ERV Retrofit to Central Air Handler
Greetings,
I am trying to figure out if an ERV can be successfully linked to my central heat pump HVAC system.
Many of the installation manuals specify minimum distances between components, but my HVAC system is pretty compact compared to others I’ve seen.
I came across a VanEE ERV that seems to have installation options without minimums (option T3 and T5 here: https://www.broan-nutone.com/getmedia/96607274-7035-42aa-87c7-c63ad3b61a5d/Installation_Guide_AI_N_Series.pdf) but am not experienced enough to know if these will work out well and can’t really trust an HVAC installer either, since the ones in my area don’t seem to care and just want to make a sale. I may even try to install this myself as it doesn’t seem too complicated, once the correct design is determined.
Here is a pic of my central unit. The return is very simple, has an opening for the basement and continues straight up to the main floor and 2nd floor. The supply plenum has 2 supply trunks (front of house and back of house).
Thanks in advance for your insights and advice!
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T3 & T5 injects into the supply side of the furnace. The manual states this requires synchronization with the blower to avoid mold growth.
This AI series with auto-balancing might work better for supply-side injection. Concerns about pressure differentials in the supply-injection scenario are often raised- you might look into that.
If I remember correctly you are looking to add the ERV partly for radon mitigation. In that case I would go with T2 or T3 option and have the stale air pickup near the floor in the basement. If there is a main floor bathroom without an exhaust fan where you can easily run a stale air pickup to, you can add a T and split the flow between the two.
I like the T2 option better as ERV air flow is filtered and conditioned by the air handler.
Make sure to spec an auto balance unit to maintain constant flow when the air handler cycles.
Option T4/5 would work as well as you are not directly exhausting the radon but diluting it. This does help but not as much.
P.S. I doubt any residential HVAC respects those distances. Usually way these are installed is the T4 with both lines connecting right by the air handler.
Thanks Akos! It sounds like this might work. I have a basement bathroom as well, I was thinking of using as a pickup location, so that I could re-use its exterior vent hole for the ERV exhaust... The main floor powder room already has its own vent.
With the pickup being in the basement would this create 'return' airflow from upstairs to basement? That actually may not be a bad thing in the winter.
I am looking at the VanEE (https://www.broan-nutone.com/en-ca/product/freshairsystems/v150e75ns) or the Panasonic Intellibalance.
It is nearly impossible to isolate a basement from the rest of the house. With the amount of air flow for an ERV you are talking about you won't be able to measure any pressure change between the two places. There is not need for any makeup air or additional ducting.
Basement is part of your conditions space, so you want airflow from the house to the basement. This gets more fresh air from upstairs into the space and the ERV exhausts any basement smells outside. This is an idea setup for basement IAQ.
You can re-use the existing bathroom vent wall cap for one of the ERV connections, but this would only be good for about 100CFM max. You probably want to upsize it to 6".
Try to locate the outside fresh air pickup somewhere where it is not likely to draw in smells from the outside. Mine are near the soffit which works well but hard to do in case of a retrofit. You might be able to use your old chimney chase if you can get a continuous length of pipe in there.
There is no need for a 2nd ERV. With the unit connected to the HVAC you'll get fresh air distributed to the whole house. The only thing you might have to do is run the HVAC fan a bit during shoulder season to get some more mixing.
I like to keep things simple so I'm not a fan of interlocking ERVs and air handlers so my recommendation is for T2 or T3 install.
Just checking- if you re-use the existing exhaust penetration then you would need to disable the existing associated exhaust fan.
Taking all the exhaust out of the basement will de-pressurize the basement. The air will be made up from the upstairs of the house, any leaks in the basement, and any supply registers in the basement. You can consider closing a supply register if it is close to the exhaust point to avoid negative short-circuiting effects. The basement can potentially leak in a lot of air at any windows and at ceiling above the foundation- you would want to check and seal obvious leaks there. Also air can infiltrate from the ground- that would be high in radon and reduce the radon mitigation benefit you are looking for. You can set the ERV with a bias to deliver more supply and less exhaust- this would help a little with de-pressurization issues anywhere in the house.
If your furnace is in the basement taking exhaust air from next to it could be good for energy usage. It cools the furnace and there can be significant waste heat/cool that ends up leaking out of the basement rather than heating/cooling the house. However, when the outside is for example 70 and the basement is 65, the outdoor air will get cooled by basement air rather than warmed, potentially increasing energy usage.
