Venting a condensing boiler/furnace

Ken C.

I went to a service call yesterday (it was a furnace, but the problem could apply to a boiler) where the furnace's burners only came on some of the time during calls for heat. It was a condsening furnace with a 2" PVC vent. The draft inducer turned on with every call for heat, but the hot surface igniter wouldn't glow. When I disconnected the Fernco-like boot that connected the inducer to the PVC vent, the HSI glowed and fired the burner, so it was definately a vent problem that was allowing the pressure switch to make only part of the time. Now I'm not familiar with venting guidelines for condensing furnaces, but I noted several questionable things about this vent: 1) Since the furnace was in the middle of the house, the vent piping had a long run (about 30 ft. developed length) before it terminated out the side of the house. 2) It had several elbows (I'm talking six). It's my understanding that each elbow has an equivalent piping length. 3) I think the pipe was improperly pitched. On the horizontal run, it was sloped away from the furnace. 4) There was water trapped in the piping. I listened at the vent terminal while the unit was running, and could hear water gurgling. After I opened up the ceiling (this was a finished basement), I saw that the pipe went from horizontal, to a six-inch vertical offset with two elbows, before it made another 90 degree turn to exit through the rim joist. Given the location of the furnace and layout of the basement (e.g., ducts, finished ceilings), I couldn't see any better or more direct route for the vent. The homeowner would have been better off with an 80% furnace venting into a chimney, which was two feet away. In any event, can someone please give me some general guidelines about running a vent for a condensing furnace/boiler (this was not a sealed combustion unit with a combination intake/exhaust pipe). Generally, what is the maximum developed length of the run, the maxium (or preferred) limit on the number of elbows, proper direction of pitch, etc? Also, what about drains/traps at the furnace?

Earthfire

Chocker

Get a hold of the installation manual for the furnace. 2" for that long a run sounds wrong.The manufacturer has a maximum equivalent lenght for the vent. Is anything else vented into the chimney? We have on occasion dropped properly sized pvc into the chimney and vented at the roof. Any reenginering from the factory specs needs to be done with consultation with the factory. The factory tech guys have probably had just about any kind of problem thrown at them, even if some won't admit to it.As for water trapped in the vent, that needs to be corrected. By the way, the WATER in the vent is CORROSIVE. Be very careful when you drain that vent.

Kal Row

i read a lot of manuals

on line and the longest i seen allowed was a 3" 40ft, now your 30ft with 6 els is effectively way past 40ft and with a 2 inch pipe, fo-gi-a-bow-eet - never work

also most require a pitch of 1/4" every 3" horizontal, so that the condensate can run downhill, as long as there are drains with traps (i mean accessible traps) - that is ok,

you need to look at the units manual, because if they modulate the combustion fan down 50%, you might have a real problem, in which case you need to drop co-ax down the old chimney

and they ran this in enclosed ceiling without coax tube or 6" air space surrounding it? - it's still a flue for god's sakes, and if you get a control failure - it's going to get hot, real hot, like "fire truck red" hot!!!!! – “the stupid stuff you see out there…”

i did a job where the customer wanted that, but what he got was a natural draft slant-fin sx-150 with two 6" fresh air ducts run to the outside, with in duct boost fans driven by a relay off the ignition module - the 2 6 inch ducts alone were borderline for the boiler and water heater - so it would still work if i got a fan failure

a boiler is no different from a human - we burn hydrocarbons into water and co2 and so does the boiler, and we both need to breathe

Ken C.

Yes, this vent was enclosed in sheetrock! I had the homeowner open up the ceiling to expose the vent piping. I suspect it may have been installed by a "handyman" or maybe even a homeowner, who knows. The furnace is located right next to the chimney, I don't know why they just didn't install a conventional 80% furnace and vent to the chimney.