HRV without using existing furnace fan?
Hello everyone,
Following the sage advice in this forum, I have done a number of green energy upgrades to my house, including sealing the house really tightly, and switching my heating cooling from a forced air propane system/central AC to ductless mini split electric.
It seems like I may have done too good job sealing up the house, because I have begun measuring carbon dioxide levels, and they now regularly hit the 3000 mark with just a 4 people in the household, when no windows open.
Because of this, I’d like to install an HRV. The HVAC person who came said that the only option they have really heard of for an HRV is to tie them into the existing ductwork and air handler used by my forced air propane heating and central AC.
My only issues with that are that I don’t think the central air handler fan is particularly efficient, and we’d be back to using our old dirty forced air ducts.
I was wondering if anyone here has heard of options for installing an HRV without using an existing forced air furnace fan? I am thinking, this must exist, because many people who install HRV‘s might not have any central forced air system.
The HVAC technician said only a giant central forced air fan is capable of moving the air from an HRV through the house. On its own, he said that an HRV fan does not have the power to really bring air into the house.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
JWolfe,
No that's just silly. HRVs are routinely installed in houses which don't have either ducts or furnaces. However the problem in existing houses is that installing a separate set of ducts for the new ventilation system involves a lot of disruption, so from that standpoint using the existing ones can make sense.
Thanks for the reply, that makes sense!
Forgive my basic knowledge - are you saying that the built-in fans that come with HRVs are powerful enough to drive air into a house (and through custom-built ducts if new ducts are run for an HRV) all on their own? If so, I imagine this HRV fan would be more efficient than the large central furnace fan.
My house is a very open rectangular layout consisting of a semi-finished basement and a first floor. The basement is completely open, and the first floor is completely open with a few bedrooms around it.
Because of this open layout, we are actually able to heat the entire house with a single ductless mini split located in our finished basement, when we leave the door to the basement open. We leave all upstairs bedroom doors open to facilitate air and heat flow.
Because of this, I am wondering if any ducts are needed at all with an HRV in our house? In other words, I am envisioning a single HRV installed in the open basement, with perhaps one single added duct running up to our very open first floor? From these two single points in the basement and first floor, I think fresh air would filter out to the rest of the house. This would allow me to use the built-in HRV fan, rather than the large central furnace fan.
Does this plan make sense at all? Thanks for any and all insight!
Jwolfe,
The default assumption is that the fan on an HRV/ERV will provide the airflow necessary for a ducted system.
Whether your your largely un-ducted design provides adequate ventilation I'm going to leave to other posters who have a much better understanding of how these systems perform. My gut feeling is that you will still have problems in the bedrooms at night.
These are good overviews of the options:
https://www.greenbuildingadvisor.com/article/a-beginners-guide-to-ventilation
https://www.greenbuildingadvisor.com/article/revisiting-ventilation
"The HVAC person who came said that the only option they have really heard of for an HRV is to tie them into the existing ductwork"
I ran into this exact same mentality when I was building my house in 2017. It's comical that it's still going on. The contractor, who was a Lennox dealer, let me know that Lennox actually stated that you absolutely cannot install an HRV on its own, it simply won't work.
I would suggest that ducting is, if not necessary, certainly advisable. Heat distributes more readily than fresh air, because it moves by conduction and convection, as well as air exchange. It's very different for fresh air, because not only are you lacking the drivers of conduction and convection, you have a waste product you need to evacuate that is absent in the heat model. Yes, you have cold air that you ideally want to return to the minisplit, but if your cold air return is not that great, the cold air that doesn't make it back gets heated by the warm air; the cold air literally becomes warm air. Carbon dioxide and other air pollutants won't become oxygen and nitrogen, they HAVE to be expelled from the house. From experience I can tell you that CO2 builds up very quickly and takes a long time to remove by dillution, even with a fully ducted system. It's going to be a lot slower without all the ducting.
If you wanted to go with single supply and return, at the very least you'd want the supply at one extreme of the house (say basement, one end of house) and return at the upper floor ceiling, opposite side of the house.
HRV/ERV fans are typically pretty strong. I have no air handler and my ERV has no issues with ducting. I also have an in-line HEPA filter box on the fresh air supply which is pretty restrictive.
In my setup, I have the ERV delivering fresh air to the living space the and the stale air pickups in the bedrooms and bathroom.
You'll have no issues with a standard unit with ducting. Try to use larger ducts and try put some flex (or in-line muffler) between the unit and the house.
In colder climate, unless you are using a post heater, the fresh air supply will be colder than house air. Watch where you install this supply this as it can create some comfort issues. Mine is sent near the ceiling peak which helps a bit with stratification.
If supplying into a bedrooms, best stop is up high by the door where it can mix with warm house air. Avoid floor registers.
I would read Dennis Wood's comments on this thread where he describes his own retrofit system with one main fresh air supply and stale air exhaust point. I really like his use of an exterior fresh air filter that saves interior space and makes it easier to use a large filter. https://www.greenbuildingadvisor.com/question/plans-for-external-erv-intake-filter-box
I did a relatively simple independent duct setup with stale air coming from my two upstairs bathrooms via 6 inch duct and fresh air going to the upstairs hallway via 8 inch duct, positioned near the doors of all 3 bedrooms and the upstairs return for my central heat pump. I didn't want to always need my heat pump air handler on since it uses 500 watts. I can run my 210cfm Broan ERV at 65cfm continuously and use only 23 watts.
Thanks for the feedback everyone! While I sort this out and think about how to move forward, I’m going to upgrade my bathroom fan and go with an exhaust-only ventilation strategy for now.