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Old 05-08-12, 12:41 PM   #1
SimpleManLance
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Default home made blower door?

Im not sure if this have been done before of if it could be dangerous if there is to much negative pressure.

I have a couple good size furnace blower motors that ive collected over the years. one in particular has 4 speeds on it. i am thinking i could take a piece of plywood a little bit bigger then one of my window openings and mount the blower to it. that way i could seal the window up and have it so it is sucking air from the house and blowing it outside. i would have 4 speed setting and i could use it to make negative air pressure in the house to find air leaks. Can a furnace blower cause to much negative pressure that it could pull caulk or foam out of cracks? im talking like a thin layer of caulk that might not have a super strong bond.
you wouldn't be able to measure your air exchange or anything like that but i think you would be able to create a good amount of negative air in your house.

anyone ever done something like this and had good luck?

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Old 05-08-12, 01:11 PM   #2
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While it will work, without being able to calculate CFM I don't see this as being incredibly useful. You could perhaps hook it up and run it at X speed and measure the pressure differential. Then, do some improvements to the house and run the test again and see what the new pressure differential is to get some kind of compairison. But, I'm still not sure how useful that really is.

I did write a blog a while ago on doing your own air pressure test to help find leaks, DIY Home Air Pressure Test | EcoRenovator.org. The technique works pretty well.
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Old 05-08-12, 01:27 PM   #3
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Since I already had my house blower door tested, I know how much air it takes to get to 50 pascals and have a vague idea of how much air was coming down through the open flue vent on my furnace when I put my hand inside. I thought of doing something similar but decided to use two box fans in each room, sealing off the rest of door space(s) or openings to the room and pressurizing the room to look for leaks.

It won't be a diagnostic tests without the duel magnahelic setup if you use a fan or blower so you really don't need a blower for this. If you use a furnace blower, you have the ability to put quite a lot of pressure on your house as they are designed to push well over 1 inch water column, which in itself is 249 pascals. If you don't have a way of measuring the pressure you could cause an issue. Also be sure that your furnace, water heater, etc gas appliances are all turned off(pilot on a modern water heater should be fine as long as you check it is still lit after you've used the blower door). If you have a fireplace, clean out ALL soot as you'll make a terrible mess as it backdrafts past the flue you though was sealed well.

My recommendation, don't do it. Use a standard axial fan setup (like two box fans taped on top of eachother with the rest of the door plastic sealed) because they can only push a small amount of pressure due to their design, that's all you need. A furnace blower can pull quite a bit of pressure with no real way of knowing how much pressure is really there.
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Old 05-08-12, 05:56 PM   #4
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daox- It would be just like your "home air pressure test". No real data that you can collect other then knowing where leaks are in your house.

MN Renovator- I can see the issue will to much pressure that's why i liked the fact that there is 4 speed setting. I cant believe on low that it would make to much pressure for a entire house. I guess you could always open a window some to lower the pressure or block off the intake to the blower some. That's how my draft blower for my wood furnace gets regulated.

I have a box fan screwed to plywood that i sit in my attic opening in the summer to help move the hot air out of the house(kinda like a whole house fan). Maybe i will turn that on and shut the windows to see how much pressure that creates.

I liked the furnace blower because you wouldn't have to move it from room to room.
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Old 05-08-12, 06:24 PM   #5
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The fan on a blower door is pretty intense. In fact, if the door isn't installed properly it will suck itself out of the door jamb. That being said, if you can get a powerful enough fan/fans, I recommend pressurizing the house to find air leaks. That way your smoke pen or whatever smoke source you use, will make a B line to the leak. The depressurization is strictly for testing the overall leakiness of a house. For spot testing everyone uses pressurization. You turn the blower door fan around. There are a few exceptions to this rule. Houses generally have the thermal boundary at the sheathing except in the attic which uses an airtight drywall approach. In this case you would depressurize the house and test for ceiling leaks from up in the attic to see the smoke scoot down into the house. Once more exception is when it's bitter cold out and you can use an IR camera to see the cold air being sucked in. That's pretty cool.
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Old 05-09-12, 02:19 AM   #6
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I did a little test because of this thread. I took two box fans and mounted them in a second story bedroom and opened the kitchen window on the opposite side of the house. Turned on the fans and walked outside, good breeze coming out. So I closed the kitchen window and listened to the fans load up a bit since they weren't able to push the same amount of air. I went to the basement and grabbed a small wad of spider web and held it near the draft of the water heater and it was sucked right in. That's all it took to put pressure on my house. I have a total of four box fans that I usually use when trying to circulate a whole bunch of air in one side of the house and out the other when the temperature outside is far more favorable than the temperature inside and it works well. I figured I'd need all 4 to get good pressure but I'm thinking that it is probably not needed and I think I was underestimating what two cheap box fans can do with a house that tested out as 1500 cfm50. I'm still not sure how accurate two magnahelics, a tarp, a chart to do the math for CFM, and a fan that looks like it came out of a hardware store used on a house with atmospheric ventilation can be compared to a blower door that can use progressively smaller openings to get accurate results for less CFM. Makes me wonder how these can cost $3k when they are so simple.

Either way, your makeshift attic fan might do the trick to pressurize the house blowing in to watch a smoke pen suck the smoke out, if you can't see air getting sucked out of a known leaky area, add your furnace blower and turn it up until you can see the smoke exiting your known leaky spot and I think that should be fairly safe if you are careful about it.

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Old 05-11-12, 08:49 PM   #7
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S-F & MN Renovator- great input. pressurizing the house does make more sense now.

Ii will probably come up with a better fan location then my attic if i will be pressurizing the house. I don't think it would be a good idea to be sucking all the nasty stuff that might in the attic into the house. I do have a little dual fan setup that stays in our master bedroom window to help cool it down at night so we can sleep better. Ill turn that on hi and go to the basement hot water heater vent and see if that makes any pressure. Its worth a try seeing as its already in the window. ill probably have to add another fan somewhere else.

the more i think about this, the more scared im getting of the results i might find.
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Old 05-11-12, 09:19 PM   #8
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In MA you can have a free energy audit through the utility companies and they will preform a blower door and combustion safety test. Is there anything like that where you are? I think you may have a hard time getting meaningful results with a couple of those little fans. With a blower door you would probably discover more even with just depressurization. Doors and windows will whistle and you can walk around and notice drafts. But if all you are concerned about is combustion in the basement you can stack two box fans at the top of the basement door,seal off any openings and just pressurise the basement. Small fans will have an easier time working on a smaller area.
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Old 05-15-12, 10:22 AM   #9
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I found a picture of the blower door that my energy auditor used on my house. Looked nothing more than a tarp with a way to lock it into the door with two magnehelics and a high velocity fan with a variable speed knob near the magnehelics.

He then took the pressure of the building and used his cheat sheet for the pressure drop through the blower to come up with his figure. Mags are under $100 on ebay and a tarp and a few metal shower door bars would be a little over $20. Add a variable speed blower fan to get the pressure you need and it could be done for $300 probably. These things really don't look complicated at all.

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Old 05-15-12, 08:33 PM   #10
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The beauty of a blower door is how it measures the pressure. A real one is pretty sophisticated. It measures inside, in the center of the fan and outside. It can compensate for varying outdoor conditions. Once it's done it can give you a number. Personally I only care about them for their ability to pressurize and depressurize a house. Tomorrow I am on a job where we have to run five a the same time and we connect them so they think as one. Pretty high tech. I'd have one if I could just get the fan and the frame. They have tons of uses. Let's say you have vermiculite in you attic and want to remove it without paying thousands of dollars. Pressurize the house, put on a HEPA respirator and vacuum it up.

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