You are brining in too much humid outside air and your system cannot remove it, it was not designed for that. Get rid of the outside air connection you may be causing mold (the smell) and damaging your system.

If you absolutely need (asthma etc.) to be rid of the dust then there may be several other options. It sounds like you have already had a pressurized door test is that correct?
If so you could have it done again use smoke to find and seal the leaks, but your house may become too tight and that can be bad as well.
Otherwise you may be looking at a substantial investment with some larger equipment or some controls. Where are you located? Do you have more details?

 

Your house is at a negative but your ducts are tight.

I'm confused.

 

LOL.

Why not find the cause of your negative pressure.

Can't be in a negative pressure if nothing is taking the air out.

What exhaust fans do you have running.
Do you have a powered attic fan.

Have a contractor install the proper fresh air damper and set it up.

 

Is there a damper in that 6-inch FA (fresh air) duct? If you are getting too much outside air in, a damper in that 6-inch duct would allow you to calibrate the amount of fresh air input. I've read research from Joe Lstiburek of Building Sciences Corporation, which documents his errors in sizing such ducts, until he devised and patented a gadget to pull in a measured amount. Somewhere in my bookshelf I have some detailed reports of air quantity vs. duct size at a given plenum vacuum.

I would not eliminate the entire duct if I were you, it is a source of positive house pressure and is supposed to keep down infiltration and dust. It remains a puzzle where your negative house pressure comes from, if not from supply duct leakage to outside. Can you shed any light on the cause?

Best of luck -- Pstu

 

thanks for the quick replies

Thank you all for responding to my question. I'll try to address your comments as best I can.

I don't know what the cause of my negative pressure is and I don't know how to find that out. The fact that I have a tight house and tight ducts, my air handler doesn't leak (I sealed it myself and tested same with smoke), and I have negative pressure is bewildering to me also. One hvac contractor assured me that the cause was insufficient return air ducting. I had a 16 inch main trunk. I had him install a parallel 12 inch trunk. No change in negative pressure.

I don't have any exhaust fans at all.

I added the fresh air 6 inch flex duct at the suggestions of two hvac contractors and one public building inspector. The inspector said that he did same at his home and that it worked wonders.

I did consider that the 6 inch diameter ducting was bringing in too much damp air. I taped over the duct opening at the window and cut a one inch square hole in the tape as a way to damper down the flow. Same result as before, dank smelling air.

 

You don't have a bathroom fan?
Your range doesn't have one?
Is there an access panel to the attic from the house?


Close off the fresh air completely, and see if the odor goes away.
If it doesn't you have more problems then you know.


Don't cure the symtom, cure the cause.

 

A large percentage of household dust is internally generated. Carpet fibers, fibers from bedding, clothing, window treatments, pets, human skin cells, etc. In fact, many IAQ people will say that the solution to indoor pollution is outdoor air dilution, which seems to be the strategy you took by adding an outside air intake duct.

To address the dank smell concern, are you running the indoor blower continuously, or does this odor only occcur when the cooling system is running, with blower off between cooling cycles? A continuously running blower with an outdoor air intake will pump a lot of humidity into the house and ductwork when the compressor is not running. That can lead to an acceleration of microbiological growth on the coil and duct surfaces, especially as air warms during the off cycle.

It appears your entire adventure with outdoor air intake began with the premise that infiltration was the source of dust in your home. It is actually only a percentage of dust generation within a house. You should consider addressing other factors of dust generation such as carpet fibers, etc. I personally will someday see a carpet-free house. Carpets are humongous dust collectors; every time you walk on it you're stirring up clouds of invisible dust particles, as the majority of dust particles are not visible to the human eye.

 

does your house have a ridge vent? could be pulling from the house if the soffet vent arent big enough. just a thought not a sullution.

 

You need to hire a professional that specializes in IAQ.

You're reading forums and taking stabs at this yourself and failing.

I suggest a properly balanced ERV/HRV installed by a professional. Also a blower door test on the house.

 

How badly depressurized is your home?
Did they document any pressures?
What instruments were used to arrive at the findings they gave you?
Do you have any can lights?
Do you have any holes that communicate directly to the attic or outdoors like soffits or chases?
How do you know your house is tight, any blower door readings taken?
Where is your ductwork & air handler located?

