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Question DetailsAsked on 6/1/2016

Who to trust and how much to pay to assess whether there is an airflow imbalance in a home?

The home has elevated humidity, and when the A/C is running it fails to decrease the humidity level (but does decrease the temperature). I believe it is caused by inadequate number/size of intake vents throughout the house, but want to confirm this before attempting to fix.

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3 Answers


To assess the functioning of the system, you need a Heating and A/C (your Search the List category) contractor, and one experienced in HVAC system design, not just maintenance.

He may be able to visually check the system out and find a basic flaw in the installation or design - or may have to run an ACCA (Air Conditioning Contractors of America) Manual J, S and D calculation for your house to see how the current installation matches up to the capacity and ducting it should have. Probably about $150-200 something to do this inspection and assessment. [J, S, and D calculations determine the heating/cooling load, the HVAC unit sizing or capacity needed, and the duct sizing and layout respectively).

Cause could be inadequate overall airflow (as you seem to be indicating) due to restricted or inadequate return vents or ducting (or registers stopped down, maybe from winter heating adjustment), an air filter restricting airflow through the ducts, undersized fan unit for the combined HVAC system, evaporator plugged with dust (and myabe mold) which needs cleaning, improper settings on duct dampers or registers, improperly sized HVAC unit, etc.

One thing to remember - when air cools, if moisture is not removed from it, then the relative humidity will go UP - cooler air can hold less moisture, hence the frosting in your freezer or being able to "see' your breath on a cold morning, for instance. So - the air conditioner evaporator may be removing moisture from the warmer return/outside air on every air exchange through it, but actually end up with the same or higher humidity in the chilled air. In a tight house the cycling of air through the evaporator will eventually decrease the humidity in the house, but in an open house (not very airtight, leaking attic air into house, open windows, etc) continual exhange of high-moisture content outside air can result in a constantly high interior humidity with air conditioning.

An air conditioner that is oversized can also cause this - because it may run such a short time (called short-cycling") that while it cools the air down to the shutdown point, there are not enough air passes through it to remove signficant moisture. This is a common problem where owners or builders or HVAC contractors use a rule-of-thumb sizing guideline or oversize an A/C unit thinking that will work better - whereas it can actually cause high humidity in the house and mold growth on the evaporator because there is not enough airflow through the evaporator to remove the excess moisture as condensate, which can also promote mold if the evaporator stays wet at the end of the airflow cycle.

Or it freezes up the evaporator because it is too cold (due to an oversized A/C unit) or because the fan is undersized, reducing airflow and not removing moisture permanently from the system.

Even a condensate drain pan under the evaporator which does not drain out correctly can add a lot of moisture back into the system as evaporation from the pan - it should not retain water, it should immediately drain out the water that drips into it.

Another possibility is the return air does not pass through the air filter right back into the air intake chamber - if the return duct was improperly (or decades ago) returned to open air around the furnace/air handler, it may be pulling more outside air in through the makeup air vent than it needs, constantly introducing new humidity into the airflow.

Answered 2 years ago by LCD


Thank you, LCD, for your extremely insightful reply. I live in the Chicago area and have had no success finding any qualified firm or individual willing to perform the evaluation you recommend. Could you provide recommendations of firms capable of performing this HVAC analysis in Chicago? I would greatly appreciate any guidance you could provide.

Answered 2 years ago by SeekingAdvice


Not working in the Chicago area, I can't help you with specific vendor recommendations. Other than looking for good ratings and some reviews talking about airflow balance work on Angies List (Heating and A/C is the Search the List category), here are other possibilities:

If you have used an architect in the past, they might be able to give you a recommendation - or larger Architect/Engineer firms commonly have a mechanical engineer on staff who designs HVAC systems for new construction and might be able to help solve your problem (though likely several hundred $ more expensive than an HVAC contractor).

If you go to a couple of larger HVAC equipment/materials wholesalers in your area who are not in the business of HVAC installations/repair as well (google or yellow pages to find that) you could ask who they would recommend for an airflow imbalance and humidity issue assessment (aka building performance assessment).

In larger areas, you will find google or yellow pages (then cross-check for Angies List reviews) indoor air quality and ventilation assessment consultants who specialize in this sort of thing - though most deal with commercial buildings mainly, because that is where you get themost drastic imbalance issues, and also the most $ impact from them.

I would go with an ACCA (Air Conditioning Contractors of America) member for an HVAC contractor - here is their locator page,

because one of the first things he will need to do is run an ACCA Manual D (duct sizing and layout) calaculation to look at what you need before determining how that fits in with what you have now for an HVAC system. Part of that assessment should include an air pressure check - checking the plenum (outgoing) air pressure both in front of and behind the A/C evaporator and also at the air filter (on both sides) to look for excessive pressure losses at either of those two points. He will also need to check for any dampers (manual or electrically controlled) in your system for settings and correct sizing and controls, and also look at your thermostats. This assessment will include determining the needed size and location of ducts and the size/location of supply and return registers - so just this step alone would answer your question about whether you have an airflow imbalance - either generally undersized returns (or supply) or an imbalance between different areas/floors of the house.

Thermostats are important because occasionally they are miswired - wrong floor thermostat to damper, or even both wired together so if either comes on the system turns on - amazing how many times I have seen that one. Also, all but the very simplest thearmostats have delay period set into them depending on the type of devices you have, as different type systems have different delay and lag times - the amount of time it takes for the system to warm up (very short ofr hot air furnaces, quite long for steam and hot water heating), and the amount of time the sysstem keeps putting out heat or cold after the thermostat turns off (again, very short with forced air or mini-split A/C, quite long with liquid heating or cooling system. Having the wrong delay set (programmable in the more advanced thermostats) can result in excessive heating or cooling each cycle. Thertmostat location is also important - it is all too common to put the upstairs thermostat right at the head of the stairs, meaning in winter it senses warm air rising from the downstairs and turns off too early, and catches cool air flowing down the stairs in summer and turns off too soon. I have seen cases with thermostats at both top and bottom of stairs that ended up having the two zones fighting with each other because they were measuring the airflow up/downstairs rather than the average temperature on each floor.

