Thread: High efficiency furnace

Hey....Most of the High E furnaces I've seen have two pvc exhaust pipes going to the outside.... this one only has the one... there's a smaller one coming out and venting inside... is this kosher..b

I've added another angle.... you can see the bigger pipe is exiting out of the house... the little one is not..b

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It can be. It's better if the system is using direct vent to the outside, to balance the combustion air. If there are enough combustion vents into the room for those two water heaters plus the furnace (and any other gas appliances), and the room is sealed from the rest of the house, it may be okay. The problem occurs when the intake starts back drafting the water heaters.

Most likely no. But it depends. I cannot see the entirety of the piping, from this singular angle I have concerns but cannot see what I need to see to be sure, and your description leaves me wanting. I see two draft hooded water heaters as well, I see several things which are concerning.

Could create a negative pressure situation for the WHs (back drafting) and combustion air calcs would be necessary. We do not know much if anything about the furnace or this assumed mechanical room. (example an air exchanger for the room?, other applicances present besides the WHs we see, size of the room, rating of the appliances, etc.).

Most Cat IV furnaces have sealed combustion chambers and require outside air for combustion, the intake being on the same pressure plane as the exhaust termination for them to operate correctly (and to protect the combustion chamber from corossive chemicals intaking from the home.

Ultimately calculations would be necessary in conjunction with a review of the manufacturer's instructions (not only for venting and combustion air but for the site location installation) to be definate regarding specifically and only the furnace.

I believe the word you are looking for at your close is concentric venting. You stated in the middle of your post that you usually see two exhaust vents terminating outdoors - that's not quite right. You've likely seen a combustion air intake (open to the outdoors) which terminates at the appliance and an exhaust vent terminating at the same plane outdoors.

In general,

High-efficiency Category IV furnaces are vented with Schedule 40 or PVC plastic pipe. This pipe must be joined securely together since the combustion gases are uner a positive pressure and would leak into the living area. They can be vented to the outside by going through the foundation, the sidewall, or through the roof.

Please keep in mind that you must follow the manufacturer's installation instructions.

These instructions will state the proper size of the plastic pipe to use, the allowable lengths, and other information regarding the venting of the unit. Most manufacturers require that the exhaust and intake pipes be installed on the same pressure plane. This means both on the same side of the building, or both through the roof.

Always reference the manufacturer's installation instructions when installing high efficient units regarding site location.

An unsealed combustion unit (one that has the combustion area open and uses indoor air for combustion) cannot be installed in areas where bleach, or other household chemicals are stored or used according to some manufacturers.

These chemicals can get into the heat exchanger by being pulled into the unit with the indoor combustion air. Corrosion of the metal will then occur, thus shortening the life of the unit. The majority of the mid-efficiency units will be the unsealed combustion type.

A sealed combustion unit uses OUTSIDE air for combustion and therefore can be located in many areas.

The majority of high-efficiency units will be sealed combustion However, there are some high-efficiency units that are not sealed combustion units. These have to be evaluated for proper installation locations.

A sealed combustion unit should be installed in homes with tight construction, since the possiblity of the appliance backdrafting and causing a safety hazard to the occupants is reduced. Also, a sealed combustion appliance would eliminate an exhaust device that may cause indoor air quality problems. This gets into Building Tightness Limits (BTL) and determining how tight should a home be, based on the exhausting devices within the home and pressures, in otherwords Depressurization Tightness Limits (DTL).

A sealed combustion appliance should be installed when the volume of the combustion appliance zone does not allow the proper amount of combustion air for the BTUs within the zone.

The Confined/Unconfined Space definition located within NFPA 54 (National Fuel Gas Code) determines the amount of BTUs that the volume of the zone can support with combustion air. NFPA 54 is the standard that is most often referenced (sometimes by its AGA or ANSI reference number)by gas fired appliance manufacturers.

Hope that helps. Without more information from you couldn't be more specific.

The only real concern here is whether the basement in question has the size to provide combustion air to all appliances,,1 cubic ft of open space is required for every 1000 btu of combustion

David.... the basement is enormous.... so probably ok... I was just concerned about the small intake as opposed to the bigger exhaust... I thought they should be around the same size... b

the power vent on the furnace is 2",,it should be increased to 3" if the run is longer than a combined length of 35 ' 1) 90 degree elbow has the equivilent length of 15 ' of straight pipe.. The reducer is placed correctly as to allow condensate back to unit.

Looks like they end up the same size.

Iam wondering about that hanging CSST jungle in back?

Is that for the kids (damn)
I hope you wrote that up.

Originally Posted by David Bell

The only real concern here is whether the basement in question has the size to provide combustion air to all appliances,,1 cubic ft of open space is required for every 1000 btu of combustion

think David ment to say 50 cu ft per 1,000 btu of input would be required (not 1 cu ft per 1,000) to be consdered unconfined space which would allow enough space to use combustion air fron the space. (IMC sec 202)

I agree on the csst installation. The ball valve connecting the gas flex connectors should have been clamped to some framing. (CSST installation booklet)

I don't like it.

the distance and elevation between intake for furnace and the extremely questionable venting for the WH closest to the furnace combustion intake is a recipe for incomplete combustion for the furance and exhaust spillage into the basement from the WH.

The short connector two elbows and extremely LONG horizontal run with high ceiling basement is just bad. Sizing seems questionable for draft hood vent connector too.

Proximity of csst which is hanging and unsupported drops which is wire tied to the TPRV discharge, multiple unsupported drops and no wet trap/sediment traps for the NG supplies for furnace or the two WHs. Its not like you don't get lightening strikes out there ... enough said on that.

With such large, expansive and high-ceiling basement real estate in Boulder Co., expect the area would be finished at some point in time soon if not already. There should be high and low combustion air for the water heaters in the deep basement according to the local ICC chapter handouts. The HE furnace should be supplied with outside air via sealed PVC from the same plane as the exhaust outlet.

At least a mile high or so - likely higher - prevailing winds, pressure changes, the weather you have out there with front pressure changes quickly, etc. and this installation now understood to be in basement and the second picture you've shared on the original post, it should have been done right. I'd be suspicious permits weren't pulled or finalized on the replaced WHs and HVAC.

Too much in this mechanical room smells DIY without benefit of a level, ac condensate running trap, the csst & the clear tube drain line zip tied to the further out WH, unsupported valves ccst, clearances to the WH vents not maintained (combustibles).

I'd call it all out.

I don't know of a carrier branded HE that allows for taking combustion air from inside a basement next to Cat I appliances.

Whats the rating for this furnace - seems 2" would be small for such a large space being heated.

sorry for the mis-information on combustion air 50 cubic feet per 1000 btu is the standard calc when infiltration rate is unknown

Hey guys thanks for the insite.... out here in Colorado it seems CSST is a big problem.... I don't think anyone out here know's it's supposed to be stable... they usually just hang it from the joists and call it good.... I'm going to call out the furnace... have an HVAC have a look... it's a short sale so may or may not happen... thanks and MerryXmas ya'll b

I know when I read the installation instructions on the gas flex pipe-One thing stands out and that is the pipe is not to be supported in a concealed area,wall cavity, and the support requirements are not the norm. as compared to water lines etc...