Pretty sad that the company who uncovered the problem can't recommend any solutions.

 

I concure!

Watch that clothes drier- it will cause a neg psi too.

 

An air exchanger may be whats needed for the dank smell.
You have to vent out what you bring in.

The air handling system alone isnt going to cause a negative pressure unless a duct is open in an attic or crawl space. Its only going to move the air in the space.

Use better filters.

A simple way to check for in home pressures is to close all the windows etc.. with AHU on. using a cigarette lighter simply crack open a door or window just a touch and check the air flow across the flame at the opening and see wich way the wind blows. It aint perfect science but it will help to begin determining building pressures.

 

thank you all again

Thanks folks. Here's some more info.

I live in Ocala Florida where it's hot and damp.

I don't have any carpets in my house at all. First thing I did when I purchased house was remove all carpets and install hardwood floors throughout.

I have perforated soffets completely around the house.

I have a ridge vent but only half of it is open. Other half seems blocked with roofing paper.

The negative pressure value given to me was 1.6 pascals. I'm an old "pounds per square inch" guy myself and don't know what the conversion factor is. Incidentally I was testing the pressure myself by cracking the door with incense burning and watching which way the smoke went. Then one of the hvac contractors showed me that that test was invalid because the smoke would blow in at the top of the door and blow out at the bottom. After that I hired a professional recommended by the "Energy Conservatory" to do the testing.

I have an access door to the attic in the bedroom closet and one in the garage.

The range fan does not vent to the attic but is restricted to the kitchen.

I only added the fresh air intake ducting a couple of days ago. Prior to that the ac air was cool and crisp, I would just get an abnormal build up of dust. This is as compared to other peoples homes.

I don't run my ac fan continuously and I don't get the dank air smell when the system cycles off.

My air handler is in the garage and my ducts are in the attic.

 

The 1.6 pascals converts over to about .00643" water column.

Are those attic access panels sealed well?
Did they try sealing them off to eliminate them as a source?

Any can lights?

Did they test the house pressure with any wind blowing outside that day?

 

Could the dust be from when you had the hard wood floors installed.

Did they use the garage as a work area for cutting.

 

Any other details about the type of smell?

 

Negative Air Pressure Dust in the House

Do you have a fireplace? (Warm air going up the chimney will cause a neg. pressure) Do you have a gas water heater? Same as above.
Be sure you have adequate combustion air for fossil fuel appliances.
If its the dust in the house thats bothering, I suggest a high efficency pleated filter, and constant fan operation, which will keep dust in suspension, and will also tend to keep a closer temperature set point by eliminating stratification while filtering the air.

 

I have many can lights. I sealed them with newer inserts. That was before I started reading this forum and learned that's a no-no but what's done is done.

I have a fireplace. I blocked off and sealed the opening to isolate same while trying to solve this problem.

My hardwood floors were installed seven years ago. They did their cutting and sanding inside the house but that dust should be all gone by now.

I have an electric water heater. No gas appliances whatsoever.

I have one of those April Aire pleated air filters. I forget which model. I've been advised by the hvac contractors that here in Florida one should not keep the fan running all the time. When compressor cycles off fan then blows moisture from the evaporator coil into the ducts resulting in mildew growth.

Attic access panels aren't sealed well. That's on my to-do list. Interestingly, I don't have a dust problem in my garage.

They tested the house pressure with the a/c system turned off and with the wind blowing outside. Negative pressure then too. Value fluctuated a lot but it seemed to hover at a value greater than two pascals.

The only way that I can characterize the smell would be that of dank but fresh air.

 

have you sealed between the elec boxes & wall | ceiling?
have you caulked ALL wall seams?
have you caulked all trim edges, inclucing above & below baseboard?
have you sealed all pipe & wire penetrations into the house?
have you sealed all wire & pipe penetratins in the attic?
have you sealed between the cans & ceiling?

 

Apparently whoever performed your blower door testing did not offer much help as to the source of your infiltration. Seems to me while a house has a blower door running, that's the best time to go around and pinpoint areas that are allowing air into the home through the envelope, note each one and a recommended course of action to correct it.

