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Passive House Ventilation Design

by Chad Ludeman on January 20, 2010 · 22 comments

in HVAC,Passive House

We thought it would be a good idea to review our new ventilation strategy and design implemented in the recent Passive Project. The ventilation strategy in the 100K House was different than most typical houses and the Passive Project took things a step further. We’ll start with the outline:

  1. Use a very efficient mechanical ventilation system (ERV or HRV >90% efficient)
  2. Design an efficient duct run layout to maximize efficiency of your ventilation system
  3. Eliminate all dedicated local exhaust systems ducted directly outside
  4. Eliminate all dedicated appliance exhausts that are ducted directly outside

High Efficiency Mechanical Ventilation (ERV or HRV)

The Passive House standard requires that you install a ventilation system with > 75% efficiency and a low electric consumption of 0.45 Wh/m3. When we say ventilation system, we are talking about either a Heat Recovery Ventilator (HRV) or Energy Recovery Ventilator (ERV). HRV’s exchange heat only while ERV’s exchange both heat and humidity.

A 75% efficient unit will be exchanging 75% of the heat from the indoor air with the cold air coming inside. The closer that figure is to 100%, the closer the fresh, incoming air will be to the existing indoor temperature. Efficient electrical consumption is basically referring to the type of motor used in the ventilator. European models typically are using the most efficient DC motors available, while unit made in the US will suck a bit more power.

If you are shopping the globe, you have a couple of options for mechanical ventilation that meet the Passive House reqs. If you want to buy in the US, you have one choice – The UltimateAir RecoupAerator.  This bad boy comes in black, runs at 95% efficiency and sucks a paltry 40 Watts while delivering 70 cfm to your home (250W @ 200cfm). This is what we spec’ed on 100K and continue to use in Passive and for all foreseeable projects in the future. We set it on its lowest setting which usually measures about 60cfm in each room vent and let it run 24/7/365. Simple.

This unit also comes standard with a MERV 12 filter which gains us LEED points and keeps our client’s air very clean. Lastly, it features an “EconoCool” feature that can be used during the summer to reduce cooling loads. Flick this switch on and the unit will recognize when the temp drops below 65 degrees F and automatically shut off the energy recovery and begin swooshing that cool night air directly into your bedroom.

Maximize Efficiency with a Well Designed Duct Layout

Now that we’ve got the correct ERV for our needs, we need to pay attention to how we run our ductwork to maximize efficiency. The ERV will suck out stale air from the home and deliver fresh air to rooms at the same time. Below is how our PHIUS consultants recommended that we locate our suckers and blowers (technical Postgreen terms):

Stale Air Exhausts (Suckers)

  • Bathrooms
  • Kitchen
  • Near Washer/Dryer

Fresh Air Inlets (Blowers)

  • Bedrooms
  • Living Room

Duct run for ventilation system
Now we don’t just want to throw the ductwork in any old pattern to get to these rooms. We want to minimize 90′s and have straight runs as much as possible. Basically a 90 degree turn is equivalent in resistance to airflow as 25′ of straight duct and a 90 degree register termination can add as much as 80′ to your calcs. The main pointers given by both PHIUS and the manufacturer of the UltimatAir are as follows:

  1. Keep it SHORT
  2. Keep it STRAIGHT
  3. Keep it SMOOTH
  4. Test after the system is commissioned

The diagram above is a 3D layout of our proposed duct runs for the Skinny Project. Laying these out in Sketchup is pretty quick and helps us really think about how each duct run needs to get to it’s final destination with the fewest turns. As a result of this exercise we added another small chase wall next to one of the closets in an upstairs bedroom. This chase will not be seen, and could be considered a minor detail, but it will greatly improve our duct efficiency and reduce labor time for our installers.

Other tips given to us for optimal ERV system performance:

  1. Always use smooth, hard duct and avoid flexible duct as much as possible.
  2. Insulate the two lengths of duct running between the exterior and the ERV. This will prevent condensation from forming on these lengths of ducts.
  3. If noise is a concern, add one 3′ section of insulated flex duct to the supply side of the ERV. Install this section as straight as possible and it will act as a silencer without needed to buy an expensive silencing duct section.

