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Community and Q&A

Electronic Dampers for ERV

wiscoguy | Posted in General Questions on

I have an erv installed with my main heat system which I have now learned is more than likely not the correct way however the house is completely done and I don’t think it’s feasible to change what I currently have going on.

the heating guy setup the erv to run all the time along with the furnace to always be set on low fan.

I don’t think this is correct because the erv running nonstop is just bring cold air and moisture into the house constantly and or heat in summer.

The furnace fan always being on low doesn’t bother me to much because it keeps the air moving and gets it running through the HEPa filter and from what I can gather doesn’t cost much more then few dollars a month to run at low speeds.

The problem is with the furnace hooked directly to the erv it is always pulling cold or warm air. Even with the erv off it will still pull in air to the furnace.

My solution was to add a percent timer to the erv and to install electric dampers on the fresh air intakes so that it only allows the cool or warm outside air to come in when the erv runs which would then keep the cold and warm moist air to a minimum allowing the system operate more efficiently.

this is the one part of my build I haven’t been the happiest with. I did a natural gas furnace but I got a modulating unit. It goes all the way down to 8000btu even according to there literature for the carrier infinity it was the best option I could afford.

now the heating guy is telling me his design firm aka the company he buys stuff from doesn’t recommend a electronic dampers on the intake and exit from the outside air and this just doesn’t make sense to me.

can anyone tell me why this could be a bad thing and if these commercial design firms understand more tightly built homes. I do have 2 1/2 exterior insulation with 2×6 walls essentially an r40 wall and I also taped alls seams and have r60 in the ceiling. All casement windows and taped everyseem everywhere it’s sealed up well. With no heat at all my temps stay good for about three days in the house even in the dead of winter.

please help if you can the cool air coming from outside in winter and the humidity in summer is horrible

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  1. jberks | | #1

    I dont see what the issue is. The point of the erv is to provide conditioned fresh air into the house, in this case managed through your HVAC ductwork.

    Yes they're not 100% efficient, somewhere between 70-80% based on whatever erv you have) But that's the breaks when you want fresh air supplied in your house.

    I have two erv's in my house running constantly. And I rather enjoy it.


  2. Expert Member
    Akos | | #2

    How big is your place (sqft and # of beds) and which ERV do you have?

    In the heating season it is usually the best is to run the ERV+air handler 24/7 on low speed set to deliver the required ventilation flow. During the cooling season, you only want to run the ERV during cooling calls as running the air handler all the time will cause re-evaporation of water on the coil which hurts efficiency and humidity control.

    1. wiscoguy | | #6

      My house is 1750 sqft normal living space 3 beds two bath and the baths have separate exhaust fans.

      The erv is a renewaire 90v it’s probably sized a little bit small for my house but only because of the upstairs above the garage otherwise it’s perfect.

      There’s also a bonus room above the garage that’s heated and cooled from the same system adding another 400sqft.

      Appreciate the comments.

      Do you think it would be beneficial then to wire the erv directly to the furnace aka air handler so that in winter when it it’s on low the erv is on low and when it kicks into high mode the erv does as well?

      This would also then help with Summer time cooling making sure the unit only runs when it’s cooling instead of all the time. I had this problem in summer where humidity seems like it was always high even with the whole house dehumidifier. It seems like that thing runs. On stop still. Albeit new build there some drying out the house is going to do no matter what.

      1. Expert Member
        Akos | | #10

        Taking a quick look on the manual (I'm assuming you have the EV90, can't find a V90). The unit is pretty basic, there is no speed control, the unit runs at full speed when on. The only way to reduce the flow is by restricting the ducts.

        Since the unit is on the small size for your place, running at lower speed (even if possible) would not be best as indoor air quality will suffer.

        The best setup would be the one suggested by Danwood with the thermostat driving the ERV and furnace fan. With a bit of tuning you should be able to avoid the re-evaporation issue I mentioned earlier during the cooling season and provide better ventilation during the heating season.

        When the ERV is not operating you do want a damper on the supply and at least a spring flapper on the exhaust (even better a damper on that as well) when plumbed into the HVAC directly. Without the damper it is a pretty big energy penalty to draw in outside air when the ERV is not running.

        Generally it is best to pick ERVs oversized for the application. This is a good thing as a big ERV running at low flow rate is more efficient and quieter. This also gives you the option to trigger the unit to run on boost when needed to clear the house for example when frying fish or a wet dog gets in the house.

