Thread: Venting furnance & water heater

Does this vent system look correct? if could you explain. Thanks to all that reply.

No wrong,
While the oversized Tee connection was probably a nice attempt by someone its still a problem. Based on the A coil box I'm going to assume this is a newer furnace with a vent fan. The basis question then of course is what incentive is there for furnace flue to only make a left at the Tee and go up the chimney; rather than make a right and come out the water heater draft hood
Beyond that, the set up can create a draft block for the hot water tank flue if the tank is trying to vent while the furnace is running.
In situations like this its very helpful to check the flue draft at the water tank while nothing is on, then when only the tank is on, and then while both the tank and furnace are on. The usual backdrafting you can demonstrate to the client makes the problem all too clear.
Always look at the top of the tank to assess heat buildup and get pictures of heat or rust damage.

Thank so much for your advice and tips. They confirmed my concern. Thank again. Happy Easter

If, as Markus says the furnace is 'fan assisted', does it tie into a masonry chimney? If so, it might not be allowed if it's an exterior chimney. Also, the vent connector looks too big for a fan assisted unit and the water heater may have to be moved closer to the chimney.

Darren, thanks for your help. You would think Hvac guys would know better. Happy Easter to you and your family

Originally Posted by Darren Miller

If, as Markus says the furnace is 'fan assisted', does it tie into a masonry chimney? If so, it might not be allowed if it's an exterior chimney. Also, the vent connector looks too big for a fan assisted unit and the water heater may have to be moved closer to the chimney.

Thanks for the articles!

Originally Posted by fidel gonzales

Does this vent system look correct? if could you explain. Thanks to all that reply.

What is the vertical white pipe in the picture? Is it connected to the furnace?

What you are looking at is one of the most dangerous installations you can find: a fan assisted furnace common vented with a natural draft gas WH. First, the code:
The chimney or vent must be suitable for the class of service-period. If not, fix it before anything else. Reline with a listed liner properly sized and installed with all the listed components required including top support plate and rain cap with animal guard. The liner must extend into the CAZ. Next, the vent connectors:
The furnace and Wh can be manifolded by code, which is what you see here. The theory Markus puts forth is an old one but is more subjective. Often, this effect can be negated simply by the angle at which the WH enters the manifold. A wye is better than a tee as it acts as a venturi. The manifold must be sized by the code but generally the larger appliance + 50% of the smaller. Note, when using B-vent, you get some relief in sizing while reducing your clearances to combustibles from 6" to 1". You must properly support all pipes and use draft hood connectors when transitioning from B-vent to single walled pipe.

Note the elbow right off the draft hood. This should always be red flagged as its is a recipe to spill flue gases. The code sizing charts start with a 12" vent rise for good reason--draft hoods unfortunately work as designed. They spill. The flue is not connected to the appliance. So, increase your connector to 4" on a 40K input gas WH with less than a 3ft vent rise at min.

Ok, so let's say you've relined the chimney and installed all B-vent connectors with draft hood connectors with the maximum vertical vent rise before any offsets maintaining your 1/4" per ft slope well supported, etc. Done? Nope.

A furnace typically has three primary safety controls-vent pressure switch, flame rollout switch or fusible link and a plenum high limit switch. Let's say the chimney gets blocked in the case shown in the pics. Will the furnace shut off? Probably not. It will probably continue to fire venting back out the draft hood thereby defeating the three primary safety controls. Is it legal? Yup. Smart? Nope. What about the safeties on the WH? What safeties? Natural draft WHs are not required to have spill switches, vent pressure switches, flame rollout switches (unless FVIR) or high limit switches.

So, what to do? Well, you can recommend replacement with CAT IV furnace and power vented WH using polymeric venting out the side of the house and abandon the chimney and cap it off. Alternatively, there is a modification taught at NCI where you remove the WH draft hood, install a bullhead tee with double acting barometric damper with spill switches to both the furnace and WH( using a thermocouple interrupter on the WH). Anything else? Sure. Seal the ducts per code using UL 181 tape or mastic then perform combustion analysis on each appliance then do a Worst Case Depressurization Test to ensure no backdrafting. Provide MakeUp Air (MUA) as needed. Note that some versions of the code requirements for MUA can kill you. You must test to ensure it works and the CAZ is neutral and no spillage of flue gases.

Anything else? Well, aside from annual inspection by a qualified contractor, yes, install unlisted low level CO monitors on each floor level and within 15LF of each sleeping room.

HTH

Originally Posted by Bob Harper

What you are looking at is one of the most dangerous installations you can find: a fan assisted furnace common vented with a natural draft gas WH. First, the code:
The chimney or vent must be suitable for the class of service-period. If not, fix it before anything else. Reline with a listed liner properly sized and installed with all the listed components required including top support plate and rain cap with animal guard. The liner must extend into the CAZ. ....
HTH

(Was hoping to get conformation from Fidel that the furnace is high efficiency with the white pipe in the picture being the intake and the exhaust being standard stovepipe.)

To add to your recommendations I would like to throw this into the hopper. Note the first application in the picture. Power assist on the complete system. Although this may be overkill in this situation where one device is power assisted, it certainly will prevent combustion gases entering the living space.

Power Assisted Vent.gif


Gas Side Wall Power Venters : Damp Crawl Space Moisture : Chimney Fan : Tjernlund Products, Inc.

Although this for side wall venting, if this is a tight situation that does not meet the height requirements for proper unassisted venting using a chimney, this might be the solution.

Originally Posted by Rich Goeken

(Was hoping to get conformation from Fidel that the furnace is high efficiency with the white pipe in the picture being the intake and the exhaust being standard stovepipe.)

To add to your recommendations I would like to throw this into the hopper. Note the first application in the picture. Power assist on the complete system. Although this may be overkill in this situation where one device is power assisted, it certainly will prevent combustion gases entering the living space.

Power Assisted Vent.gif


Gas Side Wall Power Venters : Damp Crawl Space Moisture : Chimney Fan : Tjernlund Products, Inc.

Although this for side wall venting, if this is a tight situation that does not meet the height requirements for proper unassisted venting using a chimney, this might be the solution.

You have to be careful of certain power venters. If any portion of the venting is under positive vent pressure, the entire venting system must be listed. In general, a power venter should be mounted on the exterior to where is sucks the exhaust out. It must have a vacuum switch interlocked to the burner(s), which would prove impractical but not impossible for a typical WH. Regardless, both appliances would require a double acting barometric damper.

(Was hoping to get conformation from Fidel that the furnace is high efficiency with the white pipe in the picture being the intake and the exhaust being standard stovepipe.)

If that were the case, then the whole set-up is ?"$%^$&#! No?

Originally Posted by Darren Miller

(Was hoping to get conformation from Fidel that the furnace is high efficiency with the white pipe in the picture being the intake and the exhaust being standard stovepipe.)

If that were the case, then the whole set-up is ?"$%^$&#! No?

Well, Yah!
It looks like they used whatever they had kicking around the truck bed for a while.