Sans that being done, with your ducts in the attic you likely have ceiling supply air boots and registers. Pull a register off and see if there's a gap between the supply boot and the drywall. 99.9% of the time there is, and sometimes it is HUGE. One might say that since this gap is at the supply boot, when the system runs it will actually pressurize this area and blow air back into the attic, so other than a little air loss, what's the big deal? Big deal is any air lost from return to supply will put the house into a negative. You may be blowing air back into the attic at these locations but the make-up air is coming through any other compromise in your building shell. Seal this gap wherever you find it.

One other thing I'd check is the return chamber below your upflow furnace in the garage. If any side of it has open stud bays, you're likely pulling attic air in that way. Not only does that add to dust load (filtration can't catch it all) but your heat load as well. I personally like to line return chambers below an upflow with ductboard or insulation board, and seal the joints with tape and mastic. This assures that you're getting return air strictly from the house, not the house and somewhere else.
One other thing about an air handler/furnace being in a garage...I'd want that thing airtight out there. That location is an excellent source for introduction of pollutants if the return chamber/plenum, furnace/air handler, cooling coil, and supply plenum are not sealed well.

Also, exactly what is an "insert" when it comes to can lights? My thoughts about can lights is that they should either be the sealed type from the factory (type IC - insulation contact), or they should not share space with an attic or other non-conditioned area. As an alternative for non IC I've heard of duct board boxes being made with adequate clearance from the fixture and then placed over the fixture and sealed to the drywall to form an airtight box. I would only try that if I could be assured it would not cause problems to the fixture or pose a potential fire hazard. Personally I'd be looking for a cooler burning bulb to put into any can light. Some of the new compact fluorescents are becoming pretty versatile in shape and color...no ghastly pallor.

 

>>...many can lights. I sealed them with newer inserts. That was before I started reading this forum and learned that's a no-no but what's done is done.

I have had can lights sealed with inserts too. It was a fairly high price tweak sold by the guy who worked with "Comfort Institute" from Bellingham Washington. I was told mine are UL listed, are they a no-no too?

I've seen a published report documenting a sheet-rock box made to enclose canister lights from the attic. Designed with a couple inches clearance in all directions, looks like a sound plan to me. Would anyone say that is a no-no?

Probably the best idea for most people is to just buy the sealed canister lamps in the first place, or replace the old leakmaster types with new sealed lamps. But there is a labor and convenience factor there.

>>I have one of those April Aire pleated air filters. I forget which model. I've been advised by the hvac contractors that
>>here in Florida one should not keep the fan running all the time. When compressor cycles off fan then blows moisture
>>from the evaporator coil into the ducts resulting in mildew growth.

The moisture part is true according to my best info, but I have not heard the part about mildew growth from this practice. Please tell me where this mildew would be. Ducts or in the house? On the other hand, even Trane's "Comfort-R" includes a 3-minute airflow at 50% speed after the compressor shuts off, for "Efficiency" reasons. This from a procedure whose entire purpose is to combat humidity -- perhaps they feel they give so much on the front part of the cycle, they can afford to compromise humidity at the back end?

How bad should one feel about minus 1.6 Pa pressure anyway? That is not in the range documented to cause backdrafting hazard with combustion appliances. Some people with much building experience (not necessarily building science pros, but not althogether ignorant either) have mentioned casually that *most* houses operate at negative pressure. Could one possibly dismiss 1.6 Pa as not harmful?

Best wishes -- Pstu

 

you guys are a tremendous help

Thanks again y'all. I really appreciate your suggestions.

cem-bsee, the only thing in your list that I have done is that I have sealed
between the cans and the ceiling.

shophound, I personally reconnected all of my duct joints and I caulked all of
the register boots at the top of the ceilings. The guy who measured my pressure told me that out of over 1000 homes that he tested I had the tightest
duct system he'd ever seen. At the air handler I personally rebuilt the plenum
box and caulked same, caulked the drywall behind the box, and gave the system the incense smoke test to assure air tightness. Same with air handler. Those do not leak.