Eliminate Dedicated Local Exhaust

The big difference between the 100K and Passive ventilation strategies was increasing the amount of intakes/exhausts inside and eliminating the local exhausts in the bathroom and kitchen. PHIUS highly recommends doing away with any local exhaust that is basically sucking conditioned air directly out of the homes and replacing it with whatever outdoor air can be sucked through any cracks in your envelope. Eliminating the local exhausts also reduces your exterior penetrations through the envelope by at least two (more if you have more than one bathroom).

To compensate for the lack of dedicated local exhausts, we install suckers in the bathrooms and kitchen. On top of that, we throw an ERV boost switch in each of these rooms so that occupants can boost the ERV fan to max setting (around 200 cfm for UltimateAir) while they are in use. These switches are also useful for parties or times when a lot of CO2 producing bodies are inhabiting your space.

In the future we plan to take this boost switch a step further by linking it directly to the bath lights with an auto shutoff delay. Basically when you turn on your bath light, the ERV will automatically boost to high. When you leave the bath and turn off the light, the ERV will remain on boost for another 10-20 minutes to fully clear out any fumes you may have generated during your visit. This gets us a LEED point also.

Eliminate Exhausts for All Appliances

This is less of a ventilation system design aspect and more of a whole house envelope and mechanical design strategy, but I like to include it here as well for good measure. By using electric water heating backup for our solar thermal and a condensing dryer that does not require venting, we have eliminated all appliance ducting to the outside. Less thermal bridging and warm air being sucked out of our homes. To compensate, we locate a sucker near the condensing dryer that can tend to get a little steamy at times. In the future designs, the washer/dryer is moved into the bathroom to allow the bath sucker to serve two functions.

That’s it. I hope this helps with any mechanical designs you may be considering for your next green building project. Comments are below.

One last note:
One question we still get a lot is if our homes are too tight and cause indoor air quality problems or pressure balance issues that would cause the windows and doors difficulty in opening and closing. This is a bit of an outdated concern from when production builders first started making their homes tighter without adding mechanical ventilation. Now, this issue is a thing of the past unless you have a very poor builder…

If you enjoyed reading this post I can promise you'll love our new writing over at Postgreen Homes. Yeah, we know that's the same thing your favorite band said and their new album is nowhere near as good as their early stuff, but seriously, we are actually still getting better.

There also isn't much conversation to be had here . . . at least not with us. So come on over to the Postgreen Homes Blog and tell us what you think of our new(ish) digs and crazy ideas. We will be sure to tell you what we think of your opinion.

{ 22 comments… read them below or add one }

1 Andy Torchia January 20, 2010 at 5:13 pm

The auto shutoff delay for the bath exhaust tied to the bath light switch is exactly what I want to do for the baths in the house I am renovating. Is that an off-the-shelf switch or do you have to rig it up yourself?

Can a standard dryer be exhausted into that ERV?

2 chad January 20, 2010 at 5:31 pm

Andy – It’s an off the shelf switch. Here is one I bought to put in the 100K bath –


I would NOT exhaust a normal dryer into an ERV or HRV. This would be bad…

3 Peter Troast January 20, 2010 at 6:25 pm

Chad–really excellent post. Thanks.

Do you know why the Recouperator is the only one that meets Passive House reqs vs Fantech or LifeBreath or some of the others?

4 Shawn Busse January 20, 2010 at 8:07 pm

Hey Chad,

What’s the price-tag on the HRV you are using?

5 chad January 21, 2010 at 11:33 am

The UltimateAir ERV retails for over $1,200. I’ll respect their desire to not show exact pricing on their website. You can call for pricing based on the quantity needed.

As far as other units, the Fantech units range in efficiency from 45% – 76% which is well below the UltimateAir’s 95%. The Lifebreath is 45% – 72% and uses about 50% more energy on low speed compared to UltimateAir.

