Question about combustion air input on a high efficienct furnace.

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About three years ago we moved into a 40 year old house. Sometime during the last few years prior to us buying it, the previous owners replaced the forced air gas furnace with a Rheem high efficiency model (92, I believe). The exhaust line is standard 2 inch PVC pipe vented out the side of the house. The question I have is regarding the input for the combustion air. There's no piping for this, just a two inch hole in the side of the furnace. The furnace is in a full unfinished basement, so it's not starving for combustion air. The house is not sealed as tight as some newer homes, so I'm not worrying about oxygen levels in the house, but I would think that by using the interior air for combustion it would cause cold air to be pulled into the house via leaks. Also, it's sending the warmer house air out the exhaust.
Would it be of any benefit to run a PVC pipe to the inlet of the furnace? The total length of the run would be about 12 feet, and would involve one 90 degree bend and a 45. The pipe would come out of the house about 30 inches away from the exhaust (is that sufficient?). Also, would I (or should I) put a final fitting (90 or 45) tilting downward on the end of the pipe where it sticks outside to keep rain & snow out? The current exhaust pipe has nothing on the end, it's just the pipe sticking out about a foot.
As I said, I'm working on other issues to help the heat in and the cold out, but I'm thinking every little bit I can do to keep the cold out would help. I did some internet searching, most of what I found seemed to indicate it would be a good idea. Most concerns I found were about NOT having piped input air, but it seems to be more of an issue the tighter your house is sealed.
Mike O'Donnell
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Im no pro but believe outside air is what it was set up for and you would benefit greatly taking outside air. Also get it cleaned for full eficiency. Talk to Rheem for pipe specs and location of inlet vs exaust
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start with installing a carbon monoxide detector in the basement with a digital parts per million display. then continue reading....
your answer is not in just the manual you need to read for your specific model, but also in what the installer did, and add to that what the previous homeowner might have done. if it is presently ok by the installation manual to be sucking on your stale basement damp radon air for its combustion, you might be happy to let it continue doing so. if on the other hand your basement doesn't have sufficient make-up air and the furnace is sucking on the hot water chimney flue and causing exhaust gas corrosion of the flue pipe atop the gas hot water heater, you have a needed repair. you will need to know the requirements for that furnace model.
....if the CO alarm has not rung, we're safe for now...
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| About three years ago we moved into a 40 year old house. Sometime during | the last few years prior to us buying it, the previous owners replaced the | forced air gas furnace with a Rheem high efficiency model (92, I believe). | The exhaust line is standard 2 inch PVC pipe vented out the side of the | house. | The question I have is regarding the input for the combustion air. There's | no piping for this, just a two inch hole in the side of the furnace. | The furnace is in a full unfinished basement, so it's not starving for | combustion air. The house is not sealed as tight as some newer homes, so I'm | not worrying about oxygen levels in the house, but I would think that by | using the interior air for combustion it would cause cold air to be pulled | into the house via leaks. Also, it's sending the warmer house air out the | exhaust.
You are sucking "warm" air out of the house which will be replaced by leakage thru the walls -- Workable but not good. would be better with a 2" intake for outside air. One thing not usually done is to seal all pleneum joints. Sheet metal workers will cringe, but, their joints do leak and there is no reason to add this inefficiency to your furnace load. An excellent use for duct tape - the good stuff, not the cheap paper kind.
| | Would it be of any benefit to run a PVC pipe to the inlet of the furnace? | The total length of the run would be about 12 feet, and would involve one 90 | degree bend and a 45. The pipe would come out of the house about 30 inches | away from the exhaust (is that sufficient?). Also, would I (or should I) | put a final fitting (90 or 45) tilting downward on the end of the pipe where | it sticks outside to keep rain & snow out? The current exhaust pipe has | nothing on the end, it's just the pipe sticking out about a foot.
Great idea. Just remember that every 90 degree bend adds about 5 ' to the effective run. The less bends, the better. The outside end of the pipe can be either a 90 degree elbow, a "T" junction with the open ends vertical or , at worst, nothing. There are also screens with a fairly open mesh to help keep rodents and leaves out of the intake. I once had a leaf get into the intake and completely shut down the furnace. | | As I said, I'm working on other issues to help the heat in and the cold | out, but I'm thinking every little bit I can do to keep the cold out would | help. | I did some internet searching, most of what I found seemed to indicate it | would be a good idea. Most concerns I found were about NOT having piped | input air, but it seems to be more of an issue the tighter your house is | sealed.
True, with a really tight house, the furnace will not work, with a leaky house the living area will constantly be trying to offset the colder air sucked in to replace what the furnace uses. Remember that having a really tight seal on the ducting causes more air to be pushed out through the registers and less wear and taear on the furnace blower. Above all, keep your filters clean as this eases the blower effort and increases the effective efficiency of the furnace.
| | Mike O'Donnell | | | |
--
PDQ

