# condensation in furnace cold air intake



## Bob Vila

Hi- I have run across condensation in a home furnace cold air intake duct (only in the winter). The duct ties directly into the return air duct and routes to the exterior wall just above grade. The duct is insulated well. I found the the homeowner has been running the fan continuously which may have been the problem. either way I have to replace the entire duct to from exterior wall to cold air return. Not sure what I will do will be any better than the privious insulated duct. It was also well insulated at the exterior wall so im stumped. The basement is very dry- driest i have ever seen so it is not a humidity issue either. Anybody run into this??


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## comfort

well i'm no expert in hvac. but sound like dew point. if warmer air is forced back in the fresh air intake it will find a point were it will relieve it water content. By the Ho running the fan may have caused a positive pressure in the house forcing the hot air back into the system.


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## CompleteW&D

Where is the R/A duct being pulled from? I'm assuming the basement since you mentioned it. If the basement is conditioned (which again, I am "assuming" it is, since there is return being pulled from there), and they are shutting of the supply registers to the basement and pulling cooler return air, then that may help contribute to the problem.

I also question a R/A duct being pulled up an outside wall cavity. This sounds like either a DIY thing or someone added a return up the outside wall after the system was already in place. Pulling R/A up an outside wall cavity is just NOT a good idea. Insulated or not, it's just not done generally. R/A ducting is almost always as centrally located as is possible.

I don't think running the fan 24/7 is a bad thing at all. If anything, it helps even out temperature differences and even humidity throughout the home. Running the fan 24/7 is a GOOD thing, IMHO.... and I really don't see it causing the problem you describe.

If it were me, I would be looking at ways to eliminate that R/A duct on the outside wall. It's just NOT a good idea IMHO.


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## Bob Vila

Let me clarify: The return air duct is in the basement and is centrally located. It serves the floor abve in a ranch home. The outside air intake ties into this RA duct in the center of the basement. The RA duct is 8" flexible insulated duct and routes straight across the basment ceiling to the exterior rim joist where it penetrates and allows outside air in. There is a small damper at the exterior that opens when the fan is on and shuts when the fan is off. Maybe this will help? One more thing to add, In the winter thereis actually ice on the duct in between the insulation and the duct. Maybe the insulation failed?


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## rselectric1

@ "Bob Vila"

Would you mind doing an intro in the introduction section and telling us about your contracting company, or position within one?

Thanks,:thumbsup:

CT Staff.


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## SLSTech

weird setup or your missing a few items on how the whole system works - I can just bet you have more issues that just that duct with condensation, like the walls

Pressurize Only is a trick used in southern climates while you should be either doing a balanced or exhaust only... http://blog.sls-construction.com/2013/bs4d-ventilation-strategies

John - as for the fan, it depends on climate but you are right it can help moderate the temps throughout
The one time you do not want to run one 24x7 is when it is humid & the AC is running as all that moisture that got collected on the coil & needs to drain away is instead being blown back into the house


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## CompleteW&D

SLSTech said:


> John - as for the fan, it depends on climate but you are right it can help moderate the temps throughout. The one time you do not want to run one 24x7 is when it is humid & the AC is running as *all that moisture that got collected on the coil & needs to drain away is instead being blown back into the house*


If you're blowing condensate off of the coil back into the ductwork, my guess is your fan speed is too high. Slow that puppy down. Besides, at a slower fan speed, you'll have a colder coil and get even better dehumidification.

That's where a good HVAC technician is worth their weight in gold. They don't just take the default fan speed settings out of the box and assume their right. They actually take the time to check temperatures and make sure they get that coil as cold as possible without icing it completely.

:thumbsup:


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## TheLoneRanger

Bob Vila said:


> One more thing to add, In the winter there is actually ice on the duct in between the insulation and the duct. Maybe the insulation failed?


To better understand you, please clarify.
1. Where *exactly* is the ice located? Example: coupling at interior duct to outside termination.
2. Is the interior duct metal with installer added insulation or an insulated flex-duct?
3. Is the exterior termination completely foamed around? Example: coupling and neck.



> Maybe the insulation failed?


That may very well be the case, in a sense. If it is flex-duct, you may have a hole some where in the outer wrap or the wrap is not completely sealed at each end. Both ends of the duct should be foamed. Otherwise, condensation can occur between the insulation and the inner duct or at the coupling interface.
If it is metal duct, insulated or not, replace it with high R-value flex-duct.
What is the dew point of the basement environment? Basically, the basement environment is coming in to contact with un-insulated fresh air components and causing condensation.
Since the damper is exterior, there may be negative pressure in the interior duct at times; pulling the basement environment in at the coupling. It is important the termination be completely foamed around, to seal. Always use a closed cell foam on exterior components.




