Author Topic: Combustion air intake drastically cooling basement - suggestions?  (Read 788 times)

Milkshake

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Hey everyone, hoping for some DIY wisdom here.

We have 2 combustion air intakes in our basement that allow fresh air to come in for the gas furnace and gas water heater. Unfortunately, with the cold we've been having, these combustion air intakes are causing the basement to get very cold (like 57 degrees F overall, maybe <50 degrees locally around the intakes). The basement is unfinished with no interior walls or separate furnace room or anything. Our laundry, workout area and cat's litterbox are down there, so closing off the basement entirely isn't really an option, and would only be a band-aid solution anyway.

Any thoughts on how to safely make these not bring straight cold air into the house? It's like having a basement window half open in the middle of the polar vortex.

My first thought was to route one intake to the furnace and one to the water heater directly, so that it could be pulled in only where it's needed. Is this a realistic solution?

Also, if we did just build a few walls and make a furnace room, I would be concerned that it could get extremely cold, and with the close proximity to water lines, I wouldn't want it to burst the piping.

I appreciate your ideas!

Montecarlo

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Re: Combustion air intake drastically cooling basement - suggestions?
« Reply #1 on: February 11, 2019, 08:36:59 PM »
Is there a reason why you have dedicated air intakes for those appliances?  Is it a very modern, air tight build?

Most houses are leaky enough that gas appliances get plenty of combustion air.  I leave a basement window cracked very slightly because I don't want a backdraft down the water heater or boiler flue when I'm running my wood stove, and my basement stays well above outside temp without an additional heat source.

Montecarlo

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Re: Combustion air intake drastically cooling basement - suggestions?
« Reply #2 on: February 11, 2019, 08:41:50 PM »
Do a google search for neutral plane pressure and how chimneys work.  I doubt bringing the intakes closer to the appliances will work.  Any air escaping from the house through the flues or leakage in the main/upper floors will cause a vacuum in the basement and bring more air in.  Counter-intuitively, the best option may be to find and eliminate air leaks in the upper floors.

I'm a complete amatuer in this, so take my advice with a grain a salt, but the googling may be worth your while

lthenderson

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Re: Combustion air intake drastically cooling basement - suggestions?
« Reply #3 on: February 12, 2019, 06:45:53 AM »
We have 2 combustion air intakes in our basement that allow fresh air to come in for the gas furnace and gas water heater.

My first thought was to route one intake to the furnace and one to the water heater directly, so that it could be pulled in only where it's needed.

I'm not sure how other parts of the country to this but here in the Midwest, the air intakes are plumbed right to the unit and sealed so there isn't any way you can pull inside air into the unit. The reason for this is to prevent accidental carbon monoxide poisoning if something in your furnace or water heater starts producing the gas inside the house.

Le Poisson

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Re: Combustion air intake drastically cooling basement - suggestions?
« Reply #4 on: February 12, 2019, 07:00:26 AM »
You made me take a walk to the basement to see what our setup looks like...

our furnace has an intake and exhaust direct plumbed to the exterior with white PVC.

Our water heater has an exhaust hood up a few inches off the top of the unit with "dryer vent" (I know it's not, but it looks like it) going to the chimney and venting up above the roof.

I suppose that we lose some heat up the chimney to that vent, but not enough to lose sleep over.

Traditionally we have an inversion in our house where the basement is the warmest floor in the winter and the coolest in the summer. HVAC is a mystery to me.


Jon Bon

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Re: Combustion air intake drastically cooling basement - suggestions?
« Reply #6 on: February 12, 2019, 11:34:48 AM »
Ok I have this same issue in a rental, I even posted on it here but I did not get much of an answer. I think there are 2 issues going on here.

A high efficiency furnace 90% or better will have separate exhaust and supply lines usually PVC and often side vented out of the house.

A traditional furnace will have only 1 exhaust line, made of metal (they get HOT) and vented straight up usually out a chimney.

This situation I believe happens only with traditional lower efficiency furnace. If you have a HE furnace and its drawing supply air from inside the house just spend $20 bucks on PVC pipe and run it outside next to the exhaust pipe.

