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

 

[MintJulep]

Just because the exhaust fan is off doesn't mean that the exhaust flow is zero.

You could control the exhaust and fresh air together.

 

[willard3]

Unless you are in a hospital or a lab where pressure balance is critical, the extra air will escape through cracks/imperfections in the construction.

 

[Drazen]

overpressure in toilet area can go anywhere through cracks, bringing odor, and in most applications this is unacceptable.

your exhaust fan should work permanently.

 

[EnergyProfessional]

you always should have the room that needs exhaust negative. how negative depends on the application (i.e. hospital vs. regular bathroom).

it would be against the intent of the code (and common sense) to knowingly allow the "dirty" air to seep into surrounding occupied spaces. it has to go out... that why it is called "exhaust rate", and not "transfer to an occupied space rate".

sure, when you open the door a bit air may migrate. this should be kept to a minimum.

 

[SAK9]

If the room is of tight construction,the room will get pressurised and fresh air intake will go down from 110 cfm when the exhaust fan is switched off.New fresh air intake will depend on how much the room can "leak".If the door is outward opening,you may find it difficult to close

 

[EnOm]

First off:
-My application is a large private residence with 14 bedrooms, each with its own toilet in addition to some washrooms, laundries...etc.
-Also there are two seasons, Summer when the A.C is ON, and Winter when the AC is OFF. But for now I want to focus on Summer when the AC is ON. Winter is a different story.
-I'm using an central AHU and Exhaust fan to serve each 3 or 4 bedrooms together.

I should have mentioned that in the beginning, my bad.

MintJulep: Thank you for the response. I can't control AHU and EXF together because in the winter time there is no need for A.C and the AHU is shutdown. But the exhaust is still needed. So for now they each operate independently.

Willard3: Thank you for the response. I'm leaning towards this. I think the most of the extra air will escape through the toilet door's undercut and then through the exhaust duct since the exhaust air grill in the toilet is the only opening with significant size. Unless the exhaust ducting has some sort of one-way damper after the fan, which I haven't yet come across. Is this reasoning correct?

Drazen: Thank you for the response. But the over-pressurization will be more in the room. The air will escape from the room (Where the air is supplied) towards the toilet? Which should suffice for a residence?

HerrKaLeun: Thank you for the response. See above.

SAK9: Thank you for the response. If the exhaust ducting is open won't the room leak enough to allow an acceptable amount of OA in?

 

[EnergyProfessional]

could you just supply air to the surrounding spaces? i mean is the bathroom is interior it would not have heating/cooling load and be the same temperature as the spaces around.

i didn't realize those are apartments. this is the way my house is built (and every other house with furnace/AC). i have an AV vent int he bathroom (which may or may not supply as much as exhaust. and when the EX is off and AC on, air will leak out. If I would build my own house, I would do it differently.... but that is the way it is.

This isn't really a problem since I don't use the bathroom that often, and have a delay timer on my bathroom fan so when I turn off the light it keeps running for 30 minutes. If that is code-approved, you may want to do that.

this is the switch, there may be other products. but get one of the silent Panasonic fans or you will get noise complaints.

Sanitary-wise residential is less critical, since each occupant will just smell his/her own smell, unlike in commercial settings.

 

[317069]

I think Your system configuration has to be reviewed.
you have 820 cfm, with 100 cfm of fresh air, in this case you can not depend on a user operated exhaust fan, you have to keep exhaust fan running as long as you HVAC system is running.
in residential usually we use natural ventilation unless your code has asked for certain cases

 

[ChasBean1]

No problem at all. Plus 110 cfm is nothing and it is good that you are providing this positive airflow with varying operation of the toilet exhaust. There will be no overpressurization of your rooms regardless of exhaust operation.

 

[DRWeig]

ChasBean1 hit the heart of it. 110 cfm is nothing.

I exhaust 200* CFM from our master bathroom. When that fan is running and the A/C fan is off, the bedroom door scoots a little bit open. If the A/C is running and the exhauster is off, the bedroom door moves a little toward closed.

If the bedroom gets smelly but nobody is in there, fine. If someone is in there, they know where the exhuast fan switch is.

The room is tight, doors are only undercut enough so that they don't hang on the carpet. Nobody notices pressurization or lack of pressurization.


*Exhaust rate high because of Korean food issues that I won't go into here.



Best to you,

Goober Dave

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[EnOm]

Thank you all for the responses.

HerrKaLeun: I am only supplying air to the surrounding spaces because the temperature in the bathroom is not critical since it will only be used for short periods of time. Also they are not apartments. It is a rather large palace.

317069: I'm leaning more towards what ChasBean1 and DRWeig said. I think 100 cfm shouldn't be too much of a problem. The reason I am not using natural ventilation is because the part of the project I am working on is in the basement, with no windows. They have a couple of bedrooms with no windows for the maids, drivers..etc. Even for the floors above the ground, natural ventilation is not as effective as mechanical ventilation. The project is not limited by budget because the owners are a very wealthy bunch and they want these kind of systems.

