combined venting

Andrew Klarmann

Greetings fellow wet heads,Iwould like to know the best options for venting 3 gas appliances. One is a power vented scorched air furnace with a 3" exhaust duct. the other appliances are 1 aquastar instantaneus water heater with a 5" exhaust outlet and lastly a conventional hot water heater with a 3" outlet. Is it possible to connect the 2 3" appliances together In other words connect a power vented furnace with a natural drafted water heater. My concern is
the power vent will backflow into the water heater.

Thanks in advance for your sugestions

Alan(CaliforniaRadiant)Forbes

Trust your instincts

You can combine the gravity flues as long as the appliances are on the same floor; the power vented appliance must be vented separately and it will probably say so in the instructions.

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Dale

Power Vent?

I wonder if your furnace if really fan - assist not a power vent. If it's a typical 80% furnace that has an inducer motor and a catagory 1 on the nameplate it is fan assist and since the vent pressure downstream of the inducer is negative it may be common vented with a standard water heater. I'm sure it cannot be vented into the same 3" vent pipe. The smallest Fan + Nat I see in the code is 78,000 max for a 4" B vent. You can build a manifold and vent all 3 together if they are all cat. 1. Standard water heaters don't have a vent catagory but are by default cat. 1. I don't know enough about the other water heater. Good luck.

Jackchips

Just an added note,

as I recall, when combining flues you take the largest and then add half of the other flues to be connected to get the size through the roof.

Unknown

Venting

What is the venting category on the furnace? If we have that we can give you a definite answer. If it is Category I fan assited then it can probably be vented with the other equipment. Give me the position in relation to the vent also. Is the common vent a chimney or is it a double wall pipe. If it is a chimney is it an inside or outside and is it lined or unlined? I need all of this inorder to give you an answer.

Chris_10

Looking for simular advice on a venting situation. Was in a basement and noticed the top of the water heater showing signs of possible spillage.
The set up is A fan assisted hot air furnace, and a conventional gas water heater, the water heater being positioned between the furnace and the chimney, A masonary chimney (on outside of house). Size of venting is 4" right to the chimney with no increase for the common connector with the water heater which ties into the 4" smoke pipe with a tee , No wye.. Maybe the chimney needs a liner and the tee connection should be changed and a right sized vent connector would help? Any input would be great, Thanks!

Jack_23

liner

the chimney absolutely needs a liner. Pre-92 gas furnaces were basically 60% eff. When eff requirements were increased to min 78% yo lost your margin for error on nat'l draft systems. NFPA54 has consistently tightened the spec for these systems and an external chimney will not draft properly without an AL liner. Just look at the cross sectional difference. 4" is about 12.5 sq in. 8x8 chimney tile is about 58 sq in. When a gas expands it cools. and a masonry chimney is the old"adobe principle", It never saw a btu it wasn't willing to absorb. Couple that with the typical oversizing and the short run times and the system doesn't have a chance to draw. You might want to put a CO detector down there and have the constomer let you know how often it goes off.