Stack Temperature

[Jeffory11]

Hello, I'm new to the site and glad to find it!!
I recently installed a unit heater in my garage and was wondering if these usually run a higher stack temperature than a residential furnace. Temperature rise is within range. Thanks

[acwizard]

There are two types of unit heaters, your basic model and seperated combustion. Heat rise temperature is very important, have you tested the combustion efficiency this would give you additional information in regards to a possible problem.

[Jeffory11]

CO= 27, O2= 9%, Stack 486, EFF= 76%, Temp rise 57* (range is up to 60*)

[acwizard]

Your numbers look good, i am assuming your heater is 80%, you may want to check gas pressure but results are in range where I would expect them to be.

[acwizard]

In reference to stack temp. 500 F is at the high side, your results show your efficiency slightly low, and stack temp slightly high, try tweaking gas pressure slightly ,this should raise your efficiency and lower the stack temp a few degrees.

[Jeffory11]

Thanks for getting back to me. I'm thinking of purchasing a draft guage. I'm venting with 5" B vent in a horizontal application (75,000 btu), as I remember draft should be around .2-.3?

[acwizard]

Is this natural gas, and which model are you working on. O2 should be slightly less. Draft would be about -.02 to -.04 measured in the stack

[Jeffory11]

Natural gas, Lennox M# LS24-75

[acwizard]

After checking installation instructions, you are allowed 5 feet horizontal maximum and one 90 degree elbow. your elevation where this is installed is less than 3500 feet above sea level

[Jeffory11]

The unit heater exits with a 4" exhaust at which I increased to 5" at the wall. I changed the flue to vent directly into 5" straight out of the heater, this in turn lowered my stack temperature to 420* (a 66* drop). I didn't check the draft but I'm sure it was increased by doing so. Thanks for the input.