ReplyAlot or most units today are either 1 or 2 pipe systems,
can u dig it ?
Regular Guest
Just realized the PVC pipe that I "thought" was combustion air intake for my new 90% furnace is actually the flu.
The combustion air is actually coming from a PVC 90 on the side of the furnace. Is that cool?
Flyboy
Professional Member
Alot or most units today are either 1 or 2 pipe systems,
can u dig it ?
If you try to fail, and succeed.
Which have you done ?
Regular Guest
It's only cool if you have sufficient combustion air in your mechanical room, or a way to let air into the area that communicates with the rest of the house. The manufacturers installation instructions will tell if there is any reason not to take combustion air from inside the house.
Grumpy Old Man
Only if you have adequite combustion air to the mechanical room and the system is either a 'single pipe' or 'dual' certified unit.
**'dual'...equals 'either' one or two pipe system**
If you post the manufacture and model number, we can tell you whether the unit is ok. But, as far as the combustion air goes, we can't see it from here!
Professional Member
I haven't put one in in over ten years that I didn't pipe in the combustion air.
Don't even ask why I started to do that.
It makes an immense difference in the unit operation adn the fuel efficiency, also, IF the combustion air is piped properly. This means EXACTLY like the manufacturer of the equipment outlines in the IOM.
Banned
In general the intake and the flue should be of near equal length, it has to do with the performance of the draft through the furnace.
Professional Member*
I always direct vent 90+ furnaces preferrably with a concentric termination. Why pull air from inside the home for combustion? Where does the air come from that replaces that air? From outside through leaks in the structure. Why turn on the furnace and suck in cold air from outside around the windows, doors, electrical outlets, plumbing pipes, etc., when the air could be piped in through a dedicated intake pipe without subjecting the house to negative pressures?
There is nothing more frightful than ignorance in action....Mark Twain
Regular Guest
Gees, not a lot of consensus here.
The unit in question is a Rheem 90 plus (RGRA model).
The manual acutally says that the combustion air source is "optional" but doesn't give a preference or make a case for either.
The dealer told me he would pipe it in if I wanted. I told him to do it. Make's sense to me.
Flyboy
Professional Member
Well you are asking a large audience. Heres what I will tell you. It depends on where the furnace is located. If its in a conditioned basement or indside the conditioned space somewhere then use two pipes regardless of room size. I mean why would you pay good money to condition air just to throw it out?
If its in the crawl space or attic its no big deal but as eddy pointed out, the pressure switch is a differential pressure switch so the resistance on both sides should be equal. I would If I were you.
first thing you should consider confined space. rule is 50 cubic ft per 1000btu. if you are not sure pipe out the intage to outside. that would solve all your problem.
Professional Member
take it from someone that installs it will work both ways but I run two out because they say it will save the home owner more money on the bills I don't KnoW I hear all kinds of sh!T.
Professional Member
The potential is there. Figure out how many cfm the inducer pushes and understand that much is coming from somewhere, eventually through the envelope of the home.
Is it rated different? No, but its pretty plain to see it is more efficient to run two.
Professional Member*
HMMMMMMMMMMM.......got a call back today on 3 month old 90 unit that is in basement with one pipe system.....the pressure switch was bad,i had a spare to get them by till i get another one next week.....but you can here the switch make but no continuity on switch......i wander if this may have something to do with the switch failing.....i rarely have seen them fail....temp. switch i put on actually closes at higher point than original??????????Originally posted by docholiday
Well you are asking a large audience. Heres what I will tell you. It depends on where the furnace is located. If its in a conditioned basement or indside the conditioned space somewhere then use two pipes regardless of room size. I mean why would you pay good money to condition air just to throw it out?
If its in the crawl space or attic its no big deal but as eddy pointed out, the pressure switch is a differential pressure switch so the resistance on both sides should be equal. I would If I were you.
Professional Member
Depending on the manufacturer of the furnae, it does make a big difference. Most manu. are looking for the resistance of the intake air pipe to match the flue side. If you look at carrier for ex. they state take the longer length of the 2 to determine flue size.
Also the biggest reason is for termination. in a two pipe system it is stated that you can be 1 foot away from a window or door. A one pipe system it HAS to be 4 feet away from window or door. this is to prevent funace pulling a vacuum on a mech room with insufficient combustion air. If a door or window were to be opened in the mech room it would then pull in flue gasses.
mo-flo.talking about press. switch. have you checked the exhaust for blockage or tubing for cracks? you are right most time switch is ok unless water got in. Ted.
Professional Member*
took pvc off motor and it still wouldn't make.....hoses are ok...hooked meter to old switch and no continuity thru switch.....you can hear the switch close,but it won't let current thru...
Professional Member*
Your unit was installed as per one of the methods allowed by Rheem, so it is proper.Originally posted by flyboy917
Just realized the PVC pipe that I "thought" was combustion air intake for my new 90% furnace is actually the flu.
The combustion air is actually coming from a PVC 90 on the side of the furnace. Is that cool?
Flyboy
Is is "cool"? No. For all of the reasons mentioned, it is not "cool". You are using contaminated, conditioned air to burn which causes a negative pressure in your home which causes unconditioned air (draft) to have to be drawn back into the home. Contaminated air in the home also causes issues with the furnace burners, ignitors, flame sensors and heat exchangers. So, no, it is not "cool".
As long as no combustion air piping is used there is no need for the vent and combustion air piping to be the same length. The most important factor when having the two pipe system is that the termination of the vent and the combustion air be within 12-14" of each other with the vent slightly higher then the combustion air. A better method is to use a concentric termination which puts the vent and combustion air termination in the proper pressure zone in relationship to each other.
Training is important!
Practical Training is a must!
Professional Member
If its in a big enough space with plenty of combustion air then one pipe is fine.
But I have that same scenario and the sensor on my 7 year old Armstrong furnace needs replacing every 2-3 years cause it pulls in contaminated air. Who knows what it is doing to the heat exchanger. There are surface rust spots at the heat exchanger near the in shot burners.
The other problem is that if I used 2 pipes the extremely cold air when it is below zero out would cause condensation and water at the burner area.
So I guess it depends on where you live and how cold it gets.
If you're installing in a extremely cold climate with sufficient combustion air then I would rather replace sensors every 2-3 years than replace burners and explain to the customer why all the water on the furnace.
If its moderate climate then I would go with 2 pipe every time.
Professional Member*
The concentric termination kits greatly help prevent that condensation issue by warming up the combustion air.
Those contaminants are doing a lot more harm then tearing up the pressure switches. Contaminants in combustion air are real killers of condensing furnaces.
The fact that there are so many things that are harmful contaminants is the real issue. Moisture is a harmful contaminant when it hits superheated metal surfaces. Anything that has any odor to it that is not from nature is a chemical contaminant that turns into really nasty oxides when exposed to the extreme heat of HSIs, burners, flame sensors and heat exchangers.
The colder it is outside, the less moisture is in a cubic inch of air making it more desirable to burn. There are also no common household chemicals hanging around outside to destroy the furnace. Major contributors of hazardous chemicals in the home are;
* any arosol
* scented candles
* laundry detergent
* fabric softeners (the worst)
* plug in smelly thingies (2nd worst)
* kitty litter boxes
* humidifiers
* deoderants
* perfumes, after shave lotions and cologne
* any product that has a chemically produced scent
If you can smell it, there is a chemical in the air.
Not to mention what this crap is doing to your bodies.
Training is important!
Practical Training is a must!
Professional Member
does it not lower the efficiency rating on the furnace by not using OA for combustion air?
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