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Musings of an Energy Nerd

Ducting HRVs and ERVs

Delivering ventilation air through heating or cooling ducts is a bad idea

Dedicated ventilation ducts are best. A typical HRV or ERV has four duct connections. Two ducts lead from the appliance to the outdoors: one duct pulls in fresh outdoor air, while the other duct expels stale air. Two other ducts lead from the appliance to grilles or diffusers in the house: one distributes fresh air to the living room and bedrooms, while the other pulls stale air from bathrooms and the kitchen.

Here at GBA, we have consistently advised readers who plan to install a heat-recovery ventilator (HRV) or an energy-recovery ventilator (ERV) to install dedicated ventilation ductwork rather than trying to distribute ventilation air through their heating and cooling ducts. That advice won’t seem particularly burdensome if your house doesn’t have any heating and cooling ducts — as is typical in a home with ductless minisplits or a hydronic heating system and no central air conditioning.

But if your house already has heating and cooling ducts, you may instinctively rebel at GBA’s advice, muttering, “Why can’t I use the ducts that I’ve already got?”

Here are the reasons:

In the past, I often endorsed the use of exhaust-only ventilation systems in small homes with simple floor plans. Increasingly, however, I’ve become convinced that we need to pay better attention to indoor air quality, especially in bedrooms. It turns out that bedrooms in homes with exhaust-only ventilation systems usually don’t get enough fresh air. So anyone who cares about bedroom air quality probably needs an HRV or ERV with dedicated ventilation ductwork.

If your HRV has dedicated ventilation ductwork — a type of ventilation system that is sometimes called a “fully ducted” system — the HRV will usually deliver fresh air to the living room and bedrooms, while simultaneously exhausting stale air from bathrooms (and sometimes from the laundry room or kitchen). With fully ducted systems, fresh air delivery and stale air extraction happen in the rooms where the system designer intends these events to occur. These systems use less energy than systems connected to heating or cooling ductwork, and they are far easier to commission and balance.

If you intend to pull stale air from your kitchen, remember that a range hood should never be connected to…

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16 Comments

  1. Duane Erstad | | #1

    I'm hoping that Bruce Sullivan who wrote about this ("Six Steps to Success with Heat-Recovery Ventilation," January 2018) will comment, reflecting Bruce Manclark and Dan Wildenhaus.

  2. User avater GBA Editor
    Martin Holladay | | #2

    Duane,
    Bruce Manclark and Dan Wildenhaus provide recommendations for distributing ventilation air through heating and cooling ducts in an article by Bruce Sullivan called "Integrating HRVs With Air Handlers."

    The approach they describe is less bad than most attempts to use heating and cooling ducts to distribute ventilation air -- but still complicated enough to raise questions about whether it's possible to find an HVAC contractor sophisticated enough to follow the steps recommended by Manclark and Wildenhaus.

    In Bruce Sullivan's first article in the series ("Six Steps to Success With Heat-Recovery Ventilation"), he recommended, "Create a dedicated duct system. Most experts agree that it’s best for an HRV to have its own dedicated duct system."

    That's advice worth listening to.

  3. user-6892763 | | #3

    Thanks Martin, excellent article as always. I'd like to second the emphasis on dedicated ductwork for an ERV installation. We encountered a new, light commercial building in VA that experienced high moisture problems when the ERV was activated, much of it due to poor ducting that was indeed tied into the main HVAC ductwork. Even the manufacturer stated the AHU needed to run during ERV operation but that's not how it was configured.

    In my personal home, I took care to hard duct my ERV as much as possible, even splitting the fresh air being supplied to the house into two rooms as well as pulling stale air from two places. This six duct arrangement led to airflows that were significantly higher than the 70 cfm that the ERV is rated to provide! I have come to the conclusion that, at least in my neck of the woods, it's preferable to let the bath fans and kitchen hoods handle the moisture loads in those locations. I give Terry Brennan credit for suggesting pulling ERV "stale" air from the closets.

    I'm concluding that my dream ventilation system in a mixed-humid climate like Atlanta and the southeast is an ERV with supplemental dehumidification . The other feature I'd like to see is the above system with temperature/moisture sensor controls to reduce/shut off the ventilation system and coast when the outdoor conditions are "bad".

