chimney problem or something else?

We have a gas furnace that vent through chimney. Last winter on cold days we can see "white cloud" (water vapor) coming out of the chimney when the furnace is on.
But this morning, when the furnace is on, I can't see any thing coming out of the chimney. The outside temperature this morning was around freezing point and I did see "white cloud" coming out of some neighbour's chimney (which were built in the same year with similar furnace).
1) Should I be worried? Or the forming of "white cloud" varies a lot from furnace to furnace. 2) If there is a problem with the chimney that causes the furnace not vented probably, is it dangerous? The furnace is 14-year-old and it's in the basement. 3) Just occurred to me that we recently installed a radon mitigation system. The system sucks air beneath the basement floor. This reduces the air pressure in the basement. Could this be the cause, i.e., the exhaust from the furnace actually come back to the basement because of the low pressure.
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The white cloud is water vapor condensing, just like when you can "see your breath" on a cold day. May factors come into play to see it.
First, there has to be vapor, second is the temperature of the air. What you see coming out is alsoaffected by the temperature of the chimney. If the chimney itself is cold the vapor may condense before it ever gets to the top. When running the heater for hours at a time, the chimney lining will heat up and allow more vapor to pass. With only occasional use, the chimney thermal mass is rather cool. Natural gas containers water so that is part of what you see, but so does the combustion air. Perhaps some of the others have more moisture in the air than you do.
You also need the light to shine on the vapor. Certain times of they day it will reflect more than others.
That said, if the radon system is sucking air down the chimney while the heat is running, that is not good. The chimney itself is a good vent for the radon so maybe that should be off when the heater is on? Check it out.
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[ 3) Just occurred to me that we recently installed a radon mitigation system. The system sucks air beneath the basement floor. This reduces the air pressure in the basement. Could this be the cause, i.e., the exhaust from the furnace actually come back to the basement because of the low pressure.
YES YES YES
Gas Engineers UK / Gas Technicians USA
Test the flue pull or chimney draw by closing all doors and windows enclosing the gas appliance and turning all air extracting devises ON...A smoke test or spillage test is performed on the gas appliance to ensure that the flue functions at its worst conditions...
In the USA folks have furnaces in basements...Something like a wood stove installed in the basement is powerfull enough to create a suction on the gas appliance...any suction or negative pressure around an open flued/conventional flued appliance is VERY VERY BAD....It can cause appliance failure leading to C/O poisening...
Even a door way to the basement left open can cause a flue pull through the house leading to a suction on the gas appliance vent pipe...
Solution ......every time you add air moving devises to the room with an open flued appliance get a gas engineer to safety check the applaince and possibly upgrade the VENTILATION REQUIREMENTS ..
p.s. buy a carbon monoxide detector for your house...its like a smoke alarm and cheap too....
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gastec


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Edwin's right about the vapor, wrong about the radon. Radon levels are usually higher in winter due to depressurization of the house (mainly from hot air rising, secondly from combustion appliances. Radon contractor should have tested appliances for backdrafting after installing the mitigation system.
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hwm54112
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