Thread: Make up air condensing on cold air return

Inspected new construction yesterday that meets Energy Star guidelines. Was snowing outside with temp in the low twenties. House is obviously very tight and , as such, the HVAC system is equipped with outdoor make up air connected directly to cold air return . My concern is the amount of condensation collecting on the bottom of the cold air return ductwork (see photo). Make up air enters directly above area that shows condensation. Infrared indicates (if I am reading it right) there may also a small amount of condensation inside the duct as well. Builder says this is normal. States other homes in developement have condensation collecting at same point. Any thoughts??

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I'm not an energy guy but I would think some type of heat exchanger would be required.

Is it possible there is high humidity in the basement (if that's where it's happening), temporarily, due to the fact that it is new construction?

You would need to find out what the relative humidity is in the basement. Likely its up above 50%.

Originally Posted by Darren Miller

I would think some type of heat exchanger would be required.

The largest mistake made when installing fresh air make up air for tight houses is when they just install the fresh air "directly" to the system and do not also install a heat exchanger.

When they do that, then, yes, it would be normal ... "Builder says this is normal. States other homes in developement have condensation collecting at same point." ... but does not need to be "normal" if a proper heat exchanger was installed properly.

The builder saying that is like the builder cuttin' a big ol' hole in the wall ... and you complaining that code air is coming in, ... and the builder says that, yes, that is normal --- it would be normal ... not right, but normal for cold air to come in through that big ol' hole through the wall.

The method of providing makeup air you have described was tried in Minnesota in the 70's and 80's and found to have numerous disadvantages, including condensation as you have shown, and the potential to "inflate" the house each time the air handler blower operates. This increases indoor air pressure which can drive warm humid air into the wall and attic cavities anywhere the "seal" is not perfect.

Most HVAC firms in Minnesota now use the passive make up air duct as shown in the attached drawing from Illustrated Home, by Carson-Dunlop.
When I encounter makeup air directly connected to the returns, I recommend having a qualified HVAC firm change the make up to the passive duct as shown.

And if you put the end of the air supply in a bucket it will act as a trap for the cold air.

The condensation is occuring on the outside of the ductwork due to dew point temperature on the air surrounding the ductwork. The condensation can be eliminated with insulation. Similar to an AC duct, except R value will need to be determined based on outside air temp vs. an AC temp of 55 degrees.

Originally Posted by Roger Hankey

Most HVAC firms in Minnesota now use the passive make up air duct as shown in the attached drawing from Illustrated Home, by Carson-Dunlop.
When I encounter makeup air directly connected to the returns, I recommend having a qualified HVAC firm change the make up to the passive duct as shown.

When using a passive make up air system in a "tight house" you also need an exhaust fan located some where else in the home. Some use the bathroom exhausts that are operated continuously or are on timers, depending on the ACPH they are trying to achieve.

I did a tight house a last fall. It had the set-up shown in Roger's pic. Generally I don't run into make-up air set-ups on residential very often. I have looked at a number of make-up air systems in commercial and high-rise buildings. The larger systems have heat coils installed in the duct run before the air hits the heating system to temper the air to avoid the condensation and potential coil freezing issues.

Eric
This is not a good installation. Just because the home meets the Energy Star requirements, does not make everything good. Remember in the 1970's a home that met this standard had heat strips in the ceilings. Most were energy consumers.
If you are bringing air from the outside directly to the furnace both in the summer and winter it is going to take more energy to heat and cool that air. And if you are pressurizing the interior of the home by forcing more air in, it has to go somewhere.
Now what is going to happen to that ducting in the next five to ten year? More than likely going to have to be replaced. Is there going to be mold growth in the duct? What other health problems are there going to be? Is the furnace going to last its intended life? More then likely not.
Of course the Builders going to say this is normal. I did an inspection on a home once in a local neighborhood that there was no sill/mud plate installed at the top of the crawl space walls. The builder told Me that that was his 100th home and the others were all the same. And the sill plate was not needed. And of course the code person had passed them. After my client contacted the Planning and Zoning department. And they went out to the home and looked in the crawl space. My client did not buy the home and the builder did not have an occupancy on the home. Normal is not always normal.

I would call for further evaluation by a professional of my clients choosing.
Bruce

Looks to me like we are talking about two different things. Make-up air is air brought into a confined space to dilute the flue gas at the hood, within 12" of the ceiling. The diagram is combustion air brought into a confined space to supply air to the flame, which is within 12" of the floor. The OP is describing fresh air intake which has nothing to do with venting.

I agree with Ken that additional insulation is needed at the point where condensation is forming to prevent corrosion.

Outside air intake ducts should be wrapped to prevent condensation.

Pressurizing a house with fresh air intake won't make it "balloon" up unless the air is brought in by a fan upstream from the common connection. If the air is mixed at the return then only the amount of air lost thru exhaust and ex-filtration will be brought in thru the OD duct. Air takes the path of least resistence and that would be thru the return ducts if all doors and windows are closed.

Pressurizing a structure is a good idea. It can stop outside air leaks (infiltration) in areas you don't want them. It is standard design practice in commercial and industrial applications using outdoor air for ventilation.