So I have a traditional furnace (as does the OP I believe) and we have furnace rooms that basically have holes in them to the outside. This might be code somewhere? but it strikes me as very stupid. You are essentially putting your furnace outside, which makes it stupidly inefficient. All your supply lines are going to be subjected to crazy cold before they can even do their job of heating the rest of the house. I think one would be fine blocking up the hole to the outside in the furnace room as it would draw from the rest of the house rather easily. I have have had several houses with furnace just sitting in the basement with zero venting to the outside.

If it were me I would block up 90% of the holes in the furnace room and see how it does.






Milkshake

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Re: Combustion air intake drastically cooling basement - suggestions?
« Reply #7 on: February 12, 2019, 11:55:22 AM »
I'm not sure how other parts of the country to this but here in the Midwest, the air intakes are plumbed right to the unit and sealed so there isn't any way you can pull inside air into the unit. The reason for this is to prevent accidental carbon monoxide poisoning if something in your furnace or water heater starts producing the gas inside the house.

We are in the Midwest as well. See the attached photo, the two red circles are the air intakes (one high, one low) and the red arrow is the intake area for the water heater. I'm not sure where exactly the furnace intake is, but I know it is not plumbed into the unit directly to the outside. We only have the one exhaust, which connects to both the water heater and furnace.

Is there a reason why you have dedicated air intakes for those appliances?  Is it a very modern, air tight build?

The house was built in '06, so reasonably well sealed.

A traditional furnace will have only 1 exhaust line, made of metal (they get HOT) and vented straight up usually out a chimney.

This situation I believe happens only with traditional lower efficiency furnace.

So I have a traditional furnace (as does the OP I believe) and we have furnace rooms that basically have holes in them to the outside. This might be code somewhere? but it strikes me as very stupid. You are essentially putting your furnace outside, which makes it stupidly inefficient. All your supply lines are going to be subjected to crazy cold before they can even do their job of heating the rest of the house. I think one would be fine blocking up the hole to the outside in the furnace room as it would draw from the rest of the house rather easily. I have have had several houses with furnace just sitting in the basement with zero venting to the outside.

If it were me I would block up 90% of the holes in the furnace room and see how it does.

I think you are correct, it looks like we have a traditional furnace but a well sealed home, so these intakes provide extra air to the gas burning appliances so that 1) CO gas doesn't build up and 2) The consumed air (and resulting negative pressure) doesn't pull in air from any small window seal gap or something, which could result in excessive icing or damage to the windows/doors/other vents.

I don't really want to block these inlets, mainly because of the CO gas buildup concern. But I can't imagine extending the ducts to connect to the actual appliances (or going very close to them) would be very dangerous. This wouldn't fix the inefficiency of heating freezing air, but it would at least prevent the entire basement from getting a frosty breeze all winter.
« Last Edit: February 12, 2019, 12:00:03 PM by Milkshake »

bw_94

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Re: Combustion air intake drastically cooling basement - suggestions?
« Reply #8 on: February 12, 2019, 12:26:32 PM »
At first glance, I'd make the following suggestions.

-Cut a small diffuser w/ damper into a section of the supply duct in the basement to temper the space.
-Insulate the intake air pipes?

Milkshake

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Re: Combustion air intake drastically cooling basement - suggestions?
« Reply #9 on: February 12, 2019, 12:40:03 PM »
At first glance, I'd make the following suggestions.

-Cut a small diffuser w/ damper into a section of the supply duct in the basement to temper the space.
-Insulate the intake air pipes?

I thought about insulating the pipes. It might help some, but with such a large opening straight to the outside, I doubt we would notice a difference in temperature. I will have to look into the diffuser+damper idea.

Based on my Google research, it looks like this whole setup is built in compliance with 2015 IRC, G2407.6.1 Two permanent openings method for outdoor combustion air. (Probably more like the 2006 IRC, but it looks the same.)

bw_94

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Re: Combustion air intake drastically cooling basement - suggestions?
« Reply #10 on: February 12, 2019, 01:06:44 PM »
At first glance, I'd make the following suggestions.

