What are the dynamics going on with my whole house fan?

[TNCobra]

We installed a whole house fan, and we love it. Here is my observation. I close all the windows besides our family room slider, which I open all the way. I turn on the whole house fan. I feel the flow of air at the exit point of the room. I then close it, 1/4, 1/3, 1/2, 2/3, 3/4, 7/8.

The feel of the air goig past me at the exit of the room feels significantly greater at 2/3 than the other points. I figure there has to be an optimal point, but not sure the dynamics that would cause an optimal point. Can anyone explain?

[bwana50]

Hmm, I'll take a stab at it.

Mass flow = density*area*velocity. Density is constant and mass flow should be constant, so as area decreases (door closing), the velocity through the door increases. There is probably a point where velocity cannot increase because the fan can't move the air that fast due to various ineffeciencies. (Theoretically, if the fan were perfectly efficient and the house well-sealed, the fan could move ALL the air out and create a vacuum.) Then mass flow would decrease and the fan stalls. It's still moving air, but not like it should. Thus the apparent decrease in velocity at the door.

[Hoosier]

I conclude that you have too much time on your hands.

[NiceGuyEddie]

I'm an engineer and I also worked for construction contractors for loads of years. once specialty I worked with was in was whole house exhaust and HVAC… they now have systems that suck IN air when you put on your bathroom exhaust fan, for example. a wire is run from the bathroom to another intake fan in another area of the house. the intake fan always exceeds the CFM of the exhuaust fan to help push it out… these systems REALLY help suck the poopie smell out!

anyway….

I couldn't really follow what you are saying with your 1/2, 3/4, 7/8 thing, but keep in mind the exhaust fans in your bathrooms are open passage to the outside, and so is the exhaust fan above your stove. even if they have a "flap" to allow exit air only, it can suck in.

the overflow passage in the rear of your bathroom sinks is also open to an air pipe in the roof. my bathroom sink whistles through the slot in the back of the sink if I have only one slider open like you described!

if your house did not come with an exhaust fan, you probably don't have ample area exhaust (vent) grates in the attic walls (at peaks of the roof) to allow the whole house fan to work optimally. those whirly-bird things help tremendously.

just more to consider when thinking about your dynamics.

andrew said you have too much time on your hands, and so do i - haha - but these are exactly the things i think about when sitting in traffic!

cheers,

[Hoosier]

Originally Posted by NiceGuyEddie

the overflow passage in the rear of your bathroom sinks is also open to an air pipe in the roof.

Wrong...

[JoeW]

You also need to be careful of gas appliances with a whole house fan. I recall reading that if you don't have enough inlet area, the fan could draw spent gases from your gas hot water heater down the flue pipe. At work, we have a LARGE exhaust fan built into the wall. It will blow out the pilot light on the gas water heater if the garage door wasn't opened first.

Joe

[NiceGuyEddie]

Hoosier:

mine does, so does the drain for the bathtub. all 2" black PVC.

may be a California thing, or a local code. the sink cavity is a "double shell", the overflow goes to the drain, and the top of the drain goes to the 2" black PVC vent system for the house.

.

[NiceGuyEddie]

Originally Posted by JoeW

You also need to be careful of gas appliances with a whole house fan. I recall reading that if you don't have enough inlet area, the fan could draw spent gases from your gas hot water heater down the flue pipe. At work, we have a LARGE exhaust fan built into the wall. It will blow out the pilot light on the gas water heater if the garage door wasn't opened first.

Joe

this is correct, fortunately, most modern stoves don't have a pilot light, and most water heaters are in the garage.

the instructions for whole house fans always have a warning that say not to run the fan unless you have a window open, otherwise you can burn out the motor. I hope that does not mean electrical fire.

[Cobra Cory]

Yeah you need to match the opening pulling the air in to the size opening exhausting the air. If you have a 3' x 6'8" screen door as your input you need to have the vents on your roof at least as big in area or it will create a mini pressure bulb in your house, after the fan. Usually the attic. At that point no more air can get in or out. So when you are hitting the 2/3rds setting is probably when everything has evened out. Just a like a car. Bigger intake and heads, bigger exhaust pipe. It is advisable to make sure that any floor drains have water in them. A lot of people that have floor drains forget to put water in the trap to keep gases coming back into the house. If you have dry floor drains and a whole home fan, it will suck in sewer gas.

