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What air cleaning equipment actually does
Filtration is the most heavily sold and least explained part of a heating and cooling system. This page explains what the products do mechanically, says plainly what we will not claim, and names the one category we refuse to install.
Two things before anything else.
We make no health claims on this page or anywhere on this site. We are heating and cooling contractors. We can tell you what a filter is rated to capture, what it costs you in airflow and whether your system can carry it. We cannot tell you what any of that will do for a person’s asthma, allergies, sleep or susceptibility to infection, and anybody in this trade who does is telling you something they are not qualified to know.
Every filter is a restriction. The finer it is, the harder the blower has to pull. A residential system has a fixed amount of pull, so past a certain point a better filter costs you airflow — and less airflow means less capacity, a less comfortable house and more strain on the blower and the compressor. The way past that limit is area, not density: more filter surface lets you go finer at the same resistance. That single idea explains most of what follows.
Pick what you are trying to achieve and what has been proposed to you. Everything below is written out in full.
“Active” air cleaners are a broad category and they are not all the same thing, but a good many of them work by putting something into the air rather than taking something out of it — ions, charged particles, or in some cases ozone. Ozone is a lung irritant. That is not a marketing position we have adopted to sell you something else; it is why we do not fit equipment that produces it, including equipment sold as producing only a small amount.
The wider problem is that this category is difficult to verify. A passive filter can be tested: air goes in, particles are counted on both sides, and the result is a rating you can look up. A device that claims to change particles in the room is far harder to check, and the performance figures usually come from a chamber test that has little to do with a house. If you want it anyway, that is your decision and we will not argue with you — but it will not be us fitting it, and we would ask you to find out specifically whether the model produces ozone before you agree.
Dust, and allergy season
This is the commonest reason and the one where filtration genuinely does something mechanical: fine airborne particles pass through the system and a finer filter holds more of them.
But two things get in the way. The first is the filter path. If the cabinet does not seal round the filter, or the return duct is drawing attic air through a gap at the plenum, then much of the air is going around whatever you fitted. Sealing that costs once; a better filter in a leaking rack costs every month and changes very little.
The second is run time. A filter only works while the fan is running. A system that satisfies the thermostat quickly is filtering for a small part of the day, which is why a continuous or circulating fan setting sometimes does more than a product upgrade and costs nothing to try.
And a great deal of what people call dust is generated and settles inside the room rather than travelling through the ductwork. Filtration cannot reach that at all.
Smells
Start by finding the source, because filtration is the wrong tool for most of them and the right answer is usually cheaper.
A musty smell that appears when the cooling comes on is usually the evaporator coil or the drain pan, and the fix is cleaning and drainage rather than a product. A smell that comes and goes with the weather is often a dry trap somewhere, or a bathroom extract that terminates in the attic instead of outside. A sharp smell at start-up in autumn is normally dust burning off the heat exchanger and lasts a few minutes. A smell of gas is not a filtration question at all — leave the building and ring the gas utility from outside.
Where a smell genuinely is airborne and general — cooking, a hobby, a new floor — the honest answer is ventilation rather than filtration. Ordinary filters do not remove gases and odours; that needs activated carbon, which has a limited life, needs replacing on a schedule, and is a real cost rather than a one-off purchase. We will fit it if it is right, and we will tell you when it is not.
Somebody in the house is vulnerable
This is a medical question and we are not the people to answer it. We will not tell you that any piece of equipment will protect someone, reduce their symptoms or lower their risk. Not because we are being careful with you, but because we do not know, and the people who do know are their doctors.
What we can honestly offer is the mechanical part, and it is worth having on its own terms. We can measure what your system can actually carry and fit the best filtration it will take without losing airflow. We can seal the filter cabinet and the return so the air is genuinely going through the filter. We can make sure the coil and drain are clean and that extract fans terminate outside the building. We can make sure the system is not pulling air out of an attic or a garage. And we can give you the ratings of what we fitted, in writing, so that if you want to discuss it with a clinician they have something factual to look at.
If someone has been given specific advice about their home environment, tell us what it says and we will work to it where it is mechanical. That is the shape of the help we can give.
Pets
Two different problems that get treated as one. Hair and larger dander are mechanical and filtration handles them well — this is the case where a deep media cabinet is most obviously worth the money, mostly because it does not need changing every few weeks in a house that blocks filters quickly.
The fine fraction is harder. A great deal of pet allergen is small enough to stay airborne for a long time and to settle on soft furnishings, where the system never sees it. Filtration reduces what is circulating; it does not clean a house.
The practical advice, which costs nothing: check the filter far more often than the label says. A one-inch filter in a house with two large dogs can be substantially blocked in six weeks, and a blocked filter starves the system exactly as effectively as a filter that is too fine for it.
It was offered and you are not sure
Then the useful question is not whether the product is good but what problem it was offered for. Air cleaning is the most profitable add-on in this trade and it is very often offered during a service visit where nothing about air quality was raised by anybody.