I am setting up an install scenario where I am going to be taking from a return that is further away from the furnace and not 100% of all the return ducting. To me that seems preferable than taking everything from the basement. You could do that plus take from the basement.
Thanks graygreen! The depressurization sounds like a concern for sure. I like the idea of tweaking the ERV to supply a bit more than it exausts to mitigate.
I also like the idea of splitting the stale pickup between the main return and the basement however the total length of the return duct in the basement is not much... It's ~6 feet, excluding the filter cabinet. The installation guidelines specific much longer spreads but I'm not sure what the practical impact would be.
My central system is indeed in the basement, but it's an air handler with heat pump as opposed to a furnace, so there isn't much waste heat as there was prior with my old furnace.
I also take your point about short circuiting the ERV. All of these points show that setting up an ERV is not easy and full of pitfalls!
Alternatively I scoped a location in the stairwell going up to the 2nd floor that is basically a big void space. Was thinking I could put in a dedicated ducted ERV there but it would only easily connect up to 2 of the 4 bedrooms, the stale air pickup would probably have to be the top of that stairwell, and would not do much for radon so might need to invest in a radon mitigation system as well...
An ERV can help but its possible it could hurt a radon situation depending on what happens in the install (sometimes a fresh air supply gets removed leading to more overall depressurization). It's good to radon test after making any ventilation change.
ERV in the basement is nice because the penetrations to the house will be low where they can be easily maintained. If you have to use a ladder to inspect an clean a bug screen it isn't going to happen that often. Also the ERV itself can be easily maintained whereas in this stairway location that seems potentially very difficult.
If you did duct work upstairs you can look at sending it down to the basement. It looks like the wall on the right there may be thicker because it is already dropping some utilities down to the basement.
Is your basement ceiling finished? Just trying to figure out where all the return ducting went. You could have an HVAC company come out to give you a quote and talk to them about duct options. Tell them you are supposed to have the 10 foot distance and see if they can figure anything out.
If it were me I would talk to an HVAC company but plan on going with your original thought of T-5 and injecting supply side. Stick with the AI model and interlock it to the furnace- the AI series is uniquely situated to handle this well. In the summer use intermittent mode to avoid running the blower all the time and ruining dehumidification.
Yeah good points. I think I'll pass on the stairway location... The wall looks thicker but that's actually a bedroom on the other side, so there's no good channel for ducts between the floors.
The basement ceiling is partly finished... some areas have removable drop ceiling tiles, some drywall, and some old school non-removable ceiling tiles. The main return goes up to a joist cavity, then over to the living room which is very close by. It also connects to a rectangular duct that goes up to the 2nd floor - this was all added in the 90s so there are drywall bulkhead sort of things that contain the duct.
My challenge is finding an HVAC company or person that actually understands this stuff. I emailed a few new companies, instead of dealers I searched for designers. I am hoping this will help me get closer...
Thanks for the tip about summer mode. This makes sense, as I am a bit concerned about humidity. Does this mean that the ERV will run only when the Cooling is running, in other words, the air handler drives the ERV instead of the other way around? Does the ERV controller have buttons for this or would I need custom electronics to be able to flip between winter and summer modes?
I think the HVAC companies will understand whether or not your request to have spacing can be satisfied or not. With the AI series I don't know that they have to understand that much about the ERV. It takes care of the balancing and even commissions itself unlike any other ERV on the market. They need to do a good job with the duct work- insulating the exterior ducts before they attach the ERV.
The "AI" in theory should handle dehumidification correctly if set to run intermittently.
From the manual:
> 3.3.3 Synchronization with Central Forced-Air System Function
The Virtuo technology allows synchronizing the unit operation with the central forced-air system operating time. It prevents unnecessary
central forced-air system operating time while providing a better air distribution. To use this function, W and Y connections must be added to R and C connections to inform the unit that the central forced-air system is running (refer to dotted lines in above diagram).
The text below the "above diagram" states:
> These connections must be done if you want the unit to force the central forced-air system blower operation when ventilating (refer to
solid lines in above diagram).
So what I am hoping is that I can set the ERV to run intermittently (20 minutes/hr). Without the LCD wall control, the minimum speed would have to be increased 3x. With the LCD wall control there might be a different way to set intermittent. My hope is that the ERV won't start until the AC turns on (and it should be on for 20 minutes to achieve dehumidification) or 40 minutes is up. I will try to ask the manufacturer for more details about this on Friday.