    Again, thanks for your excellent contributions! -mike barcik

  4. User avater GBA Editor
    Martin Holladay | | #4

    Mike,
    You wrote, "I'm concluding that my dream ventilation system in a mixed-humid climate like Atlanta and the southeast is an ERV with supplemental dehumidification."

    You should check out the CERV and the Minotair. Here are links to articles with more information:

    "A Balanced Ventilation System With a Built-In Heat Pump"

    "Another North American Magic Box"

  5. Robert Navarro | | #5

    What about products like a Clean Air Furnace?

    https://www.lifebreath.com/us/product/clean-air-furnace-6/

    They can be spec'd with ECM's and are inherently "interlocked" since they use the same blower motor.

  6. User avater GBA Editor
    Martin Holladay | | #6

    Crontab,
    I don't know much about Lifebreath's Clean Air Furnace, but I'm interested in reports from GBA readers who have installed the appliance.

    I'm not impressed with the documentation provided by Lifebreath for this appliance.

    First of all, the so-called "Clean Air Furnace" is not a furnace. It's an air handler with a hydronic heat-exchange coil. Unlike a real furnace, the "Clean Air Furnace" does not produce heat. To produce heat, you need to buy a boiler (or, in some but not all jurisdictions, a water heater) and connect it to the "Clean Air Furnace."

    That fact alone should raise questions with government regulatory authorities. It shouldn't be legal to advertise a product that is not a furnace by calling it a furnace. That borders on fraud.

    For more information on hydro-air systems and the type of air handler that Lifebreath is mis-labeling, see "Using a Tankless Water Heater for Space Heat."

    The instructions note, "The heat recovery ventilation (HRV) portion of the Clean Air Furnace, is automatic. Once set, a desired amount of fresh air will be drawn into the home while the furnace blower is activated." A designer needs more information. A designer needs to know a few basic facts about this air handler, including:

    1. Under what circumstances is exterior ventilation air introduced into the forced-air heating ducts? Is the ventilation air introduced only when there is a call for heat, or is the ventilation air introduced year round?

    2. If this appliance can operate in ventilation-only mode, without any space heating, how many watts does it draw?

  7. John Rockwell | | #7

    Hi Martin.

    Thanks as always for your deep dive into this topic. Perhaps some mention could have been made about home-run ducted systems with smaller, flexible ductwork that can fit within stud walls and floor joist cavities without elbows, boots or other increases to static pressure. You might also mention challenges with sound attenuation (and additional air sealing) with metal elbows and boots, as well as the potential for "cross-talk" between rooms with trunk and branch duct systems.

    Also, your several references to "a call for ventilation" or "no need for ventilation" seems to imply that ventilation is only intermittent. I think it's worth noting that, particularly in brand new construction, continuous ventilation (with very low Watts/cfm, of course) may provide benefits that intermittent ventilation may not. And wouldn't it be nice to know that if a damp bath towel is left on a towel bar, continuous ventilation will likely remove that moisture more effectively, and with smaller, less intrusive ducts?

    John Rockwell
    Zehnder America
    http://www.zehnderamerica.com/

  8. User avater GBA Editor
    Martin Holladay | | #8

    John,
    It's possible to meet ASHRAE 62.2 ventilation requirements with continuous ventilation, as you point out. It's also possible to meet ASHRAE 62.2 ventilation requirements with intermittent ventilation.

    For example, if your house needs 60 cfm of continuous ventilation (according to ASHRAE 62.2), it's perfectly acceptable to ventilate at 120 cfm for 30 minutes every hour. I haven't seen any evidence or data that the latter approach is inferior to the former approach.

    Moreover, many homeowners program their ventilation systems so that ventilation is minimized or off when the occupants are at work or school, in order to reduce the inevitable energy penalty associated with ventilation.

