Air intake for high efficiency boiler – UPDATED
What are the reasons why high efficiency units need outside air?
So far this is what I’ve come up with:
1. Colder air means more efficienct combustion. – Incorrect
2. The unit bases its operation on outside temp. – Incorrect
3. Prevents updraft from other combustion – it won’t cause back drafts on the water heater. <- Maybe….
4. Less moisture in the air and therefore less condensation in the combustion products of the HE unit. – Incorrect
Does that look correct?
Are there any others?
Thanks!
UPDATE: As I’m looking in Chap. 20 of the 2009 IRC, I’m not seeing anything particular for HE systems and M2001.1 basically says follow the manufacturer’s directions.
So far, I’m seeing some really enlightening things. I’m one of those people that likes to know the why of things.
Replies
A high efficiency combustion device of the sort that pulls air from outside (such as the types with dual plastic pipes running out the sidewall) does not itself need external air brought into the furnace room. However, codes mostly still require some outside ventilation of the furnace room to help dissipate any gas leakage or other fumes, etc, that might be in the area.
A unit that draws air from the surrounding room (whether "high efficiency" or not) needs air brought in to avoid creating a negative pressure situation that could cause back-drafting in the unit itself or other combustion devices in the house.
Using a HE device of the type that is supposed to be "plumbed" for external air without doing so is "strongly discouraged" because it's inefficent (draws cold air into the house) and can result in pressure differentials in the combustion process that are inefficient, hazardous, and//or have the potential to shut down the unit.
Huh??
I've seen plenty of high efficiency appliances installed in conditioned space without fresh air openings. The idea is to supply the appliance with "plumbed" combustion air from the exterior so that it doesn't use heated inside air and doesn't require you to build a leaky room for it. Why would you be expecting gas leakage or other fumes?
I'm just saying that code (as repeated here several times) still appears to require some (outside) airflow into the area. Of course, that's not to say it's done -- in our case, when the old furnace was removed the makeup air duct was removed and sealed. The furnace guys told me that local inspectors generally feel that a house of our vintage (1976) isn't tight enough to require anything, even though it's required in newer construction.
#1 sounds like BS to me.
#3 is certainly important.
I would add that you don't want to use conditioned air from inside the house. Better to pull outside air into the combustion chamber rather than through a crack in the house somewhere.
Couple reasons.
Laundry and Condensing Furnaces Don't Mix. Choose a sealed-combustion furnace that uses 100% outside air for combustion. Indoor air contains traces of commonly used household products such as bleach, paint strippers, and so on. Condensing furnaces have been known to corrode rapidly if fed indoor air contaminated by these common products. Most manufacturers void warranty coverage for furnaces operated in a contaminated atmosphere. Even fabric softener and salt from water softeners have been suspected culprits. Unless the furnace is near a possible source of contaminants, such as a dry cleaning shop, outdoor combustion air is much better for the furnace. (Salt air near the ocean would probably also cause problems for condensing furnaces, but the only salt water we see in Wisconsin is when the snow melts in the road.) There is mounting evidence that even furnaces in the 80% AFUE range have problems when using contaminated air for combustion. The contaminants cause condensation and corrosion to occur more easily. http://www.homeenergy.org/archive/hem.dis.anl.gov/eehem/94/941108.html
If you use the direct vent method there is no further consideration of combustion air necessary.
The direct venting (two-pipe) configuration is designed and constructed to provide a complete separation of the combustion system from the atmosphere of the room in which it is installed. All air for combustion is obtained from the outside atmosphere and brought in with one pipe and all flue gases are discharged to the outside atmosphere with the other.
Direct venting has many benefits such as the pipes can terminate in two separate locations or a single termination point called, a concentric vent; but the biggest benefit to this method of venting is that the termination of the vent only has to be 12” from any opening through which flue gases may enter a building (door, window or gravity air inlet).
The one-pipe method vent cannot terminate less than 4 feet below, horizontally from or 1’ above any door, window or gravity air inlet into a building. See your instruction manual for all the details on venting & draining the condensing furnace.
