4 Ways to Do Balanced Ventilation

13 Comments Read/write comments

ventilation energy enthalpy recovery ventilator erv humidity

Ventilation is a great thing. Bringing outdoor air into the home and exhausting stale indoor air improves indoor air quality. Well, most of the time anyway. Sometimes the outdoor air quality is worse than indoor air. Sometimes you bring in too much humidity and start growing mold. And sometimes you bring in the wrong outdoor air.

Probably the most common type of whole-house mechanical ventilation system in homes is an exhaust-only system. You put some controls on the exhaust fans that are already in the home and those fans are set to exhaust stale air from the home, either continuously or intermittently. The problem is this type of system sucks. Literally. And if your house is sucking from an attached garage, a moldy crawl space, or dirty attic, you could be making things worse.

The way to avoid having a house that sucks is to do balanced ventilation. You exhaust stale air from the house and you supply an equal amount of air directly rather than relying on the negative pressure of the house to bring in the outdoor air. Here are five ways to do balanced ventilation. I've put them in increasing order of cost, complexity, and efficiency.

1. Open the windows

OK, technically I shouldn't include this one because it's not a real solution for most homes. This one works only if the home is in a mild climate that needs to little to no conditioning. But if that's your situation, you don't need a fancy ventilation system. Just open the windows.

2. Pair a central-fan integrated supply system with the exhaust fans

A lot of homes get exhaust-only whole-house ventilation (fans plus controls). One easy way to upgrade is to install a central-fan integrated supply system to complement the exhaust-only side. The two most commonly used controls for this are made by AirCycler and Honeywell.

AirCycler supply-only ventilation system

These systems are integrated with the blower in the central heating and cooling system. They bring in outdoor air when the system is running and mix it with the indoor air circulating through the duct system. It gets filtered and conditioned before being introduced into the home. When tied to the exhaust-only controls, you get balanced ventilation.

The main drawback of this system is energy use in systems that don't have variable speed blowers. In addition to bringing in fresh air when the system is heating or cooling, it can turn the blower on when the home doesn't need heating or cooling. And some blowers use a lot of power. Turning on a 400 watt fan to bring in 50 cubic feet per minute of outdoor air is overkill. If you have a high-efficiency heating and cooling system with a variable speed blower, you should be able to do this at less than 50 watts.

And another drawback, pointed out by Curt Kinder in the first comment below, is moisture. In a humid climate, running the blower without the compressor on can evaporate moisture on the coil and put it back into the home.

3. Pair a supply fan with the exhaust fans

Another way to get balanced ventilation is to use the exhaust fans with controls and also install a supply fan. You can do this with a bath fan installed to blow air into the home or you can use a fan made specially for this task. I'm thinking of the QuFresh fan made by Air King.

QuFresh supply ventilation fan by Air King

They have two basic models. One has a sensor for temperature and relative humidity, and the other does not. The purpose of the sensor is to limit the amount of ventilation when it's really cold, really hot, or really humid outside. It'll still run 15 minutes an hour so you'll keep getting some ventilation air.

I like the concept and the features in the QuFresh fan. I haven't had a chance to try one out yet, but they do a lot of good things. You can adjust the flow rate from 30 to 130 cfm. It has a slot for a 2 inch filter that could be up to MERV 13. It's quiet (0.5 sone at 50 cfm). And it's relatively inexpensive.

3a. Pair a ventilating dehumidifier with the exhaust fans

OK. This really should be number 4, not 3a, but somehow I didn't think about it when I put my list together at first. The strategy here is to use controls on your exhaust fans, as in the previous two, and supply your ventilation air through a whole-house dehumidifier. Many models allow you to do this by providing two intake ports on the dehumidifier, the smaller of which attaches to a duct that goes to the outdoors. We like Ultra-Aire but you can also find good models from AprilAire, Honeywell, and others. (Disclosure: Therma-Stor, which makes Ultra-Aire dehumidifiers, advertises here in the Energy Vanguard Blog.)