Pstu, yeah I paid over 10 dollars a piece for my can light inserts and I have 14 of them. I did replace all of my incandescents with flourescents. Here in Florida we have a terrible problem with mildew and any moisture at all getting into the ductboard will cause growth. I don't know how bad 1.6 pascals is but the guy(professional) who did my tests said that any negative
pressure was no good. Then he proceeded to recommend some expensive
add on equipment that would surely "solve" my problem. I've been battling
ac related problems here ever since I bought the house(7 years). During that time I've hired at least 5 different local hvac professionals. Needless to say I've grown leery of all of them.

 

Pstu said:
On the other hand, even Trane's "Comfort-R" includes a 3-minute airflow at 50% speed after the compressor shuts off, for "Efficiency" reasons. This from a procedure whose entire purpose is to combat humidity -- perhaps they feel they give so much on the front part of the cycle, they can afford to compromise humidity at the back end?

One of these days I intend to place a digital psychrometer in the discharge air coming off the coil to record what happens psychrometrically in a post cooling "purge" cycle. My own home has a 90 second post cooling call purge cycle. Just sitting at my desk using the digital psychrometer I have (which measures wet and dry bulb, dewpoint and relative humidity simultaneously) I'm not seeing a big spike in indoor humidity. Dewpoint will begin to creep up a little but mainly after the blower shuts off and the room air circulation slows down. By the time it goes up one degree the system kicks back on again.

As for humidity control I'm a fan of going by dewpoint vs. relative humidity. Dewpoint will indicate actual moisture content of air (when converted to grains per pound of air, also known as humidity ratio), whereas a relative humidity value will fluctuate with air temperature. An interior dewpoint of 55 degrees is a good comfort target for normal indoor temperature ranges between 74-78 degrees.

I think the OP would be best served by a depressurizing test that includes locating sources of infiltration while the house is in a forced negative condition. Otherwise he just got a number that isn't much value outside of plugging into a Manual J calculation for infiltration. I would want to actually see how effective the "inserts" are doing at reducing infiltration at each can light, particularly if the house has many such fixtures, as some house designs do.

 

Did they do a blower door test, duct blaster test or both?
If the blower door test was done what is the ACH of the house, that is the important number that tells how big the problem is. The 1.6 pa might just be the reference number you start with and will change with the wind and even what side of the house the test is being done. It really may mean nothing at all, it depends.

 

Wind is the biggest pressure inducer on your home. The down wind negative pressure is much larger than the positive upwind side. Your hot attic can be a negative pressure pump, even without a exhaust fan. Hot air is much light than cold. Your soffit vent should 2X your ridge vent area. Your negative pressure level is minimal. You did not report your pressure with make-up air. Attaching a some data from a WI home in Oct. This like your winter. This is a tight home with .1 ach natural. We added 75 cfm of make-up air on a ventilation schedule. Most dust is a result from wind and on clothes.

You have a wet spot or a decaying animal in your ducts. Inspect your evap/pan for standing water or wet spot in your air handler. With your determination, you should be able to find the odor source. You need a minimum of 50-75 cfm of fresh air in your home when occupied to purge indoor pollutants and renew oxygen. If the indoor %RH gets high, add supplemental dehumidification to maintain <50%RH. Plenty of oxygen and eliminates dust mites and mold. Ideally you should dry ducts/condensate pan for several hours every day to avoid bacterial/mold growth. HRV/ERVs are balance flow and have no effect on house pressure. Tape plastic over two open windows on opposite sides of your home and observe the inward/outward pressure with/without wind and at warm/cold conditions.

 

"Your ridge vent should 2X your soffit vent area."

Wow! If that's the case then maybe that is what's causing my problem. I would
estimate that my soffit area is at least 40 - 50 X my ridge vent.

 

Does anyone else predict bad effects if the ridge vent is not 2X the soffit vent area? What specifically might happen? I have looked for but not seen any studies which connected attic ventilation to significant problems with either cooling or humidity. Am very interested if the answer, as my own ridge area is rather puny.

It is always very interesting to ask the question "If you violate this rule, then what will happen?". Sometimes I can offer an answer, many times I cannot.

Best wishes -- Pstu

P.S. I should add my question is for a cooling climate such as Florida or S.Texas. I have repeatedly heard that attic ventilation is to prevent snow dams, was developed up North and brought South where conditions are entirely different.

 

The hotter your attic is, the higher the heat gain from your ceiling.

So if you don't have enough ventilation in it, its time will be higher.