Which is better? One pipe or two pipe 90 % furnaces?

[cook42]

I've notice that they are making 90% furnaces that has one pipe for indoor combustion air and the two pipe ones that require that outdoor air be brought in for the combustion process and wanted to know what's the differents?

Me personally I perfer the two pipe system I just don't trust getting the combustion air from inside because of odors and stuff inside the home that might harm the heatexchanger.

Please someone give me the pro's and con's of this setup!

Thanks

[ga-hvac-tech]

ALL gas furnaces requires a source of combustion air. Where does that air come from? It HAS to come from somewhere...

A two pipe furnace has a route for the combustion air to follow...

Next problem is: Where does the combustion air for the water heater come from?

[cook42]

That's true but which one would you perfer one pipe or two pipe?

[ga-hvac-tech]

That's true but which one would you prefer one pipe or two pipe?

Everyone has an opinion... And each of us has to follow our instincts...

IMO the answer to your question is in my previous post. Read it and think carefully.

Here is a hint... the tighter a house is, the more of an issue providing combustion air becomes. Ii have actually worked on a house that was SOOO tight; when the furnace fired the fireplace drafted INTO the den...

Now do you see the answer???

[cook42]

Everyone has an opinion... And each of us has to follow our instincts...

IMO the answer to your question is in my previous post. Read it and think carefully.

Here is a hint... the tighter a house is, the more of an issue providing combustion air becomes. Ii have actually worked on a house that was SOOO tight; when the furnace fired the fireplace drafted INTO the den...

Now do you see the answer??? I see now if the house was real tight I'll use the 2 pipe compar to the one pipe.

[beenthere]

2 pipe.

Even if a house isn't real tight.

[ampulman]

As a home owner, I found that my 2-pipe system has virtually eliminated static shocks due to greatly reduced infiltration (no humidifier installed).

Amp

[clarkie]

two pipe is the way to go. Only the lazy and cheap contractors do the one pipe settup, trying to save a little money,please!

[bmathews]

I've notice that they are making 90% furnaces that has one pipe for indoor combustion air and the two pipe ones that require that outdoor air be brought in for the combustion process and wanted to know what's the differents?

Me personally I perfer the two pipe system I just don't trust getting the combustion air from inside because of odors and stuff inside the home that might harm the heatexchanger.

Please someone give me the pro's and con's of this setup!

Thanks

I hope you're not pulling combustion air from inside the house. If so you have to worry about contaminants rusting out the heat exchanger. In my area, that isn't allowed. But we always pull it from the outside via direct venting, combustion air pipes bringing fresh air from the attic into the furnace closet or if its in the attic, then from around the furnace.

[ubeill]

always two -pipe

[yourairman]

two pipe is the way to go. Only the lazy and cheap contractors do the one pipe settup, trying to save a little money,please!


There are furnaces designed specifically as a one pipe setup. To offer a less expensive option for a customer does not constitute cheap. It also does not constitute lazy. You could have a top of the line furnace put in that was a lazy installation.

[yourairman]

I've notice that they are making 90% furnaces that has one pipe for indoor combustion air and the two pipe ones that require that outdoor air be brought in for the combustion process and wanted to know what's the differents?

Me personally I perfer the two pipe system I just don't trust getting the combustion air from inside because of odors and stuff inside the home that might harm the heatexchanger.

Please someone give me the pro's and con's of this setup!

Thanks

Usually speaking the one pipe setup from manufacturer's are their base model 92% efficient furnace. When a 95% single or variable furnace is installed it is recommended to use a 2-pipe setup. This is to control the combustion process to maintain the 20% oxygen level in air. It is also to maintain the proper amount of draft needed for ventilation. Adequate combustion air is required by code. Most municipalities do not recognize or enforce this code properly. Very few cities require the proper application of combustion in which you have to bring in a specific amount of air from outside of the structure and terminate that air at both 12" from the ceiling and 12" from the floor. If you do not do this you will be pulling air from somewhere else in the structure. Therefore you would be pulling in outside cold air. Sealed combustion with a 2 pipe setup will not create this situation. So the efficiency question you are asking goes beyond the efficiency ratio of the furnace. You have to consider heat loss in the equation due to combustion air.

[yourairman]

As a home owner, I found that my 2-pipe system has virtually eliminated static shocks due to greatly reduced infiltration (no humidifier installed).