One drawback of dehumidifiers is the heat they put into your home. The Ultra-Aire model SD12 eliminates that problem by being a split-system dehumidifier. It removes the humidity indoors but puts the heat outdoors. That means it even provides a bit of cooling (about a third of a ton).

4. Use a heat or energy recovery ventilator (HRV or ERV)

This is what most people think of when someone mentions balanced ventilation. The photo at the top of the article shows the inside of a typical ERV. (An HRV looks the same but uses a different material in the heat exchanger.)

The operation is simple. It has two fans, one to exhaust stale indoor air, one to bring in fresh outdoor air. It filters both air streams. The two air streams pass through a heat exchanger, a capillary core in most models. The two air streams pass near each other and exchange heat in an HRV and heat and moisture in an ERV. But the two air streams don't mix.

This is a great way to ventilate a home. It's also more expensive than the ones above. Panasonic does have a small "spot" ERV called the Whisper Comfort, but aside from that model, you're probably looking at $1,000 or more for an ERV or HRV. The biggest difference between this type of balanced ventilation and the previous two is the heat exchanger. You get balanced ventilation with recovery, which means you don't need to do as much conditioning of the outdoor air you bring in.

Finding balance

There's your quick rundown of the main ways to do balanced ventilation. We're seeing a lot of creativity in the ventilation market these days because ventilation is a big deal. I think we've gotten to the point where we rarely have to fight the battle about the need for airtight houses. The old myth that a house needs to breathe, while not completely gone, has mostly been relegated to the dustbin of bad thinking.

But let’s be clear about what balanced ventilation really means. It doesn’t mean the building is always at neutral pressure with respect to outdoors. Wind, stack effect, and other mechanical systems can always unbalance a building. When we talk about a balanced ventilation system, we mean simply that that particular system doesn’t result in the building going positive or negative because it’s exhausting and supplying equal amounts of air.

Another point here is that my main motivation in writing this article was to show some ways you can convert an exhaust-only system to a balanced system. Methods 2 and 3 above show how you can do that relatively easily and inexpensively. Method 4 would be a good way to do it in the humid climates of the southeastern US.

One more thing. I was going to make this a list of five ways to do balanced ventilation but decided to save the other one for a separate article. Going a step beyond the ERV, you could go with a souped-up ERV. There are two companies making devices that include balanced ventilation with recovery, a small heat pump, better filtration, and more. One is the Conditioning ERV, or CERV, by Build Equinox. The other is the Minotair by Minotair Ventilation. We've got two HVAC design jobs going right now that will be using these devices so stay tuned for more on these later.

Meanwhile, try to stay in balance. Or at least be positive.

 

Related Articles

Getting Mechanical Ventilation Inside Conditioned Space

Adventures in Hotel Bathroom Ventilation

An Energy Recovery Ventilator Is NOT a Dehumidifier

Why Do Airtight Homes Need Mechanical Ventilation?

 

NOTE: Comments are moderated. Your comment will not appear below until approved.

Comments

Apr 3 2017 - 10:06am

Allison,

I submit that there is a 2nd serious drawback associated with solution #2 - integrating ventilation with HVAC system supply fan.

Modern high-efficiency air handlers have very large refrigerant coils with LOTS of surface area. A back-of-envelope calculation suggests that the total area exposed to air flow is upwards of 100 square FEET per nominal ton.

In all but the most arid climates, all that surface area gets wet whenever the system operates in cooling mode, and it stays wet for awhile, as in months! If the system fan is ever allowed to operate more than 30-60 seconds after the compressor stops operating in cooling mode, all the condensed water still clinging to all that surface area of the coil begins to re-evaporate into the home's air.

That effect undoes much of the dehumidification capacity we strive to furnish with central air / heat pump systems.

Data and experience in my area of operation, northeast Florida, suggest that operating the system fan independently of the compressor raises indoor relative humidity by upwards of 10%.