-Cut a small diffuser w/ damper into a section of the supply duct in the basement to temper the space.
-Insulate the intake air pipes?

I thought about insulating the pipes. It might help some, but with such a large opening straight to the outside, I doubt we would notice a difference in temperature. I will have to look into the diffuser+damper idea.

Based on my Google research, it looks like this whole setup is built in compliance with 2015 IRC, G2407.6.1 Two permanent openings method for outdoor combustion air. (Probably more like the 2006 IRC, but it looks the same.)


Oh, wow I had to look at that picture more closely. That is an odd setup... Insulating the intake won't help much. Cutting a small diffuser into a supply branch somewhere in the basement will help temper the space, something like this:
https://www.amazon.com/ADJUSTABLE-DIFFUSER-Register-Sidewall-Cieling/dp/B006J2VECQ/ref=sr_1_3?ie=UTF8&qid=1550001874&sr=8-3&keywords=supply+diffuser

Then you could just keep it shut most of the way just to blow a little extra heat into the space.

Jon Bon

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Re: Combustion air intake drastically cooling basement - suggestions?
« Reply #11 on: February 12, 2019, 01:07:59 PM »
This has got to be a (stupid) code thing. Where is @paddedhat  when you need him!

Every home from oh I dont know, 1960-1990 is going to have a traditional furnace with zero venting for the supply to the outside. There was not a mass carbon monoxide issue during this time period. I have had several houses with zero supply air from the outside and those furnaces were just fine. The idea of pumping your basement full of cold ass air is so ass backward it hurts my brain!

I dont think a 2006 house in the Midwest would have any trouble pulling air from outside under slight negative pressure. I think this would only be an issue in an Alaska house or something that is literally air tight. Cover it up and save a few bucks or leave it as is for the .000001% extra safety margin (made up stat!)

The Carbon monoxide thing is massively overblown IMO. If I remember correct the vast majority of the issues were folks using propane heaters in places without a running furnace due to new constitutions site or broken furnace. If you are that concerned get a CO detector for 20 bucks.

https://www.cdc.gov/mmwr/volumes/66/wr/mm6608a9.htm

Milkshake

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Re: Combustion air intake drastically cooling basement - suggestions?
« Reply #12 on: February 12, 2019, 01:23:24 PM »
I agree Jon Bon, it's pretty low risk, especially since we do have a CO monitor down there already.

I just saw in the G2407.5.1 section of the IRC, I should be able to calculate if our internal basement volume meets the spec of 50 cubic ft per 1000 BTU/h of gas appliance. If so, I should be able to completely seal those and still be in compliance. I think these vents are there in the event that we finished the basement, then you would need outdoor combustion air to still be in compliance (due the furnace room becoming much smaller than the entire unfinished basement). A back of the napkin calculation from memory says as long as my furnace BTU + water heater BTU is less than 120k per hour maximum, I should be just fine. I can't imagine its more than like 30k.

I definitely don't want to do anything permanent to those vents, in case we sell the house.

My only concern about the diffuser/damper option is if that would still meet code for free airflow of the combustion air if we do finish the basement.


Jon Bon

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Re: Combustion air intake drastically cooling basement - suggestions?
« Reply #13 on: February 12, 2019, 01:40:37 PM »
I agree Jon Bon, it's pretty low risk, especially since we do have a CO monitor down there already.

I just saw in the G2407.5.1 section of the IRC, I should be able to calculate if our internal basement volume meets the spec of 50 cubic ft per 1000 BTU/h of gas appliance. If so, I should be able to completely seal those and still be in compliance. I think these vents are there in the event that we finished the basement, then you would need outdoor combustion air to still be in compliance (due the furnace room becoming much smaller than the entire unfinished basement). A back of the napkin calculation from memory says as long as my furnace BTU + water heater BTU is less than 120k per hour maximum, I should be just fine. I can't imagine its more than like 30k.

I definitely don't want to do anything permanent to those vents, in case we sell the house.

