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Old 04-02-2011, 12:03 AM   #1
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Runaround - HVAC and HWH proper venting/exhaust


Hey all. I'm having my 13-year-old HVAC and 3 year old HWH moved about 15 feet from where they now sit to clear space for a basement area that's being finished. I've had 6 HVAC guys out and 2 plumbers. Every one has a different opinion. This is why I come here - to learn from the cumulative knowledge of hundreds.

The units work fine. We're just moving them. But they presently sit very near the metal exhaust chimney that goes straight up from the basement out my roof. When the units are moved - venting/exhaust pipes will need to be built to reach over to the same existing chimney. Again, it's a 15 foot run.

NONE of the HVAC guys nor the plumbers knew offhand what rise was needed per foot of venting. OK. I found PDF manuals for my systems online - and they don't note it either - they say to refer to local code or various national codes.

Bottom line - I think we can get about 2 feet+ of rise out of the exhaust over this 15-foot run to the existing chimney. Yet one plumber said he thought we'd need 1/2 inch per foot (that's only about 8 inches over 15 feet) - and he was concerned if it was possible, implying the pilot light might go out. WTF? Standing there, it's blatently clear what angle the exhaust would be, and it would be far more than 8 inches.

HVAC guys seem to be want a plumber to figure this out, then vice versa. One plumber just said to have the HVAC run exhaust, and he'll tap into it for the HWH. Another HVAC guy said don't let that happen - run a separate exhaust for the HWH all the way across the 15-foot run to where it meets the chimney entrance in the ceiling of my basement.

I even wrote to the country inspector who gave me cryptic references to code.

It will be to code. Code protects people. But forget code for just a second - I'm simply trying to understand from an engineering standpoint what is commonly correct here.

So - your opinions?

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Old 04-02-2011, 06:28 AM   #2
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Runaround - HVAC and HWH proper venting/exhaust


Code also requires a single wall horizontal flue pipe to be no more then 75% of the height of the chimney. So is your chimney at least 20 foot high? Or will this flue be ran in B vent? Code will only require a 1/4" per foot slope. But, for best draft, use the max slope you can get if it will exceed 1/4" per foot.

Code also requires the smaller appliance to be connected at the highest point of the common connector. Often hard to do when running a single vent that long.

Next, where will the combustion air come from for the water heater and furnace now that you are finishing the basement?

Also, is the air filter inside the furnace, or is an external filter bracket mounted on the furnace. If external, make sure it has a sealed access door/panel. If its not sealed, it could draw combusted gasses back down the chimney and put CO in your house.

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Old 04-02-2011, 08:14 AM   #3
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Runaround - HVAC and HWH proper venting/exhaust


In my experience 15 feet of horizontal venting is NOT a good idea. We try stay within 6 feet and use B vent horizontally for mid efficiency furnaces here. Any more than 6 feet and the inspectors seriously frown on it. Beenthere is right, they want the water heater tapped into the chimney above the furnace connection. Teeing it in to the side is not allowed. With the configuration you are talking about you may have pressure switch tripping problems or possible flame rollout issues if the venting is too restrictive. I would not recommend it. Just because it meets the minimum code does not mean it will work properly in the real world. You may be spending a lot of $$ on something which may not work.
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Old 04-04-2011, 12:40 PM   #4
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Runaround - HVAC and HWH proper venting/exhaust


Wow - thanks for the awesome replies. A few notes/comments:

1. Truthfully, the HVAC is about a 14 foot run and the HWH vent would be about an 11 foot run. But it will make one single 90 degree turn around a corner.

Also, the stack goes out my roof by about 5 feet, throught the attic by about 7 feet, and through 2 stories - so I'd say my stack is about 30-32 feet.

2. The manufacturer of the HWH wrote back and said 15 feet max and 1/4 rise per foot and "contain the least number of bends. That's all they said.

3. I hear what you're saying - it would seem the HWH and HVAC should be vented separately -that's the only way I can see it working like you say, where the HWH vent taps in near the basement ceiling at the entry point to the main stack above where the HVAC vent comes in. Don't see why that would be a prob - the plumber would just run his own vent.

4. I don't know what B vent is. The plumber, perhaps thinking we were still going to "tap in" to the HVAC vent nearby, said it might not work. He said I'd have to get a new HWH that could be vented with PVC and he'd run this new separate vent out the back of my house.

5. Combustion air - this has been another point of conflict. I know how you're supposed to take BTUs and do math, etc. I know how the high/low outside air vents are supposed to work. At this point, we weren't planning on doing them. This new HVAC "room" is actually an unfinished hall that's completely open to another storage room that's about 12x12 by 9 feet high. It has a door that leads to a finished space with a gap under it. We'd be walling off this HVAC "room" on the other side, but had planned to put a wall vent there. The space I'm finishing is about 400 square feet, and that's where the HVAC/HWH currently sits.

So does this still sound like a plan?
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Old 04-04-2011, 03:00 PM   #5
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Runaround - HVAC and HWH proper venting/exhaust


Not sure of your equipment size but didn't sound like near enough cubic feet for combustion air. Without enough combustion air your appliances will draw back down the flue and can cause all the proper venting your trying to do to be a moot point. Also could harm equipment and your family.
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Old 04-04-2011, 03:26 PM   #6
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50 CF of every 10,000 BTUs input. A 12X12X9 isn't enough for a water heater by itself.
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Old 04-04-2011, 05:04 PM   #7
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The storage room (50% full of junk) is actually open to a hall. This hall is the new "HVAC room" which is 10x5 by 9 high, and some understair space. It's just all open studs.

