Room Over-Pressurization when exhaust fan off

[EnOm]

Hi
If I have a room with a toilet attached that has an intermittent exhaust fan (user operated). The room is cooled by a fan coil unit into which treated outside air is injected to make up for the air exhausted from the toilet. Now in the case when the toilet is unoccupied and the exhaust fan is off, whilst the fan coil unit is running. Wouldn't the Outside Air being injected into the fan cause the room to over pressurize to a level considered problematic? If so, what resolutions exist?

Data:
Supply air= 820 cfm
Exhaust rate = 100 cfm
Outside Air = 110 cfm (10% larger than the Exhaust rate for mild pressurization)
Room size = 470 sqft with 12 ft ceiling gives 5640 cubic feet
In this case when the exhaust fan is off the flow imbalance between supply and return is 110 cfm.

Thank you
Best Regards

 

[ChasBean1]

317609:

First point about mold in every house, no. Mold will only occur on wet surfaces, which come about by the surface temperature being lower than the dew point temperature of the air. Without AC, this might only be a concern around the edges of your refrigerator. Mold only becomes a concern when cool surfaces clash with high dew points.

Point 2—only if you cool locally. If you have a building with a makeup air deficit, operable windows in the rooms of residences, and local recirc-only fan coil units (FCUs) such as Whalen units, the combination of infiltration and cold surfaces in units will lead to wetted surfaces and mold growth.

Point 3—have the building make up more air than what is exhausted at all times. This is tricky and can be done many ways. This will fix all your problems, keep people cool, and eliminate mold concerns. This cannot be done whimsically and requires someone who knows what they are doing.

Mold growth has nothing to do with RH being 75-80%. Mold growth has everything to do with surface temperatures being cooler than the surrounding air dew point temperature. Until these basic principles are followed, you will continue to have issues.

 

[317069]

ChasBean1: what if we have a pressurized room but the surfaces temperature still lower than dew point of surrounding, do we still have a chance to have a mold issue

 

[Drazen]

these are two different mechanisms of mold creation. the mold itself is the same, but different causes need different resolution.

mold created on surface wet by condensation is one thing, mold created in areas with high humidity, where there is no condensation is another thing. the latter is the problem i mostly come into, and is directly related with high rh, which is caused either by vapour migration through building elements in unventilated spaces or by overcooling the space, having too low space air temperature.

 

[EnOm]

Thank you all for the very informative discussion.

Best Regards

 

[ChasBean1]

317069, if the room is cool but positive to outdoors by mechanical supply exceeding exhaust, it is much less likely although not impossible. Even with a positive space, infiltration from wind can be a factor. It is just that mechanical exfiltration might defeat wind infiltration for over 95% of the year, which greatly exceeds concerns when continual infiltration is the case...

 

[317069]

Chasbean1
You are mixing between infiltration effect and room pressure, as you said when a surface temperature is lower than surrounding dew point temperature, condensation would happen and mold would be an issue, then you say infiltration is the reason of mold, before this, you said positive pressure will eliminate this problem, then it might reduce the chance of occurring,
You can not prevent infiltration, 93.32 of residential are not tight 100% with operable windows and using doors all the time and nobody saw inches of ice on windows.
Again same question, what about apartment building that use only hydronic system for heating and has two exhaust shafts, the first in washroom, the other in the kitchen.
You said RH% has nothing to do with mold issue, how come?
if your RH went up the dew point will go up too, this tell more chance to have condensation, the dew point of (75F DB, 505% RH) is lower than dew point of(75F DB, 75%RH)that mean we don't need to cool this surface(75F,75%) as much as the surface(75F,50%) to have chance of condensation then mold issue,
Drazen hit the point of mold issue

 

[317069]

oops 50% RH not505%

 

[ChasBean1]

317609, I haven’t been on here because of vacation and work…

Anyway, I’ll try to reply as best as I can:

Your first comment: “You are mixing between infiltration effect and room pressure, as you said when a surface temperature is lower than surrounding dew point temperature.” It is a fact that a surface temperature that is lower than the dew point temperature of the surrounding air will produce condensation (or frost if below the freezing point).

“Then you say infiltration is the reason of mold:” Yes, I said that. And I stand by the fact that mechanically dehumidified incoming air and slight building exfiltration will eliminate it. There are times when wind conditions will cause building infiltration even with mechanical systems balanced right, but the occurrences are few enough not to cause mold issues.

You state “93.32 of residential are not tight 100% with operable windows and using doors all the time and nobody saw inches of ice on windows.” I’m lost on this point. It might be a language barrier.

You said that I said “RH% has nothing to do with mold issue, how come? if your RH went up the dew point will go up too, this tell more chance to have condensation, the dew point of (75F DB, 505% RH) is lower than dew point of(75F DB, 75%RH)that mean we don't need to cool this surface(75F,75%) as much as the surface(75F,50%) to have chance of condensation then mold issue, Drazen hit the point of mold issue…”

Again, I’m at a loss to answer this. RH really does have nothing to do with mold, and it really is about wetting of surfaces, and wetting of surfaces really does have to do with air dew point exceeding surface temperatures. I’m only trying to help.

Best, CB

 

[317069]

Chasbean1

Mold issue is a result of condensation that happen when the surface temperature is lower than surround space dew point, I totally agree.
But, infiltration itself is not enough to have a mold issue, as I said before more than 97.5% of building are not 100% tight,
- What if we have pressurized room with no infiltration but the wall surface temperature still less than room dew point? Do we have a condensation in this case?
- What if we have a room with small negative pressure but we kept the wall surface temperature above the room dew point?
- For RH% effect, it effect on mold issue too, because if we change the RH, that will change the dew point under the same DBT.
- Again what about apartment building where they use hydronic systems, with kitchen exhaust and washroom exhaust, this case we don’t have pressurization and we have infiltration because the window and balcony door is about 40% to 50% of exterior wall area, does this mean that we have to find mold in every apartment in this case?
As you said it is all about surface temperature and dew point temperature.
Hope there is no language barrier this time; I am trying to increase my English