UVC in aircon: what the lamp reaches and what it does not

UVC inside an aircon describes two separate jobs, and the difference is not cosmetic. One aims a lamp at the wet coil and the drain pan, to hold back living growth on those surfaces. The other aims a lamp at a gap the air moves through, to treat what is passing.

The split is old enough to be written into test standards. ASHRAE publishes Standard 185.1 for testing ultraviolet sources against airborne microorganisms in air handlers and ducts, and Standard 185.2 for testing the same sources against organisms on irradiated surfaces. Two standards exist because the two jobs cannot be measured the same way.

Regulator guidance for homes uses the same two categories. The US Environmental Protection Agency names cleaners designed for airstream disinfection, which reduce the viability of microorganisms as they flow through the system, and cleaners designed for surface disinfection, which prevent reproduction on named components of it. Both get called UVGI. Buying one does not get you the other.

A quotation that says only UV sterilisation has not told you which one is fitted. What settles it is where the lamp points and what it can see. Aimed at a coil, it makes a claim about the equipment. Aimed at a gap the air crosses, it makes a claim about the air, and that claim has the harder problem to solve.

Whether ultraviolet light inactivates anything comes down to dose, and dose is irradiance multiplied by exposure time. ASHRAE sets it out that way in its chapter on ultraviolet lamp systems. Cut the exposure and the intensity has to rise to match, or the dose falls with it.

The two applications land on opposite sides of that trade. ASHRAE states that residence time in in-duct devices is on the order of seconds or fractions of a second, and gives a worked figure: at 500 feet per minute, one foot of travel takes 0.12 seconds. A surface has the opposite condition, because its exposure runs continuously while the lamp is lit.

That gap shows up directly in the intensity each job needs. ASHRAE gives about 50 to 100 microwatts per square centimetre as typical for cooling coil maintenance, against as high as 10,000 for airstream disinfection. Two orders of magnitude separate them, and that single line carries most of the argument.

Susceptibility varies too, and not in marketing's favour. ASHRAE ranks vegetative bacteria as the most susceptible group, then mycobacteria, then bacterial spores, with fungal spores the most resistant of all. Mould spores, the thing most often named on the box, are the hardest target on that list.

The strongest case for coil and drain pan irradiation comes from a trial that measured the surfaces and the people. Menzies and colleagues published it in The Lancet in 2003. They irradiated drip pans and cooling coils in Montreal office ventilation systems. The lamps were cycled on and off blind across 48 weeks. On the irradiated surfaces, microbial and endotoxin levels fell by 99%. The 771 participants also reported fewer work-related symptoms while the lamps ran.

Setting matters as much as the result. That was a built-up office ventilation system with lamps sized and placed for it. A split unit on a bedroom wall is a different object. The EPA is blunt about the residential version: typical UVGI cleaners used in homes have limited effectiveness in killing bacteria and moulds, and destroying some viruses and most mould and bacterial spores needs much higher exposure than a typical home unit provides.

For the air, the same body reports a far smaller number. The EPA summarises one study as finding a 99 percent reduction in microbial contaminants growing on exposed HVAC surfaces, against a reduction in airborne bacteria of only 25 to 30 percent. Its explanation is exposure time again. Organisms carried in the airstream see the light briefly. Organisms sitting on a surface do not move away from it.

Two authorities draw a boundary around what any of this proves. The EPA found no studies linking UVGI systems in homes to reduced health symptoms in sensitive populations. It also notes that mould is allergenic whether alive or dead. Killing it may therefore leave allergy and asthma symptoms untouched. ASHRAE goes further in its 2024 position on filtration and air cleaning. Performance data alone, it says, should not be used to claim direct health impacts.

That is the trap sitting inside a UVC quotation. A kill percentage measured under test conditions is a real measurement of a real thing. It is not a health outcome, and the bodies that publish the test methods say so in writing.

