ERV for CO2 Mitigation
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Hi…I recently replaced an HRV that was part of my build, 25 years ago with a new ERV but my CO2 levels still aren’t quite where I’d like.
Prior to this, I installed all new fiberglass doors and triple pane windows from a very reputable firm. Post audit airtightness shows ACH50 of 3.03 so by no means super air tight but was >20% improvement.
CO2 analyzer (new and seems accurate) is showing levels over 1000 with ERV on default settings of low speed and CFM. Boosting it to high speed and 500 CFM is getting CO2 down to low to mid 700’s. Close but was hoping for 600’s and will never achieve this in the winter as it slows down/stops to defrost.
I have checked outside vents and they are screened with no damage and can feel the airflow which is quite good. These duct runs are about 15′.
I have used a formula for calculating what my CO2 levels should be at 500CFM for my volume and it gives me 511.
Gas furnace and tankless HWH are fully vented to the outdoors. No other gas appliances. 4 occupants at night, 2 during the day. I calculated for 4.
Any thoughts on why I’m not getting CO2 any lower? If I open all windows, it will roughly equalize to outdoor readings within an hour or so.
If I turn on a bathroom exhaust fan, it will lower a bit further, 20-40ppm.
Outdoor readings are usually 420-440.
The ERV was (properly) connected to my HVAC existing ductwork. I have played with the furnace blower which is on 24/7. It’s default CFM is 450. Most of my testing is with it bumped up to 1000 to help ensure it’s not affecting results.
I realize passive designs have better ductwork and often put the ERV on it’s own system but I’m thinking that in itself doesn’t account for my results.
Readings in the basement are slightly better so it’s not as if there’s any ground source infiltration. There is also no mold that I’m aware of. PM 2.5 levels are always 0 indoors.
Given a passive house with 0.6 ACH is only using an ERV with 350 CFM, I’m presuming I have some issue that needs to be addressed but not sure how to further troubleshoot.
I’m very new here so haven’t read much but will try to do so as I get time. Thanks.
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Replies
How is your ERV ducted to the HVAC system? Can you draw us a diagram?
Which ERV Unit do you have?
How many people live in the house?
What is the cubic foot of the house?
Jamie
How many bends or 90's are in this 15' of ductwork?
What do the exterior terminations look like?
What type of filtration do you have for the incoming air to your ERV? If more than just the ERV internal filter, show up a pic/draw a diagram.
How many pets are in your household?
Are your gas appliances "fully vented" as you wrote or Direct Vented (ie atmospherically vented vs. direct vented)?
I hope to lower the unit and eliminate any sharp bends to the furnace ducts but the 15' of insulated to the exterior, I'm not sure. By the placement of the unit in respect to the location of the vents, I'm guessing 2 90's each.
It's a new ERV. I believe it uses standard ERV filter.
3 human adults and 1 medium sized dog. 2 people are out most of the day and the other part of the day.
Gas appliances are fully vented. Nothing is atmospherically vented.
Exterior vents look good. Screened with no apparent blockages. I can feel a good air flow.
Thanks.
The new ERV is ducted to the HVAC in a standard manner though in checking it again, I do see that the unit (which is suspended by straps from the joists) doesn't need to be so high and is forcing some sharper bends that wouldn't be necessary if it was lowered a bit so I will look at doing that if it can be done with everything still attached. Wouldn't need probably more than a half foot or so. I'm not sure what impact, if any, that will have but worth a try. Ducts to furnace are new 20" flex ducts. Those to the outside are the original insulated HRV ducts.
It's a Lennox ERV5-175-TPD. I've read through the 60 page install manual so am fairly familiar with its setup though it was installed by the company I bought it from.
It's connected to a Lennox S30 smart hub which doesn't give it all the functionality that it's optional dedicated control unit would but I already had the S30 and a SPL-98 furnace from them.
