HVAC Ducts and ERV Supply
I have some questions related to my HVAC duct location and ERV supply location.
First, the HVAC question. I am going to need to get a new HVAC system in the near future since mine is about 16 years old and seems to have a small leak in it. It also has developed mold in the supply plenum. Additionally, the trunk line is duckboard which I don’t like. So I would like to be able to replace the ductwork with metal ductwork when I replace the heat pump/air handler system. The problem is that the previous owners finished the basement and drywalled the ceiling of the basement. Therefore the ducts are completely enclosed between the floor of the main level and the finished basement.
The way I think I can solve this is by putting the new ductwork in the attic. It is vented and unconditioned which I know is terrible for energy efficiency so I wanted to run a potential solution to that by you all and see if you think it would work. In order to get the vents in the attic and avoid the energy issues I am thinking about constructing a 24″x 24″ housing for the ductwork in the attic. I would make it out of OSB. The bottom OSB board would not be insulated since it would have the rafter insulation under it, the the sides and the “lid” would have R19 fiber glass insulation running the entire length. In this way I’d created an insulated housing for the ductwork. This seems like a pretty simple solution to me but I’m sure I’m missing something. Would this work?
Second, the ERV question. I would like to install stale air returns in the two bathrooms and the kitchen/dining area. In order to simplify the ERV ductwork, I would like to install one supply. I know that if you connect the ERV fresh air supply to the HVAC return directly you have to deal with the air handler’s blower either coming on every time the ERV is on (which if I understand right is most of the time), or deal with ERV seeing different conditions whether the HVAC blower is on or off therefore making it hard to commission. Instead, I am wondering if installing the ERV’s supply vent right above or below the HVAC’s return vent would allow the ERV to always see the same condition but also make it so that when the HVAC blower is on much is not all of the ERV’s supply would be sucked through the HVAC return and therefore fresh air would be supplied to the whole house. Obviously when the HVAC is not on fresh air would only be supplied to the area where the supply vent is.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
1) It could work, but would not be much different from a good air-tight duct with a thick blanket or wrap of insulation. The only difference is in your proposed design the insulation is moved outward to the outside of the OSB box. The key to either option is really good air sealing both in terms of duct leakage and continuous insulation with no gaps.
2) I'm a little confused about your proposed setup. The ERV motor is likely not going to be able to handle the static pressure required to push air through the AC ductwork, at least not evenly. It might just all dump out at the first outlet location and not carry on through the whole system. It sounds like you want the ERV to push into the return of the blower (?), which would add even more friction to push through the fan into the supply ducts. More likely, it would take the path of least resistance and push ERV air backwards through the return ducts into the rooms. This is not a desired result and if you have filters in those rooms the ERV pushing against them will release the dust stuck on them back into the room. I think you best option is to send ERV supply to the supply ducts of the AC (downstream of AC blower motor) and just expect the ERV air to not make it very far in the duct run, likely dumping out very early in the run. Or couple it with the AC blower motor but there is an energy penalty with that obviously. They make some pretty beefy ERV's for light commercial spaces if you want to match the static pressure ratings of your AC unit, but again come at a cost and energy usage penalty.
Thank you for the Thoughts.
1) I see what you're saying. I'm not sure there is duct wrap insulation as high as R19 so I thought the box would insulate the ductwork better and thereby avoid the inefficiencies of attic ductwork. I plan on making the ductwork as air tight as I can possibly get it. The box also seemed like it would be easy to air seal since I could spray foam the bottom two corners and then leave the insulation on the sides about a half inch passed the top of the sides so the fiberglass on the lid presses against the fiberglass on the sides.
2) I may not have explained this clearly enough. I am trying to avoid actually integrating the ERV system with the HVAC system since it seems to complicate the air flow pressures. In order to avoid this while also avoiding having to run supply to every bedroom plus the living space, I thought about installing the supply vent/grill for the ERV on the wall adjacent to the HVAC's return vent/grill on the wall. This way whether the HVAC is running or not the air pressures would be the same in both the ERV and HVAC but (I am hoping) when the HVAC is on, the supply air from the ERV being pumped in the room will be sucked into the return vent of the HVAC, since it's immediately adjacent to it, and thereby distributed to all the rooms of the house. Maybe that makes more sense?
If you're going to go through the trouble of having some ERV supply ducts already, why not just route them to the places you want to supply that fresh air and be done with it? Don't worry about whether they make it to every single bedroom or living area. I would focus on the primary bedroom(s) only, if you have a guest room I wouldn't make that a priority since it is likely not occupied the vast majority of the time. If you have older kids I wouldn't even worry about their rooms since they might be out of the house soon enough anyways. There will be some minor diffusion/dissipation of the fresh air between adjacent rooms already, even without the AC kicking on. The minor benefit you get from the AC mixing some of the supply air for you is pretty inconsequential. I say "some" because the suction on the return duct is not going to be strong enough to suck ALL of the ERV supply air out anyways. If you have decent door undercuts and air circulation it will be fine.
The amount of ERV Supply air sucked through the return is definitely my main concern. It may not be a consequential amount. I wonder if I could model it with some smoke before installing and see if much of it is sucked into the return.
The main thing complicating the installation of Supply ducts is that I want to install the ERV in the basement where the air handler is and avoid running ducts from the ERV to the attic and having to build a bigger or separate insulated box for the ERV ducts. It also would drastically increase the amount of duct. The run from the ERV itself to the wall with the HVAC return on it is about 3 feet. To run the ducts to three bedrooms (Mine, and my kids who are still young) plus a living room would be about 50' - 70' of duct and much more complicated since I have to deal with the drywall on the basement ceiling. I can actually get the the two bathrooms and the kitchen from the basement without dealing with ceiling drywall (There are sections of the basement that are not finished).