You can’t talk about venting a condensing gas furnace without also discussing draining. http://www.totalairsupply.com/files/9%20November%2007%20Venting%20&%20Draining%20Condensing%20Furnaces.pdf
Wow, where to start..
"Most manufacturers void warranty coverage for furnaces operated in a contaminated atmosphere." Any examples of this? Most manufacturers warranty is only a year, except for high-efficiency heat exhangers, which are made of stainless steel and will not corrode in any environment remotely safe for continued occupancy, and they are good for 10 years, prorated only. An installation in a corrosive environement, such as a water treatment facility or a pool room, would not be warrantable for residential equipment or any other equipment not intended for that application.
"..outdoor combustion air is much better for the furnace." BS. IAQ contaminants, at any habitable levels, are not corrosive and have no effect on combustion.
The relative location of a flue discharge to any fresh air intake, operable window or the like, is not affected by the use of a concentric vent arrangment.
One FOS KIA Google-for-brains "information" dispenser here is enough.
Where to start
Are you so repulsively arrogant in real life or is this just your Internet persona?
you must be a real joy to be around.
And if you want to spew insults, quit using acronyms and just say what you mean. KIA? Sounds like a car to me.
Better yet, why don't you just put forth your opinion and explain it so any incorrect answers are discounted. If you have to insult everyone whom you disagree with, maybe you should get some professional mental help.
So the last line
threw you off from responding to any of the direct statements I made. Sorry for the confusion.
KIA stands for know-it-all. Not a term of endearment. It is intended for the few that feel compelled to put forth a constant stream of incomplete and incorrect information so as to feel useful, or to create the impression of knowlegde and importance to someone that doesn't know any better. I am the "bad" guy because I call the BS. So be it. FOS you should be able to work through without help.
As far as "in real life" should I ask if you are completely and blissfully ignorant, or just mostly incompetent? Can you addreess any of the specific misinformation I pointed out? Or are your feelings too hurt to concentrate on facts?
Feelings hurt?
I don't know you from Adam, so no my feelings aren't hurt, I don't care a whit what you say. You seem a somewhat intelligent, but belligerent AH. Sometimes I enjoy observing your type, it reminds me of the Wizard of Oz.
All I did was post some information I read and a link. If that makes you think I'm blissfully ignorant, I'm sorry for you.
As I see it, you pop up on this forum and tell posters they are ignorant know nothings, while you yourself never seem to offer any of your own knowledge.
So tell us, why do installers install a direct air intake to a HE furnace? Just to make a few extra bucks, or what?
Ok
I DO challenge many incorrect, inappropriate or worng information I see here, from time to time. There are some very prolific posters that constantly do so. My point is that if you or anyone else dosn't really know the answer to the question, why offer something that's not right or best, just a guess? Even just posting something read elsewhere, if you don't understand it or didn't completely read it, is it right? You don't know.
In some cases, if someone posts something I know to be wrong, I will call it BS and I will usually label the poster a moron for trying to float it by us. I do so frequently in a very brusk manner. That makes me the AH, right? I can live with that. Sometimes I enjoy the argument, sometimes I could care less.
If you had been paying attention, you would see a great deal of knowledge I've posted here. I don't just challenge bad answers, I add the correct information as well. IF I don't know what the right answer is, I don't say anything.
Why do installers pipe the intake? One reason is that they know no differenly. They do what the've seen and been successful with. Two is that the manufacturers (all as far as I know) show this as an acceptable or preferred installation. Three, because of the confines of the space it is necessary and since the furnace comes with provisions, why not utilize them. A final reason is, as the OP's #3 alluded to, the belief that backdrafting of other appliance might occur if they do not directly pipe CA to the unit.
All of these are valids reason for the installer to do it this way. They can (almost) never get in trouble by following the manufacturers instructions. They also don't have to answer any questions about why they didn't.
What are the reasons NOT to pipe the CA directly from outdoors to the unit? I can think of a few. Being unecessary in many instances, it is a waste of time and materials. The outside intake can and will be used as a temporary home by all sorts of creatures and when the unit fails operate at the start of the heating season, a service call is in order. Blockage by snow, ice, leaves. It is more likely to introduce garbage in to the unit with an outside CA intake. With the exception of some of the houses featured on "Hoarders", the air inside a house is much better conditioned and free of debris, than outside.