My only concern about the diffuser/damper option is if that would still meet code for free airflow of the combustion air if we do finish the basement.

Seal it up and report back to us. If you dont report back, well, uh I will just assume that I should not seal them up at my rental........

In all seriousness, I highly doubt that would be noticed on an inspection report if/when selling. But if you are going to seal it do it from the outside, or as close to the outside as possible, you dont want the thermal transfer along the entire length of that steel pipe. Glad you found that it was a code thing. I am surprised they built a house in 2006 without a HE furnace!

Good luck.




TexasRunner

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Re: Combustion air intake drastically cooling basement - suggestions?
« Reply #14 on: February 12, 2019, 01:42:08 PM »
Add a flapper inlet (Like this: Link ) to the pipe, as close to the intake at the wall as possible. It should be very easy to open, like less than 3 pounds draw.

Once that is done, see if you can isolate the furnace intake (or build a firebox around it) and draft the intake lines directly to the box.  The water heater shouldn't draw nearly as much air but you can do the same to it.

I would NOT cut off the supply air permanently or manually, as the house probably has Tyvek or spray foam, and the installers weren't comfortable with the negative pressures the furnace generated.

What you are trying to solve is (1) Free-flow of air into the basement when IT IS needed through the pipes- but it is mixing with the conditioned space, (2) air exchange inside-and-out through the relief ductwork when it isn't needed (its completely normal to have constant pressure variations in buildings), and (3) the fact that they used a typical furnace but (obviously) noticed that it created too much negative pressure for the house.

(1) You want to isolate the pathway to only go to the furnace, (2) you need to prevent constant air exchanges and direct the air direction and (3) They added that relief ductwork on purpose...  It would have been great if they finished the job with controlled intake but apparently they considered the basement 'unconditioned' and used the whole room as an air exchange from outside to the furnace.

The furnace box (if possible to add) needs to isolate the furnace to its own air space, and run the external intake directly into it.  The house exchange air intake (hopefully) is on another floor and is already ducted into the basement and directly into the furnace, which means you shouldn't have to worry about it.

Also, get carbon monoxide and NG detectors with battery backup.  I have these:  "Nighthawk Plug-in Carbon Monoxide & Explosive Gas Alarm with Digital Display KN-COEG-3"  https://smile.amazon.com/gp/product/B0002EVNJ6/ref=oh_aui_search_asin_title?ie=UTF8&psc=1


TexasRunner

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Re: Combustion air intake drastically cooling basement - suggestions?
« Reply #15 on: February 12, 2019, 01:55:29 PM »
FYI - IBC 501.4 and 504.6 is why they put in the relief air supply.  You would still be in compliance with it if installing a backdraft damper to ensure the negative or neutral condition is maintained, as well as ensuring there is no direct air exchange.

Milkshake

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Re: Combustion air intake drastically cooling basement - suggestions?
« Reply #16 on: February 12, 2019, 02:19:14 PM »
Thanks for the info TexasRunner! I didn't consider that it might be building pressure related, rather than combustion air related.

How would I confirm if the house exchange air intake was somewhere else? I don't recall seeing any other intakes in the house.

In the short term (like, tonight), I would think I could at least reduce the size of the openings. In the next few weeks I will look to see if I can install a flapper inlet.

Seal it up and report back to us. If you dont report back, well, uh I will just assume that I should not seal them up at my rental........
Lol, I will do my best to report an update!

TexasRunner

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Re: Combustion air intake drastically cooling basement - suggestions?
« Reply #17 on: February 12, 2019, 03:29:51 PM »
Thanks for the info TexasRunner! I didn't consider that it might be building pressure related, rather than combustion air related.

How would I confirm if the house exchange air intake was somewhere else? I don't recall seeing any other intakes in the house.

In the short term (like, tonight), I would think I could at least reduce the size of the openings. In the next few weeks I will look to see if I can install a flapper inlet.

Seal it up and report back to us. If you dont report back, well, uh I will just assume that I should not seal them up at my rental........
Lol, I will do my best to report an update!

