The furnace sometimes has a problem firing and the installer(not hvac) thinks it is not getting enough air to fire. It eventually does fire. He removed a plug on the side of furace (about the size of a hockey puck) to allow more air in. It seems to work better but this is not how it is supposed to work. The intake is 3 pvc, 2 elbows, 18 ft long. He also measured gas pressure at two places. He thought the gas pressure coming in may be too high(12). he adjusted the internal pressure to specs. The intake pvc is not clogged. But if you remove it at the furnace it seems to fire normally. He seems to think that removing the plug is good enough. I want to make sure this is not bad for furnace and safe to run this way. Would an adjustable gas valve fix it? Also, the gas water heater is still vented into chimney, it is in center of house. I have a slate roof so it is difficult to access chimney. The chimney has an elevated cover. Any suggestions are appreciated.
The new high effiency furnaces need to draw their combustion air from outside the house. it's all about the negative pressure like mentioned above.And also as a safety to keep the combustion sealed from the ambient air in the house. And like redbird mentioned your intake piping may be too restrictive, and may need to be replaced with a larger diameter pipe. But everybody has overlooked the obvious. Have you looked at the intake pipe outside? It could be that the exhaust and intake are too close together causing the furnace to recycle the exhaust gases. Since there is little to no O2 left in the exhaust it can cause the symptoms you've mentioned. The exhaust should be turned up and away from the intake or straight out from the house and the intake should be turned down, to keep them at a minimum of 18 apart. Check your manual to find out how far apart they should be and how they should be terminated. Is it blocked by something? Sucked up leaves etc etc. A couple of leaves in the pipe can restrict it just enough without making it appear blocked. I made a service call on a furnace one night for the exact same problem as yours and found the guys 6 yr old kid had stuffed a plastic bottle down into the pipe with a stick. Just a couple more ideas for you to look at.
I had to replace my furnace last year, and replaced it with an energy efficient one. When it was installed, they ran the PVC exhaust through a hole in the wall, even though the chimney was very close. When I asked about this, they told me the exhaust would be restricted by venting out the chimney. Now I have a chimney that isn't being used. They also talked about the air inlet. The flame needs oxygen, obviously. A lot of newer homes are so well sealed, that there isn't sufficient oxygen entering the house from cracks, etc. that a supplimental pipe is needed to bring in the needed oxygen for the furnace to operate. Your intake pipe may be too long? My house is a very old remodeled relic, so there was plenty of air leaks to the basement to supply the oxygen. He did not run a separate intake, relying on the ambient oxygen to supply the furnace. It works just fine with not one glitch. By removing the plug, you are using the oxygen in the basement. If you have an older home like me, with air entering the basement through cracks, you won't have a problem. If this bothers you, I suggest you call your furnace man and tell him if he can't fix it the way you want, he should call in someone with more experience. It's your money, make him do his job correctly.
Furnace Combustion Air Intake