Maybe he's related to Riverong? Only his way of thinking is the right way of thinking. And it's OK for him to not read a post ... skipping relevant words/statements. Then when he's backed into a corner and has nothing left to say ... his response is ... and I quote "I really don't care."
The man's
handle was RiverSong. And your quote was incomplete and out of context. I gave up trying to convince you of your misconceptions and there was no value in teaching you the difference. That is what about "I really don't care". While there is some entertainment in the challenge, there is no use trying to "teach a pig to sing".
So YOU ARE RiverSong (sorry for the mispelling ... stupid keyboard)? Wow. And again, I didn't really disagree w/ your point of view, I simply provided another point of view that in fact is as relevant as yours. And I did it for the express purpose of lending perspective to the topic, not to try to push my idea as better ... or worse than another.
Well you have a new handle or whatever, but you've not learned respect for another's point of view. It's all science to you and if my science doesn't look like yours, its simply labeled wrong. Reminds me of the ignorance humans have demonstrated over the ages (and some of the resulting horific actions they've taken) simply based on ignorance and fear of what they don't know and a refusal to have an open mind.
Your standards seem to be high, but your mind very narrow.
Actually,
I believe that I was instrumental in encouraging RiverSong to go away. No thanks is necessary. As a matter of record, I have used the same name here for the past 13 years or so. You? Your profile is absent any details or profession. What are you hiding?
In certain areas, i.e. my area of expertise, my mind is narrow when it comes to facts. There is right and there is wrong. There are issues that are not black and white. I respect points of view that have substance and credibility and that are presented well. I do not repect those based on something else. Deal with it or not. I will continue to challege the technically incorrect, whether posted by you or anyone else.
Since you seem to have forgotten the substance of our last debate, I'll refresh your memory. We we not discussing matters of science or fact, but inference of operational habits of thermostat users. Your initial presentation of that infered "knowledge" asserted that programmable thermostats do not save energy, but actually cause users to consume more energy. There is a large body of eveidence that contradicts that assertion. You offered none that supports it. With no support, I can give it no credence. There's that "narrow mind" again. Or is it the "high standards" thing?
You must be RiverSong's twin, though ... you think so much alike!! It's amazing.
"... my mind is narrow when it comes to facts." But your mind is very broad in interpreting/reading text ... so much so that you don't really read what is being said, but read what you want to think someone is saying.
Maybe I wasn't clear, but I thought I was very clear (crystal). I thought that I never said anything like "Your initial presentation of that infered "knowledge" asserted that programmable thermostats do not save energy, but actually cause users to consume more energy." I said that assuming you are reasonably religious about manually reseting your thermostat, it will in fact result in lower energy use than a programmable stat set to meet setpoint when you arise in the a.m. and/or when you return from work. So, no, I haven't forgot the substance of our debate ... but it does seem fuzzy to you.
Again, not advocating manual or programmable, just making sure that the poster understood that a programmable stat is not automatically a guarantee of energy savings (vs. manual setback). While I respect that most people these days probably don't have the discipline to control their thermostat manually efficiently and that there can be great value (and savings) with prog stats, a disciplined manual user (and again I've known many) can have a lower energy bill.
Not sure about the profile ... thought I had everything there (but mabye my SSN) ... no idea why it's not visible. Maybe it's this new format that the wise men of Taunton Press have blessed us with [sic].
I could
equally make the the case that you are DanH's (and his son, MarkH - the apple don't fall from the tree, does it) twin. You think so much alike!! It's amazing.
Yeah sensible thinkers are amazingly like minded.
1. wrong
2. wrong
3. wrong
4. wrong
First off, your premise is wrong. There is no such thing as inside air. ALL air leaving the house has to have come into the house from the outside.
Warmer air actually means better combustion, but only by a few points. Don't worry about it.
The outdoor temp sensor is outside, not in the combustion air pipe.