    1. Trevor Lambert | | #13

      Having the HRV go to low or off when away is a separate issue from intermittent ventilation. There are two obvious downsides to intermittent mode ventilation. All other factors being equal, 120cfm is going to be a lot louder than 60cfm (likely more than double). How much, or if at all, this bothers the occupants will vary. Running 120cfm at 50% duty cycle will also use more energy than running 60cfm at 100%, due to the nonlinear increase in power required as static pressure increases. This may be negligible, I haven't done the math. A third possible factor is that is to run at those higher cfm rates on an intermittent basis may require upsized equipmemt.

  9. Jeremy Good | | #9

    Thanks for the article. I installed a Panasonic ERV in the attic of my 1930s colonial (CZ4a) this year in a hybrid configuration. I agree that there are some compromises, though a fully ducted system would have added significant expense and its own trade-offs.

    On the plus side, I get air mixing and filtration via the MERV 16 filter in the air handler in the attic, which is for cooling only. Since I have a boiler for heat, I think that's a benefit in the heating season. The AHU is new and has a very low fan speed (configurable, ~425 cfm). At less than 30 W, it's not a significant energy concern. I would not have considered a blower interlock with a PSC motor. The fan speed is low enough that my wife and kids haven't been complained of cold air blowing. That was my biggest worry since I installed it over the summer.

    Balancing is a real concern with a hybrid or simplified system. My AHU operates at three fan speeds (Max dehum/Fan ON [50%], dehum [80%], cooling [100%]). It was easiest to balance my ERV at the highest fan speed. Of course, that's the mode it's in the least while the ERV is running!

    Panasonic says that an AHU interlock is required, interestingly. I have a central return, so distribution without the blower would be really poor. I'd be curious to try without interlocking if I had a different duct layout.

    The controls side is a work in progress and not entirely satisfactory. An Ecobee controls the A/C, including a "dehumidifier" which is just a reduced fan speed on the AHU. The Ecobee is smart enough to minimize fan on time when the compressor is not running to avoid evaporating condensate on the coils. But, the Ecobee doesn't know anything about the ERV. So, I ran the ERV for 40 min/hr at 60 cfm to get an average 62.2-specified 40 cfm. (I was having trouble getting it balanced at higher fan speeds, but that's a long story.) Now in the heating season I'm running it full-time at 50 cfm (lowest setting) and watching CO2 levels to try to determine a temp cutoff where stack effect is providing enough ventilation. I may try to use the Ecobee to control the ERV in the heating season to see how that works.

    Panasonic, are you listening? I'd love to see an ERV controller that would use weather conditions and a CO2 sensor to help determine ventilation rates. And ideally talk to an Ecobee/Nest/etc via their respective APIs. So far I've resisted the urge to see if I can control the ventilation rate, having previously created a proof of concept Wi-Fi variable speed controller for a Panasonic FV-05 bath fan. (The fan speed is proportional to a 0-5 V control signal.) Anyway...

    Bottom line, I think the hybrid configuration can be a good option for existing houses with ECM blowers and some due diligence about the potential trade-offs (and benefits). I think I'm way ahead of an exhaust-only scheme despite it not being an "ideal" installation. Certainly a lot better from a humidity-management perspective, which always seems like a challenge.

  10. User avater GBA Editor
    Martin Holladay | | #10

    Jeremy,
    Your design instincts and commissioning skills are well above average. I commend you for your diligence and analytical approach.

    That said, even someone with your skills has concluded that, "Balancing is a real concern with a hybrid or simplified system. My AHU operates at three fan speeds (Max dehum/Fan ON [50%], dehum [80%], cooling [100%]). It was easiest to balance my ERV at the highest fan speed. Of course, that's the mode it's in the least while the ERV is running!"

    You also note, "An Ecobee controls the A/C... The Ecobee is smart enough to minimize fan on time when the compressor is not running to avoid evaporating condensate on the coils. But, the Ecobee doesn't know anything about the ERV."

    It's important to imagine the likely results if a system like yours were commissioned by either (a) a typical HVAC contractor, or (b) a typical homeowner. The results would probably be highly unsatisfactory, and resulting air flow rates would be all over the map.

    1. Jeremy Good | | #11

      Good points, Martin. I find it frustrating that there are seemingly a lot of barriers to getting H/ERVs into more homes — installed to be effective — given that there's a lot of good they can do for IAQ while minimizing the energy penalty.

