Roof fans sucking air through chimney?

Badfish740

We began the buying process on our house in May and around early June the previous owner put on a new roof was put on along with two large roof fans due to a moisture problem in the attic as per our home inspector's recommendations. The fans are each controlled by a thermostat and a humidistat. I notice that even in the cold weather they run quite a bit due to the fact that they are also triggered by high humidity-frequently I'll notice a very dull humming coming from them at night if I'm lying in bed and all else is quiet in the house. The fans (along with a ridge vent) were recommended by the inspector due to the gable vents being undersized and no soffit vents whatsoever.

Last night we turned on the heat for the first time this year and after a few hours I noticed a strong odor of fuel oil in living room. I went downstairs and saw that there was a very fine haze hanging in the air. I immediately shut off the furnace and called the service company who had installed the unit three years ago. The tech came out and determined that the chimney was essentially drafting backwards. When he removed the flue from the wall at the base there was a strong blast of cold air coming in. He was able to force the chimney to draft by holding the draft control closed and turning the furnace on, but of course this only works if the furnace runs continually. Once it shuts down for any length of time the chimney will cool and begin to draft "backwards" again.

Could the two roof fans running be causing a vacuum in the house and pulling cold air in through the chimney? The house is nearly 50 years old and I wouldn't call it "airtight" but it's not drafty either. The previous owner also recently installed new windows, so they're reasonably tight. Since the fans were installed over the summer after the previous owner was done using the heat for the year that would explain why we are having problems and they never did. Maybe I should try disabling the fans, running the furnace, and seeing if I get the same problem? In any event, if this does turn out to be the problem, what do I do?

hvaclover

I had the very same thing happen yesterday on an annual furnace inspection. Tfumes poured from the furnace. The Customer had turned on his whole house fan to get rid of the furnace smells. The fan pushed so much air out the house that to make up for it the air was being drawn down the chimney which brought the furnace fumes with them.

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Just slow, not stupid.

Marvin Gardens

And to add to that this can be deadly. CO kills and sucking it out of the chimney is not good.

I suggest that you shut down one of the fans and see if that works. This will make a big difference since the second fan will no longer be pulling air through but will become a vent of sorts.

Also get a CO detector.

There should be a vent that allows air to come in to the attic so that it doesn't pull from the house. That is the problem here. Maybe the vent is not big enough or there isn't one at all.

If you are not sure then call in a professional to get some recommendations.

biggles

if the fans do cause a flue problem on the oil heat you might think about this fix.cheat 24Vs off the boiler and install a relay w/24V coil,and run the attic fanpower thur it.when the boiler kicks on the relay will open up ......shutting off the fans during the heating cycle.

8 Ball

You obviously have a life safety issue and need professional help. Turn off the fans and the boiler and call a professional heating company before someone gets hurt. Im thinking the tech that was there told you to leave the fans off, or turned the furnace/boiler off. You may want a second opinion regarding the fans in the attic.

ccarlisle

You're creating a vacuum inside your house and air is rushing into the void created by the fans. The system needs 'adjusting'; either bring in air into the attic via a separate air intake vent, raise the humidistat a bit so that it doesn't run as long once you've established a what level it comes on at, or change the thermostat setting. Venting at low temperatures should be a function of humidty not temperature if its an unheated space. At high temps, that's another story.

Badfish740

As it turns out the fans were the cause of the problem. I went around to each window in the house with a strip of toilet paper and found that with the window open about 1" the paper would be blown nearly parallel to the floor with air rushing INTO the house. The same result was observed when holding the paper next to the butterfly valve on the flue with the furnace off and the fans on. After disabling the fans the same test showed no air movement whatsoever in or out of the windows/flue. I then turned on the furnace and (once again using the toilet paper) observed that the chimney developed a strong updraft immediately-problem solved. For some reason when the PO had vinyl siding installed, only solid soffit was used-there are no soffit vents at all, only gable vents and a ridge vent. I'm going to try installing the little circular "pop in" vents every 4 feet to see if that fixes the problem. Right now the short term solution is to simply leave the fans off.

Curiously, there are two CO detectors in the house, one in the basement and one in the hallway outside of our bedroom and neither one went off. They came with the home and don't look to be that old-should I scrap them based on this and just get new ones? I tested both when we first bought the home and they both performed as they should have, and the batteries are good.

ccarlisle

Good one! I'm surprised that the soffit was unvented because that forms an integral part of the whole roof venting system and the soffits should have had some sorts of vents going through it. Anyway, for a few soffit vents, that seems to be that...good!

CO detectors? As opposed to taking them out, I'd leave them but remember that it might take an awful lot of CO to trigger them, plus, limited oxygen availability, and incomplete combustion. IMO if you had complete fuel combustion and plenty of oxygen available to permit that, then CO levels would be minimal and may not cause the detector to go off.

And smoke % does not = CO%. You had smoke.

Marvin Gardens

Consider this.

The human body will absorb CO at a level 200 times that of oxygen.

What this means is that if there were CO and oxygen in the same room at the same level of concentration the body would absorb 200 times the amount of CO as oxygen. CO actually takes the place on the blood cell and won't allow oxygen to bond. This is called carboxyhemoglobin for those that care.

In natural room air there is 20.9% oxygen 79% nitrogen and the rest are trace gases. If there were 2% CO in a room and the oxygen were still at 20.9% the body would absorb the CO and the person would die if not removed from that environment in time.

Then the body would "hang on to that" CO for up to 48 hours.

Bottom line is that the body loves CO and prefers it over oxygen.

As for replacing the sensors I would look at the model and check to see what the life of the models are. Some have a life of only 2 years.

The level of CO is determined over time also and that is around 30 ppm over 8 hours. If there are high levels for a short time the CO detector will go off. If there is a low level of CO over a long time it will go off.

This prevents cooking with gas that can raise the CO for a short time but not to lethal levels. It might get to 50 ppm for 20 minutes and return to normal. If the level were to remain at 50 ppm for say, an hour it would go off.

On the other hand if the CO were to rise to 200 in 2 minutes it would go off.

These are just examples as I have no idea what algorhythum they use to activate the detectors. I just know that it is high levels for a short time or lower levels for a longer time.

Replacing your detectors is up to you. Like I said, some last longer than others. Some even beep and tell you that they need to be replaced.

Don't be fooled by the test button. They are just testing the electronics and nothing else.