If downdraft is a problem, then air isn't getting into the house to replace what is leaving. You need to look at all sources of exhaust. Think of the whole house as a system. What part of the system isn't functioning properly? This is what you worry about.
Cold air can have high humidity. Right now the temp here is 8f and the humidity is 85%.
Some units require 2 pipe, some can use 1 pipe. Why? Because thats what the manufacturers require. Why? becauses they can't control #3
Cold air can have high humidity. Right now the temp here is 8f and the humidity is 85%.
Wrong.
http://www.wunderground.com/global/stations/WCZMJ.html
12.22 am. -19c and 89% humidity
So tell me how cold air can't hold moisture. Especially when I am scraping frost off of my windshield. Or when I'm up on a roof removing hoarfrost from the intake of a makeup air unit that won't run because the frost is so thick it is blocking the intake.
No offense Dan, but I might start agreeing with Tim.
Cold air can hold a little moisture, but not very much -- you're both confusing relative and absolute humidity. Warm the cold air and the relative humidity plummets, even as the dewpoint remains constant. Dewpoint is the standard way to figure absolute humidity, and the value that counts when talking about humidity transport in a structure.
http://www.usatoday.com/weather/resources/askjack/2002-12-01-answers-dp-relhum_x.htm
We ain't talking about dewpoint in a structure. We are talking about the effect of humidity on combustion. Try to stay on topic.
You claimed "Cold air can
You claimed "Cold air can have high humidity". From the standpoint of this discussion (where the air would be brought into the house and used for combustion) that's wrong. Colder air almost invariably has lower humidity.
Ok, let me rephrase if it will make you more comfortable.
Humidity in the combustion air, high, low, or in between, does not have an effect on combustion in a HE appliance.
Happy?
That's funny
"I might start agreeing with Tim."
I'll let you two sort this out amongst yourselves. In this field, Dan is usually FOS, but is compelled to respond regardless.
If you don't understand dewpoint and relative humidity, and appreciate how relative humidity is a red herring when dealing with humidity issues in a structure, you're FOS when talking about such issues.
Let me ask you this: If the temperature is 10F outside and 70F inside, and the outside RH is 90% while the inside RH is 20%, if I draw air in from the outside and warm it to 70F will the inside RH go up or down?
You need to
take this up with whomever was challenging you post. I did not. I merely thought it somewhat amusing to be mentioned in the post. I also did not "talk" about the issue of humidity, relative or specific, in any way. Disagree with me all you want, but at least stick with statements that I make. I'll do my best to defend technical stands I make, but I will not defend those of others, usually.
While I did not plan to weigh into this one, I do fully understand water vapor, moisture in air and the various definitions of humidty. Atmospheric air at 10 degF and 90% RH contains approximately 8 grains/lb of dry air. Air at 70/20 contains approximately 22 gr/lb. The outside air will drop in RH as warmed, and will have a drying effect on the space.
You said (in response to a post not directed to you) that I was "FOS". I think that that was challenging my post (even though you go on in this one to admit that I'm right).
You ARE frequently wrong, I stand by that opinion of your general knowledge in HVAC related issues. You are an intelligent man and have a great deal of knowledge of many things, of that I have no doubt. I don't understand why you can't tell the difference between what you know and what you make up or guess at.
You are right about the relative humidity argument you made and I should have been clear in my response. My intent was to let you make the argument to support your correct statement.
Of all the discussions we've had, did really think I don't understand the basics of humidity? Do I peeve you oof that much?
I'm occasionally wrong, but when I am I admit it. You just seem to react negatively to everything I post.
You post
far more than I react to, negatively or otherwise. But I get the point and will work on my negativity. A personal flaw I indisputably have.
Nice to see people admit to their flaws--I would if I had any. ;-)
I type
very poorly and dislyxicly, too.
Intakes, Codes and High Efficiency
First of all, all devices that burn any fuel require combustion air, which ultimately comes from outside. No building is so tight that inleakage will not cover the needs of residential heating equipment.
The extra efficiency comes from a secondary heat exchanger that cools the products of combustion (air, water vapor, NOx, C02 and CO) below the dewpoint further giving up heat, via conduction and the latent heat of vaporization, to the air to be heated. The secondary heat exchanger comes in contact with the coolest air in what is known as a "counterflow" configuration.