Amp

This is very true. This is also why you do not have as much humidity issues with a heat pump system. I have heard many times from various people in this industry that furnaces dry out the air more because they run hotter. I think it is more directly related to the amount of infiltration due to combustion air. You do not get rid of humidity from simply heating the air. Just look at Florida.

[ga-hvac-tech]

I see now if the house was real tight I'll use the 2 pipe compar to the one pipe.

Sorry about being so pushy, but I was trying to get you to think about the details. Most of us guys that have lots of answers... we have done a LOT of thinking over many years...

[qwerty hvac]

I always install two pipes. Make sure though that they are high enough for the snow load in your area. Last year we had a subdivision where all the pipes were to low and had fifty no heat calls!:oops:

[mo-flo]

2 pipe is best....

[cook42]

Sorry about being so pushy, but I was trying to get you to think about the details. Most of us guys that have lots of answers... we have done a LOT of thinking over many years... That's ok I like when someone makes me think a little bit and not just give me the answer all te time! That's why I love this site because of people like yourself is willing to help and not make fun of people who might not know!

So again I say thanks!!!

[dilla]

How did this thread get drudged up from the dank and dark basement?

[molabear]

As long as there is cubustion air to the furnace, one pipe will work unless the mfg. wants two.

[sirjames]

I can`t remember the last time I put in a one pipe furnace.

[mo-flo]

usually it is the difference maker from 20 year to lifetime warranty on heat exchangers also.....

[hvacvegas]

goodman likes to see 2 pipe, but you can run single.

I saw in a previous post someone said that a single pipe install is from lazy/cheap contractors. Why would you say that? We always run 2 pipe, but what if it isn't possible? What if your going up a chimney (only thing the customer will accept), and there is only room for one pipe, and the old 80% had a 6" C/A? Sounds like your running a single pipe up the chimney.

[beenthere]

goodman likes to see 2 pipe, but you can run single.

I saw in a previous post someone said that a single pipe install is from lazy/cheap contractors. Why would you say that? We always run 2 pipe, but what if it isn't possible? What if your going up a chimney (only thing the customer will accept), and there is only room for one pipe, and the old 80% had a 6" C/A? Sounds like your running a single pipe up the chimney.

Why would the customer only accept it going up the chimney. When he already has a 6" CA intake through his wall?

[toolman5999]

I agree that the two pipe systems are the way to go. At least you know what's in the air you are using and how much is available to the combustion process. We had a severe snow storm in Wi. when I worked for a Lennox dealer. I couldn't get out to anywhere but received arond twenty calls from customers who were without heat. Intake pipes were plugged with snow. 90 deg. ells sure helped. Keep them above the snowline, although in this instance even the roof-top pipes were packed.

[ga-hvac-tech]

(only thing the customer will accept)

I have seen a townhouse where the association was sooo strict the only available approach is through the roof, but it cannot show from the front or back... we figured out how to do that also...

Most times if one explains the benefits carefully, the customer will understand. Using a fireplace as an example is a good way to explain the details.

[whizkidtn]

Quick Question: Can a 92% AFUE gas furnance be put in the attic with unconditioned space? I currently have a 81% unit I am replacing and was initially told that given the units location, that was about the best I could do. If a 92% unit can go in the unconditioned attic space, can't you just pull air in there and not have to have a 2nd pipe (intake air pipe)? My current exhaust pipe is a simple affair and goes out the roof.

[ga-hvac-tech]

Quick Question: Can a 92% AFUE gas furnance be put in the attic with unconditioned space? I currently have a 81% unit I am replacing and was initially told that given the units location, that was about the best I could do. If a 92% unit can go in the unconditioned attic space, can't you just pull air in there and not have to have a 2nd pipe (intake air pipe)? My current exhaust pipe is a simple affair and goes out the roof.

High eff furnaces (90+%) are condensing furnaces, and have a water drain. If the drain freezes, the furnace will either quit, or leak... Some folks insulate it... I would be VERY cautious in this area.

[threadcutter]

Hi cook42

Two pipe is the way to go. It increases comfort in the living area by not using conditioned air for combustion and reduces a whole host of draft issues related to fireplaces and other fuel burning appliances in the building.

