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Make-up air


chrisprickett
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I'm writing an article on the good and bad of the different types of make up air practices that I see in my part of the country. I'm focusing on three main practices:

1. pulling make-up air from the attic (where there is usually two gas furnaces located)via an opening in the return air register box.

2. pulling from a roof mounted boot via an opening in the return air register box.

3. Using an auto-setting that runs the air handler fan every hour (factory set) along with practice #1 or #2.

I'd like to hear your opinions.

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In my area, there is no need for additional makeup air for attic installations - the attic ventilation is sufficient.

For a few years Micigan required a Skuttle vent supplying makeup air attached to returns

The real need for makeup air can only be determined through testing: especially testing for changes in draft during operation and especially when the blower comes on.

In my draft testing (as part of the CO Safety Analysis Proptocol I follow), the rules for "unconfined space" seem to be very conservative: I've never tested an installation where 50 cu ft/1,000 was marginal: roughly speaking, I find 25 cu ft/1000 is usually sufficient (depending on the age and tightness of the building and utility room -

For example, a typical two car garage is good for up to about 115,000 BTU. When I see garage installations, they usually run 130,000-180,000 btu for furnace and water heater. I have never draft tested such an installation where that was an insufficient volume of air.

But - of course, I still call it out.

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I think there's a little confusion going on here:

I believe Chris is asking about mixing 'fresh air' with inside building air while Bob is talking about combustion air.

For the record, I see very little 'fresh air' intakes (starting to see more amd more); when I do, it's usually when the furnace is in the attic and the duct is mounted thru the roof.

Darren

www.aboutthehouseinspections.com

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Originally posted by Darren

I think there's a little confusion going on here:

I believe Chris is asking about mixing 'fresh air' with inside building air while Bob is talking about combustion air.

For the record, I see very little 'fresh air' intakes (starting to see more amd more); when I do, it's usually when the furnace is in the attic and the duct is mounted thru the roof.

Darren

www.aboutthehouseinspectons.com

Darren is correct, I am talking about introducing fresh "make-up" air, not combustion air.

That being said, I do see builders installing systems where make-up and combustion air come from the same source: attics!

The general idea for my article is to look at the different, and often stupid, ways that builders are trying to combat homes that are built too tight.

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I have only seen fresh air come from the attic two times: The first attic had been treated with pesticide (light white powder) for scorpions. You could see where powder had gone into the fresh air supply pipe. This is not what I would call a healthy situation.

The second time, loose fiberglass insulation had fallen inside the supply pipe blocking the small filter at the bottom. The homeowner was not aware of the filter or that it needed to be cleaned from time to time.

I often see the fresh air supply coming from the roof. The only issue I have observed is when the main return air filter becomes dirty, more air is the pulled from the fresh air supply. I do not know if that makes much difference related to energy but I always highlight the importance of keeping the air filter clean.

Jeff Euriech

Peoria Arizona

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Originally posted by chrisprickett

I'm writing an article on the good and bad of the different types of make up air practices that I see in my part of the country. I'm focusing on three main practices:

1. pulling make-up air from the attic (where there is usually two gas furnaces located)via an opening in the return air register box.

Obviously this is a dumb-ass idea.

2. pulling from a roof mounted boot via an opening in the return air register box.

Seems reasonable to me.

3. Using an auto-setting that runs the air handler fan every hour (factory set) along with practice #1 or #2.

Every hour seems like a lot. In my area, the systems that do this seem to run about 15 minutes every four hours.

As someone who'd be interested in reading this article, I'd want to know the consequences of pressurizing the building with these methods. Is there a provision for exhaust along with the introduction of outdoor air? If not, what are the effects of the resulting pressure increase?

- Jim Katen, Oregon

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Agreed, there is confusion aboiut the word "make-up" air.

The HVAC sources I've looked at somethimne use it to refer only to the air introduced around the flam but not through the burner; othert talk about it as all combustion air.

Others refer to is as air introduced into a house envelope to replace that 'expelled' from the house envelop: thus, make up air for air diplaced by bath fans, for air displaced by venting of combustion gases, etc.

In my area, the only stated need for make up air is to prevent backdrafting - thus when a furnace is in an attic (rare in this area, as well, since most houses have basements) a supply of make up air such as a skuttle vent froom outside to the return isn't required.

In fact, MI but not OH has a make up air requirment at all.

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Originally posted by Jim Katen

[br As someone who'd be interested in reading this article, I'd want to know the consequences of pressurizing the building with these methods. Is there a provision for exhaust along with the introduction of outdoor air? If not, what are the effects of the resulting pressure increase?

- Jim Katen, Oregon

Jim,

That's basically my point- what are the consequences of introducing the fresh air using the various methods in my area? I'm not an HVAC expert, and my reading audience is your average homeowner. This is what makes this article challenging.