Thanks again, graygreen!
It sounds like if I go for an ERV, that the VanEE "AI" is the right equipment. Wiring it up seems pretty straight forward. Curious what else you find out.
It sounds like the ERV connection to the central HVAC ducting is pretty much resolved, but now I am struggling to figure out what the routing for the exterior vents would be, due to several barriers (garage on one full side of the house, porch on the entire front, plumbing and back doors on the back, leaving only one side of the house suitable which already has several vents from the bath fans, dryer exhaust and hot water heater vent!)
I'm still debating about a radon system as a sort of alternative although not comparable to an ERV, it would require only 1 vent penetration on the house instead of 2. I would have to live with the elevated CO2 and smells but at least radon would be under control.
With respect to the ERV vents- that is probably something an HVAC company can figure out well. They know what code is for staying away from other exhausts. Tell them you don't want it by your driveway (car exhaust) because code doesn't necessarily prohibit that.
I am planning to install my ERV near the ERV vents to minimize the insulated duct run rather than locating it close to the furnace. I am going to be taking and putting into a return that is not 100% of the return path but close to the ERV install location. If you can't do that and the runs are long it should be okay as long as the duct is smooth metal and not flex duct. Houses are designed with separate exhaust and supply systems for ERVs- those are long runs.
Are you removing an existing fresh air supply as part of the ERV install? If not, then then ERV will improve the radon situation rather than make it worse. It's possible to ventilate away your radon problem. However I don't think its economical unless you only need a tiny amount of ventilation or spend a lot of time in the basement and want it to always be well ventilated. Whenever you run an ERV you are leaking energy out of your house, whereas radon mitigation does not. Have you tested for radon? Have you got a quote or an idea of how much radon mitigation costs at your house?
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Yeah I am on the hunt for a good HVAC company or person... The first one, who did my heat pump and air handler installation, told me that I need to figure out the vent locations first... guy was not willing to brainstorm it with me.
Good tips on the ERV location, I think I could do the same. Heck at this point I feel like I could install the ERV myself, maybe just get a handyman to make the holes in the rim joist/brick since I don't have those kind of tools.
Re: fresh air supply, no, we don't have one.... But we do have air leaks at the basement rim joist area which I'd probably work on sealing up if I got the ERV. One of the HVAC supply ducts actually runs along one of these rim joist areas with more significant air leaks and it really impacts the air temperature going to that one room.
I have tested for radon yes... the results from the winter are attached. This was a basement measurement, but due to the forced air system, the whole house seems to have similar radon levels. The radon system would cost in the $2000 - $3000 range depending which contractor I use which I think is pretty comparable to the cost of hiring someone to install an ERV. Still on the fence which system to get but I do want to do something !
I think you need to focus on figuring out your radon situation. I wouldn't get an ERV in your situation, I would get radon mitigation. I would also additionally want to get an ERV, but if that isn't affordable you can crack a window where you are in the house until you have the budget for the ERV. Or install a direct fresh air supply to the furnace. Figure out what you want to do with the ERV but instead of installing it, run a single fresh air supply to the furnace. Then later when you have budget for the ERV get the ERV.
For the radon you have a monitor so you can do some experiments. Although one thing to keep in mind is that AirThings is not all that accurate. You can get an accurate one time test for ~$50.
Are you re-circulating the air from the basement? It seems like the quickest fix might be to close all the (return) vents in the basement and seal them. Just closing them might not be enough- you might need to buy a thin magnetic sheet to put over them: https://www.acehardware.com/departments/heating-and-cooling/heaters/heating-registers/4169504
Or when the weather is nice enough just don't run the blower (or heater) at all.
Then close the basement door. Then open some windows and ventilate the house to get the radon levels down. Then do the radon levels stay down or go back up? When the radon levels are low, does running exhaust fans make the levels go up?
Do you have sump pump wells? If you seal them does that reduce the radon?
You can simulate what an ERV would do by creating a fresh air supply without any heat exchange. Leave some windows cracked open. See what happens to the radon levels.
BTW, I talked to Broan and they couldn't tell me anything specific about how their AI adapts to being synchronized. However, they did say that if you set it to run for 20 minutes intermittently it will do the 20 minutes all in one contiguous block- which I think is helpful to avoid altering dehumidification.