Sizing an Exhaust Fan Replacement for Bathroom

[PlymouthTony]

I had my basement finished but now realize that I am getting a lot of condensation and dripping on walls after showering. The bathroom is about 130 sq. ft. and has a shower, toilet and sink in it. Contractor put in a 100 cfm Utilitech fan and only a 4 inch duct. Unfortunately, he has walked away from helping me fix the problem. I was thinking about replacing the fan with one rated at 150 cfm and an additional issue is there is no access from above. Problem is that most of these higher cfm fans have 6 inch vent. So a couple of questions:
1. Can I simply use a reducer from 6 to 4 inches to make this work?
2. Do you think I am getting 100 cfm from the Utilitech fan (is it any good?)
3. What impact on performance does taking the fan with 6 inch duct to 4 inch ducting have on airflow?
4. Have been looking at Panasonic exhaust fans and wondering what fans are best. The existing fan is only 6 inches deep and I likely can’t go more than 8 or 9 inches deep in the space I am working in.

Many thanks for any information you can provide. Thinking about doing this myself but not sure yet.

 

[Terry]

The Panasonic fans work well with 4" ducting. Very quiet too. For a fan to pull, there needs to be some sort of air intake too. Is there a heat duct in the room for air to draw through, or a gap under the bathroom door?
When time in a commercial building, same issue with the fan not pulling. I removed the door and trimmed off some at the bottom. After that the fan worked well.

 

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[John Gayewski]

The outlet of a fan should be full size all the way out or toy won't get the full capacity of the fan. The manufacturer should have info available about sizing the vent. If you have 4" duct I would just get a more powerful fan that is compatible with that.

As stated the air needs to come from somewhere so that needs to be planned.

 

[WorthFlorida]

As Terry suggest, panasonic is an excellent product. You want one with a drum blower, not a noisy fart fan. Also, use a timer switch for the fan and let it run for at least 20 minutes.

Another solution is I use a floor fan. To minimize or eliminate condensation and mold buildup, you need air circulation. LASKO makes a nice tower fan with a built in timer and remote. This is my main bath on the second floor and the fan sits outside from the bathroom door. It'd be a hassle to change out the exhaust fan in my home and I've been using this set up for about two years. I do use the exhaust fan and after the shower I turn on the floor fan and set it for a one hour run.

 

[Jadnashua]

130sq ft, and lets say it's 8' tall, so that's 130*8=1040cuft. You want about 8-10 exchanges per hour, so let's say 8*1040=8320cuft. At 100cuft/min that's 100*60minutes=6000cuft, so not enough if you want 8 full exchanges. 8320/60=138.66cuft/min.

Now, the HVAC system will (or should) be exchanging some air with the rest of the home, but in the summer, in a humid area, you wouldn't want the humidity put into the air from the shower in the bathroom to the rest of the home to make for a bigger a/c load. You might want more in the winter, but keep the concentration down in that room.

If you're going to dump that much air outside, keep in mind that the makeup air into the house will come in from leaks or cracks in the home unless you include an ERV in the plan. If you've got a tight house, without one, you may not be able to exhaust that much air, and especially if there's not enough room to get air into the room, likely underneath the door unless you put a grill in to allow that air exchange.

 

[PlymouthTony]

The Panasonic fans work well with 4" ducting. Very quiet too. For a fan to pull, there needs to be some sort of air intake too. Is there a heat duct in the room for air to draw through, or a gap under the bathroom door?
When time in a commercial building, same issue with the fan not pulling. I removed the door and trimmed off some at the bottom. After that the fan worked well.

Terry, thanks for your reply. There is a heat duct in the room but I likely do need to increase the space under the bathroom door. When I shower with the door open, the amount of condensation on the walls is considerably less, but still more than I want. The problem with using panasonic that work with 4 inch ducting is that I don't think I can get more than 110-120 cfm and wanted to go to 150 cfm. Wondering what the effect would be to go with the 150 cfm unit but with a reducer from 6 inches out of the fan down to 4 inches sized for the duct. Your thoughts?

 

[PlymouthTony]

130sq ft, and lets say it's 8' tall, so that's 130*8=1040cuft. You want about 8-10 exchanges per hour, so let's say 8*1040=8320cuft. At 100cuft/min that's 100*60minutes=6000cuft, so not enough if you want 8 full exchanges. 8320/60=138.66cuft/min.

Now, the HVAC system will (or should) be exchanging some air with the rest of the home, but in the summer, in a humid area, you wouldn't want the humidity put into the air from the shower in the bathroom to the rest of the home to make for a bigger a/c load. You might want more in the winter, but keep the concentration down in that room.

If you're going to dump that much air outside, keep in mind that the makeup air into the house will come in from leaks or cracks in the home unless you include an ERV in the plan. If you've got a tight house, without one, you may not be able to exhaust that much air, and especially if there's not enough room to get air into the room, likely underneath the door unless you put a grill in to allow that air exchange.

This may be a bit technical for me, but here is my take on what you are saying. I definitely need more than the 100 CFM unit that was installed. The house is very tight and we have a life breath air exchange unit to deal with this. Problem with the higher CFM units is that they are fitted for 6 inch ducting, but mine is sized at 4 inches. Can I use a reducer to deal with this or does that effectively reduce the 150 CFM to something much less? Lastly, are you familiar with the Lowe's Utilitech exhaust fan? I am wondering if it is drawing anywhere near the 100 CFM it is rated at. Thanks for your help.