Unit capacity ratings are based on standard conditions for combustion air and fuel gas, as are AFUE ratings.
The IECC International Mechanical Code defines a "confined space" as any that has less than 50 cubic feet/1000 btuh input rating of all the fuels burning devices in the space. IF the appliance is installed in a confined space and draws its combustion air from that space, combustion air must be provided via natural or forced means. The exception being for sealed combustion units that take CA directly from outdoors.
All of your reasons are entirely incorrect except #3, and that is only partially correct. This is primarily based on the myth that a house can be so well sealed that the small amount of draft a forced air unit can "pull a house negative". If the confined space limitations are observed, very unlikely.
Only #3 is a reason for possible concern, but even then, most houses are not sealed tightly enough so that other appliances, like the WH, would backdraft due to lack of combustion air.
By "outside air", I assume you mean air piped in directly to the combustion chamber of the furnace. This is good for efficiency, since it reduces the amount of air leaking into the house. No other outside air source is code-required for the furnace itself.
In my experience of testing many hundreds of new houses, the most common cause of backdrafting WH's has nothing to do with combustion air or even with the WH flue--the most common cause is due to pressure imbalances created by an improperly designed duct system, which results in de-pressurizing the furnace room where the WH is usually located.
#4 (condensation from warm inside air, which contains more moisture than cold outside air) This is not the main source of condensation within the combustion system of a natural gas furnace--water vapor is a by-product of burning hydrogen in the gas. You make about 1 gallon of "new" water for every 100,000 BTU's of heat. This is far more water than what is contained in the air that you burn in the process.
Don't most if not all of these appliances utilize a blower on the exhaust to move exhaust gases out of the system since they are too cool to move efficiently through convection?
There are obviously several Mensa members on here but I just assumed they wanted the outside air to make certain they didn't draw the air for the powered exhaust from the Water Heater flue.
Condensing, or high-efficiency, furnaces do have exhaust blowers, as you say, and for the reason you mentioned. If they have the intake air piped directly into the combustion chamber, the exhaust blower will have no effect on the air within the house, and no other outside combustion air is required for that appliance.
If the furnace room also contains a water heater, whether of the gravity vent type or the power vented type, then there is a requirement for combustion air, which can be met by normal infiltration in some cases, or by ducting in combustion air directly from the outside to inside the room containing the WH.
"Only #3 is a reason for
"Only #3 is a reason for possible concern, but even then, most houses are not sealed tightly enough so that other appliances, like the WH, would backdraft due to lack of combustion air."
Yeah, but newer construction, built tightly and combined w/ e.g. kitchen, bath, and water heater exhaust, you could easily cause badkdrafting. Add a fireplace and you've got problems.
"Yeah, but newer
"Yeah, but newer construction, built tightly and combined w/ e.g. kitchen, bath, and water heater exhaust, you could easily cause badkdrafting."
Others are clothes dryers (200 CFM or more), and whole-house vacuums (50+ CFM), and high performance kitchen hoods (up to 1500 CFM).
On top of that, literally, is the stack effect of the house, especially in tall houses. Basements in these are nearly always de-pressurized to some extent.
Each house can be unique, and it's tricky to pinpoint the source(s) of depressurization, and then correct the problems without going too far the other way, i.e., bringing in way too much outside air, thereby losing a lot of the energy efficiency that air-tightness is aiming for.
Let me take a crack at this. None of the above. I think the dedicated combustion air was provided due to the pressure of new codes that result in tight houses means that we 1) need to provide combustion air that won't be negatively affected by e.g. exhaust ventilation fans drawing a negative pressure on the house and 2) dedicated combustion air eliminates cold air being drawn into the house or furnace room for combustion purposes (those vents are often open 24/7 even though the furnace may be on only 20% of the time) and 3) if the furnace is in a confined space combustion air has to either be drawn from the house or provided to the room which is problematic as noted in the above 2 comments.
Dan's response basically reiterated this in a little different way, I think.