  11. Andy Kosick | | #12

    I would like to qualify my following statements with the fact that over the last couple years I’ve commissioned dozens of residential ERVs, most of them (unfortunately) in the simplified configuration. I’ll also vent my frustration that all of the HVAC contractors I work with do not really understand the relationship between these ERVs and the space conditioning ducts they’re connecting them to, even more frustrating is that most of them don’t seem to care. There is good reason for hope though, because with commissioning it is entirely possible to get any one of the configurations working well.

    I’d like to defend the hybrid configuration as a good way to reduce overall duct work and advocate here for always connecting the fresh air from the ERV to the supply side of the air handler in both hybrid and simplified systems. Most manufacturers allow for this (see attached image) and there are several advantages to this.

    First, it’s a best-case scenario for not running the air handler fan because the filtered fresh air distributes through the supply duct work when the air handler is not running. The lower air flow of the ventilator combined with the resistance from coil, heat exchanger and filter tend to prevent most back flow to the return side. My own testing shows this general holds up. The addition of a one-way damper or zone damper at the air handler could prevent back flow all together. I’m going to make myself a custom cape damper for my own setup and see how it performs (whenever I get the time).

    Second, preventing unintentional flow through the ERV when it’s not running can be accomplished with a simple one-way damper instead of a powered damper.

    Third, in a simplified configuration, where stale air is being pulled from the return duct and fresh air being pushed into the supply, the cycling of the air handler resists both ventilation streams (fairly equally assuming decent duct design) instead of resisting one and assisting the other. What this means is that the ventilator may flow less air overall but is still relatively well balanced.

    Furthermore, in the installations I’m seeing, with flex duct runs to outside, the ERV fans are operating at significantly more TESP than the 0.1 to 0.2 iwc in the space conditioning duct work, 0.4 to 0.6 generally, which means the ventilation air flow doesn’t actually change all that much when the air handler turns on and off. In my own ducted mini-split setup the 0.06 iwc in the supply ductwork changes the HRV fresh air flow from 110cfm to 100cfm. Curiously, this is right at the 10cfm tolerance considered balanced from the manufacturer.

    The point is, if you design and commission it, you can reduce duct work and avoid running the air handler fan for ventilation.

  12. User avater GBA Editor
    Martin Holladay | | #14

    Andy,
    Thanks for sharing your experience and tips. You've come up with a superior way to distribute ventilation air through heating ducts, and I salute you.

    However, the HVAC industry still faces a major challenge here, because (as you admit) very few HVAC contractors understand the principles you discuss well enough to commission an HRV as you recommend.

  13. Ultimate Air | | #15

    Designing the ventilation connection is always a balancing act. Obviously independent ducting is the absolute best way to achieve all the benefits from an ERV installation. However, retrofits can make this style of installation very difficult, mainly because routing branch ducts around an existing home can be nearly impossible. Especially if the customer has a budget.
    In new construction; we (as a manufacturer) always recommend an independent system exhausting from bathrooms and supplying to living spaces.
    In retrofits; we try our best to find a balanced approach that solves IAQ issues while also keeping installation costs as low as possible. Often this is done with hybrid systems. Exhausting from a central location (preferring Bathrooms and Kitchens) and supplying into the larger furnace ducting network. One option is to have the ERV tied to the furnace fan; while also having the furnace fan linked to a timer that triggers the fan 20 minutes out of each hour. This way the ERV distributes fresh air even during shoulder seasons when the furnace/AC might not be operating as often.
    Also, zoned ERV installations have become more popular in retrofits. Rather than installing a single large ERV system and routing ducts throughout the residence -- people have chosen to install 2 smaller ERV units to minimize the length and complexity of ducting. For a two story home, one option is a small ERV in the basement and another small ERV in the attic. Running independently of each other and supplying balanced ventilation to the respective floors.

  14. User avater
    user-7219340 | | #16

    In old barns around here (climate zone 7) with tons of insulation in the ceiling (hay) wooden ventilation chimneys would pull out stale air from the barn and it would be replaced with fresh cold air through the cracks in the walls, doors and windows. Could an HRV perched on the top of such a chimney perform a similar function using less energy and making less noise?

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