Also, a check of the evaporator and any filters or electrostatic precipitators will be needed - both for airflow blockage because they need cleaning, and also to check that the A/C unit is operating correctly so the evaporator (A/C coil) is running at the right temperature so it cools correctly and also so it removes humidity correctly. It has to be cold enough that its surface is well below the dew point of the air flowing over it to remove humidity, but of course can not be so low it freezes up.

You said it cools but does not reduce the humidity - which may not actually be a problem, because say it is cooling the air to 20 degrees below outside air temperature, to 70 from 90 degrees. If the 90 degree air is anything above 50% relative humidity (a level where humidity would normally just start to be considered a concern), if you drop that air to 70 degrees without any dilution the humidity in that now 70 degree air will rise to 100% and condense moisture in your house. So - if your humidity is not rising significantly in the cooler air (as compared to the humidity in the "incoming" air at the air filter say) then it is actually removing moisture at the evaporator coil, and your evaporator coil drain tube will be dripping water anytime the unit has been running more than a few minutes. The humidity % (which is a function of temperature) may not be changing - but if that is the case, the actual amount of water vapor in the indoor air is being decreased by the unit.

The charts are hard to explain outside a classroom, but if you want to measure the temperature and humidity (in calm air, not in an airflow) of the air passing thorough the air filter into the air handler, and also of the temperature and humidity in a typical room (or average of several) near a supply vent but not in the airflow itself, I can tell you what percentage of the moisture in the incoming air is being removed in one pass through the system. Respond back using the yellow Answer This Question link right under your question, just as if you were answering your own question, and I will respond back to you.

If the indoor air humidity is too high regardless, then unless there is such inadequate returns that little airflow is making it back to the air handler, the problem is more likely the operating temperature of the evaporator (which could indicate low gas pressure in the A/C unit or a malfunctioning TXV valve (which is what controls the refirgerant gas flow in the A/C system). Or if your system is oversized or the thermostat setback is too small (difference between where it kicks on and where it kicks off), it could be "short-cycling" - running for a short time to cool the house but not running long enough to pass enough air over the cold evaporator to remove a significant amount of moisture from the air.

Another possibiliity is your system is mixing in too much outdoor high-humidity air, so it basically cannot ever get ahead of the game. Checking your unit for short-cycling (should run for at least 5 and preferably 10-15 minutes per cycle) and putting a humidity gage right at a several supply registers and checking that against readings at several return registers (to see how much humidity it is picking up in the house) might give you an idea of whether overall low circulation is the problem or not. Basically, if the airflow coming in at the supply registers is enough to lift up and "float" a piece of typing/printer paper system airflow is not likely the problem - and if a piece of paper will stick to the return register (assuming it is in ceiling or on wall) that is likely OK too. Also, if A/C is the only problem, and heat is OK in the winter - adequate amount and well-distributed, your ducting and returns and blower are not likely the problem.

Another common problem is too much moisture being added froom inside the house. The exhalation vapor and skin evaporation from residents you cannot do anything about, but excessive hot baths or showers without running the bathroom exhaust fan during and for at least 20-30 minutes afterwards adds a lot of household moisture, as does having a clothes druyer venting moisture into the house, gas fireplaces that vent inside instead of outside, a lot of boiling or wok or similar open-surface cooking (especially if uncovered) and most especially if kitchen fan is not used, open surface hot tubs and pools and ponds and such in the house, moist basements or crawlspaces connected to the "conditioned space", and especially a lot of house plants (other than most cactuses) can really crank up the moisture in the house. You can get a feel for that by measuring the humidity in the vicinity (but not direct airflow) of a supply vent and then near return vents - if there is a significant humidity rise (or generally any, because that return air is warmer so normally humidity would drop) indoor moisture sources may be a good part of your issue.

Other things he needs to look at is convective airflow (is heat or cold air moving up/downstairs along a stairway without doors for instance) or are there excessive or inadequate undoor airflow out of rooms when considered in conjunction with return vents. Checking for leakage at attic entrance (if you have a hot humid attic) or from a damp basement (if not part of the air conditioned system) should also be included in his check. In many cases, hot humid outside air from the attic or damp air from a non-air conditioned basement can throw an HVAC system totally out of whack, because you can gain as much as 5 gallons per day or more of moisture from those sources - I worked one job where they were getting over 15 galloons per day of moisture into the HVAC system because the central air furnace/AC evaporator were in a damp basement with outdoor air vents but none of the return air was getting back to the basement - they had zero reutrn ducts, so the damp basement/outside air was air conditioned and ducted to the rooms, then leaked out of the house through doors and windows and fans, so there was no recirculation through the system to continually reduce the humidity.

He should also check the air filters to be sure they are not too restrictive - some people put in HEPA (hgih-efficiency) filters that are too restrictive and limit the airflow to less than the HVAC unit is designed to move.

I realize I dumped a lot on you here (and you could find more explanations at Wikipedia and Inspectapedia) but without assessing the system no way to tell from here what the issue is - but I hope I have given you some insight into the possibilities. I would start with an ACCA contractor with good reputation and reviews who does system designs, not just installations and repairs - then if there is still a problem after that assessment, yuou could step up to the pricier (typically $500-1000) indoor air efficiency assessment, which might include a blower door test to evaluate how much air you are losing /gaining through walls, vents, etc if it is suspected that your house is not air tight enough.

Answered 2 years ago by LCD

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