10% increased indoor RH is a high price to pay, so high that I constantly recommend against any device, option, mode etc that operates the system fan independently of the compressor in cooling mode...not here...not anywhere near here!

Allison
Bailes
Apr 3 2017 - 10:21am

Yes, Curt, that is indeed a serious drawback in humid climates. I should have put that in the article to begin with but will go back and add it now. Thanks for pointing it out.

Apr 3 2017 - 10:22am

Never pull the trigger on an air sealing/tightening job until there's a ventilation plan in place.

Broan makes another balanced ventilator using a single fan and a mixing strategy, which helps maintain comfort and simplifies ductwork.

Allison
Bailes
Apr 3 2017 - 10:36am

Yes, indeed, Mac! Always have a ventilation plan ready when you're building a new, airtight home or sealing up an existing home.

I did a quick search and couldn't find the Broan system you're talking about. Do you have a model or URL?

Apr 3 2017 - 1:44pm

Measurements. This is what is lacking in every discussion about IAQ and ventilation. Lots of discussion about (and firms selling) solutions without any discussion about measurements. It's why the most obvious error - outdoor humidity - pops up right away. You can get a relatively accurate digital RH/thermometer unit for $8 - measure and know. It's why there are a whole variety of people with different opinions about what is "good" (but still no measurements to verify). Humidity aside, is outside air "good"? You say "Well, most of the time anyway." I don't (blindly) agree. And how do you know indoor air is bad? What are the allergen levels like in your area? What are the pollution levels like in your area? What about all the other "stuff" that some people say are bad for you like VOCs? Where's the measurement? You can go to https://airnow.gov/index.cfm?action=aqibasics.aqi (an official air quality testing resource) and look up how "good" YOUR outdoor air is currently. Disagree with it all you want, it's the only game in town right now. It's what the media outlets report. So you can know what the AQI is outside at your location at any given time. But until the inexpensive, relatively-accurate indoor air measuring test equipment that mirrors the government outdoor air tests becomes available so that we can all know for sure if the indoor air is "good" or not (and whether or not we really need a fix), all the marketed solutions to provide "goodness" is just Wild West snake oil.

Apr 3 2017 - 1:45pm

I know the article is about *balanced* ventilation, and you give mention to exhaust-only ventilation in passing. Would supply-only ventilation be worth a mention? My focus is on hot-humid climate and I understand it might be poor for heating dominated climates... however I remember Lstiburek recommending it for new construction (2-3 ACH50) and believing it would not present problems if they built as he recommends.

In my 1989 house, I am rather pleased by the thought of working against infiltration from all those uncounted, unidentified little holes in the building structure. I am thinking the first X CFM of supply-only air will be nearly all offset by reduction in random infiltration. Do you think that is plausible?

Apr 4 2017 - 3:01pm

To Mark's point... From a pressure balance standpoint there's no practical difference between supply-only and any non-recovery balanced ventilation scheme you might come up with (unless you imagine the envelope is so tight that a supply fan can't move the desired CFM!).

The issue of course with supply-only ventilation is the negative impacts on comfort & space conditioning loads. There are various design approaches to address these impacts, none easy. But it's silly to pair an exhaust fan with a supply-only solution simply to balance out the pressure! At least in warm climates, positive pressure is a good thing.

