ERV: Fully ducted vs. sharing return duct?
In this discussion (https://www.greenbuildingadvisor.com/question/does-anyone-know-the-approximate-cost-to-run-an-ahu-blower) the thought of fully ducted maybe not being clearly best practice came up.
Fully ducted is clearly more energy efficient. But with a low static pressure duct system and ECM AHU blower is it really that great of a difference to warrant the extra costs for the extra ducting?
Fully ducted likely results in better IAQ, particularly in closed door bedrooms at night. Can running the AHU blower frequently or continuously likely result in just as good of IAQ? In my case I’m looking for CO2 of about 600ppm in closed door bedrooms.
IOW, is the cost of fully ducted greater than the benefits?
Thanks
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Replies
> Fully ducted is clearly more energy efficient.
Or maybe running the ERV with short duct runs (both supply and return connected to air handler) plus the AH fan pushing only 100 CFM through large ducts takes less energy than running only a fully ducted ERV through smaller ducts with far higher static pressures. For a given airflow, large ECM fans and larger ducts are typically *more* energy efficient than small ones.
Unbalancing the ERV (perhaps very slightly) and the cost of running a larger ECM fan can be non-issues. Continuous running of air through an AC coil if it cycles will reduce latent removal. An issue that could be solved with some dampers. Or some fancy controls that vary the ERV flow rate to match the AC duty cycle. For example, if the AC needs to run 25% of the time, use 4x the ERV air and run both fans intermittently. Or maybe latent removal is another red herring. Just run the dehumidifier a little more - you probably need it at low sensible loads anyway.
More important is air quality. A fully ducted system designed to best practices often results in poor IAQ - failing to meet even 15 CFM/person in a breathing zone.
I'd not convinced that the best system isn't a third option - some number of distributed ductless ERVs that can be turned on/off based on occupancy. I can come up with realistic scenarios where it provides better air quality with less cost to install and less cost to run.
The real answer is "it depends - run the numbers". Avoid coming to conclusions based on generalizations and claims without the figures to back them up.
"More important is air quality. A fully ducted system designed to best practices often results in poor IAQ - failing to meet even 15 CFM/person in a breathing zone."
Curious where you got this info?
If the ducting is designed properly and the ERV/HRV commissioned properly then it should deliver spec'd CFM of fresh air. I'm not aware of any way that a return duct based system could do any or much better. Poor duct designs resulting in high static pressure is a different issue.
More importantly, fully ducted delivers the air to where it is most needed - closed door bedrooms. This rather than distributed throughout an entire house of mostly empty rooms and closed door bedrooms getting only a very minimal amount?
Here is a close example. Without the atypical high CFM boost, it fails with 4 people in two closed door bedrooms. I expect you would create a better design, providing the ~30 CFM/person you want. Fully ducted can work well.
https://www.greenbuildingadvisor.com/article/saving-sustainably-installing-ventilation-system#comment-154547
I'm totally missing something in what you're saying. No matter how large of an ERV/HRV you have... You will have more fresh air in fully ducted bedrooms when that CFM is divided up only among those bedrooms than you will injecting it in to the return duct and dividing it among every room in the house?
True, that is the problem with the example. 60 CFM split evenly among 4 rooms does not provide 30+ CFM to a closed off master bedroom. Yet it follows what many call "best practice". Specifically "supplying fresh air to living areas and bedrooms" when "supply fresh air only to bedrooms" (or some other solution) would be better.
There is another issue that is more technical. Imagine you have lots of internal airflow such that you have perfect mixing between the bedrooms and the rest of the house. It doesn't matter where you supply the fresh air - the CO2 (not other contaminates) level will be the same in the bedrooms. Actually, lower when accounting for the fact that the house isn't perfectly sealed and has a larger volume. Using the air handler fan will never provide perfect mixing, but it can provide a lot, making ventilation supply register locations less relevant.
Open bedroom doors also provide quite a bit of mixing.
Lets try to calculate this.
Typically for a master bed we want to target 20CFM of fresh air supply with a fully ducted setup.
In a typical house with say a 1000CFM furnace fan, the bedrooms would get somewhere around 100CFM (that is on the high side but makes the math easy).
When the furnace is running on fan only, we set the blower to 250CFM (1/4 of full capacity), so the bedroom should get around 100/4 or ~25CFM. So that is pretty close to what we need.
How "fresh" is that air?
Typical ERV would run around 120CFM, so the air your furnace is circulating would be half house air/ half fresh air. Unless you have a lot of people and pets in the house, with continuous ventillation the house CO2 level would drop down to outside levels an hour or two after you close the bedroom door, so the the air your bedroom would be supplied with would be pretty decent CO2 wise. VOCs are a different, they would be higher with a shared duct than with a dedicated supply.
This is all theoretical, it assumes your HVAC is set up right, the bedroom register is supplying the right amount of air and your ERV is commissioned properly. If somebody closes the bedroom register because it is too hot or cold this all goes out the window.