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NeedHomeRepair 12-20-2010 12:36 PM

Exterior Furnace exhaust vent pipe
Hello DIY chatroom!

This is my first post and I have some questions about vent pipe for a ~80K BTU 80% efficiency furnace on the 1st floor of a 3 floor unit. Currently the vent pipe exit is in violation of code as it is just piped out the side of the house (previous owner did this job). I have the job contracted out which involves replacing the current 4" gas vent pipe and extending the pipe along the outside of house above the roof to meet code. There is already a interior vent pipe on the 2nd floor currently supplied with a water heater and 90K BTU natural gas furnace that is piped into a 5" type 8 vent pipe. According to the HVAC this diameter is too small to vent the water heater and furnace from the 1st floor. The total pipe distance is approximately 40 to 50ft including interior/exterior pipe and will take about 3hrs to install and cost me $650.

Does this appear to be a fair price? Considering the cost of the materials and the short labor time I thought it might be a bit pricey.

Is the interior vent pipe too small to vent 2 water heaters 2 furnaces? I tend to trust the HVAC guy or go with the safer option but I figured I would ask.

The standard vent pipe is double walled galvanized steel, will this be appropriated or does it require something special since so much of it will be exterior pipe?

How far above the the base of the roof does it have to extend? My guess would be just beyond the peak of the roof.

Since this is not a typical vent pipe installation are there any shortcomings or pitfalls I should be concerned about?

Marty S. 12-20-2010 05:17 PM

Sounds like a very fair price, maybe even on the cheap side. They are correct that 5" isn't big enough for two furnaces and a water heater. The new flue will need to be a minimum of 2' above anything within 10' so if you have a 10/12 pitch roof it's going to be pretty darn high. Sometimes the minimum isn't enough due to wind. Since they are the ones that have seen the job and they're the ones standing behind the work, trust the men you hired.

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