SLSTech said:


> Pressurize Only is a trick used in southern climates while you should be either doing a balanced or exhaust only


With proper design practice, "pressurize only" is fine for any climate. I agree it's not as efficient as a balanced design though.




CompleteW&D said:


> If you're blowing condensate off of the coil back into the ductwork, my guess is your fan speed is too high.


I believe SLSTech was referring to re-evaporation of the coil's condensate. It can take up to 20 minutes for a coil to drain.



> Slow that puppy down. Besides, at a slower fan speed, you'll have a colder coil and get even better dehumidification.


That's an excellent on-demand stage, for humid regions. However most technicians do not properly design this stage. They risk compressor damage, over time. Since efficiency plummets, a timed lockout is required after this stage. Otherwise, this stage is highly in-efficient if moisture is allowed to be re-introduced.



> That's where a good HVAC technician is worth their weight in gold. They don't just take the default fan speed settings out of the box and assume their right. They actually take the time to check temperatures and make sure they get that coil as cold as possible without icing it completely.
> 
> :thumbsup:


As professional technician, I would never design like this, especially a single stage condensing unit. Taking a coil to near icing will raise head pressure and possibly flood the compressor, TXV or not. With multi-stage condensing units, it is easier. However, a blower lockout is still required, after this stage.:nerd:


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## beenthere

TheLoneRanger said:


> As professional technician, I would never design like this, especially a single stage condensing unit. Taking a coil to near icing will raise head pressure and possibly flood the compressor, TXV or not.


Taking a coil to near icing, won't raise head pressure.


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## SLSTech

TheLoneRanger said:


> With proper design practice, "pressurize only" is fine for any climate. I agree it's not as efficient as a balanced design though.


Sorry but he11 no - you are in Texas (at least based on your name), for everyone living further up North... you know where they do not want ice forming on the backside of their sheathing... exhaust only or balanced (see the link in my last post for more)


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## TheLoneRanger

beenthere said:


> Taking a coil to near icing, won't raise head pressure.


I guess I need to clarify what I consider "near icing" here. "Near icing" is below 36F. Try that in a proper installation and watch your high side. Now imagine that in a not so ideal environment.




SLSTech said:


> Sorry but he11 no - you are in Texas (at least based on your name), for everyone living further up North... you know where they do not want ice forming on the backside of their sheathing... exhaust only or balanced (see the link in my last post for more)


I live in Michigan. With plenty of exhaust vents, a tightly sealed home will not induce moisture push through the building envelope, using *very* low positive pressure.


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## beenthere

TheLoneRanger said:


> I guess I need to clarify what I consider "near icing" here. "Near icing" is below 36F. Try that in a proper installation and watch your high side. Now imagine that in a not so ideal environment.


Running a evap coil near icing/freezing(34°F) will not raise the head pressure. 

Low suction pressure with low temp refrigerant means little load. Which means lower head pressure.

Look at any High Velocity system. Anti freeze stat is needed to keep the evap from freezing, and the head pressure is normal to low.


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## Bob Vila

The ice was on the coupling and neck and down the outside of the duct a few feet (inbetween the duct and insulation). It was also on the rim board. The duct was a metal flex duct lined with insulation from the manufacturer. The coupling and neck only had batt insulation tucked around it at the rim board. After the ice melted it would pool in the duct in small amounts and rusted the duct and soaked the insulation. No holes in the lining around the insulation because none of the water dripped. To fix the problem, I have demoed the entire fex duct back to the main return duct. Installed a new one and made sure the flex duct insulation was sealed well at both ends. I then spray foamed the coupling rim connection and down the neck. After it dried I added R30 batt insulation. I now wait till the 0 degree days this winter and see if the fix works.


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## SLSTech

TheLoneRanger said:


> I live in Michigan. With plenty of exhaust vents, a tightly sealed home will not induce moisture push through the building envelope, using *very* low positive pressure.


As an FYI that is an exhaust based system not a "pressurized" system - you might want to check the link I included as they are wildly different


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## TheLoneRanger

SLSTech said:


> As an FYI that is an exhaust based system not a "pressurized" system - you might want to check the link I included as they are wildly different


No. It's not. Thank you though. I have plenty of information. I said "using very low positive pressure". This is achieved by a filtered fresh air intake to the return trunk. The exhaust vents are not active. They are there to prevent over pressurization. A motorized damper limits full ventilation only during conditioned cycles. A combustion make up is also included in the maintenance room, to prevent any severe negative pressure situations. The vast majority of time the house is under a very slight positive pressure, as a continuous fan cycle is used. All exhaust/make up vents use simple draft preventers. Is it as efficient as a HRV/ERV? No, but it's still very efficient if implemented properly.