        Since the ERV is more efficient at low flow rate, it is better to run at 24/7 at low speed than cycle it on/off at some schedule where it has to run at much higher flow (thus lower efficiency) to prove the equivalent air changes.

        1. wiscoguy | | #12

          With such a basic unit would my furnace be able to run the erv or the thermostat that it comes with rather. I agree with the dampers on the unit I do feel as though I keep getting an energy penalty plus excessive moisture in the house in summer. Currently the erv runs separately 24-7.

          Or am I over thinking this and the erv should be tied to the furnace and then whenever the furnace runs or the air condition the erv will then kick on as well? Not really sure if I’m following it correctly.

          Would a percent timer to the erv with electronic dampers work the same or as efficent as having it hooked up to the thermostat no one I’ve talked to likes to have it hooked up like this. Especially since it’s not variable speed it seemed like it would make more sense to have it left separate and have it on the percent timer with the dampers.

          I’m trying to follow your line of though I think I understand it. This has been the difficult part of all of this is the fresh air intake I wish it would have been ran separate at this point I fear it’s to late at this point.

          So would it be better to just leave the erv and the furnace fan running 24-7 ?

          Right now the furnace has been recommended to me to always be on low for comfort and it helps keep the house evenly heated and cooled which does appear to work.

          1. Expert Member
            Akos | | #17

            The issue is you have the Carrier Infinity furnace. For this to run properly you need to use their controls and their controls don't integrate well (or at all?) with third party ERVs.

            You can check the manual of your unit and see if it has a relay output that turns on whenever the blower is running. If it doesn't have you can install a flow paddle switch in the furnace main supply trunk.

            Install in series with this output a humidistat or minutes/h timer. This signal than triggers you ERV and opens the motorized damper.

            The idea is that the ERV would only run if the furnace blower is running (doesn't matter if on heat/cool/fan) and if the humidistat/timer is satisfied. If the humidistat/timer turns the ERV off, the damper will close which will prevent the furnace from drawing in unconditioned outside air.

            The Achilles heel of the simplified ERV ducting is that it is hard to get the flow through the ERV balanced. Make sure this is checked once the whole setup is up and running. I would check at two points, when the furnace fan is running on low and when running at max. I would aim for the ERV to balanced on the low fan as this is where your system will most likely spend its time.

  3. DennisWood | | #3

    Your ERV has dampers (from 1 to 3 depending on the model), and should have a flap on the exhaust vent. Adding more dampers is not required regardless of the furnace.

    For the heating season, if your furnace has ECM motors, running it on low fan with the ERV makes sense. In summer, your furnace fan really should be set to auto as per Akos' comments.

    On a setup I helped out with, (using an Ecobee 3) we connected the stat's dry contacts (configured as HRV contacts) to a set on the ERV (used for an optional ERV remote) so that the home owner can dial in the percentage run time they want on the ERV...which in turn will fire up the furnace fan via the stat. This ensures the minimum hourly ventilation time, while taking into account the furnace's calls for heat/cool. In other words, if you set 20 minutes/hour ventilation time on the stat, and the system called for heat for 10 minutes, the furnace/ERV will only run for another 10. Works the same in summer, so in most cases you won't be running the furnace fan continuously in summer with AC operation.

    1. wiscoguy | | #4

      I have a renewaire 90v there’s three bedrooms and two bathrooms. It’s approximately 1750sqft

      Also have a room above the garage that’s a conditioned space and is heated and cooled by the same system adding another 400sqft

      Appreciate all the thoughts and comments. I have learned a great deal from this group and overall my house is extremely warm no cold spots and well done. I attribute that to what I learned and the great amount of insulation I used exterior interior quality house wrap and tapes.

      In summer humidity was extremely high in the house even with the whole house dehumidifier running non stop. Granted it’s a new build and I know there’s moisture it seemed excessive.

      Also I don’t believe my model has any dampers anywhere and when the furnace calls for heat it will always bring in cool air from outside. To me this seems to defeat the purpose of the insulation if your alway bring in cold or warm air.

      Appreciate any ideas thoughts etc.

    2. wiscoguy | | #5

      This sounds very intriguing would you care to elaborate some I’m not 100% sure I understand exactly what your saying but from what I’m getting running the erv non stop with the furnace fan on low is ideal for winter months but not for summer ?

  4. DennisWood | | #7

    What model is the ERV? Not sure I've ever seen one without dampers... It sounds like you may just have a fresh air intake and no actual enthalpy core?

