Mechanical ventilation of your home is a good idea. That means a fan running 24 hours a day 365 days a year. It provides a means of reducing indoor air pollutants and improving IAQ. It’s good for you and your whole family. We have a cute saying “dilution is the solution to indoor air pollution”


But aren’t you supposed to show me ways to save money and use less electricity? I don’t get it

True our goal is to save you money and we will show you how to use less electricity. Remember our goal is to address your safety and comfort and using methods of performance contracting and building science and our end result will be a more efficient home. Even though a fan will be running at all times these fans are very efficient and sip electricity rather than guzzle it. The benefit more than outweighs the cost.


Can’t I just open a window?

Sure you can it’s a good idea to open your windows.  We still want to mechanically ventilate your home to ensure it has fresh air at all times. We want to have control over our fresh air and not simply guess. You can open a window whenever you want but we still need ventilation.

Ok so now we have established you need ventilation. What kind and by what standard. The logical answer would be to use ASHRAE 62.2 2007 here in California s a guideline.  It is what the current California new home construction standard uses. There are some rebate programs that require other versions but we will stick with the referenced code.The formula below is watered down version

Let’s take a 2000 square foot four bedroom home as an example

The formula is pretty basic (sq ft * .01) + (bedrooms + 1 * 7.5)

(2000*.01 =)20 +37.5( =5*7.5)

So we need 57.5 cfm for our home.

So let’s get a 60 cfm fan right? Uuuummmmm probably not

The thing is that we want that amount delivered not close. The type of material used for ducting is important. The length of the run is important. How many elbows does it have is important. Proper sealing of ducts is important. Quality of the product is important.  Not accounting for these factors will have our 60 cfm producing 40 cfm or even less.

I would normally recommend that we take that size and add 15 percent and round up to the next available fan. I would recommend a 70 cfm fan in most cases for proper ventilation.


What about using more cfm would that be better?

Yesnomaybe is the answer.

Yes              It is better for IAQ

No               It is wasting Energy

Maybe       Every home is different and occupants are different. If you have 10 100 gallon fish tanks we might want to ventilate a bit more. There are other reasons as well. We call them when we see them.

For the most part we want to size according to the standards that is why they are there. We want size and install the system correctly and test it to make sure it’s providing you with what you need.


What kind of fan?

This is a good question and the answer can vary given different factors.


Balanced system’s which brings fresh air from outside and exhaust stale air from inside. Cost is the biggest issue with this style


Supply only ventilation which brings air from the outside. It puts the home under positive pressure (see does my house suck?) and ensure the air is fresh


Exhaust fans. These are already set up in many homes. The install and parts are more cost effective. However this air will be made up from wherever. Studies have shown that even though there is no control of where the air is pulling from that the air that it pulls in is better than the air in your home. This is the most common strategy not due to performance but due to price.

OK lets seal it tight and ventilate it right!

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Comment by Shade Structures on January 31, 2013 at 8:32pm

I would just open my window, install blinds if necessary, then I would have free natural ventilation.

Comment by Robert Riversong on September 9, 2012 at 3:27pm

Curt, ASHRAE 62.2-2007 was set including a default credit of 2 cfm/100 ft² for infiltration (which is the equivalent of 0.15 ACHnat with 8' ceilings). It assumes a natural infiltration rate and requires specified ventilation rates on top of that.

Because IAQ is as much dependent on building materials and furnishings as on occupant activity, it makes sense to require a minimum air exchange rate based on the size of the home, moderated by the number of bedrooms (potential occupancy).

Comment by Walter Stachowicz on September 9, 2012 at 2:58pm

Thanks for the comment, Curt.  I agree wholeheartedly, but the folks at Energy Star don't.  I recently did a 5 bedroom home with one occupant.  NO credit can be given for phantoms, nor natural ventilation, so the bath exhaust fan has to be set to run 100% of the time in this case.  (The homeowner can reset the controls if they'd like, but that's after I've done my certification.)

The Florida Energy Code requires that ASHRAE 62.2 be the maximum rate allowed for residential, and ES v3 requires it as the minimum.  Code overrules ES, so we have to get the settings right on the money.  Seems kind of self-defeating, but the contractor makes the choice as to how to achieve the required ventilation.  Hope they don't have issues in the future....

Comment by Curt Kinder on September 9, 2012 at 1:18pm

I'm in a very humid area - Jax, FL. I'm not sure I agree with specifying the fan to provide 100% of ASHRAE 62.2. Perhaps some credit should be given for a home's natural ventilation rate.