I would imagine that other US players will be catching up soon, so I’d keep checking and hounding manufacturer reps… Good questions.

6 Dan January 21, 2010 at 1:41 pm

How is efficiency defined for these units? I find it hard to believe from a thermodynamics standpoint that 95% efficiency is possible. If the volumes of air are the same and the humidity in each is the same (not always true) then the theoritical maximum efficiency would be 50%. If the outside air is 10 F and the indoor air is 70F, you’re not going to get the fresh air coming into the house at 70F without adding energy to the system. The best you can do is 40F ( = (70-10) / 2). What gives?

7 chad January 21, 2010 at 1:52 pm

Valid points Dan. This is over my head, so you’d have to contact the manufacturer. I will make a not to discuss with him next time we speak…

8 John January 21, 2010 at 11:52 pm


This is a great project and your blog has some great information.

I have researched ventilators a little and the literature I have found are all very insistent not to have a “sucker”, to use your term, within 12 feet of a stove – grease and these things don’t get along. Are you concerned at all about that? If you don’t have a hood, won’t smells and grease become an issue?

9 Jim Wild January 22, 2010 at 6:02 am

This discussion on ventilation is interesting. It seems in the UK we have many more options for units. As we have designed a few systems over the years it is noticeable how quickly the controls and efficiencies of the mass production players has come on massively. We don’t really have a history of warm air heating in this country so these systems are still looked at suspiciously by many.
It would be normal for us to be able to expect to specify controls that automatically adjust to the humidity and CO2 levels, though with a penalty for initial capital and sensor replacement costs. With these controls we are able to set the base flows to really low values.

On 95% efficiency, Dan, with counter/cross flow the and a large enough surface area this is perfectly legitimate thermodynamics as a pure sensible heat transfer.

The electrical power efficiencies are now also very good, with units available at 0.27kWh/m3. Look out for DC/EC motors with ball bearings and noise and vibration free running.

10 Jim Wild January 22, 2010 at 6:04 am

Will you have data on the energy use of your projects? I know it’s likely a very different climate, but we are hoping to get some real life data over the next couple of years and we could probably exchange/publish this.

11 Peter Troast January 22, 2010 at 9:28 am

All–Martin Holladay has a new post up this morning on Green Building Advisor on ERV vs HRV. It’s good.

12 chad January 22, 2010 at 10:52 am

John – We use a recirculating range hood with a quality charcoal filter that handles all grease and smells. Good question.

13 Dan January 22, 2010 at 12:05 pm

First, let me correct my formula in the example I gave. The new air temp will be the equilibrium temp of the two, assuming perfect heat transfer, i.e. 100% efficiency. So in the example I gave, 40F = 0.5*(70-10) + 10 is what I should have typed.

Jim, the laws of thermodynamics say that heat always goes from hot to cold. That works to transfer the heat from the hotter outgoing air to the colder incoming air up until they reach the same temperature, 40F in the example above. That transfer stops once they are at the same temp! If you can get that incoming air up past 40F to anything approaching 70F without adding energy, you’ve rewritten the laws of thermodynamics, solved much of the world’s energy problems and are a shoe in for a Nobel.

Maybe efficiency is calculated in terms of this theoritical limit? I don’t know, but was hoping for clarification. However, it is often phased by the HEV/ERV vendors in a manor that definitely implies that 95% (or whatever) of the heat is saved. Sorry, but that is simply not possible according to the laws of physics/thermodynamics. At the very least, I think it’s important for each of us to be aware of this and not expect something that can’t be delivered.

For what it’s worth, I plan on putting an ERV system in my home that I’m building later this year. I think they are probably worth it despite the misleading claims. I hope others use them as well, but also hope they don’t have unrealistic expectations.

14 greenbuildingindenverdotcom January 23, 2010 at 2:30 am


Your 50% calculations are correct based on a parallel (concurrent) flow heat exchanger. Most HRVs, including this one, are cross-counterflow. In units with large surface area, they perform like pure counterflow (countercurrent).