This setup only allows me to do programmed timed mode of up to 23h59m in high fan speed. Though I'm thinking I could reverse the high / low connections and then run it always in high speed as the always on mode defaults only to low speed. Not an ideal setup as it turns out. Why they give you less functionality from the S30 is really discouraging and had I'd known the dedicated control would serve me better, all mfg's would have been up for consideration.
I can take it back though with a restocking fee and that might yet occur but options with higher CFM's are more into the light commercial sizes and are a lot more costly with unnecessary features such as their large circuit boards. Though I'm sure if properly setup, I shouldn't need anything bigger for my space. I had calculated it should be capable of getting my CO2 to 511. It's currently at 800 in my basement office at 2020h and was 667 at 1917h when I got home. But it went out of the 24h timed mode and so defaulted back to always on, low speed which I changed on the S30 from default of 50CFM to max of 200CFM. Eventually that would climb over 1000ppm at 200CFM.
The CERV2 looks interesting though less CFM at 300. The built in heat pump would likely allow the same performance as mine in the winter or better I'd imagine. Not sure what the price is for those though I'm guessing double what mine cost. Or more.
I know these (Lennox) are being installed in homes a lot bigger than my somewhat average size but what people are getting for VO2 measurements, I'm not sure.
3 people and a dog so I'm calculating for 4 people.
Just under 30,000 ft3
Something weird going on there. A house that size with that many people should be in the 600ppm range at around 100cfm, even if it was <0.6ACH.
Seems to me that somehow the air the ERV is bringing in is going right back out without mixing with the indoor air.
Yes, that would certainly make sense and I agree that something weird must be going on as I had done a calculation that showed I should be able to get it down to 511ppm.
Attached is the duct configuration on the unit. I have triple checked the ducts as that was my first thought, that somehow it wasn't connected properly but when I thought it through, it definitely is connected properly. Though I haven't opened the unit. I will do that just to see if there's something wrong going on inside but I'd be surprised.
Was trying to upload but it won't let me.
The only thing I can think of is that it's not balanced. I don't have an HVAC manometer but might check that out further on the weekend.
Earlier in the week I opened it up and found that many of the dampers were only open a small amount. Ensured all 4 were fully open as per the install manual for balancing and the unit and furnace running on high.
I borrowed a good manometer (UEI EM201B) but without the rubber fittings from Lennox, the ones with the device aren't a perfect fit so I will seal it with tape and try to do some balancing this weekend though I likely won't have much patience for re-applying tape each time.
However, opening the dampers has made a big difference. I guess they are mostly closed by default and with use with the default low speed CFM of 50.
Tonight I've let the 23h59m custom timed mode expire and will see what the values are now at low speed with max cfm of 200.
Just to update this post, I went over the Lennox install manual thoroughly and have made some adjustments to the dampers which were about 3/5's closed and are now fully open. Also lowered the unit a bit to prevent one duct sitting on another in case that was causing any flow issues. CO2 was now quite good so stopped using the manually set "timed" setting for high and let it go back to the default low fan speed which I continue to use at its highest 200cfm speed.
CO2 is currently 615 in the primary bedroom on the 2nd floor with 2 occupants on the main floor level.
So, it would appear the dampers just needed adjusting. I also used a manometer to check the balance with all 4 fully open and it was pretty close so just left it be as I didn't have the Lennox attachments to easily move from one set of door ports to the other.
In hindsight, I guess it pays to get experienced and knowledgeable installs. If I were to do it again, I'd probably go with a CERV2 unit but it looks like this will suffice though will be interesting to see how it handles the cold in winter.
Thanks!
I'm glad you managed to make this work better, though I'm curious about the CFM quoted for the LENNOX ERV. When I look up the model you mention it appears to max at 175cfm which makes sense given the 175 in the model name. Is 500cfm the furnace air handler speed? https://www.lennoxpros.com/healthy-climate-erv5-175-tpd-energy-recovery-ventilator/p/17Y57?clear=true