    In summer, the AC evaporator in your furnace creates condensation which increases during the cooling cycle. The cooling coils can be quite loaded with condensate just after a cooling cycle. You want the moisture to drip down off the coils and into the condensate drain which may take 5-10 minutes. If you run the fan 100%, this doesn't happen, and the moisture just ends up back in the house. This is not an issue with heating, hence the different advice for cooling vs heating.

    1. wiscoguy | | #8

      The model is the renewaire 90v

  5. DennisWood | | #9

    Ah...I see. There is no provision for frost so I'm guessing your outside temps don't dip below -10F. I get that they may not recommend external dampers to avoid back drafting your house if say a large CFM kitchen exhaust fires up. And with the ERV off, furnace on, it's likely pulling air in just due to negative pressure on the furnace return. I see no provision there for speed control, so it's on or off...that's it.

    The ERV is there to bring in fresh air and exchange energy with outgoing stale air...really nothing to do with insulation, although I get your point with regard to BTU's heading outside via the ERV. I also know from experience that the KISS rule always prevails, particularly with HVAC. I would personally leave your intake damper in place, but this assumes you have on demand hot water (no open flue water heater), no gas fireplace etc. that could backdraft CO into your home. In any case, if your house is in a negative pressure situation, (with ERV off, damper in play) it will pull from the ERV exhaust, depending on how the ERV is connected to your furnace. In say figure 2.4.3, having the ERV off (with no dampers) and no backdraft damper would send warm or cool air outside...but your HVAC contractor should know better...

    On page 12 :

    which figure describes your HVAC/ERV setup? Btw, I'm not a big fan of 24/7 ERV operation if no one is home, so I get how you might want to tighten things up a bit. HVAC guys have simplicity and call backs on their mind so don't always see the logic/wisdom in messing with things. There can be some divergence of end goals at play often between contractor and home owner focused on efficiency.

    1. wiscoguy | | #11

      Ok so I will do my best to answer everything. First I have direct vent hot water heaters not on demand.

      No fire place.

      Although when my bath fans kick on I sometimes feel cold air come inside I’m wondering if that’s just air from the vent becoming open because the flapper on the outside of the house does in fact open.

      Also temps rarely get below -10 it’s not to often maybe a few times a year it’s not normal at all for this to happen.

      I do not currently have an intake damper on my erv or a back draft preventer on the exhaust or anywhere else.

      My system is setup most closely to the third picture or 2.4.2 in the install manual. Basically fresh air gets dumped into the return air Before the furnace so it can mix with the house air before being distributed back throughout the house. The exhaust is behind the intake in the same return air setup so that it’s taking air out before the fresh air is coming in. I think I explained that correctly.

      The bath fans are Panasonic and run separately.

      I have a range hood but it’s highest setting is like 350 cfm give or take 50

      It seems like maybe by putting a percentage time and or tieing the erv to the furnace so it only runs when the furnace is on could help with fresh air and allow only the amount of air needed instead of constant air inside.

      Another option would be to just run the erv on a percent timer so that it’s only bringing in fresh air 40% of the time at full speed and the furnace will also pull fresh air albeit less when it kicks In. If this makes sense anyway.

      My biggest concern is that I have high levels of moisture even with the dehumidifier running. Since the erv is always running on rainy days or any time there’s moisture outside that same level of humidity is inside as well.

      I went over all of this in detail with the guy that did it and I believe he did it to the best of his ability and helped with some stuff. Most homes here do no even have ervs at all for code just fresh air intakes with waited dampers in case the house gets negative pressure.

      I’m a general contractor but in my area there are next to zero passive houses and very few contractors that understand tight homes. We use 2-3 different guys that we always use and have for years a typical house they follow all the rules and do it to code and even better. My house is really tight though. One of them says leave it and just put a percent timer the other guy says put the electronic dampers on the erv and have them only open when the percent timer kicks on he doesn’t seem to think there should be much of if any problem with negative air unless your running big exhaust fans or kitchen hoods. Both of them recommended to leave the furnace running on low all the time to keep the air mixing and it helps prevent dead air spots seems like an unnecessary costs but I get the logic.

      Appreciate your thoughts and any more inout you can provide. It’s not like I’m freezing in winter but something just doesn’t seem to be performing up to optimal standards at this point. I feel like if I can dial I the erv a little better and bring in only the necessary amount of fresh air instead of constant I will save money and have a much more comfortable house.

