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Would a motorized AC coil bypass make sense?

patrick1 | Posted in General Questions on

I live in a 3500 sq ft house with 3 stories (inclusive of finished basement).  HVAC is  natural gas forced air furnace with an air conditioning coil and HRV.   The current setup has symptoms of an oversized AC unit – short cycles (e.g.,  3-5 minutes), uneven temperature between rooms/floors, insufficient dehumidification.  The ducts are all rigid metal ducts and we sealed everything we could access when we were finishing the basement (all the trunk lines and take-offs from them).  All the rooms in the house have at least one return, though I’m sure they’re probably super leaky since the returns use framing voids.

I have found that leaving the blower fan on continuously greatly reduces the temperature variation between room/floors but it has the side effect of driving humidity levels up quite a bit.  It seems that a cure for this would be to install some sort of motorized bypass that would direct air through the AC coil when the thermostat asks for cooling but bypass it when only the blower is on, allowing the moisture on the coil to drain out rather than getting evaporated back into the conditioned air.

Does anyone make a product like that?  Would it make sense to install one?

Our air conditioner seems to be working just fine so I’m looking for lower cost improvements I can make until we have occasion to install a new, properly sized, AC unit or maybe a heat pump and make other weatherization improvements to the house (e.g., improve air sealing and insulation in the attic).

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  1. joshdurston | | #1

    You might be able to accomplish what you're looking to do with a time delay relay that blocks the fan signal for 10-15minutes after a cooling call. I'm not sure how long a coil takes to drain though.

    1. patrick1 | | #2

      That sounds like a really good idea....worth a try at least.

      I suspect that the humidity is not just coming from water in the catch pan. Because of all the surface area on the coils, they probably remain wet for a while and any air flowing over them would evaporate that moisture. Based on the time it takes for dishes in my dishrack to air dry it could be hours before the coils are completely dry.

  2. Expert Member
    BILL WICHERS | | #3

    I'm not aware of any purpose-made bypass damper for something like this. You'd need something BIG to bypass that much air, probably near the size of the duct enclosure the A coil is already mounted in. Dampers big enough to do this are commonly used as fire dampers in commercial buildings, but you'd need three and some ductwork to make a proper bypass, along with actuators on each one. You're probably looking at $1,000+ to do all of this with new parts.

    I like Josh's time delay relay idea. You could try a simple time delay relay with a long delay range (the one at this link can do anything from under a second to over four days):

    This is an "on delay" relay, so it closes the contacts AFTER the timer times out after the coil gets energized. The one at the link runs from a 24v AC control circuit like is commonly used in HVAC systems. You'd need to set things up so that the relay is only running that delay when you're going to circulate mode after a cooling cycle though, so you don't want to just delay the fan all the time or you're going to freeze up the coil (or have issues in the heating season). You might need a few other relays to accomplish this, but it should be doable without too much effort.


    1. joshdurston | | #4

      Most airhandlers/furnaces will start the fan on a heating/cooling call without energizing the G (fan) signal. Making it safe to block the G wire between the stat and thermostat interface. But you'd want to test this.

      1. patrick1 | | #8

        I'm pretty sure that's how mine works...but yeah I'll test it to be sure.

    2. patrick1 | | #11

      For $1000+, I'd have to think twice about dampers for sure. Probably better to get correctly sized equipment first and then see if I still have problems with uneven temperatures.

      Is this the type of equipment you were thinking of?

      1. Expert Member
        BILL WICHERS | | #12

        That looks like a lighter weight version of what I was thinking of, and would probably work fine in your application. You’d ideally need three (both sides of the A coil and one in the bypass duct), but could get by with two (one side of the A coil and one in the bypass duct) to save some money. You’ll still have a BIG box though.


  3. Expert Member
    Akos | | #5

    I don't think that will do much. The humidity spike is from the water on the coil and not much of that will drain down if you wait.

    Since the coil is a high pressure, you can get away with just two dampers for the bypass. One between the blower and the coil and between the blower and the bypass duct. One normally open one normally closed wired to your thermostat's cooling output.

    You might be able to replace the bypass one with a spring loaded damper but that will take some tweeking (spring pressure) to make sure it doesn't open when the air is flowing through the coil.

    As an easier modification would be to try to reduce the airflow. Getting the flow a bit lower will increase the latent removal of the coil and increase the runtime a bit. Less efficient but would be better for comfort. Just make sure you don't reduce flow too much and freeze up the coil.

  4. charlie_sullivan | | #6

    Everyone is assuming the humidity increase is from drying the coil, and I'm sure that's a lot of it. But some of it is probably also the leaky returns and various imbalances driving infiltration of humid outdoor air.

