--108.176.xxx.xxx

Can you vent it outside and use PVC? --24.101.xxx.xxx

If you're getting condensation, there's another problem you need to solve. Your flue may be partially blocked or you may need a new chimney liner. Are you using a B vent pipe? --204.210.xxx.xxx

It is an old house, no liner in the chimney. I know it is suppose to have an aluminum liner in the chimney. I'm not using B-vent, is it appropriate. My vent pipes are 4 inches, and 4 furnace feed into it, becomes 6 inches before entering chimney. I looked in the chimney yesterday and it is clear. What is B-vent for? I have never used that. I thought it was just for oil appliances, or ones that burn real hot, and I assume it is more expensive., but may be more rust proof? --108.176.xxx.xxx

The flue gas is hot.. not sure why you have condensation before it enters chimney.. too efficient? --66.186.xxx.xxx

John,don't know too much about gas furnaces I have all electric..but Lowes have dual lined stainless steel pipe in various sizes.you may be able to get a size to fit.I'm wondering about the moisture and where is it coming from.Even with moisture,it shouldn't have much effect on the stainless.I used an eight foot section for a pellett stove....charlie.................................... ............. ...... --174.199.x.xx

I think your vent might be oversized, which could be why you're getting rust on it... let's see what the HVAC brains in the room say... is your vent cap on and in tact? --173.25.xx.xx

The 6" section is slowing the velocity of the gases, it may help to make it all 4". I posted a response that is held up by the moderator. But google...condensation inside flue pipe and look at the SFgate article. --68.60.xxx.xx

4 feeding into 1 so lets do math - 1 4inch is 12.6 sq in (this is the easiest math cause pie times the radius squared is simply 4 times pie), now you have 4 of these 12.6 delivering flue gas so that is 50 sq in, this means you need 8 inch flue, however first you may try adding a metal divertor inside the sheet metal wye so the flowing gas does not go backwards into the adjoining equipment outlet, to get the correct chimney liner you need to know the rise, this plagues the 80 percent equipment installs so the liner has become the defacto problem solver even though you will continue to condense at least you will not have mortar decay leading to carbon monoxide going into the rooms next to the chimney, seams on the bottom will exhibit the drainage of condensing moisture. --76.188.xxx.xx

wait you mean 4 furnaces not 4"? --68.60.xxx.xx

Four 80% furnaces venting into 4" pipe, and last 6 feet before chimney is 6". Usually this is sufficient in my other rentals. Maybe it is a draft issue, and like one of you mentioned, some of the condensation is flowing backward to a furnace that is off now cause unit is empty. There has been loose bricks and debris in their before. Most chimneys in this side of town, no flu caps, no liners, and no crowns. --108.176.xxx.xxx

The flue size is based on the BTUs of each boiler / water heater, the distance it has to travel, the number of elbows & the over all height of the chimney. Your plumber or boiler supplier should be able to size them for you. --72.93.xxx.xxx

CALL THE GUY !!!!!!! - dont fool around with gas or C02 - "DANGER DANGER WILL SMITH" that is a saying from "lost in space" for those older folks in the audience. This is simply an area where a landlord should NEVER be cheap. --24.34.xx.xxx

Ill look into sizing, but im pretty sure tjey are sized correctly according to instructions. They are 60,000 btu output each. Water heaters have separate flus. They have inducer fan motors to assist blowing the exhaust thru vent pipe. It was once explained that it is good to keep the vent gases hot so they rise, so the vent pipe is kept small. With a larger pipe, the gases cool too much and all the moisture would consense before reaching the flu. Ill look into b vent, and sizing again, and that flu is clear.and dradting properly. --108.176.xxx.xxx

I am SHOCKED that the Republic of NY would allow anyone without a gas license to touch a Gas system. Please check your laws 1st --24.34.xx.xxx

--8.38.xxx.xxx

I have alot of experience working as a plumber, and ive taken heating training in the past so im comfortable with gas. The only license is in the big cities for heating, or running gas you have to be a plumber. The condensation is normal, the furnace does that, ny extracting moisture from the heated air, and sending it up the chimney. Ideally you want it to stay hot until it exits the chimney, so it doesnt start to condense inside flu pipe or chimney. Using bigger vent pipe would allow it to cool faster and condense sooner. It is an abnormally long run in this circumstnace. That probably contributes to the problem. When all 4 furnaces are going, the pipe is very.hot, and i think i have less condensation inaide the house, more in chimney, or outside. Often waterheaters are vented with furnaces cause.they keep the flu hot. If you only have one furnace going, i have two right now, it isnt as hot as it could be probably why it's condensing too soon. I saw aluminum vent pipe today, which wouldnt rust, but i think its not meant for furnace, just for a dryer. Only the chimney liner can be aluminum. --108.176.xxx.xxx

I solved the mystery of dripping condensation. The draft up the chimney ensures the exhaust is vented outside properly. Any opening in the pipe may allow it to leak, and cool prematurely. A aluminum chimney liner is recommend, or the chimney may suffer damage from the condensation. When I fixed a drafting issue, and replaced some rusted vent pipe, the pipe stopped leaking, or flowing back to unused furnaces. --108.176.xxx.xxx