In another thread (that grew quite unwieldy) the question came up about exhaust venting

Robert Riversong:  "the most foolish way to duct an exhaust fan is straight up through the roof"

John Brooks: "It seems to me that in a heating climate (or during the heating season)... It would be desirable to have the "passive NPP"  high. This would make the house more "negative" and would reduce (or eliminate) positive pressure near the top of the enclosure."

At this time I brought up the issue with condensation, wind, and dampers & that they should be vented out through a gable end if possible, if not then the roof  but never through a soffit. This was replied to with a "I did say "During the Heating Season"" so I thought maybe we should take a look at this


First, condensation is a big concern during the "Heating Season" as these fans are transporting humid warm air up & passing through a super cold area

Wind is a big issue which is one reason why chimneys have to be vented so high

The final issue is the dampers, if one was to get stuck open, you would have a nice big hole or open chimney transporting all that nice conditioned hot air straight out of the house 24x7.


So what say you???

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John B. - I might be blind, but I don't see where John N. said it makes the house more positive though I did catch where he said it runs 24x7 - which is not always true & bad to do in many cases

An HRV/ERV does not make a house more positive or negative as it is a balanced system (which does not mean that it will always exhaust the same amount air coming in or vice versa as other factors come into play).

Next layouts are not done that way, it is based on amount needed & optimal layout (where are the bedrooms, kitchens, baths, etc...)

Also, the pascal chart you guys are working from is only good for a lab / theoretical modeling & not indicative of real life - summer stack effect, no stack effect, wintertime stack effect, the wind all throw that model right out the door 

Sean wrote:

"I might be blind, but I don't see where John N. said it makes the house more positive "


John Nicholas indicated that his ventilation system would lower the NPP....

During the heating season...If the NPP is lowered...the house becomes more positive.

John B,

I threw out a quick comment about changing the NPP, which you commented on. Changing does not mean reversing (making the house positive).  

Disclaimer:  I believe Air Sealing is more important than insulation. I believe the ASHRAE position of mechanical ventilation in every home. I also believe uncomfortable people do lots of things that are energy intensive to obtain comfort.

A 3 story home.  Mine.  Prior to the addition of the ERV (CZ 4) the difference between my office on level 1 and the top level where our bedroom is, would average 8 - 12° F.  Usually, a significant amount for comfort and a cause of increased energy bills. After the ERV install (and change to continuous) the average difference is 2 - 4° F. We haven't had any nights below 35°F since the project finished. So actual winter conditions are not considered.

In this case it was a comfort issue, in addition to the IAQ reasoning.

At the same time; the walls were insulated, the attic went from R-11 to R-50.  ACH @ 50 went from 8.1 to 5.5.  My goal is 3.0; which is Dr. Joe's magic number meaning I have all the big holes.  Since we finished the major work, I have some additional minor work to complete. I believe I will get 1 to 1.5 ACH at 50 reduction from those small items.

I recognize the change in the ranger of ∆T are not solely D/T the ERV, because the house is a system and other work was done.  I do attribute the effects of the ERV to an ultimate reduction of energy usage and an increase in comfort.

The ERV discharges 100% of the incoming air at the top floor. It takes 100% of exhaust from the bottom level. The utility room on the bottom level according to my wife is less musty smelling since the install. The standard E/H RV install into the return and supply ducts would not result in the energy decrease I expect.  Two weeks ago, we had a week of very temperate weather. Lows in the 50's and high 40's. Highs in the 70's.  The heat pump was set to cool at 85 for the entire 168+ hour period. The highest temperature the house reached during the day was 74 - the lowest was 70. Without the continuous running ERV - the temperature set would have been bumped daily to kick the AC on.  Musty, stuffy dead air in many homes results in Energy Intensive interventions, such as changing the 'stat set point to freshen the air; or continuous running of the blower fan, instead of the E/H RV fan. The unit is a Fan Tech 704 model.

John N.,

I agree with your disclaimer.

and thank you for the new details.

In my last post I should have said "your suggestion to make a 3 story house MORE positive".


you said:"

Using an H/E RV to put fresh air in high and exhaust low would lower the NPP and decrease the temperature difference."

This made me think that you saw an advantage to lowering the NPP ?

Since I don't know much about HRV's ... I just assumed  they could be adjusted in order to make the house more or less Negative ?

In other words you could manipulate the location of the NPP by adjusting the HRV ?

During the Heating Season... why wouldn't you want almost all of your enclosure to be at a slightly negative pressure WRTO?

and thank you for trying to help me understand

Most Recovery Ventilators have a fixed capacity. You can decrease that with dampers. You may wish to use one large (ex 130 cfm) unit, a smaller size (ex 40 cfm) or several depending on the needs of the home.

You need to look seriously at the existing leakage, the proposed decrease, future efforts to decrease and apply the ASHRAE 62.2 calculations to all these numbers.  Typically a 2K sf home with an ACH at 50 of 5 - 7 will not require many CFM in mechanical ventilation. Get down to 3 ACH at 50 and you will need around 30 CFM.

Then look at what is going on in the home. What comfort concerns? What activity?  My first call out to a high humidity problem in a house related directly to lifestyle choices of the family.

1.  She drank hot tea as I drink coffee, continuously. So Teapot on the stove and boiling; no hood, no spot ventilation.

2.  She dried her husband and her son's overalls in the basement.  Summer, winter, didn't matter. Discussed using a clothes line outside. Nope!  

Recommendation:  Install Spot ventilation in kitchen, 2 bathrooms and in the Family room with the clothes drying activity.  (They had a supply side air cycler in place.) 

I have not been called back to verify anything. My feeling after presenting the report was they were not happy at having their lifestyle choices pointed out as the problem and not something simple like a new furnace or more insulation to solve the problem.  I don't drown horses.

John N. you said a mouth full there. "My feeling after presenting the report was they were not happy at having their lifestyle choices pointed out as the problem and not something simple like a new furnace or more insulation to solve the problem."

Finding workable solutions that improve the situation and WILL BE installed is a real challenge some times.  Whether it is cost of convenience, we have to be able to convince them to do it.


I always use examples of how lifestyle choices effect energy usage.  One of my favorite is if the family has 3 HS football players, they are standing at the open refrigerator most of the time, and when they are not, mom and dad have the door open reloading. compared to 3 HS cheerleaders, in the bathroom with hair dryers, curling irons etc. I did in this case and they followed that one. But the attitude changed when we moved on the humidity causes.

John N.,

were they having humidity problems in the winter, summer or both?

since the home already "had a supply side air cycler in place"....

did you measure the flow at the supply side and make sure it was calibrated?

assuming the problem was during winter......why didn't you just increase the run time of the air cycler?

please pardon my ignorance...I am just trying to learn

The home was having humidity problems summer and winter. They were getting worse. They had gone from uncomfortable and 1 dehumidifier running continuously to 2 dehumidifiers running continuously and still uncomfortable.

I would have tested the air cycler, if they had elected to proceed. I recommended it testing be done.  

I have had no further contact from them. 


I think you're overthinking this passive NPP issue, since it's "passive" only in strictly static conditions, changing all the time under real conditions, and far less important than almost any other design consideration. Active NPP, however, is always worth considering in ventilation system design.

Robert wrote:"John, I think you're overthinking this passive NPP issue...."

Robert, I know ... I am aware of my stacking disease....


Now, if you really want to "stack" the variables, consider that the outside air is dynamic, with falling high pressure ridges and rising low pressure troughs, falling air becoming warmer and rising air becoming cooler (yes, meteorologists speak of rising warm air columns), and wind dynamics....


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