I need to explain what is going on, why it's often done wrong, and the consequences to the average Joe. I have to do this while keeping it around 600 words and making it entertaining enough for someone to actually want to read!

Your point about where the exhaust, or lack thereof, is a very good one! I'm guessing that once the house is pressurized, the fresh air opening becomes useless, and the auto fan is just recirculating stale air.

In many cases that recirculated air is partially the crap that is pulled from the attic, thus making the air even nastier.

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Originally posted by chrisprickett

Jim,

That's basically my point- what are the consequences of introducing the fresh air using the various methods in my area? I'm not an HVAC expert, and my reading audience is your average homeowner. This is what makes this article challenging.

I need to explain what is going on, why it's often done wrong, and the consequences to the average Joe.

Well, before you can explain it to the average Joe, you've got to get it explained to you by the above-average Joe at: http://www.buildingscience.com/resource ... elopes.pdf

I have to do this while keeping it around 600 words and making it entertaining enough for someone to actually want to read!

Sounds like a fun challenge.

Your point about where the exhaust, or lack thereof, is a very good one! I'm guessing that once the house is pressurized, the fresh air opening becomes useless, and the auto fan is just recirculating stale air.

Not quite. I'd guess that within a few seconds after the blower engages, the house reaches a point where the increased pressure forces a finite amount of air out of a thousand little (and big) openings in the house & the duct system. Once it reaches that point, the make up air opening will be replacing that finite amount of household air with exactly the same amount of fresh air.

If the thousand openings are large (in aggregate) then lots of air will be exchanged. If they're small, then only a tiny amount of air will be exchanged. Unless there's someone who's able to calculate exactly how much air is moving in & out of the house, no one has any idea of what the effect of one of these systems is going to be.

In your climate, the penalty for too much air exchange will be higher air conditioning costs.

In my climate, the penalty would be a mold farm in the walls.

If I were really concerned with stale air in my house, I'd install an energy recovery ventilator. The fact is that most houses aren't nearly tight enough to need the type of make-up air scheme you're talking about, vast quantities of air move in and out of them through loose construction.

- Jim Katen, Oregon

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Hi,

Around here, they do it a couple of ways. The one I see the most is where the windows have little inlet vents built into the frames and a larger-than-normal ceiling fan (Usually a 2nd story bathroom fan but sometimes a laundry fan or even a separate fan at a hallway ceiling) somewhere in the house is coupled to a 24-hour timer and exausts to the outside. I show my clients how to set the timers and open the vents and emphasize that they must use them. I tell them to keep the window vents open year-round, if only just slightly cracked, because we don't get really cold winters here and the drafts these create is negligible.

The next method utilizes the return air ducting on the HVAC system. There will be a separate fresh air duct with an inlet grill on an outside wall that's connected to the return air plenum. A 24-hour timer on the side of the furnace is used to simultaneously turn on a ceiling fan somewhere in the house and actuate a damper on that fresh air duct. The timer kicks on the fan, the damper opens and fresh air dumps out of the return air intake grills into the home. Here I make sure they understand how to use the timer and emphasize that they need to keep the fresh air intake clear of debris.

The next utilizes fresh air vents installed in the exterior walls and a 24-hour timer in a laundry or bathroom somewhere. The fresh air vents are adjusted to let in the desired amount of air according to the climate and are manually opened and closed. Here the procedure is the same as the first method.

As you can see, only the second method ensures that there is fresh air automatically replacing the air the fan is removing from the house. The first and third methods must rely on the homeowner's good sense to open the window vents or the fresh air vents, to allow fresh air in, or they are essentially depressurizing the entire house. It's not unusual to find one of these homes where the air inlets are never opened and the perimeter of the carpeting and walls around electrical receptacles will be stained dark gray where dust was being sucked into the home under the wall plates and through the wall plane. It's really, really common to find that homeowners haven't a clue and don't bother to set the timers. Many even defeat the timers because they don't like the noise that the fan makes or they are afraid of spending energy dollars.

I've had a 3-year olf home about a year ago where the homeowner had defeated the timer, closed off the window vents and kept the home toasty and close all the time. It was an Asian family. They cooked by boiling a lot and this increased the amount of moisture. The underside of the roof was covered with gray fungus with white mold, like peach fuzz, growing all over the place. All of that moisture bottled up in the home was diffusing thorugh the ceiling plane via air passages and was condensing on the underside of the roof and feeding the fungus. Inside the home, the air was so close that my sinuses were killing me. Although I didn't see any, I bet there were a few micro-organisms propagating inside that home.

The OSB roof decking was ruined. The listing agent told the buyer's agent that I must have been high on crack. I don't know what ever happened about that. I know my client bought the home. Not my problem. He was very well informed about how the system had been defeated and the roof decking was likely to be toast in less than a year.