Bob Vila said:


> I now wait till the 0 degree days this winter and see if the fix works.


As long as you took the foam to the rimboard, it should be a good fix. Did you use a high R value flex duct?


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## Bob Vila

i used the highest r value i could find- i spray foamed a few inches on the rim board and then stuffed r 30 batt insulation in the entire pocket. The part that I worry about, is now I may have moved the dew point further into the duct ( all the way to the return duct). I am hoping the air movement will evaporate any moisture thta does occur.


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## TheLoneRanger

Bob Vila said:


> The part that I worry about, is now I may have moved the dew point further into the duct...


You do not have to worry about this in the winter. You're not worried about the air temperature. You're worried about the surface temperature. Keep the surface temperature above the dew point and you'll be fine. A high R-value flex duct will achieve this. I always stress flex duct. There is a reason. Metal duct is thermally conductive. It will transfer its thermal load across the span. This becomes an issue in the summer when the unconditioned air (warm & humid) reaches dew point on surfaces near the cooler R/A trunk. Use a thermal break between the R/A trunk and the take off collar. This can be achieved with duct board or rigid XPS and a plastic take off. A metal take off can still be used, as the thermal transfer across the screw is minimal. The other end of the F/A duct stays well above dew point, from the outside heat. Remember to use a constant fan cycle and always keep a *small* amount of air flowing through the F/A intake. This is achieved by leakage or very slightly opened damper. You will over ventilate the house if left open. Using a lockout for F/A, when outside conditions are not acceptable, is ideal. Upgrade to an BPM motor when running constant fan cycle, if not already equipped. If your trying to reach ASHRAE ventilation standards, then a HRV/ERV is recommended over this design.


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## flashheatingand

TheLoneRanger said:


> You do not have to worry about this in the winter. You're not worried about the air temperature. You're worried about the surface temperature. Keep the surface temperature above the dew point and you'll be fine. A high R-value flex duct will achieve this. I always stress flex duct. There is a reason. Metal duct is thermally conductive. It will transfer its thermal load across the span. This becomes an issue in the summer when the unconditioned air (warm & humid) reaches dew point on surfaces near the cooler R/A trunk. Use a thermal break between the R/A trunk and the take off collar. This can be achieved with duct board or rigid XPS and a plastic take off. A metal take off can still be used, as the thermal transfer across the screw is minimal. The other end of the F/A duct stays well above dew point, from the outside heat. Remember to use a constant fan cycle and always keep a *small* amount of air flowing through the F/A intake. This is achieved by leakage or very slightly opened damper. You will over ventilate the house if left open. Using a lockout for F/A, when outside conditions are not acceptable, is ideal. Upgrade to an BPM motor when running constant fan cycle, if not already equipped. If your trying to reach ASHRAE ventilation standards, then a HRV/ERV is recommended over this design.


That will jack up the electric bill fairly significantly. I am not hating on your take. You obviously are a smart guy. Just think it's ironic how much time and energy is spent on keeping the utility bills low. and in the long haul, how much money is really saved?


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## TheLoneRanger

flashheatingand said:


> That will jack up the electric bill fairly significantly. I am not hating on your take. You obviously are a smart guy. Just think it's ironic how much time and energy is spent on keeping the utility bills low. and in the long haul, how much money is really saved?


While I understand your point, it is completely dependent on the construction techniques used. When a home is built very tight, a fresh air intake is a requirement. Otherwise, a fresh air intake is just an energy waste, as normal leakage and drafting does this already. All of my previous posts, in this thread, have been regarding very tight construction. To be clear, I'm not describing constant fan cycles over 600CFM. With the low ESP produced, negative pressure in the fresh air intake is extremely low. We're talking 20-40CFM continuously. This does not meet ASHRAE standards, but is does give a very energy efficient ACH. If you have not yet surmised, I'm all about IAQ. I do not recommend any windows be opened in the tight construction home. If you want to open a window, go outside. *Controlled* ventilation is the key.


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## Bob Vila

Well its zero today and I went down to the basement and still have the problem. A very small amount of condensation on the outside of the flex duct barrier where it transitions to spray foam after the collar. However- I can feel ice inside the flex duct or between the flex duct and insulation about 2 feet into the house. I have had my fan on continuously so im going to shut it off and see if it evaporates, if not I might eliminate the air intake and cap it at the cold air return. Let me know your thoughts.


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## comfort

This may sound crazy. could there be a flashing problem? that water may be not from condensation


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