In this episode of the Ask Derrick Cole Show, Derek Cole answers two homeowner questions: which air filter is best for a house and why a return grill might whistle. He explains that filter choice depends on the home’s construction tightness and the capacity of the duct system. High‑MERV filters can restrict airflow in tight, energy‑efficient homes, while cheap low‑MERV filters fail to capture fine particles. He recommends considering whole‑home air cleaning solutions that capture sub‑micron particles without adding excessive resistance. He describes symptoms of an over‑restrictive filter—air bypassing the filter media, reduced efficiency, and a whistling return grill. The whistling is usually caused by an undersized return duct or grill, which creates high velocity air flow. He notes code considerations (e.g., NC requirements for multiple returns in larger homes) and advises homeowners to have a technician inspect the system during annual maintenance to select the appropriate filter and ensure proper return sizing. The episode ends with a plug for the 1HourAirCarolina website, which hosts related articles on filters, vent closing, and air cleaning.
Questions this video answers
Use the best filter that your home’s duct system can handle. In tight, energy‑efficient homes, a very high‑MERV filter can restrict airflow, so a lower‑MERV or a whole‑home air cleaner may be preferable. In older, leaky homes, higher‑MERV filters can work because the system isn’t as restricted. The safest approach is to have a technician evaluate your system during annual maintenance and recommend the appropriate filter.
Whistling usually means the return side is too small—either the return duct or the grill is undersized—causing high‑velocity air flow. It can also be caused by an over‑restrictive filter that forces air through a tighter path. Check the filter and consider enlarging the return grill or adding additional returns, especially in larger homes where code may require multiple returns.
Chapters
Hey, on this episode we're talking about your air filters. Which one should I have for my house? And why is my return grill whistling? You ask your comfort and energy questions and they get answered. This is the Ask Derrick Cole Show. Hey everybody, this is Derrick Cole. Welcome to episode 62 of the Ask Derrick Cole Show, answering the home energy heating, air conditioning, air quality questions. And we have some today. So, I don't know, I mean, power went out yesterday here. It was kind of, it was bad. Yeah, it was a little depressing. Yeah. But anyway, let's get into it.
What filter should I use in my house?
You should use the best filter for your home. And let me explain this because it's kind of hard to understand. It depends on your house and your duct system as to which filter you need. Now, I do always recommend air cleaning, which is a different type of filtration than a filter. But if you're going to use a filter, you want it to be the right one. Now, you go to the box store, they got like 2 rows of filters— I'm sorry, 2 aisles of filters really— because you get the cheap one for like a dollar, and then you got like the $30 $50 filters, you're like, "Well, that one must be better because it's that much." But your system will determine what filter it can handle. So if you have a tight home, a newer home, and it's a tight construction, energy-efficient construction, and your duct system is tight, then putting a really high MERV-rated filter in that system could cause more problems. And good. So you could put in a filter that stops up the airflow, which is a problem. On the other side of the spectrum is a cheaper filter or a cheap filter, 80-cent, dollar filter you put in there, and it's not very good at all because, you know, only thing it's stopping is sandwich bags and Coke cans and anything else you throw at it. Yeah, it's 100% efficient at stopping that, but 98% of what we're trying to stop are the particles you cannot see. So if you look at your TV or your cabinet and you see dust on it, we're not trying to filter that because it's so heavy it's sitting there. You're trying to filter the particles in the air that are coming through, and those you cannot see. So you need a better filter for that. Once you get into air cleaning, then you actually stop filtration and go into air cleaning. And the reason you want to do that is because now you're getting the small particles you cannot see and you're not stopping up your duct system. So if you go to the filter on the return grill and it's too tight and the filter is getting sucked in, then the filter is too much for that system to handle. So you need to go down in filters. I mean, it's a case-by-case basis. I mean, some houses are older, they're very open, the duct system is very leaky. They can handle a MERV 15 filter in there because they're leaking so much air, very few of that, very few, very small amounts of air are coming through that anyway, so it's not stopping it up. It's kind of a complicated answer, but it really depends on what kind of system you have. The best thing to do is get the technician when he does your maintenance every year, It's just check your system and recommend what filter is better for you because some are good, some are bad. It really depends on your situation. I don't even think I answered that question. I went around in circles. It depends. I mean, it really does depend. I've been to a house before and his filter was too good. So I opened the filter grill and the filter is getting sucked in. And the air is bypassing the filter and going through the 4 corners and not through the filter. So you're defeating the purpose of what you're trying to do, which is filter the air. So it all depends.
Why is my return grill whistling when my air is on?
It's whistling because your filter is too good. No, it could be that. It could be your return side duct system is too small. Your filter grill is too small. Bigger is better when it comes to your return. So you really, for all purposes in a residential application, you really can't have too much return or too big of a return grill in your house. So the return grill is too small, therefore your return duct is too small, and therefore the air is whistling as it goes by because it's a tight fit. So check your filter, Probably your return duct is too small in that situation, therefore your return grill is too small. We go to a lot of homes where we go to change out a unit and the return grill is too small. It's not sized properly. Actually, in the state of North Carolina, if you have an over 1,600 square foot house and you change out the duct system, you're supposed to have more than one return. So we've run that a few times and we run into the situation of the return grills is too small. So you're not returning enough air back to the system. So the system itself is sucking in air from outside, under the house, in the attic, through the walls, whatever it has to do is getting the air to the unit. But kind of go back to the filter question, you want it to be clean conditioned air going back to the unit and not hot humid or cold outdoor air going back to the unit, which is not filtered. So it goes hand in hand, filter and return. All right, that's it. That's episode 62. Hopefully I answered some questions. There's quite a few. If you go to our website, 1houraircarolinas.com, There are 40 LinkedIn articles you can read. Some of those are on filters as well as closing vents and air cleaning. So check all those out. If you have any other questions, hashtag them #AskDerekCole and we'll get them on the show.