Furthermore, many of my clients don't have a full home - sometimes it is just 2-4 people in a 4-5 bedroom house, so it doesn't make sense to ventilate for phantom occupants

Bearing in mind the limitations of negative pressure, I do like the Panaosonic Whispergreen bath fans. Their low speed is user adjustable, very quiet, highly efficient, and delivers the rated CFM within a range of back pressure

Excessive ventilation is costly in that it adds to cooling load and creates a comfort issue with the addition of latent load

Comment by Robert Riversong on September 7, 2012 at 10:28am


In hot humid zones, positive pressure is what is needed to prevent infiltration condensation. So a supply-type ventilator is appropriate. Stand-alone dehumidification is typically less energy-intensive to operate than relying on the AC for dessication. If your 110 cfm bath fan is pushing only 60-70 cfm, then there's too much static duct loss (or the fan was over-rated).

Comment by Walter Stachowicz on September 7, 2012 at 9:35am

Does anyone have any suggestions for hot, humid areas? The exhaust only ventilation is going to bring in humid air through myriad little leaks.  With an RH of 73 in the summer, this can (and does) cause condensation on grilles and drywall.  ERVs don't work well, as they get overloaded with moisture.  BTW, with we've found that a 110 cfm bath exhaust is only drawing 60 - 70 cfm. Maybe the house is so tight that we're setting up too much negative pressure?  My conclusion is that supply only ventilation is the way to go, making sure that the fresh air is filtered and de-humidified through the coils before coming into the home.  There needs to be an override for the heavy pollen or smoke days.  Any comments or ideas?

Comment by Robert Riversong on September 6, 2012 at 2:03pm

Code requires that sufficient continuous or intermittent mechanical ventilation be installed, but does not require the occupants to use it. There's no way to legislate individual behavior. We can only include an "operator's manual" and educate the homeowners.

While 219 kWH may not seem like much, once you multiply that by 130 million homes it becomes a substantial additional load on the grid and global warming contributor. And an HRV/ERV can consume four to six times that wattage, particularly with a HEPA filter. 

Unless the house is in an area in which outdoor pollution is a problem, it's hard to justify running a whole-house ventilation system during the seasons when windows remain open (and window fans may be used for summer cooling).

Comment by Glen Gallo on September 6, 2012 at 1:56pm

@ Randy

Sorry did not see your other comment until now.

Through wall systems should not be a problem in my area but have to wonder at is wisdom in your area. I live in an area with little temperature swings and low delta t. I would be concerned about the in wall systems comprising the wall system and providing a unnecessary bridge of outside temperatures to inside. I would reconsider if it were my obvious best option.

As a tradesman you know there are things we want to do  that are thwarted by existing conditions. We have to be fluid and adapt and overcome. Analyse the issue and come up with the best solution possible on a project by project basis

I would think sight unseen ventilation in kitchen bathroom and laundry areas.

An HRV/ERV for fresh air ventilation

Comment by Glen Gallo on September 6, 2012 at 1:09pm


It is already the code here in California for new construction and it is continuous ventilation. It is based on ASHRAE 62.2 2007. There are some workarounds with intermittent but it gets into a grey area. I have some questions regarding the new code and will be in a workshop soon.  

Our windows are open almost everyday so you are preaching to the choir.  

While I would not disagree with the energy loss point you make. The question is will the homeowner turn it on after turning it off? How do we make sure that ventilation is assured once the windows are closed? Therein lies the proverbial rub. At 20-30 watts for a good low sone high quality exhaust fan the penalty is less about 33-64  dollars at 25 watts and 219 KwH per year in my area. 

Having the fan exhaust with windows should aid in removing pollutants and improving IAQ which is the whole point of continuous ventilation.

We might need a better moustrap

Comment by Marie McMahon Meehan on September 6, 2012 at 12:21pm

I respectfully disagree regarding running the mechanical ventilation when the windows are open.  Many people live in regions of the country where they open their windows in mid-April and leave them open until mid-October.  A common practice in some high performance homes is to run the exhaust fan continuously whenever the windows are closed (winter and if air conditioning is used) and to use the exhaust fan intermittently as needed (for showers, etc) during the temperate months when the windows are open.  Depending on the region, this could reduce the fan run times by half or more.  


Granted if properly sized EnergyStar fans are used, these are relatively small energy cost differences to the individual homeowner, however it would be unfortunate if continuous year-round ventilation requirements ended up in the building code and where it would result in significant energy wasted in aggregate.

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