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:
- Use a very efficient mechanical ventilation system (ERV or HRV >90% efficient)
- Design an efficient duct run layout to maximize efficiency of your ventilation system
- Eliminate all dedicated local exhaust systems ducted directly outside
- 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)
- Near Washer/Dryer
Fresh Air Inlets (Blowers)
- Living Room
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:
- Keep it SHORT
- Keep it STRAIGHT
- Keep it SMOOTH
- 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:
- Always use smooth, hard duct and avoid flexible duct as much as possible.
- Insulate the two lengths of duct running between the exterior and the ERV. This will prevent condensation from forming on these lengths of ducts.
- 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…
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