mins.

Thanks for the links to that Broan unit. That's the second one I've found with any type of controls listed in the manual
Unfortunately it's twice the price of all the others and out of stock till winter.

This pansonic intellibalence is exactly what I need, features wise.

https://www.supplyhouse.com/Panasonic-FV-10VEC2-Intelli-Balance-100-Energy-Recovery-Ventilator-Cold-Climate

I have no idea if it's big enough tho. I looked back at my notes, and I have two companies give me "quotes" for a 130 cfm unit and a 218 cfm unit

So of the 5 different ways to calculate howamy cfm I need, I'm now at 70, 130, 150, 218 and 232.

Should I pick one of those numbers out of thin air? Mean? Median? Mode?!

I'd really like to buy something already. I've had my windows open all fall and winter. I've done 50 plus house of reading on these silly things. Can anyone just tellw if the 40-100 cfm panasonic unit is enough Ventilation? I can go find my blower door test results if needed.

I think I am going to get 2 of those or the equivalent for about 1,800 ft^2, or about 18,000 ft^3.

I don't know how those numbers are calculated but one will be for 1-3 people upstairs and one will be for the shop space downstairs.

I will need more ventilation if I am doing TIG welding, plasma cutting or mist spray coolant on my milling machine. I will probably get a straight exhaust fan for that and turn off my shop AC zone when I am doing that. Being shaded by the second story, starting cool and being well ventilated, it shouldn't be too bad even in the summer if I try to work in the morning or after it cools in the evening.

It has been mentioned to have the inlet in the bathroom, I put my inlets right along side and at toilet level. I also have Smartthings automation so I have motion sensors in the bathrooms that turn on the HRV.

The moisture in the bathroom dissipates through the whole room in a ~5 minutes so my runtime after there isn't any motion is 10 minutes.

Has worked great for 3 years. with not moisture problems. I did buy the Fantech 70 cfm version and which pulls under 50 watts at max. Some states regulate there size directly by their current/power they pull. I think the next size up might have been better for smells of some people but it still works.

I have three bathrooms with 4 inch duct to each.