High efficiency furnace issue?

[mrcaptainbob]

A friend recently had a high efficiency furnace installed. It has the PVC inlet and outlet through the old chimney stack and is properly sealed and such. All seems well. The problem now is the fireplace smoke that used to go up the chimney when the old furnace ran now gets sucked back into the house so bad that they get smoked out! The fireplace has been in use throughout the winter for the past 6 or 7 years never having this problem. I thought about the possible reasons and figure it must be either the flame side 'circuit' or the house side 'circuit. We first checked the in/out pressure by comparing with our hands at the inlet/outlet. Both appear to be pretty darn close to equal. We eliminated the fuel burn as causing negative house pressure. We then checked the furnace attachment to the floor, as it was installed in a mobile home and the ducts do run from the bottom of the furnace. All is well there. So, the last possibility on my list would be a hot air duct not in full contact with a floor vent. We crawled around down there and all appears fine. So now we are stumped. I would expect a high efficiency furnace would cause neutral pressure in the house versus the older furnace that would of coarse cause a negative one. What am I missing here? Any suggestions?

[Java_Bob]

It has the PVC inlet and outlet through the old chimney stack and is properly sealed and such. All seems well. The problem now is the fireplace smoke that used to go up the chimney when the old furnace ran now gets sucked back into the house so bad that they get smoked out!

So it's getting a smoke smell? with no fire going? if thats the case I would say there is a problem with the fresh air PVC line

[jnaas2]

You have a negative air pressure in the house which you already know that, there are a couple of things that I can think of that you might check. Depending on how far they ran the pvc for intake on the new furnance and the number of 90s in the line determines the size of the intake. If the only time the smoke is pulled into the house is when the furnance is running then the problem is the furnance. They are suppose to install fresh air vents in the mechanical room for fresh air make up to replace the air the furnance was using for combustion, I have seen these closed off when new furnances were installed with there own fresh air intake. And sometimes when you make a house too air tight with new replacement windows, doors and weatherstripping the house cant get any makeup air thats needed for a fireplace to burn and vent fans in bathroom to exhaust air. Another thing can be the duct work, if the velocity has changed in the ducts and the fan is running at a higher speed the return airduct may not be able to keep up and the furnance may be pulling air from the chimmney, The ducts should be sealed good to prevent this from happening but you may check it out, finally if the furnance is blowing air under the house it will be starved for return air and pull it from the easiest place it can. Good luck on finding it, I personally would check and make sure what was causing the problem and if it was the furnance I would call the people who installed it and let them fix the problem

[mrcaptainbob]

Furnace issue...

The smoke incident happened the one time with the new furnace. The fireplace was lit and all was well until the furnace kicked in. The smoke was pulled back into the room. Even closing the fireplace doors did not help much as the smoke was being sucked past them! The people have not and cannot use the fireplace any more until this is resolved. The furnace inlet is on the roof, as is the outlet. The PVC pipes were fed through the existing chimney stack and all is very well sealed. There is considerable negative pressure on the inlet portion and appears to be the same value as the positive pressure on the outlet. I'm leaning toward leaking duct work, but I think at this point a call to the furnace installer is in order. It's all the more strange, as this NEVER occurred with the older less efficient furnace that drew it's fire burn air from the living quarters.

[mrcaptainbob]

Furnace issue update:

My neighbor was about to adjust the thermostat a few days ago and noticed the wall there was VERY warm. The hall floor next to the furnace was also very warm, as was the under sink cabinet in the nearby kitchen. We cut a 2' x 2' hole in the hall floor over where we estimated the first line from the furnace was. It's about a 3' long section that sat atop the main trunk run. Well, the end of that 3' section was merely a strip of galvanized with two 90* flanges on the long sides. This slipped over the end of that 3' run from the furnace. It was only taped using the foil tape. there were no mechanical tie clips fastening it to that trunk end. Needless to say, a LOT of hot air was being exhausted out past that loosened plate. The bottom of the mobile home was quite toasty, while the furnace cold air intake was sucking make-up air from every possible source. That is why the fireplace could not be used! YIKES! Well, after turning the proper flanges on that trunk end and properly fastening the cap, it sealed it up quite nicely. He and his wife are now enjoying the fireplace once again.

[HayZee518]

if you look on most sites that have high efficiency furnaces that have pvc vents - they say that most should have make-up air and exhaust venting independently of an existing chimney. just like if you have a working fireplace, DO NOT exhaust water heaters, furnaces, in the same flue.

[mrcaptainbob]

Furnace issues....

Well this furnace sure falls into that category, as the furnace in/out air for the flame is wll past twenty feet away from the fireplace and it's chimney. What was the puzzler on this is that the negative air pressure in the house did not occur when the flame was on, but rather when the blower kicked in! I was under the impression that the blower can't push any more air than it can pull from the cold air intake. COnversely, it can't pull any more air from the cold air intake than it shoves through the registers. Well, I guess it can if it's dumping air to the outside. In effect, one register was exhausting to the outside. The air for the cold air intake had to come from somewhere. It was sucking it from every nook, cranny and, of coarse, the fireplace.