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Old 08-16-2020, 06:03 PM   #1
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air exchange rate a house


Can anyone tell me a formula you use for estimating the necessary blower cfm for an energy recovery ventilator (ERV) for house that is not built yet.

What I want to know - How do I plan/size an ERV during planning stages of construction, prior to having a blower door test?

Using some "guidelines" I've found online, I come up with as low as 30 cfm or as high as 150 cfm (2 bedroom/~1800 sf). I want to plan for this without having a too high rate of ventilation, which might cause excessive energy (heating/cooling) loss?

If you have any advice or know of an information source I would appreciate you sharing information.

Thank you!

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Old 08-16-2020, 06:10 PM   #2
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Re: air exchange rate a house


For sizing I imagine local codes should specify minimum required.

EVRs and HRVs can be installed with a controller that allows you to set the ventilation duty cycle. These units can also be run at different speeds.

It's better to have too much ventilation than not enough because you can always cut it back.

I would never go below 100 cfm.

With heat/humidity recovery, the energy penalty shouldn't be that great.



To need a ventilation unit, the house has to be pretty tight. You may be able to get away with exhaust fans at a lower cost unless the unit is required by code.

Not sure if blower door testing is commonly done on new homes in your area. Just my opinion -> if the house tests below 3 air changes per hour at 50 pascals below atmosphere (what they're commonly tested at), you should have an air exchange unit.
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Old 08-16-2020, 07:12 PM   #3
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Re: air exchange rate a house


Our local code requires 3 air exchanges per hour (the house's volume x 0.35=CFM). A blower door test is not required.

The part I don't understand is that some ventilation occurs through air leakage. And the quantity of leakage depends on how well the house is air sealed and also the quality of the windows and doors, etc. How is that leakage accounted for in the calculation to size an ERV?

If it is not accounted for, then the house is over ventilated.

The reason I don't want to use just fans to bring in fresh air is that during summer in the south we have a really bad problem with warm exterior air condensing its humidity as it enters the air conditioned interior. If I would use fans and create a negative pressure, the condensation could occur inside of a wall, at a leaking weather strip, etc.

ps:
I forgot to metion that I will have a separate air damper that will provide makeup air for the kitchen range exhaust hood.

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Old 08-16-2020, 07:31 PM   #4
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Re: air exchange rate a house


Natural ventilation is not factored in when sizing because air leakage greatly varies depending on weather. Even a very leaky house can have very low leakage when the winds are calm and the temperature difference between outside and inside isn't great.

The HRV/ERV is there to provide controlled ventilation and is sized for the worst case scenario.

The larger unit needs to run less and and or or at a lower speed to deliver the same ventilation.

Do not worry about oversizing. There's no penalty like there is for a/c -> it's just a box with a couple of fans and heat exchanger.


It's better to have more capacity than you need.


Quote:
Our local code requires 3 air exchanges per hour (the house's volume x 0.35=CFM). A blower door test is not required.
I think you meant 0.35 air changes per hour, not 3.

Typical 2000 sq ft plus basement.

3000 sq ft x 8 ft = 24 000 cu ft / 60 = 400 cfm per minute x 3 = would be 1200 cfm.

No residential hrv/erv ventilation unit does that.

0.35 x volume is just over one third of an air change per hour.

Going back to my example, that's 140 cfm which is within reason.
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Old 08-16-2020, 08:05 PM   #5
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Re: air exchange rate a house


My comments are in red
Quote:
Originally Posted by user_12345a View Post
<<snip>

It's better to have more capacity than you need.
I am concerened about too much air exchange because of energy loss. When I look at the cost of ERV units, it increases incrementally as CFM exchange rate increases. The varriable speed units are twice more than a single speed. I want to do it economically as possible., that's why I am trying to plan efficency.


I think you meant 0.35 air changes per hour, not 3.
YES, you are correct!

<<snip>>
ASHRAE says 15 cfm per minute per person, though that's not code in my area. That would be 45 cfm for a two bedroom house, 3 people. (ASHRAE assumes 2 in the master and 1 in the small bedroom)
There is a chart in NC Residental Code that says a house the size I am building must have a 60 cfm per minute minimum exchange rate.

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Old 08-16-2020, 09:01 PM   #6
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Re: air exchange rate a house


I recommend you follow the ASRAE recommendation and err on the "slightly more" side for best results.

I prefer to do that by air induction through a good filter rather than exhaust it since it keeps the house cleaner from infiltration of dust laden air. Trust me, it'll find a way out. I'm not all that oversold on ERV units.

