furnace filter merv ratings?

[The Cobbler]

Looking at furnace filters. my setup is honeywell 16x15x5 filters with merv 11 filters
looking on-line, there are merv ratings from 13, 11, 10, & 8 in my size (maybe more)
the higher the merv #, the finer it filters ... is there really that much noticeable difference between say 8 & 13?

 

[justinjoyal]

Well, the 13 will obviously catch smaller particles/dust, but make sure your system can keep up with it!

Simply put, it will create more restriction and your blower may suffer and overall performance could decrease.

 

[bonneyman]

^^^What he said^^^

MERV ratings on filters are alot like MPG ratings on cars. A filter has to grab and hold such and such sized particles at a certain percentage rate in government tests. But out in the real world, that should only be used as a guide - not a guarantee.
Filter manufacturers will put a high MERV rating on the filter packaging and make it sound all grand, but read the small print. For instance, some expensive filters say "lasts up to 3 months". For a 1" thickness, I strongly recommend my customers change the filter every month. But that filter retails for $16, and most customers balk at the idea of paying almost $200 a year for filters. So they leave it in. But it's pretty much loaded beyond design after 4-6 weeks, so the system is struggling the rest of the time.
And if you ever blow a compressor due to clogged filters and complain to the filter maker, they themselves will say, "We said it would last UP to 3 months. Your locale and operating conditions may require a more often change".

My standard procedure was install a pleated MERV 8 filter and spray Endust on it, and have them change it once a month. Rooftop units got 2" or 4" pleated filters installed, and I set them up to have me come out every 3 months and change it. If there were allergy concerns, I'd put MERV 11 electrostatically-treated pleats in and tell them once a month religiously.

 

[yeldogt]

There seems to be this idea that somehow the furnace is cleaning the air ... it's actually often disturbing the air. You want a filter to protect the equipment first ... getting a bit more is great as long as the flow is there ... the simple fiberglass 1" filters are really best for oil furnaces where you need a lot of flow. Even the 11 will cause issues with some.

I use the 4" 11's -- they seem to be a happy medium. It also depends on the unit and flow requirements.

 

[PWC Repair]

The average Joe won't notice the difference in the air betwen using a MERV 8 or a 13. Now, if you have allergies, pets in the house, always opening windows on a nice day, etc......you will notice it. Many people seem to forget also that the more dirt the filter collects, the more often it would nee changed. Most whole house 4" thick filters will last about 90-120 days.

 

[SGKent]

California is implementing a standard that has to be on every filter. It will show MERV and RESISTANCE. If too much resistance is added to a system the fan can overload and have its life shortened. Control boards that control fans can burn up. So before you add a high MERV filter you better know what the resistance is inside the furnace now. That said, the 3M Filtrete 1900 is a MERV 13 filter. It also has the lowest resistance of any high MERV filter (even the MERV 4 and 5 basic ones) according to all the filters I compared. The state of California study is out there online if you look for it, and they also found that the 3M Filtete 1900 1" had about the same low resistance as 2" and 4" filters. All that said, the day of snake oil filters is coming to an end now that everyone will have to print the ACTUAL specs on the filter. It is a rectangle with the MERV and air flow resistance at different CFM printed. Typically a ton of AC is 400 CFM if properly designed. A 3 ton AC uses 1200 CFM. You can have a HVAC person measure your system. You can go to the manufacturer's site and down load the specs as to CFM and switch or wire settings to see what it is. The problem can be, like I discovered on mine, that the CFM was set high and the filter ducting poorly designed. Adding a high MERV filter increased the resistance to unacceptable levels. I had to modify the system to get it within an acceptable range for higher MERV filters. The HVAC people who have worked on my system didn't really want to get involved. Much of the industry uses the fiberglass MERV 1 filters because they don't overload the system. But they do catch moths and dog hair - and keep the kids from crawling into the duct.

 

[Jackfre]

I was the Purolator filter Rep for a number of years. We parted ways just as the merv rating were coming in. In a long career I have never seen a category of product that better demonstrated that “figures don’t lie, but liars figure. There were efficiency and effectiveness numbers that were what you wanted them to be. The old fiberglass filters did not filter until they got dirty. Merv ratings are a first day number so you got what you paid for. Many homeowners want the best filtration but did not understand its effect of the furnace, HP or air handler. Merlin’s info is interesting and I was unaware of the pressure drop ratings being added, but even those numbers assume an adequate duct system and they are frequently rare birds as well. If I had a decently operating system and wanted better filtration I would get a manometer and do before and after readings prior to settling on a higher rated filter. You need to prove the system can handle it.

 

[rharman]

My HVAC guy said "It's all about air flow. Get a low MERV and just change it regularly. You probably won't notice the difference but the furnace will. DOn't waste the $$ on a high MERV."

 

[justinjoyal]

My HVAC guy said "It's all about air flow. Get a low MERV and just change it regularly. You probably won't notice the difference but the furnace will. DOn't waste the $$ on a high MERV."

Not a bad thing to tell customers to avoid problems/callbacks.

I saw the difference when I went from merv8 to merv13 though. Way less dust on furniture and stuff.

 

[PelicanPines]

I use merv 13... hvac guy said my system can support it... is there different quality blowers?

 

[yeldogt]

I use merv 13... hvac guy said my system can support it... is there different quality blowers?

Some motors ramp up -- so they are working harder to maintain the same CFM.


The filter is really there to protect the equipment ... the old 1" fiberglass type are from the oil furnace days and belted blowers. Those systems had no AC coil to clog - open heat exchangers that required lots of flow -- large ducts. Put a high Merv filter on one of them and the system will be air starved.