The coil application carries a second justification unrelated to breathing. ASHRAE notes that airstream systems fitted in air-handling units also hold back growth on the coils. It reports lower maintenance cost and lower energy use as a result. The society funded field research on coil irradiation aimed at energy use alone. In commercial plant that is often the case being made, and the field work sits on built-up systems.

Output falls with age, and the lamp gives no honest sign of it. ASHRAE rates UVC lamps in effective hours of UVC emission, not in hours of electrical life. Many are designed to emit 50 to 85% or more of their initial intensity by the end of useful life. The same chapter warns that current models carry on emitting blue visible light long after they pass that point.

So the glow proves nothing. A lamp that still looks lit may be well past the intensity its design assumed, and there is no gauge on a fan coil that reports ultraviolet output. ASHRAE puts typical useful life at 9,000 hours of continuous running and suggests it may be prudent to simply change lamps annually, which is 8,760 hours at that duty. Frequent switching can shorten life further depending on the ballast, and dust settling on the tube lowers output on its own.

An aircon is also the one environment that holds a lamp back. Output from a low-pressure mercury lamp depends on the temperature of its coldest spot. ASHRAE notes that in moving air the cold spot of a standard lamp can sit too low to reach the required output, which is why windchill-corrected lamps exist. Cold moving air is the normal condition inside a running fan coil.

This is where an unreplaced lamp quietly turns into decoration. ASHRAE's 2024 position asks that devices be judged across their whole service life, not just when new. It counts maintenance, ageing and wear among the things that decide how a device performs in place. If nobody holds a replacement interval for the lamp, the brochure claim expired on a date nobody wrote down.

Ozone depends on the lamp envelope, not on ultraviolet light as a category. ASHRAE explains that radiation below 200 nanometres can produce ozone in air, and that low-pressure mercury lamps emit mostly at 253.7 nanometres with some output at 185. Most UVC lamps used in HVAC work carry an interior coating or a soft glass envelope that filters the ozone-producing 185 nanometre line out. Quartz lamps can be built either way, which is why the envelope is worth asking about.

A label settles it without argument. ASHRAE's 2024 position says every powered air-cleaning device should be tested and labelled for ozone. The named test is UL 2998, or an equal international standard. On systems already measured, the EPA cites two studies of lamps aimed at air-handling unit surfaces. Neither found a rise in ozone, VOCs or other by-products.

The light is hard on the parts around it. ASHRAE states that UVC energy in HVAC use can be very damaging to organic materials. The damage shows up as weaker filtration, failed seals and broken components. Synthetic filter media are singled out as vulnerable. A wall unit is mostly plastic housing, a plastic drain pan, foam insulation and a synthetic filter. All of it sits a short reach from wherever a lamp was fitted.

Then there is the reason none of this is a casual retrofit. UVC at 254 nanometres injures eyes and skin, and the injury announces itself late. ASHRAE describes symptoms starting 6 to 12 hours after exposure and clearing within 24 to 48 hours. Because ultraviolet is invisible, it notes, eye damage may go unnoticed at first. Its position document on infectious aerosols records that this wavelength can cause severe and lasting eye damage. That is why it is applied in ways that keep building occupants out of the beam.

Containment in those documents assumes a chamber only trained people open. ASHRAE calls for in-duct systems to be fully enclosed. Warning labels go on every access panel. Disconnects are wired so that opening an access cuts power to the lamps. Viewports are made of ordinary glass, which absorbs UVC. A homeowner opens the front panel of a wall unit to take the filter out. Whether that panel cuts power is the first thing to establish, and the answer belongs to the installation.

The EPA carries one more boundary worth taking into any retrofit conversation: ultraviolet equipment not specifically designed for indoor air disinfection is not recommended, and can in some cases expose eyes and skin to hazardous levels. That rules out improvised fittings. It does not rule on the properly engineered ones, which are judged on where they point, what dose they hold, and who is booked to change the lamp.