The air around here is planty dirty... YMMV
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Old 08-16-2020, 09:03 PM   #7
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Re: air exchange rate a house


Quote:
I am concerened about too much air exchange because of energy loss. When I look at the cost of ERV units, it increases incrementally as CFM exchange rate increases. The varriable speed units are twice more than a single speed. I want to do it economically as possible., that's why I am trying to plan efficency.
You're going in circles. A bigger unit does not increase utility bills.

Lets say the house needs 75 cfm minimum and you put in a 150 cfm.

The 150 cfm just needs to run half as long as the 75 cfm to do the job. Big deal.

Energy loss is the same. It does not run all the time, it's controlled.

Putting in under 100 cfm is silly and I would be tempted to go for 150. A cheap bathroom exhaust fan can easily move 60 cfm -> it's not a lot of air.

Minimum means minimum, when showering, cooking, mopping floors or having people over a tiny hrv/erv won't cut it.

You don't need a really fancy unit with variable speed/ecm style fan motor, 2+ discrete speed motors are common.

You just need to have it installed with a controller that allows you to select run-time/duty cycle.

As for the cost of the unit, it's tiny compared to the cost of the house, and if you undersize you'll be wasting money on another hrv of the correct capacity. The difference between an undersized builder's special and a proper unit shouldn't be more than $500.

Don't skimp on mechanicals because you'll be stuck with what you buy for 15-20+ years - light fixtures, counter tops, paint on wall you can easily always re-do later.
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Old 08-16-2020, 09:03 PM   #8
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Re: air exchange rate a house


The debate between enough and too much has been going on for awhile. Not sure where they are now but last I read they are not counting natural infiltration and requiring the exchange system to provide some number whether you need it or not, which I think is silly.

Rick Karg and Charlie Holly from Residential Energy Dynamics, they are in Maine, have done a lot of calculating on indoor air quality and how much exchange is needed. I get their notices but long past trying to keep up with all of the changes.
https://www.redcalc.com/?utm_source=...0b392ccba7ca5a

And here is a link on the debate:
https://www.greenbuildingadvisor.com...your-home-need

Bud
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Old 08-16-2020, 09:17 PM   #9
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Re: air exchange rate a house


^the capacity of the unit just determines maximum possible ventilation - the best way to tell is to actually live in the house and see what works.

It's an educated guess at best.
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Old 08-17-2020, 11:20 AM   #10
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Re: air exchange rate a house


Since this house that I am working on does not exist, except for a few drawings, I can't live in it for a while.

I am planning/designing right now. When I go for a building permit, I will have to provide plans for review.

Calculating what size ERV will be required when the time comes is my goal.

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Old 08-17-2020, 11:45 AM   #11
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Re: air exchange rate a house


I want to let each of you know that I appreciate your responses.
Some of my replies might seem to be argumentative, that I did not appreciate your thoughts.
I really do value your opinions, even those that are not the same as mine.
I'd like to learn from you!
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Old 08-17-2020, 11:50 AM   #12
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Re: air exchange rate a house


Quote:
Originally Posted by AWWarn View Post
Calculating what size ERV will be required when the time comes is my goal.
You should be working with an hvac designer who knows the codes in the area to get the minimum.

Quote:
Some of my replies might seem to be argumentative, that I did not appreciate your thoughts.
Your arguments don't make sense, you still seem to think the house will be over-ventilated if you install a larger unit than the minimum required.
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Old 08-17-2020, 11:51 AM   #13
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Re: air exchange rate a house


There is lots of design information out there and since you are in the planning stage the air leakage is also part of that design. Basically you plan on tight and then add extra fresh air to get up to the minimum recommended. Built right your resulting infiltration will be so low you can forget it.

BUd
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Old 08-17-2020, 01:36 PM   #14
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Re: air exchange rate a house


Quote:
Originally Posted by Bud9051 View Post
<<snip>> Built right your resulting infiltration will be so low you can forget it.

BUd

I do plan on building right - sealed tight, with more insulation than is required by code, and duct work inside the thermal envelope.
A low infiltration rate is the reason mechanical ventilation is needed.
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Old 08-17-2020, 01:49 PM   #15
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Re: air exchange rate a house


If you read the second article I provided one proponent wants to include natural ventilation while the other does not. The big difference occurs in older leaky homes. If they already leak a lot then why add more. But in a tight home omitting any natural leakage does not make a big difference. You said you need a blower door measurement and you don't. If a number is needed just plug in a very low number and calculate fan flow. Note I have never used a calculator for that.

What are you missing?
Do you have a calculator?

Bud
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