[Hoosier]

Originally Posted by NiceGuyEddie

Hoosier:

mine does, so does the drain for the bathtub. all 2" black PVC.

may be a California thing, or a local code. the sink cavity is a "double shell", the overflow goes to the drain, and the top of the drain goes to the 2" black PVC vent system for the house.

.

I've never seen or heard of it done in such a fashion. You should encounter a near constant sewer gas odor problem if it is as you say...

[Cobra Cory]

Yeah, ummmm, that don't sound right in any zip code......
Not to mention if it goes straight to the vent stack you can get critters and bugs coming down the pipe into your house. Or Santa even.... and your tub wouldn't even drain if it wasn't vented properly and if you think about it, every time it rained, water would come down the vent line into your tub. Not much but it would....as well as allow cold air into your bathroom through your fixtures.

Not to hijack, but this is a proper code bathroom vent line.



Now to bring it back on point, you could hook up a whole house fan and exhaust it up your tub vent.......

Just messin' with ya.....

[Jeff R]

Mass flow through the house and fan should be constant.

Mass flow through sliding door = mass flow through exit of room = mass flow through fan = mass flow out of attic

If the sliding door is closed to 2/3 the velocity will increase through the sliding door, but the mass flow should still be about the same. Is the exit of the room directly inline with the sliding door? Is it possible you're feeling more velocity at face level, and the areas near your feet have a slower velocity so the mass flow is still the same? You've got me curious about my house fan, I typically have a handful of windows open. Tonight I'll try reducing them and see if I can feel an increase in flow.

[The Italian]

Yeah ,the sink "trap" is always filled with water to block the stink and if you are pushing that water out with home air pressure ,you got a problem-won't happen.
I solved the cooling problems when I did my remodel
VELUX skylites

I installed 4 of them with auto sun shades.
they work by remote and you can open them as much as you like and the hot air that is naturally at the highest point at your ceiling rapidly escapes and if you have a window open (in my case downstairs), you create a suction or breeze that continues until the house either cools or until you close the skylite.
They also close automatically when it rains
They can also be installed if you have an attic by using a box up to the roof.

[TNCobra]

Thanks for the replies. No stink problems, no gas problems. The sqft of the exhausting air was well in excess of what was required for the size of the fan.

There is definitely a point of opening the door that maximizes the flow and then falls off. Too much time, nah, just a curious mind.

[jem44357]

Maybe when you reach the closing point that the flow drops off it is using some of the other points of exit previously mentioned which offer less resistance than the door wall opening.

Jim

[Buck70]

Originally Posted by NiceGuyEddie

my bathroom sink whistles through the slot in the back of the sink if I have only one slider open like you described!

,

The water in the trap that stops sewer gas should prevent this.

I wonder if the whistle is really coming from the wall cavity where the
pipes go through the drywall? Duct tape the vent, turn the fan on and see
if it still whistles at you.

[Buck70]

Originally Posted by TNCobra

The sqft of the exhausting air was well in excess of what was required for the size of the fan.
.

Most attic fans suggest twice the fan size for free flow area to allow for the heat in the attic and to prevent static pressure in the attic. If you don't have enough exhaust you can stand under the fan and hear the blades paddling against the air above it.

[ClemsonS197]

Originally Posted by TNCobra

The feel of the air goig past me at the exit of the room feels significantly greater at 2/3 than the other points. I figure there has to be an optimal point, but not sure the dynamics that would cause an optimal point. Can anyone explain?

You've found the sweet spot in the fan curve. The more you close the door, the more static pressure (SP) you are putting on the fan, and simultaneously you're increasing the velocity through the door opening. As SP increases, air flow decreases. However, the SP vs. air flow is on a curve. So you've found the and ideal spot of SP vs. Air flow that is also combined with the velocity through the door, which is what you feel.

Along the same lines, the more windows and sliding doors you open, the less resistance you have on the fan which will increase the amount of air the fan is moving. There is a point of diminishing return for each fan, as they will only move so much air based on motor hp, RPM, and fan blade design.