Venting question


Old 03-29-04, 07:30 AM
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Venting question

I'm getting ready to install the furnace (93% Rheem) and PowerVent water heater (GE, actually also made by Rheem). They will both be sidewall vented with PVC. I have 2 questions.

The instructions manual for the furnace (90K BTU) specify that for my distance (about 30 ft) a 3" PVC pipe needs to be used, but it needs to be converted to 2" prior to exiting the house (basically 2" will exit the house). It also states that the air intake pipe needs to be sloped up 1/4" per foot, but says nothing about the vent pipe. The HVAC contractor I use for guidance (and who's going to do the AC part of the work) says that he never decreases the vent pipes back to 2" (3" exits the house), and the 1/4" slope is excessive. I have to problem with the slope, but was wondering what the purpose of decreasing the pipe back to 2" is.

The second question is about the water heater. It will be vented via 2" PVC for about 14 ft. The manual says nothing about slope. I would assume that it needs to be sloped also. It does say something about a trap being installed next to the unit (on the vent pipe) if the distance is too long, but won't the forced air just push the water out of the trap? The other "puzzling" difference is that the furnace's vent pipe needs to extend for 12" away from the house, but the water heater's pipe needs to be basically next to the house. Why not both 12" away?

As a background, these are both LP unit, being fed off a 500 gallon LP tank via a 1/2" copper line between the tank and the house regulator, and via 3/4" black pipe inside the house (1/2" drop to each unit). According to the little table that came with the furnace manual 3/4" pipe is good for about 200KBTU (feeding a 90KBTU furnace, 40KBTU water heater and LP stove).
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Old 03-29-04, 08:06 AM
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Hello: trinitro

Excellantly worded and well described post. Thanks. Helps us to help you better...

Regarding the furnace venting and intake combustion air, contact the mfg and ask why the slop is required and why the exhaust vent needs to be 2 inch at the final end run section.

There may be a reasoning behind it. Air flow charactoristics are involved somehow do to the side venting system. May also have something to do with the climate zone in you area.

Not all instructions are the same for all areas of the country. May also contact the building and safety department in your area on the venting matters. Another view point may help to calrify the issue or cloud the issue. Worth a try maybe.

Verical vent the type of furnace out this way requires only the intake air to be angled downwards, for obvious reasons. The exhaust vent points directly upwards. Our local area does not get snow which would or could block the pipe when facing upwards.

Power vent water heaters, (using OEM included venting double walled system venting pipe and outside double venting exhaust cover, does not need any sloping.

However, regarding the above, I assume from your description, there will be two seperate pipes for the water heater? One intake & one venting?

Assuming such based on the question you asked about pipes being 12 inches from the house. You said why not both, indicate two pipes?

Venting pipes need to be away from the structure as a fire safety prevention measure. Heat generation in the pipe and heat transfer to wood surfaces, (facia boarding etc ) most likely reasoning.

Based on the gas info, should be well sufficient for the purpose.
Think I covered each item above to hlep answer your questions best as possible, based on my knowledge and experiences locally and as read and understood.
Old 03-29-04, 08:34 AM
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what the purpose of decreasing the pipe back to 2" is.
The decrease is to help the exhaust "Blow" away from the home more to help avoid any "steam" getting built up along side of your home witch may lead to siding damage. Also, 2" pipe is not much an eye sore vs a 3" pipe outside.. Have the pipe set up outside like the book shows.

pipe needs to be sloped up 1/4" per foot, but says nothing about the vent pipe
1/4" seems much too, But you need to slope for exhaust pipe to help avoid any "standing" water in the pipe. You want the water on the exhaust pipe roll back to the furnace. So just make sure you have enough slope for the water to roll back. As for the intake, there won't be any water in there unless a kid puts a garden hose in the pipe outside.

The manual says nothing about slope.
Right, Water heater exhaust is not cooler like the 90% furnace is. So, there is no water condensation in the exhuast.

As for the trap? Never heard of it.. I think it must be a typo, or misunderstanding about putting a trap in the gas line before the valve.

I"ll look in the book tonight when I go to work and see if I see what you are seeing on the GE Water heater install about 12" away from the house.
Old 03-29-04, 09:31 AM
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Thanks to both of you for the quck answers.

First the furnace:
The 1/4" slope is for the intake, not the exaust pipe. There is no mention of specific slope requirements for the exaust pipe, other then sloping upwards (there is a drain for the exaust blower comparment in the furnace). The manual also specifies that a trap be placed on the intake pipe if the pipe is long or goes through unconditioned areas. The intake pipe simply goes into the cabinet, it's not really connected to anything (this is a GRK (I think( Rheem furnace).

Now the water heater:
There is only an exaust pipe (2" PVC) for it. It makes no mention of slope, but it specifically says a trap should be installed next to the water heater to prevent condensation from dripping into the blower cage (exaust blower) if the vent distance is too long. But, unlike the furnace (which specifies the vent pipe is to extend for 12" straight away from the house (vinyl siding)), the water heater says that a 30 or 45 degree elbow pointing down needs to be used right after the pipe exits the house (which basically puts the exaust right next to the house). This is the "standard" LP power vent heater sold by Home Depot under the GE brand, although the manual says Rheem. The manual actually came with a separate sheet labeled "important", "read first", "product change notice" detailing a change in the start up procedure (ignition type changed from spark to pilot). Not only that I found it add that a new model would actually go "back" to the pilot type ignition, but when lookng at the heater I could clearly see this is a spark ignition. Kind of confusing.
Old 03-29-04, 02:29 PM
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i cannot comment on the water heater, but as for the furnace, the slope must be 1/4" per foot as stated due to the fact it is also a drain, as you are aware. never heard of the intake needing slope, only the exaust. NEVER have i reduced the vent size, although the statement of causing it to blow away from the house makes sense. if it says it in the manual, do it..if not, i would leave it 3 inch
Old 03-29-04, 11:58 PM
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I believe the 1/4" per foot for horizontal venting is pretty standard even for non-condensing gas fired appliances. For me it is required by local codes. As I understand it it is to prevent the flue gases condensing into water near the outlet particularly in winter. The slope helps the water to drain back into the heater where it is burnt off again.

Never heard of sloping the intake although it may make sense to slope it away from the appliance to prevent moisture and other things entering the appliance intake.

As for where the vents can be placed - again this is covered by the gas codes and there are some restrictions on how close it can be to windows, doors etc. Again some local code may apply.
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