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Question DetailsAsked on 1/13/2014

Does a UV light on a furnace really reduce allergens & mold? How does it differ from a HEPA filter?

I've looked up UV furnace lights online and found conflicting results. I have heard that HEPA filters may be as good or better, but have not researched that yet. My situation: 3 cats in a 2400 sq' ranch, and a husband with allergies.
Looking for a way to decrease the dust/mold/allergens in the house. All suggestions welcome.

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8 Answers


UVC light will only treat the area within a foot or so of the light, positive results for preventing "growth of stuff" on the indoor coil, not much help in your case.

Read about the REME Model here;

We have installed many for allergy relief, with mostly positive results. If you interested, call the best of the best, HVAC contractors on Angie's List to find one that will install this for you.


Answered 6 years ago by BayAreaAC


The UV light only stops things like mold from reproducing they won't work to reduce exposure to pet dander. HEPA filters are what you need for pet dander and someone with allergies sharing the same house. You may want to look into getting a good quality room air purifier as well.

Answered 6 years ago by Guest_92983222


UV kills organic light - so for mold and bacterial growth, and HVAC units are only for disinfecting the A/C condenser, which tends to grow mold. You can get medical grade units to put in ducts for disinfection, but very expensive to run and change bulbs in.

For dust and allergens, filtering is what you want. A HEPA (High Efficiency Particulate Air) filter goes a far better job of removing particulates suspended in the air, However, they do so by having very small openings and much more circuitous airflow paths through the filter, which creates a much larger air pressure drop that a normal filter. Therefore, less airflow in the system, so longer time to heat or cool, and possible overheating of the blower fan and the heat exchanger, which are designed for a certain airflow rate for heat transfer.

An electrostatic filter is another solution - works particualarly good for some dusts and hair and dander, but need frequent cleaning in that kind of environment.

Therefore, you cannot just put in a HEPA or electrostatic filter - you need an evaluation by an HVAC contractor of your airflow - may take larger fan installation, or a local enlargement of the duct to a larger cross-section so there is a lot more area for the air to flow through, or both. An electrostatic filter runs on electricity, so that takes an installation by an HVAC contractor.

For your case, I would think a good HEPA filter would give you the most allergen reduction for your buck - also cheapest to put in.

One other suggestion - consider a rhumba - will clean your flooring of dander and hair - might even grab one or two of those pesky cats.

Answered 6 years ago by LCD


First I think LCD means ac Evaporator coil, as that is the one the indoor air passes through, not the condensor,as it is outdoor air.

HEPA filters are great, however they are very restrictive and no residential system has the fan static(think power) to have 100% of the air filtered through one. They are added to a system and have their own Fan power, costley and consume a lot of space.


Answered 6 years ago by BayAreaAC


BayAreaAC is right - slip of tongue (brain ?) - I did mean evaporator, which is the "radiator" in your forced air duct system where the A/C refrigerant runs through its coils, picking up heat from the passing forced airflow, thereby cooling the air passing over it. Because it is very cold at the evaporator, it condenses moisture from the passing warm airflow, so it is commonly damp on the surface (at times to the point of runoff, which is why it has a drain tube) which of course promotes mold growth which UV lights can alleviate, though for most aftermarket residential systems not entirely effectively unless high-wattage units mounted on both sides of the unit.

Several points BayAreaAC makes regarding HEPA filters are very important - and the subject of several recent comments on this site,so I would like to elaborate for your benefit, and for other readers with similar questions. A HEPA filter catches much more of the airborne contaminants, primarily the particulates suspended in the air. It does NOT filter any noticeable amount of chemical fumes or odors, and only about half the very fine smoke particles in a house with smokers. To remoe those you need a carbon filtration system (for odors/chemicals) or ionizing unit (for odors/smoke) - which can be installed but are rare in residences.