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There would be some benefit, how much? Don't know, but I would recommend it too.. Before you attempt this I would get a manual for the furnace. I work for a Ruud dealer, which is the same furnace, different label. The air inlet requires a condensate catch "T" to keep moisture in the fresh air from making it's way to the furnace. Also the air inlet is supposed to be close to the exhaust, within a few inches, generally slightly below, with an elbow pointed down. All this info will be in the manual. find one, read it, and follow the directions to the letter! It may look easy, and it really is, as long as the instructions are followed. I have be out on calls to solve problems with furnaces that are not running properly. Often it is just a problem with the install. Too much pipe, too many elbows, wrong diameter of pipe, and so on.
What size of furnace is it? Larger furnace require larger pipe. Longer runs require larger pipe. It may be possible that your furnace is not installed properly right now! I can e-mail you a manual if you give me the model of the furnace. E-mail me if you like. Greg
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you don't receive it, the furnace is a Rheem "90 Plus" upflow natural gas furnace. I think it's in the85K to 90K BTU range.
The previous owners didn't leave any documentation. I've done some searching, and have found a spec sheet and a "use and care guide", but haven't located anything relating to the inlet air piping, so if you can point me to site with more info, I would appreciate it.
Mike O'Donnell
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Call Rheem
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Re-send the e-mail. I did not get it, or it got deleted by mistake. I NEED model numbers!! The numbers will be found on the inside of the furnace. There are stickers with the info on the right or left side. Send it to Hotmail addy! Greg goo1959 @ hotmail.com (leave out the spaces!)
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Mike O. wrote:

This is Turtle.
get with Grego and check this job out.
Yes put a air intake from outside will be a big help in heating cost by burning out side cold air and not the houise room air that is heated already. if you are going to run a intake air for a 92% afue Rheem furnace. I suggest you run it with 3" PVC pipe to make sure your not starving it in some way. i looked at the chart sometime back when installing one and I had on a 100K btu heat furnace to run the vent out and in with 3" pvc pipe and could have 2 -- 90 degree elbows and 90 foot run.
Now you need to get a Rheem Venting chart to see the size, length, and btu rating for all to work.
TURTLE
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If 100 cfm of 130 F air leaves the flue in either case, you can either a) heat 100 cfm of house air from 30 to 70 F with about 4K Btu/h from the furnace, then heat it from 70 to 130 with 6K Btu/h, total 10K Btu/h, or b) heat 100 cfm of 30 F outdoor air from 30 to 130 with 10K Btu/h.
Where is the energy savings?
Nick
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Nick the obvious energy savings is in not pulling in inside air to the furnace, which the house makes up by sucking in cold outside air, you need to learn some basics.
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I disagree. There is no obvious savings.
Nick
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Nick, yes the uninformed would disagree,
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You seem to be in arrogant dimwit mode again :-)
Where is the energy savings?

Nick
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Nick you are the dimwit, call any heating equipment manufacturer to learn the benefits of using outside air, your math is irrelevant again.
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Call Lennox about winter humidification :-)
Nick
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m Ransley wrote:

But the informed understand and agree with Nick's math, with the caveat that the inside air was heated at less than 100% efficiency. Here's a re-accounting of that original analysis.
Given a 92% efficient furnace, if 100 cfm of 130 F air leaves the flue in either case, you can either
a) with sealed combustion, heat 100 cfm of 30 F outdoor air from 30 to 130 with 10K Btu/h, or
b) heat 100 cfm of outside air "leaked" into the house from 30 to 70 F with about 4.3K Btu/h burned in the furnace, then heat it from 70 to 130 with 6K Btu/h, total 10.3K Btu/h.
In case a, 10K Btu/h is being exhausted, but remember that this furnace exhausts only 8% of the energy it burns. That means the heat output into the living space is 115K Btu/h, with 125K Btu/h total energy input to the furnace. 115/125 = 92%
In case b, same 125K Btu/h input, but only 114.7K Btu/h real output (10.3K is lost to support the combustion process). 114.7/125 = 91.8%.
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The gas furnace output would be about 500K Btu/h, enough for 10 houses :-)
Nick
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snipped-for-privacy@ece.villanova.edu wrote:

The outside air is "dry" so you'd have to add moisture to get it within the "comfort zone", which requires additional energy. Using outside air for combustion reduces or eliminates this energy sink. It also decreases the potential for air infiltration in certain areas which would make those areas "feel" cold, inciting the occupants to increase the thermostat setpoint.
But I do believe you are correct that you must heat the combustion air from 30 to 130 F overall, whether it comes through a pipe from the outside or by way of the interior space. No free lunch. I'd be curious if anyone could come up with an reasonable explanation otherwise.
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The air in the house was heated at 90 percent efficiency (by the furnace heat exchanger) before it is sucked into the combustion chamber to burn. If you draw outside air directly into the combustion chamber, it is heated at 100 percent efficiency (no losses in the heat exchanger). Also you don't have the effect of drawing humidity out of the house into the flue and you improve comfort by reducing drafts.
Sealed combustion also reduces standby losses that would otherwise go up a chimney. Such systems often require balanced pressures between intake and discharge to work properly. Check with your equipment manufacturer.
Stretch
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