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This Little Ventilation Mistake Can Lead to Comfort Problems

Ventilation Energy Recovery Ventilator Erv Electronic Damper

If you’re designing a ventilation system, first you have to determine how much air the house needs. You can use ASHRAE 62.2 or the new BSC-01 for that task. Then you have to decide what type of ventilation system to use: positive pressure, negative pressure, or balanced. In many green homes, the balanced system is becoming a popular choice. I’ve seen some installations lately, though, that are missing a key component.

If you’re designing a ventilation system, first you have to determine how much air the house needs. You can use ASHRAE 62.2 or the new BSC-01 for that task. Then you have to decide what type of ventilation system to use: positive pressure, negative pressure, or balanced. In many green homes, the balanced system is becoming a popular choice. I’ve seen some installations lately, though, that are missing a key component.

The balanced system shown in the photo above is one such case. It’s an energy recovery ventilator (ERV) that’s connected in with the ducts for the home’s heating and cooling system. The duct with the fresh air from outside is connected to the return trunkline of the heating and cooling system.

If the system is designed to run continuously and factors in the negative pressure created by the duct system it’s attached to, this can work well. A typical ERV, though, can move 100 to 200 cubic feet per minute (cfm) of air. A typical house needs maybe 50 cfm (depending on which ventilation standard you go by). What many designers do is set the ERV to run intermittently, say 20 minutes out of each hour, to meet the ventilation needs of the house.

The missing link

If the ERV is designed to bring outdoor air into the house only 20 minutes per hour, there needs to be a way to close the hole to the outside for the other 40 minutes per hour. Such a device is the Turbo Thermo-Encabulator Max.

Oh, wait. Sorry. That’s for something else. What you really need is a motorized, zone-control damper, like the one shown below. Every HVAC supply house sells them. The motor is wired to the ERV controller so that it opens and closes when the ERV turns on and off.

hvac duct motorized zone damper ventilation

The sin of omission

If the motorized damper isn’t there, the negative pressure from being connected to the heating & cooling system’s return ducts will cause more outdoor air to be pulled in through the ERV. (The same will happen for an HRV.) The result is that the house will be overventilated, especially during winter and summer when the heating and cooling system runs more often. If the system is right-sized, it will run more often than a typical over-sized system.

Another case where this creates problems is when the home has a ducted mini-split system. Those air handlers run all the time, though they do ramp down to low speed when the thermostat setpoint is met. Still, we’ve seen a system like this that brought in excess ventilation air and made the house uncomfortably humid in summer.

A standard heating and cooling system that shuts off the air handler between runs can cause problems, too. If the occupants set their fan to the on position, the system will pull in more outdoor air if there’s no motorized damper.

Even if the ERV or HRV has its own duct system and is not connected to the heating and cooling system, you need to be able to close the hole when it’s not running. Stack effect, wind, and other mechanical systems in the home can pull in outdoor air when it’s turned off otherwise.

If you’re designing or installing ventilation systems, make sure you get to know this little device. It’s an essential component.


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This Post Has 12 Comments

  1. Until that little device
    Until that little device fails, probably in the closed mode, which they have a reputation for doing. Nobody will realize that it has failed and it can go on like that for years. 
    Also, we have found that the ventilation rate changes drastically depending on whether the air handler is running or not when the ERV/HRV is in operation. The negative pressure in the return side of the HVAC dramatically increases ERV/HRV flow. 
    We have found that it works much better is to connect it to the supply side of the HVAC system. That way you just need a dumb (more reliable) backdraft damper to keep HVAC supply air from backdrafting into the ERV/HRV. 
    But won’t that also dramatically change the ERV/HRV flow depending on whether the HVAC system is running or not? Not in our tests. I see about a 5 cfm difference. I am told that this is due to teh venturi effect. 
    *Warning* I could be not entirely correct, or even dead wrong! I did not do a study to determine underlying issues, differing results with different static pressures, etc. It could be that the readings I’ve taken from the return-installed systems have had returns that are too small. But it’s what I’ve seen in the field. I’ll be interested to know if anyone else has more insight.

  2. I see this a lot as well,
    I see this a lot as well, Allison. It probably negates the energy benefits of the heat recovery…and then some!  
    Side note for HERS Raters – if the ERV is set up to run intermittently, you’re not allowed to tape off the exterior hoods for a blower door test (or duct leakage test, for that matter, I think).  
    Another side note regarding ducted mini-splits – the ones I’ve dealt with (Mitsubishi SUZ/SEZ series) CAN be set up to have the air handler shut off when the thermostat temperature is satisfied. It’s not the factory default, but it can be set up through the thermostat controls.

  3. It is a truism that the fewer
    It is a truism that the fewer moving parts, the more reliable and inexpensive is the system. You are suggesting one more moving part, for a good reason, but we should be careful with that. 
    Your argument that you might over-ventilate with a fresh air duct to the return plenum, is quite sound. There is a powerful fan aiding this ventilation, it would not happen without that fan. 
    Your argument that the same happens when an ERV has its own separate ductwork, is not as strong. You must propose stack effect, wind and other mechanical systems in the home (what does that last one mean) to drive air flow. I submit that each of these factors is far weaker than the central fan, and some objective studies to measure this, would be valuable. 
    Lastly in a hot-humid climate I question the humidity effects of an ERV vs. supply-only ventilation. It would seem that people would design the ERV for much higher ventilation rates than a supply-only system… feeling comfortable because the ERV “removes outside moisture”. But what percentage of that outside moisture is actually removed (transferred to the exit airstream)? If the proportion is 50% then you can go no higher than 2X the airflow before your humidity input is actually higher than compared to the simple and cheap supply-only system.