Stay Safe

[deux]

I go one pipe on rentals. Less chance of freezing the house when it is unrented and not checked after snow storms.

I go two pipe on the rest with the one piece concentric for owners. The one piece just came out here, I have not heard of one freezing ever. I might go two pipe on rentals this year as the concentric works out so well. I am getting lazy and like the one bigger hole!

[cook42]

Hi cook42

Two pipe is the way to go. It increases comfort in the living area by not using conditioned air for combustion and reduces a whole host of draft issues related to fireplaces and other fuel burning appliances in the building.

Stay SafeThanks form your comment!! I was thinking about Two pipe systems myself I will only use one pipe if I really need to!

[mo-flo]

Hi cook42

Two pipe is the way to go. It increases comfort in the living area by not using conditioned air for combustion and reduces a whole host of draft issues related to fireplaces and other fuel burning appliances in the building.

Stay SafeThis post hits the nail on the head....

[Big]

2 pipe

[praha99]

Wow how did 80%`s ever work without that extra pipe!?!?!? lol

Wonder how much exhaust gets sucked back in with those excentrics or even with two holes and the wind blowing the right way.. Those nuisance calls for frozen intakes are a waste of time too.Even if above the snow line when it gets cold here in Chicago they get freeze up if the wind the blowing the right way.

[hvaclover]

2 pipe.

Even if a house isn't real tight.

wHAT HE SAID. Two pipe trumps on pipe any day.

[hvaclover]

There are furnaces designed specifically as a one pipe setup. To offer a less expensive option for a customer does not constitute cheap. It also does not constitute lazy. You could have a top of the line furnace put in that was a lazy installation.

Tell me one mfg that has a dedicated one pipe set up.

They all have some models that will let you use one pipe but in some cases will not give you the full warranty. Armstrong for one.

[hvaclover]

Wow how did 80%`s ever work without that extra pipe!?!?!? lol

Wonder how much exhaust gets sucked back in with those excentrics or even with two holes and the wind blowing the right way.. Those nuisance calls for frozen intakes are a waste of time too.Even if above the snow line when it gets cold here in Chicago they get freeze up if the wind the blowing the right way.

Installed correctly they won't recirculate exhaust. I alwys go a litttle more than the vent table call for at terminations just to be sure.

[beenthere]

Wow how did 80%`s ever work without that extra pipe!?!?!? lol

By sucking lots of dry fresh air into the house, and making the customers house dry out.

Wonder how much exhaust gets sucked back in with those excentrics or even with two holes and the wind blowing the right way.. Those nuisance calls for frozen intakes are a waste of time too.Even if above the snow line when it gets cold here in Chicago they get freeze up if the wind the blowing the right way.

Not many of them have a freezing problem. If you don't put the screen in.

[sammy37]

Wow how did 80%`s ever work without that extra pipe!?!?!? lol

Wonder how much exhaust gets sucked back in with those excentrics or even with two holes and the wind blowing the right way.. Those nuisance calls for frozen intakes are a waste of time too.Even if above the snow line when it gets cold here in Chicago they get freeze up if the wind the blowing the right way.

Depends on where you live. Most of the places I have lived, required the furnace to be in a closet with an outside air intake for combustion, if the furnace was installed in a living space.
I was surprised the first time I went to Utah and saw a furnace sitting in the basement using indoor air for combustion.

[ga-hvac-tech]

Wow how did 80%`s ever work without that extra pipe!?!?!? lol



You will laugh at this one: My company installed a basic system (80% furnace, 13 SEER air) at a lady's home who is a piano teacher. She called me back and complained about the noise the flame made when it lit off, said it sounded like a rocket taking off.

I went out to her house and looked at it... yes it was noisy. The furnace and WH are in a closet with louvered doors, in the middle of the main floor (2 level townhome). On Rheem/Ruud 80% furnaces, one can use a max of 10 ft of PVC and a max of 2 wide sweep elbows to duct combustion air to the furnace... however one is not allowed to use outside air.

On this house, I ducted crawl space air to the furnace, total of 6.5 ft and 2 wide sweep elbows... it hushed the roar, customer happy.

Note: Always check delta temp rise to be sure the furnace is not over-firing.

Sure looks funny to see an 80% furnace with a PVC intake and a 'B' vent flue... Should have taken a picture... and placed it in the 'wall of pride' thread...