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We do a lot of work for a realtor. We get a call today because the utility company red-tagged the furnace while he was negotiating a short sale. We show up and find this beauty. The vent connection is rotted through. The house needs to be sold ASAP, so a half hour later, here's the repair.
Standard stuff. The real point of this post, while running our combustion test...
80% efficient. 80%!! And only 5ppm CO. Who the heck knows what the actual thermal/system efficiency would be. We didn't get that far into it.