At one end there's a doorway to the finished space - but the other end is planned to be walled off. But we planned to put in louvered vents in that wall which opens to the new 400 square foot new finished space.

So it's not like an airtight closet.

But I was thinking - if I run supply air (if needed) into this HVAC space - that's outside air. Won't it make the whole storage room hot/cold? I mean, if it were an HVAC closet that was closed, I could see feeding it. But this is a pretty big open area. It stays about 65 degrees all year round with no temperature control. Now, wouldn't I be bringing freezing winter or scaling summer air into the space?

Moreso - I noted there is a door leading to the finished area - there's a 1.5 inch gap under it leading out across finished tile. I'd imagine icy cold outside air would race out if I put in suppy air from the outside.
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Old 04-04-2011, 06:05 PM   #8
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Runaround - HVAC and HWH proper venting/exhaust


Round a pipe in to a bucket, and the room won't get cold.
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Old 04-05-2011, 08:00 AM   #9
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Runaround - HVAC and HWH proper venting/exhaust


Quote:
Originally Posted by beenthere View Post
50 CF of every 10,000 BTUs input. A 12X12X9 isn't enough for a water heater by itself.
Mind if I jump in with a question? Does the above mean that if an oil burner is rated 100,000 btu, it needs 500 cubic feet of ambient air, minimum?
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Old 04-05-2011, 02:37 PM   #10
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Mind if I jump in with a question? Does the above mean that if an oil burner is rated 100,000 btu, it needs 500 cubic feet of ambient air, minimum?

Duh. that was suppose to be 1,000 BTUs of input. So a 100,000 BTU input oil burner would need 5,000 cubic foot of free air/volume.
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Old 04-05-2011, 04:25 PM   #11
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Runaround - HVAC and HWH proper venting/exhaust


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Duh. that was suppose to be 1,000 BTUs of input. So a 100,000 BTU input oil burner would need 5,000 cubic foot of free air/volume.
Wasn't being a smart-alec, just a guy who has just learned that he has 60% as much air space as required.

Is this a code requirement?
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Old 04-05-2011, 04:29 PM   #12
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The total square footage of my whole basement is about 1000 square feet.

So my "HVAC area" which is open to a storage space is about 250 square feet. I plan to put a large wall vent to one part of the finished area and another in a door on the other side to a wraparound of the same finished area. So I figure techincally the entire basement is "open" and acting as supply air.

The ceilings average 8 feet. So 1000 square feet x 8 feet high would be about 8000 cubic feet. Of course, there is furniture and stuff down there, which would subtract some of the cublic feet.

The HWH is 40,000 BTUs. The furnace is 50,000. 90,000 total BTUs.

Still - per the figures noted here - I'd think I'd be OK. 90,000 total BTUs would need just under 5,000 cubic feet. I'd have 8,000 minus the space "stuff" takes up.

I suppose my only issue/concern should be these wall/door vents. I know there is math needed there too to determine their size. But I'm thinking like 18"x18" or more for both. Plus, my HWH and HVAC also do have that 250 square foot area to breath from first. In other words, it's not like a sealed HVAC closet - everything will be fairly open.
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Old 04-05-2011, 05:44 PM   #13
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Runaround - HVAC and HWH proper venting/exhaust


It is not always so much as how much free air space you have but rather how airtight your house is. If you have real good door gaskets/weatherstripping and sealed windows and attic hatch etc etc all the free air space in the world won't help. Your combustion air is supposed to come in around cracks in doors etc. That is how those formulas were thought up 30-40 yrs ago. Modern homes need a combustion air pipe from outdoors to the furnace area. If you turn on 2 bathroom exhaust fans, kitchen fan, clothes dryer and central vacuum you exhaust LOTS of air and it needs to come in somewhere.
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Old 04-05-2011, 06:31 PM   #14
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Quote:
Originally Posted by diy888 View Post
Wasn't being a smart-alec, just a guy who has just learned that he has 60% as much air space as required.

Is this a code requirement?
Yep, its code required. Your local may or may not enforce it as strict as other areas. Weather it does or not. Its in your best interest to make sure you have proper combustion air for your furnace. It will burn cleaner for a longer period of time, and save you money on your heating bill. And help to avoid injury to yourself, your family and home.
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Old 04-05-2011, 06:35 PM   #15
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Runaround - HVAC and HWH proper venting/exhaust


Quote:
Originally Posted by denemante View Post
The total square footage of my whole basement is about 1000 square feet.

So my "HVAC area" which is open to a storage space is about 250 square feet. I plan to put a large wall vent to one part of the finished area and another in a door on the other side to a wraparound of the same finished area. So I figure techincally the entire basement is "open" and acting as supply air.

The ceilings average 8 feet. So 1000 square feet x 8 feet high would be about 8000 cubic feet. Of course, there is furniture and stuff down there, which would subtract some of the cublic feet.

The HWH is 40,000 BTUs. The furnace is 50,000. 90,000 total BTUs.

Still - per the figures noted here - I'd think I'd be OK. 90,000 total BTUs would need just under 5,000 cubic feet. I'd have 8,000 minus the space "stuff" takes up.

I suppose my only issue/concern should be these wall/door vents. I know there is math needed there too to determine their size. But I'm thinking like 18"x18" or more for both. Plus, my HWH and HVAC also do have that 250 square foot area to breath from first. In other words, it's not like a sealed HVAC closet - everything will be fairly open.
Requires 2 opening into that utility/furnace room. 1 high and 1 low. Each having a 1 sq in opening for every 1,000 BTUs of input.

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