      Thank you so much for all the help to you and everyone chiming in.

    2. wiscoguy | | #13

      Essentially my erv is not tied directly to my furnace it runs independently.

      However because of the way it is hooked up any time my furnace runs it does draw in air through the erv not as much as when it’s on but it definitely does bring in some air from outside anytime it runs.

  6. DennisWood | | #14

    No worries. Hopefully some of the real experts will chime in here :-)

    "My biggest concern is that I have high levels of moisture even with the dehumidifier running. Since the erv is always running on rainy days or any time there’s moisture outside that same level of humidity is inside as well."

    1. You are 100% correct in that running the ERV with furnace off, makes no sense given the way it is connected to your system. That's a waste of energy, and it's not doing much for ventilation, nor is it moving excessive moisture outside. With no dampers, negative building pressure AND ERV off (if furnace is running) you very well may be seeing air flowing through the ERV via the outside air vents, so no exchange of heat or moisture would be taking place. However, with a tight house and two bath fans plus kitchen exhaust (not connected to your ERV system in any way) you may want a path for incoming air that is not blocked by a damper.

    2. Keeping the furnace fan in "Auto" mode during AC use, and the ERV tied to the furnace, should really help with summer moisture levels. I'd also use the dehumidifier if needed as your new build's moisture levels come into line. I mentioned earlier that a stat like the Ecobee which can "guarantee" a certain runtime per hour is not a bad idea so in that case your furnace fan and ERV could run together as required to meet the minimum vent time. In that scenario the Ecobee would run your furnace fan (if the furnace itself has not run the minimum set ventilation time) but would also control your ERV (turn it on with the furnace fan) via the dry contacts on your ERV for a remote switch. This may not be documented in your ERV manual, but it does have dry contacts. The science on constant fan use vs AC condenser drying seems pretty clear to I would not do it.

    2. Your ERV is there to not only exchange heat, but moisture vapour as well. That said, the moisture exchange is nowhere near 100%..but you do need fresh air in your house. In winter, the RH inside should be greater than outside (along with the temp delta) so you will likely have less issues with moisture, providing you run the ERV/furnace fan an hourly set amount...that amount you'll need to figure out for your house.

    Given your concerns with efficiency and also the fact that none of your appliances (assuming you don' t use a gas range) can back draft CO here is what I would do in your position and climate:

    1. Use a passive flap on the ERV exhaust outside, and use an active damper on the ERV fresh air intake tied to ERV use.
    2. Tie the ERV to the furnace fan via the furnace itself, or a stat like the Ecobee with dry contacts for HRV/ERV control.
    3. Use Auto fan mode in summer, making sure you meet your minimum air quality requirements for fresh air. If you look at a few of the studies, this is important to reduce humidity in your home during summer.

    Along with humidity you may also want to take a look at a USB Co2 sensor which I find helpful if you want to assess your indoor air levels at different parts of your house. Start with bedrooms, at night. You might be surprised at how high these levels can get, which may in turn change your programming for ERV use during the day vs night. I've seen levels as high as 1800 ppm in a closed bedroom with one occupant.

    I have a few of these which can log to a laptop, or just power via USB for live viewing. They are not insanely accurate, but close enough in my book for comparative testing:

    This is a blurb the Co2 sensor guys did on our commercial build:

  7. greenright | | #15

    The infinity series return air should not be tampered with as the air handler uses information from the thermostat AND the thermistor in the air handler to drive the modulation. If you are dumping cold air in the return it will throw off the modulation by a lot. In this scenario your need erv air injected in the supply air (with an elbow) and controls to run air handler fan on low when the erv is running and a damper to close it when the thermostat is calling for heat/ cool
    Fan operation. If you don’t have a damper to close the path to the erv when you run the air handler fan on higher speed it will overpower the erv and backfeed air into the erv and out of the house.

    1. wiscoguy | | #16

      It’s not right into the air handler but it is like three feet away.

      I appreciate you chiming in and letting me know. It didn’t seem to be effect to much like the heat barely is running I there a spot i can look up in an infinity manual that says anything about this. If I go to my heating guy I need to be able to speak intelligently and back up what I’m saying with some facts. I believe you but it will be hard for me being not a heating guy to coni once someone else.

  8. DennisWood | | #18

    Green, I would think in this scenario, modulation would adjust to temper the air correctly given that ERV air is mixing via the return air duct. Maybe a 2-3 degree drop to the handler, maybe less.

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