In tight building (home) designs - introducing OA to eliminate "stale air" and for ventilation is recommended and should be introduced at a rate higher than what is exhausted to allow for some pressurization. It can be done by balancing the makeup and exhaust air or by supplying outside air and exhausting through a relief air damper that has a calibrated weight for pressurization.

I agree with Vern, there are two different operations/systems being discussed. The one I am assuming Eric (and Ken addressed) is talking about has to do with ventilation, i.e. not creating a negative pressure in house and moisture problems etc. because the house is so tight. The following link explains it very well: Home Ventilation Options - Oikos The other is combustion air for the furnace/water heater. Rodger's illustaration is how it is done around here. They used to require a high and low air but now just the high one. I always let people know that the cold combustion air will make the mechanical room cold and that it's not a bad idea to weatherstrip the door to room and insulate walls.

Originally Posted by Ken Amelin

When using a passive make up air system in a "tight house" you also need an exhaust fan located some where else in the home. Some use the bathroom exhausts that are operated continuously or are on timers, depending on the ACPH they are trying to achieve.

Just want to make sure I understand this.... You make the house energy efficient and generally seal up all areas where heat can escape, or in the summer, heat can enter.

Then you add a large opening to the outside, allowing unconditioned air to enter the house to "make-up" the air (because you have a tight house.) Then to make sure this is happening---you have a fan on a timer to suck the cold (or hot) air into the house. Doesn't this defeat the whole purpose of a energy efficient home? What happened to heat exchangers as discussed in previous postings? This should be pointed out to the client.

Rich

Originally Posted by Rich Goeken

Just want to make sure I understand this.... You make the house energy efficient and generally seal up all areas where heat can escape, or in the summer, heat can enter.

Then you add a large opening to the outside, allowing unconditioned air to enter the house to "make-up" the air (because you have a tight house.) Then to make sure this is happening---you have a fan on a timer to suck the cold (or hot) air into the house. Doesn't this defeat the whole purpose of a energy efficient home? What happened to heat exchangers as discussed in previous postings? This should be pointed out to the client.

Rich

Point taken Rich but you ain't the first one to question it.

The real issue is being able to control the ventilation rate, not eliminate it.

A heat exchanger may help lower costs but in reality they are a ways out there still. They are not all that efficient and they are pretty costly, enough so that any "typical" home construction will not permit it. When a potential home owner is offered a fresh air heat exchanger or an additional powder room guess which gets picked most often.

Great points everyone. Thanks. Will be doing a radon test this week at same property so will check relative humidity at that time. Also, the make up air ducting is insulated which hasn't stopped the condensation. Client was referred to a HVAC contractor familiar with "tight homes" Client is a neighbor. Will post when he informs me of HVAC contractors take.

Originally Posted by Eric Williams

Great points everyone. Thanks. Will be doing a radon test this week at same property so will check relative humidity at that time. Also, the make up air ducting is insulated which hasn't stopped the condensation. Client was referred to a HVAC contractor familiar with "tight homes" Client is a neighbor. Will post when he informs me of HVAC contractors take.

Insulating the fresh air intake duct will only make the condensation problem at the return air duct worse. The return air duct is what needs additional insulation where the condensation is forming.

Originally Posted by Vern Heiler

Insulating the fresh air intake duct will only make the condensation problem at the return air duct worse. The return air duct is what needs additional insulation where the condensation is forming.

Vern,

Makes sense. Thanks.

Makes you wonder what other condensation related concerns we will find on new "tight" construction going forward.

Originally Posted by Eric Williams

Vern,

Makes sense. Thanks.

Makes you wonder what other condensation related concerns we will find on new "tight" construction going forward.

Sometimes common sense goes out the window when we think we are making things better.
Things that have been found with this type installation. Condensation causing the ducting to rust out shorting its life. Mold counts have been known to be higher. That 80 or 90 percent furnace or heat pump, having to work harder to heat and cool the air in the home. That condensation is more likely to form on the heat exchanger as well as those ducts. If you are having to heat or cool the air that is passing through that heat exchanger 30 to 60 or more degrees or more. What is happening to the Furnace.
More insulation is not going to help on the Ducting. As the air passes through the ducking at the outside temperature. The temperature is going to migrate through that insulation as it does when you have no heat on the inside of your home. The air needs to be conditioned before it passes through the ducting. There has been other things found with the the tight home concept. The good thing is you are filtering the air you breath. There are better ways of doing this.

To me make up air implies air to a combustion appliance. It appears what you have is a fresh air intake that is ducted into the return vent. That is a cheap way of doing it and not a very good way either. The super cold air be a problem for the furnace if it is not diluted before getting to the heat exchanger.


Because there is condensation I suspect there is not control of the incoming air. A constant flow of air is keeping the metal cold and you have condensation. It is more a matter or temp than excess humidity.

If the house is built tight then this system provide no control on the amount of fresh air. The more the furnace/ac runs the more fresh air that is brought into the house. When the furnace/ac does not run much or at all then the home is being under ventilated.

This setup will also pressurize the house because the blower is pulling in outside air. It forces air out through leaks which can be a bad things in cold climates. Warm moist air is being forced into areas where it can condense, not actually condense but sorption by the materials that may be prone to mold.

Other approaches are an HRV/ERV or an exhaust only system using a low volume continous fan.