Posts Tagged ‘indoor air quality’

What goes out must come in

December 5, 2010

On my Linked In page, I passed on a link to an article titled, “Makeup Air for Range Hoods” (http://www.greenbuildingadvisor.com/blogs/dept/musings/makeup-air-range-hoods).  The author, Martin Holladay, digs into the details of adding a larger range hood to a home.  In particular, he investigated whether consumers were warned about what could be the impact of adding such a hood to a home.  Mr. Holladay’s blog is a fair warning to consumers that they might not be warned about products that could have a negative impact on your home’s environment, and that impact could be hazardous.

In this post, as I promised on my LI page, I am going to dig into more detail about the impact the fan or other devices that change home pressurization could have on the home and potentially its occupants.  Let me start the same way.  Consider your home–or any building for that matter–as a box.  When air is removed from that box, replacement air has to come into the box to replace the air that is removed.  This replacement air is normally called makeup air by building professionals.  The issue is from where that makeup air comes.

If the home is air leaky, makeup air will come through openings in the building’s exterior shell, i.e. walls, roof, windows, doors, etc.  Leaks are usually a major issue because the air can come from many places.  Rehabbing older homes cuts down on the leaks, but usually cannot totally eliminate them.  But, even newer, so-called tight, buildings have air leaks.  The problem with air leaks in all buildings is mainly a thermal comfort issue because leaks cause drafts, and few people enjoy the feeling of a cold air stream inside the home on a cold day.  Another just as important issue is that if air can come in through a leak, it can go out through a leak, which translates into loss of heated or cooled air.  But, air streams flowing past hoods, fireplaces, ventless heaters, etc. can impact their operation.

Air is a lot like most people in that it takes the path of least resistance.  So, air coming through leaks can mess with the home’s return air system.  For a heating and air conditioning system to work properly, adequate air of a certain temperature has to be supplied to an area to compensate for the heat gain or loss from that area.  What may no be known is that air has to also be removed from the area to help the supply air side of the system work right.  In buildings with poor return air systems, large temperature differences could exist from one area to another even if the supply air flow is ideally designed and installed.  What can happen is that air entering through leaks has the path of least resistance to the air handler.  That air, then, prevents the return air system from returning air back to the air handler from some (or all) of the building’s areas.  Those areas that do not have proper air return will be hotter or colder, depending on whether the air handling system is in the cooling or heating mode, respectively.  Alternatively, those areas through which the leak air is traveling could be too hot or too cold.

What about buildings that are pretty well sealed?  If the exhaust fans in the home do not have adequate makeup air, they will either not move as much air as designed or makeup air will come from wherever it can, which might be from undesirable places, such as through flues or chimneys.  The condition of air flowing in the reverse direction from which it is intended is called backdrafting.  When backdrafting occurs in flues and chimneys, combustion gases, including carbon monoxide, could be pulled into the home.  When wood is not being burnt in the fireplace, backdrafting air can pull creosote emissions into the home, which believe me is not a desirable fragrance. Backdrafting can also impact combustion of some appliances, making them less efficient.

What if a combustion appliance, such as a water heater or furnace, have flue booster fans that are supposed to push the exhaust gases outdoors.  If the appliance is a closed system, such as many of the higher efficiency furnaces, no impact should be expected because closed systems pull air directly from outdoors.  Appliances that pull air from the general building air, as do the water heaters with flue fans, may be affected, depending on which fan in the building moves more air or other characteristics.

The impact that exhaust fans could have on other combustion appliances is not unknown to building professionals, though.  Some homes with furnaces that used general home air for combustion have a duct that runs from outdoors to somewhere in the vicinity of the furnace.  That is all.  This duct is not connected to any fans or any other devices.  This duct is simply a path for air to travel from outdoors to indoors to makeup air removed from the home.  Do they work?  Maybe.  But, as I said earlier, air takes the path of least resistance and, if the duct is not the path of least resistance, it will not work.

The concern as stated in the blog post cited at the beginning of this blog in particular was discussing the installation of large kitchen exhaust fans that move over 1000 cubic feet per minute (cfm) of air.  That is an extraordinarily large fan for a home.  Will the usual kitchen exhaust fan cause the same issues?  If it doesn’t exhaust air outdoors, which is normal for these fans, the answer is most likely no.  If the fan exhausts air outdoors, the answer is maybe.  By itself, the fan likely will not cause backdrafting at the least.  That fan in combination with other fans might cause backdrafting.  Again, if the home is air leaky, any exhaust fan can pull air through leaks, causing thermal comfort and/or heating or cooling efficiency issues.

Are exhaust fans the only things that can cause air leakage or backdrafting issues?  Well, no.  All buildings have a chimney effect where heated air rises within the building.  As this air rises, air is pulled into the building at lower levels.  The chimney effect is for the usual home is definitely not as great as that for a high rise building.  In most homes, backdrafting due to the chimney effect is likely not an issue; but the chimney effect can cause drafts in lower parts of the home.  The only way to know whether a backdrafting or draft issue exists is through investigation.  A homeowner might be able to investigate the issue using smoke from an incense stick or other air flow indicator.   However, a professional will know more about the conditions to which to test under and will have more sophisticated instruments than a homeowner.

