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Old 11-19-2012, 08:59 AM   #1
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venting water heater and furnace together?


We just bought a house, the inspector didn't say anything except we should get an outside fan for that room. The water heater and the furnace are vented together up to the roof. We have just been leaving the window open, but it is now winter, and it is pretty cold in that room, and going to just get colder. So, is this safe for them to be vented together? Can I close the window?
Water heater is ---GE GG40T06AVG00 BTU is 36,000 Manufactured in 07/2004
Furnace is a Lennox G50UH (X) series. This also has a whole house humidifier on it, but not sure if it works, or what to do with it as we have never had one before.
So, is this safe? can I shut the window without too much worry? Thanks!

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Old 11-19-2012, 12:14 PM   #2
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Why did he say you need an exhaust fan? Was the water heater spilling fumes into the house? If so that is INCREDIBLY dangerous and you can get CO poisoning!! Post some pics of the venting and yes they can be vented toegether but there is special procedures to be followed. Also post where you live as those procedures can vary from one area to another.

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Old 11-19-2012, 12:41 PM   #3
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venting water heater and furnace together?


I am unsure of why he said we needed a fan. We did the raedon test, and it was a little high, so we have the raedon kit installed, it tells vents the air under the house or something up to the roof. I will take a pic right now.




Thanks!

Last edited by mfam99; 11-19-2012 at 12:51 PM. Reason: adding pics
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Old 11-19-2012, 01:09 PM   #4
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venting water heater and furnace together?


My guess is the venting looks OK. But I would wonder if you have sufficient volume in the room (does the door have a grate?) to safely supply combustion air to both, or even one of the units.
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Old 11-19-2012, 01:54 PM   #5
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venting water heater and furnace together?


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Originally Posted by raylo32 View Post
My guess is the venting looks OK. But I would wonder if you have sufficient volume in the room (does the door have a grate?) to safely supply combustion air to both, or even one of the units.
The room is actually a huge bathroom. I was just close to these to get a pic. It is an unfinished room with a toilet, and a sink, and those. It is probably at least 10x10. The door is an outside door with no ventilation as that door leads to a bedroom. We are in UT.
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Old 11-19-2012, 02:07 PM   #6
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There are tables and such for working out room volume for combustion air based on total appliance BTUH/CFH. My house that had a 40000 BTUH water heater and 80000 BTUH furnace (both vented together with type B galvanized like yours) in a 10x10 room that had a grated door to communicate with a much larger room for drawing in combustion air. Note I didn't do the calculations and design, that was just how my house was built. You might do some quick research on that and/or maybe one of the pros here can advise.


Edit: I think the rule of thumb is 50 cu ft volume for every 1000 BTUH so if you have a 100,000 BTUH furnace and a 40,000 BTUH water heater you would need ~ 7000 cu ft room.... or have louvers opening up to a larger inside room or from the outside like you are apparently doing through the open window.
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Old 11-19-2012, 03:10 PM   #7
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Where would I find how to do that? I only know the BTU on the water heater. I didn't see one on the furnace.
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Old 11-19-2012, 04:05 PM   #8
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venting water heater and furnace together?


Why is that door even there? It's useless now with the vent and the furnis in the way.
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Old 11-19-2012, 04:54 PM   #9
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venting water heater and furnace together?


The door opens into the bedroom and you need it to access the front of the furnace for service.

Remove the front door of the furnace and get the model # for us. Usually a G50-36B-070 or something. The number after the 36B will be the BTUs. Lennox usues 70,000, 90,000, 110,000 on those units.

Venting looks OK but definetly you need some combustion air into that room and I am not sure what US or your local codes are. I would call your local gas co and ask to have an inspector come down and give you the proper info. That way you are covered legally too for your ins company etc.
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Old 11-19-2012, 06:34 PM   #10
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Combustion air is a big deal. Not enough and the carbon monoxide levels rise in the flue gas and it can cause the water heater to back draft. Can't take it from a bedroom so it will have to come from outside.
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Old 11-21-2012, 10:07 AM   #11
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venting water heater and furnace together?


Okay, so the furnace has 110,000 BTU, and the water heater is 36,000 BTU. How large does the room need to be so we can close the window without worry about combustible air? Also, this is attached to a bathroom with just a toilet and a sink in it. The door is almost always open on that side.
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Old 11-21-2012, 10:27 AM   #12
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venting water heater and furnace together?


Your situation is pretty close to the example I provided above. The NFGC says you need additional combustion air when the volume of the confined space is less than 50 cu. ft. per 1,000 Btuh. input of all appliances installed in area. You have: 110 + 36=146, that then requires 146x50=7300 cu ft. if it is to be a confined space.

So for a confined space, assuming an 8 foot ceiling you'd need a room a little bigger than about 30 feet by 30 feet. (30x30x8= 7200)

If you don't have that volume (and you clealry do not) there are formulas for determining how much air you need to supply (how big of openings, etc).

But as Yuri said, though you need to check you local code.

Here is something to read up on:

http://contractingbusiness.com/feature/cb_imp_5734/
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Old 11-21-2012, 10:45 AM   #13
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venting water heater and furnace together?


What I found online is that if I put grates in the wall on the other side of the furnace, in the hallway, that the rest of the house is open, so it should be fine. At least that is what I am understanding from here:
http://www.houseofcraig.net/combustion_air_calc.html
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Old 11-21-2012, 11:26 AM   #14
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venting water heater and furnace together?


The surface area of the louvers and the sizes of the connected rooms both matter. Without a lot of research I am not able to tell you what is adequate and what is not. Maybe one of the pros here can....

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