    Considering that, I like Akos's idea of lowering the fan speed to the extent you can, as that could also decrease the amount it drives infiltration.

    But ultimately I'd be looking at moving to a better sized system with modulation capability, heat pump capability so you can get off gas, and ideally mini-splits or maybe even a Chiltrix system so you can get rid of the leaky ducts.

    1. patrick1 | | #10

      Yeah an equipment upgrade is definitely the long-term plan. With mini-splits or the chiltrix system, how does one route the lines to the outdoor unit? Do people usually just punch holes through the exterior walls in each room and run them outside the house? ...I'm not particularly enthusiastic about having a bunch of refrigerant lines snaking all over the outside of my house. We're not planning on doing major interior renovations anytime soon so it seem like running them outside would be the only option.

      I was thinking of getting Aeroseal out to try to reduce duct leakage before upgrading equipment. Hopefully that would get the ducting to an acceptable state.

      1. charlie_sullivan | | #14

        Aeroseal is a good idea. I'm not sure how that works with framing void ducts, but it would be worth asking someone who does that if it's up to the challenge.

        The general idea with minisplits and Chiltrix hydronic lines is that they are much easier to snake through walls than ducts are. And with hydronic, you don't want it outside where it can freeze. It can take some scheming to figure out how to do it in a particular house, but it should be doable if not always easy.

  5. Jon_R | | #7

    For longer run-times, consider a thermostat with more hysteresis/dead-band.

    1. patrick1 | | #9

      I think I can adjust that on my thermostat. I have a Honeywell VisionPro IAQ. It has a setting called "Deadband" but the description doesn't seem to match up with what you're suggesting. From the manual: "The deadband indicates the minimum number of degrees that are allowed between the heat and cool settings when in auto changeover. For example, if the deadband is set to 3°F and the cool setpoint is 75°F the warmest heat setpoint allowed would be 72°F."

      There's also a setting called CPH and it has options between 1 and 6, with 3 being the recommended setting. If I understand correctly, lower CPH causes the AC to cycle on/off less frequently.

      And there's a setting called "Temp Control Cool" : "2=standard, 1=Less Aggressive (may cause temp undershoot), 3=More Aggressive (may cause temp overshoot)"

      Seems like lower CPH might allow more droop before turning the equipment on while the temp control is about how quickly the equipment is turned off once the temperature sensor detects that it's at target. So setting setting a lower CPH with more aggressive temp control would seem to cause less cycling. Is that correct? ...or should I just adjust one of those two settings?

      1. Jon_R | | #13

        The only one that's seems clear to me is CPH (I assume cycles per hour). Try lower.

        I've putting a bag around the thermostat - worked as expected.

      2. charlie_sullivan | | #15

        I agree with Jon--lower CPH is the one that will reduce cycling. I think the less aggressive/more aggressive just changes whether the wiggle room it makes use of around the set point is mostly above the set point or mostly below, without changing the amount of cycling.

        The low tech bag solution is probably more suited to a less sophisticated thermostat.

  6. patrick1 | | #16

    Thanks for all the help everyone. I'm going to try playing around with the thermostat a bit and see if that helps...maybe the fan speed also, though I a bit nervous about causing the coil to freeze.

    I'll keep the bypass on the backburner for now. It seems like an idea that could have merit in the long-term but given the cost I might just accelerate moving to a better HVAC system and consider having it installed at the same time if I maintain a centrally ducted system.

    As Charlie suggested, Mini-Splits or Chiltrix would obviously fix all the comfort issues and be much more efficient but I need to get my head around how to make that work and look good in our house.

  7. walta100 | | #17

    If I read your posts correctly and your ductwork in in your conditioned basement Aerosealing your ducts will do very little or nothing for your humidity problem.

    Years ago I had a similar Honeywell stat (perfect climate comfort system) as I recall “dead band” was for auto changing from heating to cooling and you could adjust cycles per hour for heat and cool. If you are willing to pay for the fuel your thermostat can operate your equipment to give you museum grade control of your temp and humidity with your current equipment look for “reheat” in your manual. It will operate the furnace and AC at the same time to reach your temp and humidity set points. Not a very green way to go but will deliver perfect control. If you can’t bring yourself to use reheat (could not)your system can control a dedicated dehumidifier and use the AC fan and ducts to deliver the dehumidified air. I did not read all 130 pages of your manual but I am guessing in there somewhere is an over cooling option for humidity. My old system also had a minimum run time per hour setting for the blower that would run the blower the last 10 minutes of every hour if there had not been a 10 minute call to run the system and even out the room to room temps.

    My wild guess is your house leaks a lot of air to the outside and is poorly insulated. Blower door testing before and after 20 hours of air sealing would totally change your house. infrared photography would show you the thin spots in your insulation.


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