Help with HVAC filters

We bought a house a few months ago and it has a Trane XB80 furnace in it. With all the fire pollution lately, I went to check the filter, and it’s this thin plastic mesh thing. Good enough to keep lint out of the blower, but it’s not going to clean particulates out of the air.

I’d like to put a better filter in there (it looks like I could fit a 1x20x25" filter in), but the installation instructions (and some things I’ve found online) have given me pause. The instructions mention that you should only use a “high velocity” filter, and that using one that impedes airflow too much will void the warranty. The furnace is 15 years old, so I imagine the warranty is long gone, but I don’t want to starve the thing of air and break it.

I have tried contacting the manufacturer a number of ways, but all I get is the runaround and a refusal to give me anything like specs on how much airflow the furnace needs. Of course, even if I had that, it’s hard to find out how much any given filter would impede it.

I tried a few local hvac places (including the one that has their phone number on the unit as a previous servicer) but none of them will seem to answer that question either. They’ll tell me that I can put a filter on it but when I ask about whether that will impede airflow too much, they’ll say that they can put a bigger filter box (more surface area), but they don’t seem to know if that’s necessary.

There is also a 20x20" return grille that I could replace with a grille that takes a filter.

Any thoughts or help is appreciated.

I’m not sure what a thin plastic mesh filter is, but if it’s the spun fiber, non pleated but has some thickness (loft) to it you have the best filter for a furnace.

For a/c it’s more important but not to be overlooked for furnaces either. It needs air flow, and the filter is deigned only to protect the heat exchanger, thus everything is designed around that, it was never intended to filter the air for you as well. Less air flow is less efficient and should lead to a shorter lifespan of the unit, as well as higher energy costs. Now some are made to run with such filters, but not your 15 yr old unit that requires a high velocity filter.

Can you do it, yes you can, expect sub-optimal results.

Thanks kanicbird

To clarify: I realize the furnace doesn’t need a crazy filter, but I was hoping to piggyback on the system that already blows air around our house to clean it for us.

Would it be safe to only get the grille filter and only install the filter when the heater/AC is not running (just the fan)? I don’t think we need to filter the air all the time, just when the surrounding countryside is on fire.

I do not have an answer about what kind of filter your system can handle. But to deal with the smoke…

Got a 20 inch box fan? Bungee a 20 in filter to the back of that and use it to filter your inside air.

It’s made a difference here. Today was our first smoke-free day in what seems like forever.

Yes if your not using the furnace for heat (nor a/c for cool), you can run the fan, get the filter you want, even hepa style filters, and run to your heart’s content.

Being in the line of smoke ourselves, I recently looked into this very issue.

What I concluded … from some pretty salient web pages [see below] … was that keeping the (much better filtration, aftermarket) filters clean – even if it means replacing them weekly for this short period of time – was really all I needed to do in order to maintain near-normal CFM levels that our HVAC system prefers.

If any of your registers in the house are closed or partially closed, you can temporarily open them as a way to incrementally reduce static pressure, maybe offsetting any losses the new filter causes.

Filters are pretty cheap. With luck, these air quality conditions won’t last too long (and these fires will stop devastating people, property, and ecosystems).