 

[John Gayewski]

This may be a bit technical for me, but here is my take on what you are saying. I definitely need more than the 100 CFM unit that was installed. The house is very tight and we have a life breath air exchange unit to deal with this. Problem with the higher CFM units is that they are fitted for 6 inch ducting, but mine is sized at 4 inches. Can I use a reducer to deal with this or does that effectively reduce the 150 CFM to something much less? Lastly, are you familiar with the Lowe's Utilitech exhaust fan? I am wondering if it is drawing anywhere near the 100 CFM it is rated at. Thanks for your help.

If you reduce the pipe size you will reduce the capacity of the fan. The 150 cm is only applicable for running full size pipe a certain length with only a certain allowance for fittings.

4 inch pipe is about half the size (actually a little less) of 6 inch pipe. It's more complicated than just that, but a 150 cm fan with reduced pipe will reduce the capacity by about half.

 

[PlymouthTony]

If you reduce the pipe size you will reduce the capacity of the fan. The 150 cm is only applicable for running full size pipe a certain length with only a certain allowance for fittings.

4 inch pipe is about half the size (actually a little less) of 6 inch pipe. It's more complicated than just that, but a 150 cm fan with reduced pipe will reduce the capacity by about half.

Last question. Does that mean that the 100 cfm fan on this 4 inch vent is not operating at its 100 cfm rating?

 

[John Gayewski]

Last question. Does that mean that the 100 cfm fan on this 4 inch vent is not operating at its 100 cfm rating?

The 100 cm rating is applicable when hooked to the proper pipe size. That can get chocked down when the pipe gets too long or has too many bends. The fan manufacturer should have literature available that simplifies the requirements. Some will even show you how much capacity you loose as you add elbows and linear feet.

 

[PlymouthTony]

The 100 cm rating is applicable when hooked to the proper pipe size. That can get chocked down when the pipe gets too long or has too many bends. The fan manufacturer should have literature available that simplifies the requirements. Some will even show you how much capacity you loose as you add elbows and linear feet.

Many thanks for all your help.

 

[Jadnashua]

Any time you're trying to move a material, be it gas or liquid, you need to take into account the effective length. That includes both the linear length, and a factor for any changes of direction it may take. Also consider that a corrugated hose versus a solid-metal, smooth duct will have more resistance to the flow, and reduce the effective throughput. The type of outlet grill can make a difference, too.

Many of the Panasonic fans are smart, and can adjust based on the head (resistance to flow) they see, so may end up better than many others out there.

 

[PlymouthTony]

The Panasonic fans work well with 4" ducting. Very quiet too. For a fan to pull, there needs to be some sort of air intake too. Is there a heat duct in the room for air to draw through, or a gap under the bathroom door?
When time in a commercial building, same issue with the fan not pulling. I removed the door and trimmed off some at the bottom. After that the fan worked well.

Thanks, Terry. I am going with a panasonic fan as that has worked very well in our other bathrooms, that were in the original build. And I do think that I need to increase the gap underneath the door to increase airflow. There is a heating duct in the room. One question I have is how do I determine if one of my issues is lack of strength or quality of the existing fan or if the duct has been poorly installed with many sags, bends and turns? If I use a manometer to check static pressure in the vent, will that isolate the issue? Many thanks for all your help.

 

[PlymouthTony]

Any time you're trying to move a material, be it gas or liquid, you need to take into account the effective length. That includes both the linear length, and a factor for any changes of direction it may take. Also consider that a corrugated hose versus a solid-metal, smooth duct will have more resistance to the flow, and reduce the effective throughput. The type of outlet grill can make a difference, too.

Many of the Panasonic fans are smart, and can adjust based on the head (resistance to flow) they see, so may end up better than many others out there.

Thanks, Jim. I wish I had seen what type of ducting the contractor was putting in but believe that it was not solid metal, smooth. Given that there is no attic space above the ceiling, I have no easy way to check or replace without taking out part of the ceiling. Given what I am reading about the existing fan, a Utilitech 7115-01, and its poor performance, I am inclined to first change out the fan for a Panasonic. But am also considering putting in a higher CFM fan, say 150, with a reducer to fit 4 inch vent. Wondering what my effective CFM will then be but seems that it has to be higher than putting in a 110 CFM fan fitter for 4 inch duct.

 

[PlymouthTony]

As Terry suggest, panasonic is an excellent product. You want one with a drum blower, not a noisy fart fan. Also, use a timer switch for the fan and let it run for at least 20 minutes.

Another solution is I use a floor fan. To minimize or eliminate condensation and mold buildup, you need air circulation. LASKO makes a nice tower fan with a built in timer and remote. This is my main bath on the second floor and the fan sits outside from the bathroom door. It'd be a hassle to change out the exhaust fan in my home and I've been using this set up for about two years. I do use the exhaust fan and after the shower I turn on the floor fan and set it for a one hour run.

View attachment 80085

Thanks. If after replacing the fan and increasing the gap under the door, I don't get the results I need, I will try the floor fan route. Many thanks.

 

[LLigetfa]

For a fan to pull, there needs to be some sort of air intake too.

You have to look beyond just the room make-up air supply. Basements can have low pressure due to building stack effect. Folks often seal up anywhere there are cold drafts letting in air but overlook sealing where the warm air is leaking out upstairs. This raises the NPP (Neutral Pressure Plane) and can have detrimental effect on more than just exhaust fans.