Three questions will sort it out quickly. What did you measure? If the answer is nothing, that is your answer. What will this cost me in airflow, and what is the static pressure before and after? Anybody proposing a filtration change should be able to say. What does it cost to run and maintain? Media elements, carbon and lamps all have a replacement cost and a schedule, and that is frequently the part left out of the conversation.
It is entirely possible the answer is yes. A media cabinet in a house that has been blocking one-inch filters every month is a genuine improvement. But it should follow a problem rather than a sales visit.
A higher-rated one-inch filter
The cheapest option and the one most likely to do harm, because a one-inch slot has very little surface area. Going up in density in that small area increases resistance sharply, and the airflow loss is often enough to cost you comfort and put the blower under strain. The system does not complain. It just delivers less, runs longer and wears faster.
If the rack has room, going to a four- or five-inch cabinet at a similar rating is usually better in every way: finer, less resistance, and changed once or twice a year instead of monthly.
If a one-inch filter is genuinely the only option, the sensible thing is a moderate rating with good pleat count, fitted in a cabinet that actually seals, and changed on time. We will measure the static pressure before and after so the decision is made on a number rather than on packaging.
A media cabinet on the return
This is the upgrade we recommend most often, because it is the one that works on the physics rather than against them. A four- or five-inch cabinet has many times the surface of a one-inch filter, so it can be considerably finer at the same or lower resistance.
The practical benefits are real and unglamorous: better filtration, less strain on the system than what you probably have now, and an element changed once or twice a year instead of every month, which matters because the commonest filtration failure in any house is simply not changing it.
Two things to check before agreeing. There has to be physical room at the return for the cabinet, and in a tight closet or an attic platform there sometimes is not. And it has to be sealed properly into the ductwork, because a cabinet with air bypassing it is worse than the filter it replaced. We measure static pressure before and after and give you both figures.
An ultraviolet lamp in the air handler
There is a narrow use for these that is mechanical and defensible, and a much wider set of claims that is not.
The defensible use is a lamp positioned to shine on the evaporator coil and the drain pan. Those surfaces are wet and dark whenever the system is cooling, which is a good place for biological growth, and growth on a coil is both a hygiene matter and a performance one because it fouls the fins and blocks airflow. A coil lamp is a maintenance device, and on that basis it can be worth having.
What we will not tell you is that a lamp in your ductwork sterilises the air in your house. Air passes a lamp in a fraction of a second, and what that achieves depends on the organism, the intensity and the time, none of which anybody is measuring in your utility room. We will not make a claim about illness, and we would be sceptical of anybody who does.
The practical points: lamps have a service life and lose output before they stop lighting, so they are a recurring cost; and they must be installed where the ultraviolet cannot reach anyone’s eyes or degrade nearby plastics and wiring.
An ioniser or “active” air purifier
See the note above, which is the whole of our position: we do not install devices that work by producing ozone, because ozone is a lung irritant, and we treat the category as a whole with caution because its performance is difficult to verify in a real building.
If somebody has quoted you one, the single most useful thing you can do is ask, in writing, whether the device produces ozone, and ask for the independent test that supports whatever it claims. A confident answer with a document behind it is a different situation from a confident answer without one.
What we would rather sell you instead is dull: a correctly sized media cabinet, a sealed filter path, a clean coil, and extract that goes outside. It is less impressive and it is the part that is measurable.
Where to start
Seal the filter path and check what the system can carry, before buying anything. Then consider a media cabinet rather than a denser one-inch filter, and try the circulating fan setting — it is free and it doubles or triples the hours the filter is actually working.
Where to start
Find the source before buying any equipment. Coil and drain pan, dry traps, extract fans terminating in the attic — those account for most of it and they are cheap. Carbon is a real answer for a genuinely airborne odour, and it is a recurring cost rather than a purchase.
Where to start
Ask the clinician what they want, and bring us the mechanical part of it. We will fit the best filtration the system can carry without losing airflow, seal the path, clean the coil and drain, and give you the ratings in writing. We will not tell you what it does for anybody’s health.
Where to start
Check the filter monthly regardless of what the box says, and consider a media cabinet. In a house that blocks filters quickly it is worth the money on convenience alone, and convenience is what actually keeps a filter working.
Where to start
Ask three questions: what did you measure, what does it cost me in airflow, and what does it cost to run. If the first answer is “nothing”, you have learned what you needed to know about the offer.
What this page refuses to do
No health claims. Not one, anywhere on this site. Filtration is not a treatment and we are not clinicians.
No percentages. No capture rates, no “removes 99% of” anything. Those figures come from a laboratory test on one product under one set of conditions, and repeating them about your house would be dishonest.
No prices. A cabinet retrofit depends on what is there and how much room is around it.
And we turn down a category of product that is one of the more profitable things sold in this trade. That is the most expensive sentence on this website and it is the one we are least willing to change.
If you want the measurement
A visit, static pressure measured at the system, the filter path inspected and the coil and drain checked, and a written note of what your system can carry. Whatever you decide afterwards, the figures are yours.