  4. We’ve had problems with the
    We’ve had problems with the mini-split continuous running air handler in our humid climate, in very low-load situations. The compressor shuts down but the AHU keeps running, re-integrating the water on the coil back into the air. 
    There is a resister that needs to be cut and then the wireless wall-mounted Honeywell thermostat is used to allow the AHU to cycle with the compressor.

  5. not sure how it works in my
    not sure how it works in my buiding but i do know its noisey along with hot spots in floors and very dusty and feels like water or air under floors ….like my floors have movement 
    very scary

  6. Sky D.:
    Sky D.: Indeed, maintenance is a requirement with any mechanical system. Thanks for posting about your measurement experience. We need more people who don’t just install these things but measure what the air flow is actually doing. You’re right that more air will flow through the ERV when the air handler is running. If you measured both sides, you’d probably find that the system isn’t balanced either. 
    John S.: Yes, the HERS Standards specify whether or not to cover ventilation openings based on whether it’s continuous or intermittent, not whether it’s balanced, positive pressure, or negative pressure. Unfortunately, those Mitsubishi controls are far from easy to use. 
    M. Johnson: Would you want a 6″ hole in your living room wall? If you don’t have a way of closing off the ERV, the only difference is that the hole passes through the ERV instead of directly through the wall. 
    bernadette w: The problems in your building may be due to more than the ERV. 

  7. First, no I would not want a
    First, no I would not want a 6-inch hole in my living room wall. But by applying some measurement and judgement I can distinguish between big problems and small problems. 
    A blower door test on my home told me 5.3 ACH50, with a “measured leakage area” of 393 square inches. A 6-inch hole measures 28 sqin. The air it takes in is relatively clean and shaded even before the filtering, this is not from a hostile air source like crawlspace or attic. The filter itself tends to inhibit random airflow. 
    What I actually have installed is two 5-inch ducts on two systems. Since installing I have observed no detectable increase in either humidity costs or energy costs — possibly because I had a contractor install a new 13 EER system and seal some leakage at the same time. So this is not a high quality experiment but anecdotal evidence.

  8. @Mark, you raise an important
    @Mark, you raise an important point. ERV manufacturers don’t generally split out sensible vs latent efficiencies @ cooling conditions. The HVI database just lists total recovery efficiency (TRE), and few companies go beyond that in their specs. Seems like 50% may be typical of TRE (which BTW, excludes the impact of fan energy consumption). Latent would obviously be a lot less than TRE. 
    As an important aside, the fans on the most efficient recovery ventilators only operate at about 1 CFM per watt versus, say, 10 cfm/watt for WhisperGreen. This is because of the huge static drop incurred by the recovery core. It’s therefore highly doubtful many ERVs and HRVs recover enough energy during extreme cold weather to offset the fan efficiency penalty during the bulk of the year when recovery efficiencies stink. 
    Not only does thermal efficiency (Apparent Sensible Effectiveness) drop off at moderate temperatures, but the Sensible Recovery Efficiency (this rating includes impact of fan) actually goes negative for a large portion of annual hours — when delta T is relatively small. Just imagine a bell curve of the BIN data to understand what I’m talking about. This is the ERV/HRV industry’s dirty little secret. 
    Unfortunately, it’s not easy to model this since there’s no such thing as a seasonal efficiency rating for ERVs/HRVs. Thus, the entire industry focuses only on rated efficiencies, which are based on 32F, 0F and -13F, and 95F. This not only is misleading, but deceptive in my opinion.

  9. The sin of omission [Ha!]&amp
    The sin of omission [Ha!] 
    “though they do ramp do to low speed” 
    Should this be? 
    “though they do ramp down to low speed”

  10. M. Johnson
    M. Johnson: If those two 5″ ducts are part of a positive-pressure ventilation system, they definitely need motorized dampers to close them when the ventilation system isn’t running. Otherwise you have two 5″ duct leaks. 
    David B.: Yes, until you told me about this recently, I hadn’t thought about the seasonal effects on ERV/HRV efficiency. For occupants who open the windows in the swing seasons, it should be moderated somewhat. Still it’s good to keep in mind, as with all equipment, that your mileage may vary, despite what it says on the label. 
    Donald B.: Yes. Thank you for pointing it out so I could fix it. 

  11. I find a very enlightening
    I find a very enlightening general rule of education is to ask “What are the consequences if you VIOLATE this piece of advice?”. If the advice is valid, one can give an articulate answer. I think in this case you cannot.  
    Both my AC bills and humidity control are better than ever, with two summers of experience here. Even I find it incredible the AC usage measures to 30% of what it was the first year in the house. But it’s been measured since 2003, and it’s true. Am I supposedly measuring the wrong things??? If there is a downside, in approximate terms it rounds down to zero. 
    A researcher should be glad to hear measured results which tend to disprove what he believes, as it gets him closer to the full truth. 
    That is why I earlier said there are big problems and there are little problems, and I submit only by measurement are you able to distinguish between the two.  
    By accepting the minimal downside in theory on this project, it economizes on the cost of two mechanical devices and their installation costs. Let me put it this way: It would probably cost me $500 to add the features you say. I do not benefit by investing $500 to save zero. 

  12. Mark wrote, referring to
    Mark wrote, referring to motorized dampers: “It would probably cost me $500 to add the features you say.” 
    Or $10 ea for spring loaded butterfly dampers.  
    Not sure why Allison suggested motorized. In any case, the potential energy loss we’re talking about here is more an issue in cold climates, where the winter delta-T is much larger than you’ll ever see in summer.

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