So, if you have this favorite nook in your home that sometimes feels comfortable and then other times feels drafty, consider that the reason could be an exhaust fan or the home’s air handler pulling air through a leak.  What to do about it from there is up to you.

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Central vs. Portable Humidification Systems

November 14, 2010

In previous posts, I discussed how humidification systems work and how effective a central humidification system might be.  In that post, I hinted at differences between central (whole house) humidification systems and portable (local) humidification systems.  In this post, I want to discuss more about the differences between the two systems.

If you have not realized yet from previous posts, I am biased–I am not a fan of central humidification systems.  I see them often in the homes I inspect; but I suspect that most do not work as intended.  Yet, there they are.  I have to wonder if the installation company actually analyzed the need for a humidification system or was just selling a product that is quite profitable for the company.  Simply asking the homeowner whether he and/or she wanted a humidification system is not the analysis about which I am talking, by the way.

Following are the reasons that I believe a PHS is better than a central humidification system.  For simplification, CHS is used for central humidification system, while PHS is used for PHS.

First is the big picture.  Why try to humidify a whole home when all of the home occupants are not in all of the home’s areas at once?  Let me put is bluntly, injecting a gallon of water into a room is going to be more effective at raising the humidity than injecting a gallon of water into the whole home.

Second is effectiveness.  I believe that a portable humidification system providing spot moisture would likely work better than a CHS.  With a PHS, moisture from a judicially placed unit can be directed into the air around a person’s head (the area that industrial hygienist types call the breathing zone or when they really want to be cool—the BZ).  Even if the PHS cannot be aimed to direct moisture into the occupant’s breathing zone, it can be located close enough to elevate the moisture in the air people are breathing.  If the room can be closed up, such as a bedroom, humidification will likely be more successful than trying to humidify an entire home.  In fact, in the previous blog where I discussed virus and humidification, a portable humidifier was used in that research.  As my previous blog post showed,, CHSs are probably not effective at significantly elevating humidity levels in some homes, particularly air leaky homes.

Home leakiness leads to the third reason portable is better than CHSs—losses.  Moisture in the air is in the form of water vapor, which behaves like the other gases in air.  As such, if the amount of airborne moisture in one area is greater than in another area, moisture in the first area will travel to the second area as long as the two areas are connected.  On cold winter days, the amount of moisture in the outdoor air is usually lower than the amount in indoor air (a subject for another post).  In air leaky homes, indoor moisture will likely move outdoors, even if outdoor air is moving in the opposite direction.  So, most of the water a CHS is putting into the air could be traveling directly outdoors, barely elevating the indoor humidity levels.  Over an entire home, the total area of the air leaks is less than those in a single room.  Therefore, the amount of moisture being lost from one area will be less than throughout all areas.  If a PHS is supplying more moisture to a given area than the central system is supplying to the entire home area, moisture levels in the area with the PHS will be greater.  Even if the moisture from the PHS is also traveling outdoors, it has a better chance of being effective as it travels through people’s breathing zones on it trip outdoors.

A fourth reason is condensation areas.  Most homes have cold surfaces in the winter where condensation can occur.  The chances that condensation surfaces are in the same room with a PHS are less than the chances of moisture from a CHS seeing a condensation surface.

A fifth reason is better humidity control.  Some PHSs now come with their own humidistats.  Basically, the portable unit is sensing the humidity right in the space where the person is.  CHS humidistats are installed in the return air duct in an effort to sense the “average” humidity in the home.  What happens if the return air system is pulling more air from some areas of the home than others, meaning that it is not sensing the true average humidity levels?  That issue is more common than you might think.  Some central system humidistats are placed on a wall in the home; but those also have the same issues as far as sensing the “average” humidity levels.

A sixth reason is operation.  CHSs, if they are working right, only humidify air when the air handling system is working.  During the rest of the time, the CHS is at the mercy of the thermostat.  The central humidification control system has to wait until the thermostat calls for heat before it can work.  Just in case someone is thinking that the CHS can be set to operate without the furnace, remember that the CHS needs air moving through it to work.  Someone is likely also thinking that the thermostat can be set to ON so that the fan is operating all the time.  Then, if the humidistat calls for humidification, air will be flowing through the CHS.  It could; but, the reason air downstream of the furnace is passed through the CHS is because heated air can hold more moisture than cooler air. So, more water will be lost with the CHS if unheated air is passed through it than heated air.  With PHSs, the unit is always injecting moisture into the air without the need for moving air to transport the moisture.