Counterflow heat exchangers theoretically CAN achieve 100% efficiency because the outlet temperature of the warm stream can equal the incoming temperature of the cold stream. For a picture, see

Chad, I was relieved that the PassivHaus rules changed just in time for you, and you didn’t have to use that goofy underground air inlet. I suspect Dr. Feist will eventually be browbeaten so badly he’ll relent on the “subslab overkill” as well.

15 Dan January 25, 2010 at 9:59 am


Thanks. That makes a lot more sense. I’ve asked a lot of vendors selling these units a similar question. Not one has been able to answer how this was done. Glad I now know the “trick”.

16 shawn October 15, 2010 at 12:15 pm

i am looking at an erv for my new home. i contacted ultimateair – they recommend not using it to vent bathroom/kitchens. ive also found a best practices paper on ervs that also recommends not venting bathrooms/kitchens through the erv. (

whats your reasoning for venting through the erv.

17 Jim Wild October 18, 2010 at 6:59 am

Your post and the report makes interesting reading.
It is not always easy to put new kit and technology into old systems. The model shown by postgreen is very much the one we see in Europe, where we don’t have any other ducted HVAC. It’s all about fresh air and recovery of heat, not cooling.
HVAC solutions tend to be climate specific, though no matter where they are passive houses (not passivhaus) tend to need very little HVAC so perhaps look at how you can design out HVAC as much as possible with lots of insulation, thermal mass, moisture balancing materials, minimal solar gain, etc (it’s all out there as DIY guides) and then you may find you don’t need anything to complex to condition the space.
As an engineer I can design all sorts of HVAC to condition “poorly” designed buildings, but I love the challenge of proving that you can design a building to not need HVAC. ERV/ HVAC as the report on advancedenergy site kind of says is just too complex for domestic installers and home owners. Do you think they will ever be educated in these systems? I’m degree educated and 10 years in the job, it’s still a learning curve for me.

18 shawn October 18, 2010 at 9:57 am

ive been reading your house blogs for the last year. trying to incorporate passive designs into my new house. i am not an architect/engineer but did design our house that will be breaking ground in a month. i bounced my ideas off architects/engineers i know: fixed windows,sealing strategy’s,radiant heat in basement, rain water collection,solar water heat,superior walls,white metal roof,etc.

i was thinking about this all weekend – you are right. i am building in north carolina with humid summers and mild winters. i am more concerned with air quality and cooling rather than air quality and heating. ultimate stressed being concerned with just air quality. im also thinking that the erv will be pressurizing (pressuregaurd) the inside, for the few showers a day, its worth the few times id heat incoming air. versus pushing conditioned air out the vents all the time. jeez full circle on it – i have about 2 months to figure this out :)

19 cgaff July 14, 2011 at 9:23 am

Great information here! Would you still use an ERV if you had operable windows?

20 STUART October 13, 2011 at 10:05 pm

I really liked your very informative webpage, including the “question and answer” format blog postings. I am hoping to build my retirement home in the next few years, and I am heavily researching every aspect of “green” construction in preparation. I have discovered that there is a world of eco-friendly information and construction supplies and components out there. I just have to find the right combination for my particular needs and wants. I’m sure that you know many good information resources on the internet. Feel free to suggest any or all of your favorites with me and your other readers. Thank you!

21 Rajesh Patnaik June 2, 2012 at 1:38 am

I want ventilator design of my house

22 Shaun November 21, 2012 at 7:40 am

Im researching these units and am surprised you recommend a US manufacturer when as you say, they use more power. You have to ask yourself running ground source heat pumps, rainwater harvesting pumps, appliances, HRV etc when do these start to be more of a drain than a gain?. I am not on a gas main so will probably opt for induction cooking, this alone is more than my PV array can supply.
Starkey Systems make a 96% unit and are UK based.
Out of interest, what insulation and thickness did you use?

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