A same-size HEPA filter as your current air filter, because it is more tightly constructed, restricts the ariflow (amoutn depending on filter efficiency), which at least reduces system capacity, and cause cause fan and heater heat exchanger damage due to decreased airflow across them. Therefore, as Don was leading up to, you have basically six choices -

1) One solution is to filter incoming fresh air, and recycle none of the used air. This is done in some medical and hazardous materials environments, but wrecks havoc on your utility bills, as all cooled/heated air is used only once through the system, so that is out for all practical purposes in all but very temperate climates. This has been used with some frequency in areas like coastal southern California and other Mediterranean climates where the need for heating or cooling is relatively minimal, but increasing energy efficiency regulations are eliminating that practice.

2) A second one is to install a signifiantly increased filter area for the air to pass through - commonly 3 or more times as big a cross-sectional area for HEPA filters - I have seen as high as 10 times with very high efficiency installs, which means dimensions of duct at the filter increases three-fold, or you can install multiple banks of lower efficiency ones with less area increase. Basically makes a wide point in the airstream, which means some duct modifications and can take up a fair amount of room.

3) A third choice is to put in a supplementary or larger fan to force the air across the filter - more up-front expense, and permanently higher (though not exhorbitant) electric bills to run the fan, and less energy efficient in the cooling mode because you are burning up electricity that is wasted in air turbulence and friction at the filters.

4) A fourth option is to use lower efficiency filters, but increase the air changes per hour so the air is cleaned more often (makes more passes through the filters in a day). This is commonly done in commercial installation, particularly bathrooms and kitchens and animal care facilities like shleters and vet clinics and such, but is expensive to do as a retrofit, as it generally requires larger fans and ducts.

5) A fifth option, as he implied, is either to take only part of the air in the ducts through the HEPA filter, in a parallel duct or "filter box", thereby reducing pressure and airflow reduction. In the case of outside allergens, the incoming fresh air can be filtered this way before being fed to the HVAC system. In your case, probably the return air from the house would be filtered to remove cat hair and dander. Such a system can be configured to swap back and forth between which source (outside make up air or insicde return air) it filters, by means of a flapper valve. However, these units do take up additional floor space (though can be ceiling mounted like in a garage or utility room). Can take up about as much space as your furnace.

6) Another option is a stand-alone filtration system, similar to the small portable air purifiers (which don't filter enough air to make any difference house-wide), but on a much larger scale, and are usually installed in attic or basement with other utility appliances. I have been involved in a few creative flat-lying installations between floor joists or in ceiling air plenums where space was at a premium - basically built multi-filter air boxes where the air took a serpentine up and down path through a large number of flat-lying filters - this arangement is not uncommon as retrofits in infectious disease medical facilities and electronics fabrication plants, but rarely used in houses.

Obviously, whatever you do will require a demand and sizing assessment by an HVAC specialist - not just a repair tech.

Answered 6 years ago by LCD


UV lights are not however, as effective at killing mold spores. In fact, the spore must be present in the “kill zone” of the UV lights for an extended period for the light to have any real effect on mold. And that just isn’t fesible.

Picture this: A mold spore slips past the air filter and then heads through the coil. Since it’s traveling on air currents, it is going to go past the UV light in a fraction of a second. If the UV light was efficient, it would kill the spore during that “light contact time.” But the reality is that the limited exposure time of the light does not kill spores. This is supported in research performed by Penn State.They modeled microbial decay using various powers of UV lights. The results showed that the mold spore survival rate varied depending on the power of the light. In one example, some spores had to be exposed for up to 1,500 seconds (that’s 25 minutes) before they were killed. "

Answered 4 years ago by Sheffer


Try placing a 20x20 high end disposable filter on the back of a box fan on low in all bedrooms. Allergist said this creates a "safe zone". Considering I live along, have no indoor pets, don't generally wear shoes indoors AND my bedroom is the furtherst point from any exterior doors, it is amazing how much "stuff" this filter collects!

Answered 4 years ago by pat78750


I good filter rated with 10-13 MERV rating is the best way to reduce allergiens in a home. UV lights help to control and eliminate mold spores and viruses. A combination of both is the best. Lennox makes a system call the Lennox Pure-air that combines, Fresh-aire also makes a system called the Aire-Purity that also combines high MERV rated filter with UV technology

Answered 4 years ago by Aaronkie

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