A seventh reason is maintenance.  If a valve on the CHS sticks open, water will be dumped right down the drain when air is not moving through it.  In some cases, the CHS drain is plugged up, and water is dumped into the air handler and then ultimately onto the floor outside the unit.  If that water travels to nearby furnished areas, moisture-related damage can occur.  Sometimes, the damage is extensive, such as shown in the photos below.  The fact is that CHSs are usually not inspected very often and problems may not found until a serious malfunction occurs.  If a PHS malfunctions, it is usually right in the same room with the home’s occupants, who can then see that a problem is occurring.  Maintenance of a PHS is usually so easy that the home owner can do it.  For most homeowners, a HVAC technician is needed to service the CHS.  Remember too, that the more debris that collects on the media inside a CHS, the less air can get through the CHS and the less effective it will be.  The PHS, on the other hand, can be kept clean of debris.

 

Water damage caused by a malfunctioning CHS

 

 

An eighth is bioaerosols.  One of the more well-known cases with PHSs is humidifier fever caused by a PHS that was not properly cleaned and reservoir water was allowed to sit in the unit and grow yuck.  When the unit was operated, the yuck was injected into the air that people were breathing.  But, CHSs are not without the same problems.  In fact, they are essentially operating as a back-up filter to air handler filter.  The debris collected on the media inside the CHS stays there until the media is replaced and that debris contains bioaerosols that may find the conditions inside the CHS a very nice place to grow.  Nowadays, most PHSs are made so that the owner can readily clean the unit and all of them recommend using fresh water every time the unit is operated.

A ninth reason is cost.  A homeowner can buy a lot of PHSs for the cost of one CHS.  For sure, the initial cost of a CHS is much more than a PHS.  But, consider also that if you are not happy with the operation of the humidification system, replacing a PHS is a lot cheaper than replacing a CHS.  I have found cases where homes had unused or disabled CHSs along with PHSs that were being used.

A tenth and final reason is choice.  For residences, homeowners are mainly stuck with one option—the wetted media CHS.  Although residential steam injection CHSs are available, they are even more expensive than the wetted media CHSs.  With PHSs, the owner has not only choice of the method of humidification, as explained in a previous post, a number of manufacturers produce the various types of PHSs.  Having a range of options also means more competition with PHSs than with CHSs, which further means price and feature competition.  I have seen several CHSs and they appear to be amazingly similar, while I have seen a range of PHS designs and those designs continue to evolve.

I guess I could be faulted in this post for not finding more advantages of CHSs over PHSs.  The truth is, none comes to my mind other than the fact that water is supplied to the CHS, while the owner has to carry water to the PHS.  Even though that difference could be considered an advantage of a CHS over a PHS, I think it can also be considered a disadvantage because when changing water, the owner actually is inspecting the PHS and likely keeping it clean.

If you have another opinion, let me know.

Humidification and Virus

September 29, 2010

The Environmental Health Journal published an article this past month titled, “Modeling the airborne survival of influenza virus in a residential setting: the impacts of home humidification“.  The results of the research taken from the article are, “The presence of a portable humidifier with an output of 0.16 kg water per hour in the bedroom resulted in an increase in median sleeping hours AH/RH levels of 11 to 19% compared to periods without a humidifier present. The associated percent decrease in influenza virus survival was 17.5 – 31.6%. Distribution of water vapor through a residence was estimated to yield 3 to 12% increases in AH/RH and 7.8-13.9% reductions in influenza virus survival.”

What the article states that a slight rise in relative humidity in the room caused a slight drop in the influenza viruses in the air.  I have a couple of criticisms about the article, though.  The authors derived this conclusion based on limited data using modeling instead of actual sampling of the virus.  The lack of data does not really show how the trend would go for humidity versus virus concentrations.  Without data from actual virus, the question also exists whether the model is realistic or whether the results are repeatable.  I cannot really fault them because sampling for viruses is difficult at its least, particularly infectious viruses and research is expensive.

But, I am still concerned about whether the risks of a humidification system are worth the beneficial effect on virus.  Other studies have shown that hygiene–hand washing and avoiding sick people–are quite effective in preventing or minimizing virus-related illnesses.  Flu shots are also effective in preventing flu for which the sera are developed.  I can personally attest to that fact.  I also tend to believe that flu shots, even past ones, may help minimize the effects of other virus.

I will confess, though, that I am not a big fan of some humidification systems, particularly central humidification systems.  I have observed too many issues with them from basic design to maintenance-related issues.  In future blogs, I am going to go into more detail about humidification systems and their pros and cons, as I see them.  I even got off on that tangent writing about those issues for this blog.  I decided to reign in this post to stick with the topic of humidity and virus.

What can be drawn from this and other research.  I believe that humidity levels could have an effect on virus levels in the air.  I have known for a long time that when those sloppy wet infectious particles that are sneezed into the air, or fomites as the medical gang call them, they have a better chance of drying in low-humidity air.  Drying of the particles means that they become smaller and lighter, which in turn means that they can stay afloat in the air longer.  They can also be breathed deeper into the respiratory system where they can more easily set up shop.  The weird thing is that some virus actually survive better in the dried out state.   (Here are a couple of examples of research pointing to that conclusion:  “Dry air might boost flu transmission” and “Flu Mystery Solved? Why It Flourishes in Winter“.)  Elevated humidity is a good thing.  My dispute is with the way to humidify, and that will be the subject of other posts.


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