Ice on furnace flues

This morning’s inspection. 10:00 am, 23 degrees F, clear and 64% humidity. When I arrived I could see both furnaces were running from the steam. As I approached the front of the house I could see ice on the flue pipes. I am thinking the hot flue gasses are condensing on the flue cap and then running down the pipe and freezing. Has anyone seen this before?

only in the winter,


Yup, that is what that flange at the bottom is for.

Combustion gas has a huge amount of water. The lower the efficiency of the furnace, the more moisture/ice you will see.


That kind of condensation and resulting freezing at only 23 degrees, seems like the flue gases are cooler than they should be. The warmth of the gases should hold the moisture in suspension until the gases clear the flue. Heating systems don’t have to be designed to never condense moisture at any low temperature, but a properly constructed system should clear the humidity out of the flue at 23F. I would (and do) call for a HVAC tech to evaluate and repair.


Burning gas produces a lot of water as a byproduct.
To avoid condensation inside the chimney they will specify a smaller flue size to keep the exhaust moving fast enough to exit the top before it cools below the dew point where condensation will form.

As you drive around (be careful) notice the moisture vapor exiting the chimney on other homes.
If the vapor forma a clout above the chimney then it is warm enough to avoid condensation.
If it has already formed a vapor cloud as it exits, it can be condensing inside the chimney.

Both furnaces had a single wall flue pipe for the first 1.5’-2’ above the cabinet before converting to type b double wall. Could that lead to cooling the flue gasses?

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Many factors can cause the gases to cool off too much before they exit. Need a smart HVAC to assess. For example, lack of proper draft will slow down the gases too much before they exit, resulting in condensation. What can cause poor/improper draft? many things. Two feet of single wall in a cold basement will cool off the gases some, too, and can contribute to the issue.

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I have a theory. Good ol’ fashion dew point; which may have to do more with atmospheric conditions than excessively cool exhaust gases.

I look for these types of indicators which are a sure sign of problems. I do not think the ice is enough for concern IMO.

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If the flue is sized properly, then no. It sounds counter intuitive, but sometimes upsizing the flue is the “fix” for this, but we defer to a HVAC tech to determine what to do.

Thanks all, good info. I did refer it out since there were other issues that needed correction.

That will never be the case in a residential gas furnace with a metal flue.

BTW: Flue temperature is only this critical in oil fired equipment.

No matter how hot you make the combustion gas, it is going to condense on startup. It is the temperature of the flue pipe that causes condensation, not the temp of the gas. The furnace does not run all the time, and when it’s off, the flue gets cold. Heat of the gas can never warm the flue before condensation occurs.
Condensation may occur in the flue above the roof, but it evaporates as it gets closer back to the furnace (if it doesn’t leak out, like it did in this case).

Even the furnace is cold before start up. Condensation happens even at the furnace single wall pipe when it first starts. This is why you all see the white moisture marks at joints on the exterior of the interior flue.

If you are not capable of taking psycrometric measurements of the flue, you have no basis to call it out. There is no old wives tale or rule of thumb to determine what is right or wrong when it comes to this, without the tools. There are too many variables. Flue gas at 35% water vapor has a Dew Point of about 165F. Anything below this temp will condense the combustion gas vapor. What exterior metal flue maintains this temperature at 23F OA?

How do you “Clear” humidity out of the flue? By adding heat?
Humidity can change state at different temperatures, but the amount of humidity in that flue never changes.


It is not a matter of gases hot enough to warm the flue before condensation occurs. It is a matter of gases hot enough to minimize condensation in a metal flue. HVAC techs cannot and do not try to prevent any condensation against the cooler metal surface. The “idea” is to try to limit the condensation. In cold weather, condensate can accumulate enough inside the flue to build and drain or drip back toward the furnace where the water encounters warmer gases that evaporate it back into the rising flue gases (I’ll call it the “drip line” past which there is not enough condensate to drain or drip back toward the furnace). As the furnace continues to run, the temperature of the flue surface rises which decreases the amount of condensation. As the flue warms, the flue gases have warmer temperatures higher up the flue and continue to hold in suspension higher humidity further up the flue. The “drip line” rises up the flue until it is at the top of the flue (at least in moderately cold weather). Below this drip line, condensation still occurs on the metal interior, but it is not enough to drain anywhere, kinda like fogging a mirror with your breath.

Except, we don’t see that white scale all the time or even half the time. That white scale (that resembles efflorescence) is also an indicator of condensate not being controlled properly, as are stains down the face of the induction fan cover. A little white scale or stain on the condensate cover is not a concern. A lot of either or both is the concern. And since this condensate is slightly acidic, excessive condensate can be excessively corrosive.

No. The flue temps are what the flue temps are. You “clear” the humidity by maintaining sufficient gas temps to the top of the flue, but you know that. The flue gases need to stay warm enough to hold sufficient humidity in suspension to keep excessive condensation from forming.

Okay, you have successfully totally confused me here.

I think I understand what you’re trying to say, but how you’re saying it is not correct, and very confusing.

The reason that condensation occurs is that it comes in contact with an item that is colder than it’s dew point temperature. A furnace flue that has been off for a while becomes quite cool relative to the temperature of combustion gas during operation.

Raising the temperature of the combustion gas does not change the quantity of moisture in that gas. It may change its state from a solid, liquid or to a vapor. It may be visual based upon temperatures relative to the air temperatures the gas is being discharged into.

Raising the temperature of the combustion gas also raises its dew point temperature, which means condensation will occur at higher flue pipe temperatures even as the flue pipe temperature is warmed at startup.

You cannot isolate combustion gas from coming in contact with the flue pipe. Nothing more to say about that.

The short of the long is that, as you stated, condensation occurs in the flue and eventually evaporates back to a gas and is discharged to the exterior. Just because some of it leaked out as a liquid and turned to a solid is not an issue unless it affects the structure.
When you take this to extremes, is when there is too much moisture, which cannot be controlled. To simply state that you see ice on a 23° flue pipe, therefore the combustion gas temperature is wrong, cannot be substantiated through observation.

Proper construction and design prevents excessive problems like this, and those designs are visibly identifiable. Ice up on the roof has nothing to do with it.


@dwigger could you supply the model number of the furnace, or at least the type? A natural draft furnace compared to various condensing model makes a huge difference in vent behavior, and often the old flue is left with a new furnace.

The rakish cant angle of the flue pipe is also worth a mention.

The ice seems pretty minor.

They were both Goodman natural draft units. One 2020 model and the other 2017 model.

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Given that, I would just move on. The natural draft unit put a lot of moisture up the stack. Weather does the rest.

That sentence confused me. Ice on the roof? Not sure what that is, but ice on the flue above the roof at 23F indicates improper construction and design of the furnace system.

The pictured ice appears to be on the flange, which will be colder than the pipe. The pipe is crooked, but what kind of design do you see as wrong?

For ice to form on the storm collar, excessive condensate accumulated at the flue cap and dripped/flowed down the warmer flue until it got to the cold collar. That should not happen at 23F.
We don’t call 80% furnaces “condensing furnaces” even though we know that some condensation will form. In any gas furnace, condensate and moisture in the flue gases must be controlled and removed. 80% furnaces remove moisture in the flue gases out the end of the flue. In our example here, occasional excessive condensate at the top of the flue in very cold conditions is tolerated.
I live in a very cold area. Most winter days, I would see ice on my own flue, not to mention all the ones I inspect, if ice at 23F was intended. I typically don’t see this sort of thing until temps are below zero.

You can not use this flue in a “condensing furnace”. Besides, how do you hook up a PVC Vent to the metal flue properly (other than duct tape). :wink: