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

My Ecobee just said to replace my air filters, but are they dirty enough, or just look dirty.
I think the real issue is blockage as it traps stuff.
Anybody automated the process by measuring the change in pressure as they get dirtier and dirtier?
I was thinking about using a Zigbee temp/humidity/pressure sensor to see the drop in the duct as the fan runs.
I mentioned this in the devices area but figured the lounge might be a better forum.
Aqara Temp/Humidity/Pressure Sensor and HVAC Filters

I use Filtrete smart air filters. They have a zigbee Bluetooth pressure sensor attached to the furnace side of the filter and measure drop. You pair them to a phone app and can link it to Alexa for automated replenishment if that's your thing. I definitely save money doing this, as I typically can use the filter for about 4 months instead of 3 as suggested for the dumb filters.

1 Like

Sure they are Zigbee and not BT or WIFI?

Oops! They're Bluetooth. Sorry about that.

Also, the sensor is discarded with each filter and you get a new one with each filter, so the sensor must be pretty cheap to make.

Interesting concept. Another way to think about this is in hours of use. The less a furnace runs, the less it needs to be changed, right? It should be pretty simple for us to log furnace time with smart thermostats, but unfortunately I can't find how often to change the filter in terms of hours. Everything just says every 60-90 days.

Editing to add: Looks like most manufacturers recommend changing filters every 200-300 hours, but that can change based on air duct leakage, pets in the home, etc. Measuring pressure would remove these variables...

1 Like

Nothing definitive, but web search results seem to indicate that the device counts down the time of use by sensing the pressure drop and NOT the actual flow restriction amount.
Better than just a clock since it only runs while in use.

I’m looking at measuring the blockage by comparing the air pressure drop while running versus the ambient air pressure when fan is off.

Drop should increase as filter gets dirty.

1 Like

at any given time I have 4 or more dogs in the house. They get replaced monthly. low tech, but keeps the air clear.

1 Like

As I said in the other thread, there are several factors to consider:
— Some antiallergenic filters can cost $25 each
— Over replacement adds to the landfills
— Forgetting to replace them is not good for your health or your HVAC
— An most importantly, it’s a challenge to implement and to get it right :innocent: :

1 Like

I have worked on large systems, with differential meters connected to return and supply.
Equipment supplier and engineering want no more than X static pressure.
When dial hits X", replace the filters.
Readings on the start up worksheet will show no filter/filter pressures, to prove that you put it in according to spec!
Easy for maintenance depart, and as cheap as you can run, the amount of energy used vs filter costs are easy to determine.

1 Like

I use very fine Filtrete 2200 premium filters in an attempt to remove as much contamination as I can from the air. My HVAC system has a variable speed fan that will increase speed if it detects a pressure drop across the filter so that air flow is maintained. However, it does so at a higher energy cost.

Although 3M says their filters are designed to be used for 3 months, I flip them upside down after 1 month as there is a brace that blocks some of the filter. Then I replace the filters after two months of use, more often than recommended. During the summer and winter, when the HVAC unit is running more often, they get dirtier than they do in spring and fall, but I still adhere to the 2 month schedule. That regimen costs me about $10 per month. I do not consider that unreasonable for improved air quality.

1 Like

Ummm... you flip them over?
Please confirm that.

Be smart and know what your system was designed for with regard to the MERV rating for the filter. It's a very common problem in my rentals where the tenant thinks that the highest MERV rating is ideal for the filter. They use the wrong filter with a high rating and it severely impacts the system. Most of my older systems get MERV 8. One can't handle more than a Merv 4. One day it will get a new system.

As noted above if you know the correct filter and you set up a manometer monitoring correctly you can absolutely use it to tell you when to change the filters. It's just not all that common for people to know what their residential system requires.

I have a couple of Dwyer Mark II manometers.

If you need to filter more or use a higher MERV rating the best thing to do is likely to modify your returns and use a thicker pleated MERV filter OR install a V bank filter box that takes two filters.


Pretty sure they meant end to end and not face to face.

1 Like

The DOGS get replaced????? Busy house!


Yes, you read me right.

I flip them so the top goes to the bottom, but the air flow is still going in the correct directly as marked on the filter.

My filter is immediately prior to the HVAC unit. There is an brace that goes across the filter opening to keep the plenum rigid. When I check the filter after one month, there is always a white stripe across the filter about 1 1/2" wide where the air flow is blocked by the brace. Thus, I flip the filter upside down so the clean area is now exposed to air flow.

My older HVAC used a 16 x 25 filter and there was no such brace. When I upgraded my system, they converted to a 20 x 25 filter to provide more air flow. That is when the brace was added. With the older filters, there was no reason to flip the filters. now there is. It all depends on how your filter compartment is set up.

Lots of good stuff here.
@JumpJump , how can you determine the proper MERV rating? I have a 2 zone system with 3 ceiling returns. 20x25 in main great room, 14x14 in downstairs master bedroom, and 14x20 in upstairs hall, all combining as returns for both zones.
The downstairs is 2600sq ft. Upstairs is 800. When only upstairs zone is running the bypass is forced open because of the lesser amount of ducts.
Single speed fan. I also run the fan a minimum of 10 minutes per hour for each zone using Ecobee thermostats.

And I think that it will be a fun project to attempt to automate the replacement timing.

Haha, actually yes. That’s the minimum. We foster rescue dogs. The four are the permanent residents. The most we have had (and I try really hard to stay well away from this number) is 10. That’s just pure chaos! We did also install a UV light in the air handler. That’s has seemed to help with keeping biological growth down. We used to get a funky smell, particularly when the AC ran. We haven’t had that issue this year.

1 Like

I'd recommend potentially getting an HVAC professional do an inspection and cleaning on your next schedule and then also measure the system and tell you the proper filter rating for the system. Basically, do a bunch of things at once for economies of scale.

An unrestricted system is more efficient and costs less money to run. If you have an older system with 1" filters like a 25x16 you probably should stick with no higher than MERV 8. Just a rule of thumb and less scientific like you would get from a pro.

1 Like

I 100% agree.

Going too high of a MERV on 1" filters is a (common) mistake. as it creates too much restriction. If you really think you need more air cleaning, you need to get a standalone HEPA filter external of your HVAC system.

Many brand new systems are designed for/tested on up to 13 MERV filters (I just replaced 2x units this year and looked at that on the models I was looking at), so that is usually where I stop. But that 100% depends on the system, the filter thickness (different answer on 4" media vs 1" media), etc.

Again, if you really want/need "super clean air" you need a separate HEPA filter(s). The HVAC system is the wrong place to do that level of fiiltering.


Haven't you all figured out that it is the challenge of using HE to tell me when to replace the filters is all I "really" care about? :smiley:

1 Like