DRWeig: Thank you.

Anyhow what I've come to after some discussions and thinking:
I will use continuous exhaust through a central roof exhaust which should make sure the toilets get constant air changes. This will reduce my AC loads because less make-up air has to be brought in through the AHU, and it will also provide the required air changes in the basement dwellings.
Coupled with this are local exhaust fans in each toilet that provide a high exhaust rate right after usage. This way my system is more efficient and will have much less air balance issues because the only time it is imbalanced is when the local exhaust fan is ON.

Any further comments are welcomed
Thanks for the help

Regards

 

[Drazen]

when i read that my esteemed and very experienced colleagues from north america say such an arrangement is quite customary, i will take it as granted.

i am surprised, though, when having in mind that us is "mother" of hvac trade, as such an arrangement i can see here only in military barracks or field accommodation for construction workers.

allowing toilet air to go out uncontrolled as toilet is at overpressure is not considered sanitary solution here. toilet space has to be on underpressure or at zero. there must be transfer grill between main space and toilet, so much of that excess will go to toilet before finding its way through cracks.

the issue is emphasized further if energy-efficient tight carpentry is used - the odor can escape from one apartment to another instead of leaking to outdoor. of course, that can occur at partition between living spaces, not between toilets, but the problem is that we have uncontrolled situation.

the issue of having too cold air in bathroom during summertime could be a trouble only if fan coil supply is too close or oriented toward door transfer grill. this should be in mind during design phase.

 

[ChasBean1]

90% of the residential spaces out there are burdened by more exhaust than supply because of the kitchen, toilet, laundry exhausts. So many have negative pressure issues and OA infiltration, leading to mold concerns. If you have an excessively positive pressure issue, you are in the minority. You'll have a very healthy building and might only have worries about security doors not latching because of building over-pressure. Congrats.

 

[317069]

Chasbean1
if we stop HVAC system in a house or even remove it, and opened windows allowing air coming in going out, summer and winter, will it cause a mold issue,
kitchen, toilet and laundry are not 24/7 running,

 

[DRWeig]

317609, I know you asked Chasbean1 -- but your location has a lot to do with the answer.

Mold from windows open?

Phoenix: no
New Orleans: yes
Vostok Station: no
Singapore: yes

Etc...

Best to you,

Goober Dave

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[317069]

- in Singapore where not every house has HVAC system, are we going to find mold in every house has no air duct system,
- if windows are opened we would get mold inside just because fresh air came in directly from outside then left out through exhaust fan, is that mean we have to find mold on outside surfaces of exterior walls too, because it always in touch with the outside air condition.?
- what about apartment buildings in north Amerca, as you know most the apartments have one exterior wall with more than 50% of glass and balcony doors, with exhaust shaft in washrooms and other exhaust shaft in kitchens, they use hydronic for heating and many of them do not have an AC, but they use just a fan, in addition, the code also ask for a master exhaust fan running 24/7 in every residential unit, so a building in New Orleans with this conditions must have mold everywhere, but it is not the case.
I agree with that the pressurization is a good idea, and extra 100 cfm will not cause a disaster.

 

[Drazen]

well, i see there are pros and contras based on practical experience.

theory says that mold grow when rh comes at 75-80%, and in practice i see it only in unventilated spaces. sometimes i specify minimum ventilation for luxury houses which works when there are no occupants and so far that was always working, i mean where mold existed before ventilation is introduced, it ceased to be problem after ventilation is installed. exceptions are "dead spaces" in a house which occur because some or the other limitations does not allow sufficient air distribution over the whole area.

i actually never see or design fcu's which receive ventilation air in residences. that is reserved for hotels and commercial premises which continous central exhaust.

in homes, fcu's are usually used only to deal with space gains/losses, and ventilation system is mostly separate - be it exhaust only with some make-up air openings eventually built, or it is more and more used small residential ventilation centrals that include heat recovery unit, filters, both fans, and heaters for colder climates.

 

[DRWeig]

317069, I can't explain the un-aircondtioned apartments in New Orleans being un-moldy too. I'd like to know the answer to that one too.

Drazen said good things. Our experiences are different. I was in the humidity controls business for a good long time, and we had the wetness issues to solve. Without a cooling coil or dehumidifier, I'm not sure what I'd do.

As for the apartments again, I lived in one like you described but it had A/C. My mold came in the winter time when the boiler failed while the weather was below 10°F. I made the place warm with an inappropriate use of the gas stove in the kitchen and a couple of plug-in fans. After two days, the ice on the inside of my windows was inches thick. The mold followed the thaw.

Best to you,

Goober Dave

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[317069]

Ice was inches inside of you windows, what about outside, how many meters?