Our state's model energy code describes about half a dozen makeup air methods and recommends that a house be ventilated 8 hours out of every 24. I think that's excessive. I've been recommending 3 to 4 hours every 24 hours for the past 10 years and have never had it come back and bite me.

ONE TEAM - ONE FIGHT!!!

Mike

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Since your asking our opinion:

I can't see pulling air either from the attic or the roof into the return air system. Wouldn't this lower the efficiency of the furnace?

In fact I would never use air from the attic for anything but exhaust air.

Around this region it seems like fresh air intakes and balanced air exhaust are all located in the basement/crawlspace. The better designed systems like the heat recovery systems do have ducts in the attic, but the ventilator unit is the basement.

You might also want to inform your readers; that for the most part air make-up units are designed for each individual system. Just like anything else now a days, if you get a "Jack of All/Master of None" type of guy to install, the efficiency can be greatly compromised.

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Here in my neck of the woods the local municipality has tried on a couple occasions to intrudoce fresh air intakes into the code. For awhile the standard was to have a 6 inch duct plumbed into the cold air returns. My experience is with our cold climate the warm basement air condisates on the cold duct at below freezing tmepratures. Even with insulated duct work to the main trunk the problem is just moved down stream. Lately they have been just leaving an open pipe into the unfinished basement areas. Some of the upper end builders invest in mechanical ie. air exchagers/HRV's to supply fresh air.

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Sorry, I can't seem to load a picture here (Mike, there's a picture floating around labled 'fresh Air').

Anyway, go here:

http://www.aboutthehouseinspections.com/download.asp

type in: air

This is what I've seen. As I recall, I did several inspections in a townhouse development in Flemington NJ; they all had this set-up.

Un-insulated flex is run to roof mounted intake.

Of course, all the intakes were leaking water.

Darren

www.aboutthehouseinspections.com

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Hi Darren,

Are you using Netscape or FIrefox? If so, open and Internet Explorer browser and post it. If you're using I.E., make sure that you keep it under 100Kb by resizing it if you have to and make sure that the name of the file doesn't have any spaces or special symbols like parenthesis in it and then try posting it again. If that doesn't work, shoot it to me by email to hausdok@msn.com and I'll post it to your post.

ONE TEAM - ONE FIGHT!!!

Mike

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Mike, you might want to press your BB vender to work on the firefox incompatibility (&/or check you usage statistics)

IE is now reported to be under 80% usage in the US.

I use Mozilla myself, and I really have to want to post a pic to open IE and manuever back to the thread just to post a pic

Most times that I think of posting one I don't bother.

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Hi Rob,

That may be true, but TIJ was built around IE and displays best, and full-screen, on IE. My computer handles having IE and Firefox both installed and I can open one of each and not have a problem. The new IE 7 looks and functions much like Firefox. In fact, when it came out, I was on here talking about how cool it was and others who use Firefox were going, "What's he talking about? It looks just like Firefox." After that, I downloaded Firefox to see what it looks like and discovered that they do function almost identically - except TIJ displays best on IE.

Why haven't I changed over yet? Because doing so will take the programmers several weeks of dinking around with the site to make everything work properly in Firefox. That costs a whole lot more than what TIJ has in the budget, which is $0. As long as IE can be run concurrently with IE without doinking either up, I'm not going to change it.

As for posting a picture, you don't have to exit Firefox or Netscape to do it, although it's easiest that way. Just click on the upload a file icon below the composition box, browse to the photo, and click on the upload button. Once it says that it's successfully been uploaded, do not close that message window. Instead, highlight the URL displayed there and copy it. Then, close that message window and paste that URL into your post and submit it. The photo will display.

OT - OF!!!

M.

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On a couple of new homes i am montering here in central Indiana the HVAC installers ran two 6 inch metal duct lines through the side wall of the house into the furnances return air plentium with no dampers or anything. I have been told the reason for this is to pressureize the home and stop unwanted air infilteration. Now there is another question,maybe a good place for a new thread; these homes are built pretty tight, icysonene insulation with no vapor barrier.It sounds like a moisture problem waiting to happen. Also,the entire interior of the home was insulated includeing the roof deck. Any feed back would be great.

Bryan

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Originally posted by Bryan

On a couple of new homes i am montering here in central Indiana the HVAC installers ran two 6 inch metal duct lines through the side wall of the house into the furnances return air plentium with no dampers or anything. I have been told the reason for this is to pressureize the home and stop unwanted air infilteration.

Close, but no cigar. Those ducts might decrease the negative air pressure in the area of the returns if they are poorly sealed and localize/"focus" makeup air supply when exhaust fans are run.

OTOH, if those installers have really figured out how to pressurize a house simply with 6" duct work, find out how they do it and file a patent and you can retire as the richest man on earth.

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