There should be a time delay on blower startup vs heater startup.  Also, more pics of the furnace would help.  If it is taking in basement air to spit out of the ductwork above in the rest of the house, that is **technically** a code violation depending on the year and the fact that they made the bsaement 'unconditioned space' with the way they provided relief air.  2006 might have been fine- I don't know when that change happened.  Either way, its not a safety issue in your situation.  Hopefully, there is a grill intake somewhere up above where the furnace takes in air to be heated and distributed in the ductwork and doesn't use basement air.

The air intake for the duct work in the house is going to be large.  Very large.  Like 24 x 36 at least.  If it is in the basement, you'll see an intake grill headed into the furnace.  That grill will 'suck' air for several seconds (after the blower starts) before the combustion kicks on.  Once the combustion kicks on, you will feel the air flow coming in the relief duct work that just goes in the basement, which is eventually being sucked into the combusion chamber on the furnace. 

Overall, photos would help.

lthenderson

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Re: Combustion air intake drastically cooling basement - suggestions?
« Reply #18 on: February 12, 2019, 04:30:41 PM »
We are in the Midwest as well. See the attached photo, the two red circles are the air intakes (one high, one low) and the red arrow is the intake area for the water heater. I'm not sure where exactly the furnace intake is, but I know it is not plumbed into the unit directly to the outside. We only have the one exhaust, which connects to both the water heater and furnace.

I will politely bow out of this conversation. I have never seen anything setup like that. It sounds like you have some good advice up above though. Good luck.

Milkshake

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Re: Combustion air intake drastically cooling basement - suggestions?
« Reply #19 on: February 13, 2019, 07:23:28 PM »
There should be a time delay on blower startup vs heater startup.  Also, more pics of the furnace would help.  If it is taking in basement air to spit out of the ductwork above in the rest of the house, that is **technically** a code violation depending on the year and the fact that they made the bsaement 'unconditioned space' with the way they provided relief air.  2006 might have been fine- I don't know when that change happened.  Either way, its not a safety issue in your situation.  Hopefully, there is a grill intake somewhere up above where the furnace takes in air to be heated and distributed in the ductwork and doesn't use basement air.

The air intake for the duct work in the house is going to be large.  Very large.  Like 24 x 36 at least.  If it is in the basement, you'll see an intake grill headed into the furnace.  That grill will 'suck' air for several seconds (after the blower starts) before the combustion kicks on.  Once the combustion kicks on, you will feel the air flow coming in the relief duct work that just goes in the basement, which is eventually being sucked into the combusion chamber on the furnace. 

Overall, photos would help.
So, here's a few pictures. We have 2 of the roughly 10"x32" intakes, one in the upstairs living room and one in the basement. Both photos shown.

We also have four 10"x16" intakes, 2 are back-to-back of each other, one in the master bedroom and one in the hallway. The other 2 are in the other bedrooms.

All 6 of these intakes eventually combine and run into the filter, which I assume then gets heated and blown back around the house again. I don't see any larger intake anywhere inside the house.

When the furnace starts, what sounds like a blower kicks on for about 10 seconds (no air comes out of any vents though). Then combustion begins for another 10-15 seconds, then the actual blower kicks on and warm air comes out of the vents. Even when the furnace and water heater are running, I don't feel any actual cold air coming in from the outside relief ducts, just the same constant cool air, like opening a refrigerator door. It feels this way whether the furnace is on or not.

Side note, last night I plastic wrapped + rubber banded the relief ducts, as we met the IRC for cubic ft of air for each 1kBTU/h requirement. Just to stop airflow, no real insulation there. Air temp in the basement went up only 1 degree (58F now), but over by the furnace was also 58 degrees, rather than the frigid 50ish that it was a few days ago. And I moved the CO monitor over to near the furnace just in case, and it didn't go off.

TexasRunner

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Re: Combustion air intake drastically cooling basement - suggestions?
« Reply #20 on: February 14, 2019, 07:35:48 AM »
So, here's a few pictures. We have 2 of the roughly 10"x32" intakes, one in the upstairs living room and one in the basement. Both photos shown.