Apr 4 2017 - 5:53pm

I found that number 2 simply does not work. I've had the opportunity to test a few of these and they don't balance out. Here's what I think is going on. In a tight house (3ACH50 or less) a well design duct system operating at 0.1 inches (or less at 50 watt fan speeds) is intended to move air around inside the house and does very little to influence movement across the envelope. Because no fan is driving air across the envelope, or to look at it another way, because there are other openings to the house in the return air duct the pressure difference between the house and the outside becomes part of the equation whether we like it or not. For the exhaust fan to make a difference in the supply air flow they would have to create a significant pressure difference across the envelope, which does typically happen, and if it did would negate the whole purpose of balanced ventilation. At that point there is no good reason why the air would come through that supply duct as any imperfection in the air barrier and that's what my modest sample set of testing has shown. The house is essentially part of the fresh air supply duct, a really big leaky part. Typically situation (actually looked one up) fan speed low = 22 CFM at inlet, fan speed high = 36 CFM at inlet, add 150 CFM of exhaust = 39 CFM at inlet. This house was 1.5 ACH50 and 61272 cf (big possibly a factor) At this point I've been turning people away from this strategy because as supply only it does move enough air and with exhaust fans it amounts to a sophisticated duct leak, with most of the air coming form else where. Any thoughts or data on one of these working would be appreciated. Really enjoy the blog by the way.

Apr 6 2017 - 8:09am

Is an HRV capable of making up the difference in air pressure between the inside and outside of a home? Or is the exchange of air equal coming in and going out?

I moved into a new home in Syracuse, NY. The blower door test shows 1.5 ACH. My water heater and furnace burn indoor air. I have a 1200 cfm kitchen vent hood and 6 bathroom fans.

I know that I need to provide for make up air and I would like to exchange stale air for fresh air on a regular schedule.

Would an HRV accomplish the MUA task? Is there another product that would? Thanks

Apr 7 2017 - 5:14pm

@Dave, atmospheric furnaces and water heaters must have dedicated combustion air vents (high & low), sized according to BTU capacity of each appliance. There's an exception when the appliance is located in an unconfined space, but that clearly doesn't apply in a home that tests out at 1.5 ACH. But even with code-required venting, that honking range hood can easily backdraft your combustion appliances, creating a dangerous situation for your family. I hope you have really good CO monitors!

An HRV will help dilute contaminants, however, by definition it does nothing to offset the imbalance created by exhaust fans, especially that range hood. That imbalance is what creates such a dangerous situation, literally sucking poisonous gasses into your home. And if you have any return-side leaks, your furnace could transport CO and other combustion gases throughout your home.

I'm unfamiliar with NY codes but the i-codes require dedicated MUA for range hoods rated >400CFM. Also consider when outside temp is 0F, your hood on high speed adds ~90k BTU/hr to the home's heat load.

Atmospheric combustion appliances (including open combustion fireplaces) have no place in a home as tight as yours, and I would make the same argument against a commercial sized hood, although in that case it can be accommodated at a cost. My advice would be to replace your furnace & water heater with sealed combustion models (unless they're located in an unfinished basement that can be properly vented and isolated from the house).

You asked about a MUA product for all your exhaust fans... Bath vents generally don't require dedicated MUA since they have low CFM and are not likely to all operate simultaneously.

MUA for your range hood can be as simple as a passive barometric damper (huge) or a smaller fan-assisted MUA system tied to your hood controls with or without reheat. And finally, there are different strategies for locating MUA diffusers depending on your home's layout and circumstances. I recommend that you hire an experienced mechanical designer to come up with a makeup air system for your home. It should be based on zonal pressure testing.

Apr 7 2017 - 10:03pm

"I recommend that you hire an experienced mechanical designer to come up with a makeup air system for your home."

Where would I find such a person in Syracuse, NY? My builder, HVAC contractor and appliance salesman know less about this subject than I do. HVAC subcontractor recommended a MUA system but I had no confidence in him and figured that I would deal with it after closing - which is now.

Thank you.

Apr 11 2017 - 10:13pm

Dave S., I live and work in the Syracuse area and help train at the NYS Weatherization training center. I can help you get connected with the right folks. David B. gave you good advice. NY State adopted IECC 2015 Codes on October 3, 2016. Section 505.2 of the NYS Mechanical Code addresses makeup air for kitchen exhaust hoods. Above 400 CFM, makeup air must be approximately equal in flow to the exhaust rate.

https://codes.iccsafe.org/public/document/code/449/8079886

There are several possible solutions, an onsite visit for a little diagnostics will get you on the right path. Contact me via email and I'll help get you started.

Add new comment