Anyone use pressure drop to determine when to replace HVAC filters?

I agree, but I just want to point out that in the US there is a lot of variety on how the filter frame is applied to a forced air system. Some air handlers have that 1' fitler area built in as an option to use. Others are built into the stand or added between the air handler and the return etc etc. There are so many variables with vertical and horizontal etc etc. I just wanted to maybe color what you said and add that you don't need a separate HEPA filter so have good filtering, but it's a super idea. You can sometimes just have the filter frame changed to a 4" from a 1". It's not simple most of the time, but it's possible. As a mentioned above you have add a new V style to your return if you have the space.

We know that. It's just that this is a discussion forum and community and while you started the topic you aren't really in control after you lob the ball over the fence :slight_smile:

You can try putting a CT on the power for the fan motor. When the filter is blocked it will probably have a different current draw. Assuming you don't have a multi stage or variable speed fan that might be simple.


All fair comments. I was probably oversimplifying too much.

It also very much depends on where your air returns are (ceiling vs floor especially). If on the ceiling you will likely not get a ton of debris/dirt in the filters ever, as it will stay primarily at ground level - etc. Mine are all ceiling returns in my primary home, and I think I could probably go years without changing the 4" filters. I don't, but likely could.

In any case, I have 4" filters on all of my systems now. :man_shrugging:


For sure ceiling vs floor or wall will impact how much enters the return. I renovate houses and do a lot from the 60's and 70's. You would be amazed and just how poorly ducts and return ducts are sealed. In many places returns are chases made from blocking floor joists and are very leaky. When this happens dust and whatever gets pulled in along the way. Also, rodent and squirrel damage in attics and crawls open ducts to all sorts of nasty things...

4" filters are the way to go.


Great! After I posted I thought about it and assumed that you did some kind of fostering and had alot of "dog traffic", so my comment was a little stupid... My apologies!
It just sounded funny at the time....

The UV light is a great idea to kill the airborne bacteria and odors as you don't want to get people and the dogs sick!

Interesting $60 device. Would love to learn more.

  1. Do you use one on each side of the filter to calculate the differential, or do you just use one on the the side of the filter closer to the air handler, and you know your max static pressure?
  2. Can you mount this on the outside of the duct so you can read it without removing the filter - ie how exactly does this install?
  3. How do you calculate what the proper static pressure is supposed to be, or do you just change the filter when the pressure between a new filter and current filter = x differential?

Nah it wasn't. Kinda funny to me too!! no worries!! Late last year we had a litter of GSD puppies that had Distemper and Ringworm.. They all had upper respiratory infections in the shelter, which is what led to the distemper (at least 3 got it, we lost two, and one survived, so we kept her) and the one that survived is also the one that had ringworm and they were passing that back and forth for a while. For what it's worth Hydrogen Peroxide is great at wiping out ringworm. Learned all kinds of cleaning tips with that litter! That is also where the idea for the UV light came from.

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It's a neat science project. Once you've characterized your system for a given filter rating and brand, however, it's doubtful things will change much and a simple timer will do.

Filtrete are expensive but have lower pressure drop than most brands for a given rating (I think they have more pleats, thus more surface area). The more surface area the better, so if you can go to 2" or 4" you should.

They even increase filter area (more pleats) as MERV goes up, so for a range of different MERV filters, the resistance is similar.

If y'all like the Filtrete filters, check out the Costo branded Kirkland ones. Only available in 1", but more pleats (=more filter area) than even Filtrete and much less expensive.

You can find the instructions on the website and videos on youtube. You have a hose for the high side and another for the low side.

Yes. You mount the hoses to fittings on either side of the filter.

This device can do more than differential, but for the purpose of using it for filter changes you can set it up as differential. Shows you the delta P in inches of water column. You calibrate with a new filter and then determine your delta for changing the filter.


I have the 25X25X4 filters, and I use Marv 13. Through Filter buy, i think I pay about $20 per filter. I but them six months at a time (keep in mind I'm replacing them monthly)

My Ecobee tells me when to change the filters LitterRobot.

Nah, too high tech for me :sweat_smile:

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@JumpJump I measure the delta P for a fresh filter in my system at about 0.1 kPa which I think is about 0.4 inches of water column.
I’m curious as to what you would expect the delta to be for a very dirty filter?

My sensor has a resolution of 0.1 KPa. No idea as to its accuracy.

The new filter and my 4 month old dirty, but light shining through, filter BOTH indicate 0.1 KPa drop.

Not sure your conversion is correct. I replace filters at 0.4 - 0.5 iwc. These units had this device already installed when I purchased the properties.

The numerical conversion of 0.1 kpa is 0.4 in. water column, I do agree with you that the dirty filter pressure drop of a 1" pleated media filter should be around 0.4- 0.5 in. w.c. Of course this depends on the efficiency rating of the filter and the air velocity across the filter and assumes that the filter is properly sized for the furnace/evaporator coil.

I found this on"A 1” MERV 13 pleated filter has a pressure drop of around 0.27, and a 1” MERV 8 has a pressure drop of around 0.14."

One rule of thumb for the dirty filter pressure drop is to double the clean filter drop which would give you a dirty filter pressure drop of 0.4-0.5 in. water.

@TArman says his sensor has a resolution of 0.1 kpa, so the actual reading is probably at the low end of the scale and is likely less than 0.1 kpa for a clean filter.

The problem with reading filter pressure drop is that the numbers are so small. That is why an inclined manometer is often used to indicate filter pressure drop.


Ok, that’s all agreeing with what I have been reading. I think the resolution of this pressure sensor is not fine enough for the job. It reads 0.1 KPa delta on both a clean AND a very dirty filter. Thanks!

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One kPa is equivalent to about 1/4" inch of water level differential. Thus, a sensitivity of 0.1 kPa should be sensitive enough for the application. However, if your system has air leaks around the filter chamber, which is often the case, this might cause inaccurate readings.

Since "nature abhors a vacuum", air is going to leak in through any available orifice. Most filter slots are not well sealed. You might have air blowing out on the inlet side of the filter and sucking in on the outlet side. Either/both will reduce the differential measured.

I have an Ecobee smart thermostat that will keep track of the hours of run time. Thus, I could change the filter more frequently during periods of high use (summer and winter) than periods of lower use (spring and fall). However, I just set up a schedule in my Google Calendar to change it periodically. Since I use a high MERV filter (Filtrete 2200), I change mine every two months on the same date. If you are using a lower MERV filter, then change it every three months.

You can purchase a couple of filters for the cost of the instrumentation to monitor pressure drop accurately.


I think we have established that 0.1 kPa = .4 i.w.c. and that the instrument used had a resolution of 0.1 kPa which means that it can only count in 0.4 i.w.c increments which isn't granular enough to measure accurately. In other words if what has been said up thread is true then the tool can only measure 0, 0.4, 0.8, 1.2 etc in inches of water column.

I'll restate the challenge with filters and measuring filters in a system. If you have the wrong filter you can still measure the differential and use the increase to know when to change the filter. I hope that makes it more obvious of the problem. If you stick a MERV 13 in a system that needs a MERV 8 and you change it every 2 months you really aren't treating the system right. It's a little like sticking a dirty MERV 11 filter in the system and running it until it performs like a dirty MERV 13. You can still measure the degradation of the filter after baselining a new filter, but do you have the right filter? Are you wasting energy using a high MERV filter on a system that wasn't designed for it?

So to the OP's question though. Can you measure with a sensor connected to Hubitat to tell you when to change the filter? Probably. You just need to decide if that meets your ROI.

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