We also have four 10"x16" intakes, 2 are back-to-back of each other, one in the master bedroom and one in the hallway. The other 2 are in the other bedrooms.

All 6 of these intakes eventually combine and run into the filter, which I assume then gets heated and blown back around the house again. I don't see any larger intake anywhere inside the house.

When the furnace starts, what sounds like a blower kicks on for about 10 seconds (no air comes out of any vents though). Then combustion begins for another 10-15 seconds, then the actual blower kicks on and warm air comes out of the vents. Even when the furnace and water heater are running, I don't feel any actual cold air coming in from the outside relief ducts, just the same constant cool air, like opening a refrigerator door. It feels this way whether the furnace is on or not.

Side note, last night I plastic wrapped + rubber banded the relief ducts, as we met the IRC for cubic ft of air for each 1kBTU/h requirement. Just to stop airflow, no real insulation there. Air temp in the basement went up only 1 degree (58F now), but over by the furnace was also 58 degrees, rather than the frigid 50ish that it was a few days ago. And I moved the CO monitor over to near the furnace just in case, and it didn't go off.

Ok, that sounds and looks normal as far as the conditioned air intake goes.

There is a TON of thermal mass in that basement.  I would probably take upwards of 4 or 5 days for it to warm back up after closing off the tubes.  As long as you have a carbon monoxide alarm in there, and aren't experiencing other negative-pressure related issues (slamming doors, window edges icing up excessively, etc) then I would keep those tubes sealed off for a few days to see what that does.

It appears that the (completely normal) negative pressure state of the house (due to exhausts and chimneys through the roof) is continually bringing in cold air through those relief ducts.  Hence the constant breeze.  I would expect them to get a little stronger when the water heater and furnace are on, but considering the size of them (and that there is two) it may be hard to feel a discernible difference.

Leave them closed for a few days and see.  That might just be a fine short term solution until you finish out that basement space or sell the house (in either case the relief duct work would be wise to keep).

Also if that basement isn't insulated, I wouldn't expect it to get above about 60 or so, based on your local ground temp.

Prairie Stash

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Re: Combustion air intake drastically cooling basement - suggestions?
« Reply #21 on: February 14, 2019, 09:02:04 AM »
In my part of the world we have air to air exchangers to avoid Radon buildup in basements. Its a naturally occuring gas commonly found in basements.

Your system would mitigate Radoon very well.

Milkshake

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Re: Combustion air intake drastically cooling basement - suggestions?
« Reply #22 on: February 14, 2019, 03:42:26 PM »
Thanks TexasRunner! I really appreciate all the input. I'll try keeping them sealed for probably the rest of the winter and monitoring any changes.

Prairie Stash, in regards to radon, the previous owners actually had already put in a passive mitigation tube (in the first picture, it's the PVC pipe above the sealed sump cover with the blue U-shaped thing). The basement failed radon testing during our inspection though, so we made them install an active radon fan on that tube in the attic. Our last radon test came back as nearly equivalent to outdoor air levels.

zolotiyeruki

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Re: Combustion air intake drastically cooling basement - suggestions?
« Reply #23 on: February 14, 2019, 04:07:16 PM »
After looking at your setup and comparing it to mine (also midwest, built in '05-'06), a couple thoughts come to mind:
--You may have a lower-efficiency furnace and water heater, as evidenced by the sheet metal exhaust.  The exhaust on both of mine is PVC, meaning the exhaust isn't very hot when it comes out of the appliance.  In addition, our furnace has its intake air supplied via a parallel run of PVC from the outside.  I would be very surprised if your furnace isn't designed to accept that as well.  From the conversation above, that would allow you to at least get rid of one of the inlets.  Why the builder didn't do that in the first place is beyond me.
--Our water heater draws air from the basement like yours, but it's an induced draft model, i.e. when the burner is going, there's a blower extracting air out the top and expelling it through a PVC pipe out the side of the house.  I suspect the blower is required because there's not enough vertical run in the exhaust to get natural draft. Induced draft water heaters are also waaaay more expensive than the traditional type, which annoys me.