With AC -- it's best to have a bit more filtering. The 4 and 5 inch type are that way to increase the life and allow greater for over time. Some (most) of the 1" high Merv are really too restrictive

 

[bonneyman]

There are some customers who are hyper-sensitive to odors and allergens, so much so they almost have to live in separate rooms from the rest of their families. As I recall situations like that require the whole house HEPA filtration and all sorts of other precautions have to be performed so the person doesn't go into respiratory shock. I don't even think MERV 13 is enough for these cases.
One lady I knew couldn't even have electronic thermostats in her small guest house where she lived separate from her family, as the capacitors charging and discharging in the stat would set her off! Her husband had to use very specific bar soap and shampoo so he wouldn't make her sick when he went in there. I couldn't go into the living space when servicing the units - I did all the outside stuff, and he went in and brought the filter out to me and adjusted the stat when I needed it. Man, that's gotta be tough!

 

[Notch1988]

I bought two of the K&N washable filters a couple months ago to try out. Figured with two I can swap them and have no down time while I washed and dried one.

 

[BCreekDave]

Easy solution, install a pressure drop meter around the filter. Most HVAC system manufacturers will say how much pressure drop the system is designed to allow. Then change your filter when it needs it, not according to an arbitrary schedule. I found there are too many variables for a set schedule.

 

[SGKent]

the pressure across the filter is only part of the resistance. There is also internal resistance. Many units have an allowance of .5" water column plus .1" water column if the internal filter is removed. The typical air handler uses up about .4" of water column of the .5" wc total, or .6" wc allowable with the internal filter removed. If your filter is .3" wc and the internal .4" wc -= .7" wc total, then you are already over the limit set by the manufacturer.

What I have learned is that very few HVAC companies even look at this area, or know how to do the calculations. They install a basic MERV 1 filter and know that will give the best air flow. If the coil gets dirty the worst that will happen to them is another profitable service call cleaning the coil, and maybe the ducts too. If you put a high MERV filter in and the system isn't designed for it they get to sell you a fan motor, maybe control board, and at worst a whole new system.

I'll give you a quick example using my system. The HVAC installer in 2005 suggested we put in a second filter to pull from the back of the house in the ceiling. Done. 14 x 24. They ran a 12" flexible duct from it to the area above the air handler and then a 3.25" x 14" wall stack down to the return. The 14 x 24 added 338 sq inches of filter area - perfect. The 12" flex duct will flow up to 480 CFM - perfect. But the 3.25" x 14" wall stack will only flow about 100 CFM - or very little so it wastes the whole purpose of the additional filter.

Since California has gotten involved in this area trying to improve energy efficiency, it will be harder for the HVAC and filter industry to hide behind advertising campaigns and pretty packaging. Normally I don't like government intrusion but in this case I hope it will be a plus.

You can get duct flows here: https://www.buildingincalifornia.com/wp-content/uploads/2014/03/duct-sizing-chart.pdf

Here is a good video on measuring static pressures in your system. It makes the understanding easier.

And the chart on your filter will look something like this if the manufacturer is already for the new code. Remember that the total you have left to work with after including the internal furnace air resistance is .1" to .2", which combined with say a .4" internal resistance would give you .5" to .6" total. In the case of this filter, what ever size it is, you would need 2 or 3 of them for the average 2 - 3 ton AC from my understanding.

 

[Stuart in MN]

My HVAC guy said "It's all about air flow. Get a low MERV and just change it regularly. You probably won't notice the difference but the furnace will. DOn't waste the $$ on a high MERV."

The vision comes to mind of trying to drink a thick milkshake through a straw that collapses. I buy the cheap filters and replace them regularly.

 

[Jackfre]

the pressure across the filter is only part of the resistance. There is also internal resistance. Many units have an allowance of .5" water column plus .1" water column if the internal filter is removed. The typical air handler uses up about .4" of water column of the .5" wc total, or .6" wc allowable with the internal filter removed. If your filter is .3" wc and the internal .4" wc -= .7" wc total, then you are already over the limit set by the manufacturer.

What I have learned is that very few HVAC companies even look at this area, or know how to do the calculations. They install a basic MERV 1 filter and know that will give the best air flow. If the coil gets dirty the worst that will happen to them is another profitable service call cleaning the coil, and maybe the ducts too. If you put a high MERV filter in and the system isn't designed for it they get to sell you a fan motor, maybe control board, and at worst a whole new system.

I'll give you a quick example using my system. The HVAC installer in 2005 suggested we put in a second filter to pull from the back of the house in the ceiling. Done. 14 x 24. They ran a 12" flexible duct from it to the area above the air handler and then a 3.25" x 14" wall stack down to the return. The 14 x 24 added 338 sq inches of filter area - perfect. The 12" flex duct will flow up to 480 CFM - perfect. But the 3.25" x 14" wall stack will only flow about 100 CFM - or very little so it wastes the whole purpose of the additional filter.

Since California has gotten involved in this area trying to improve energy efficiency, it will be harder for the HVAC and filter industry to hide behind advertising campaigns and pretty packaging. Normally I don't like government intrusion but in this case I hope it will be a plus.

You can get duct flows here: https://www.buildingincalifornia.com/wp-content/uploads/2014/03/duct-sizing-chart.pdf

Here is a good video on measuring static pressures in your system. It makes the understanding easier.

And the chart on your filter will look something like this if the manufacturer is already for the new code. Remember that the total you have left to work with after including the internal furnace air resistance is .1" to .2", which combined with say a .4" internal resistance would give you .5" to .6" total. In the case of this filter, what ever size it is, you would need 2 or 3 of them for the average 2 - 3 ton AC from my understanding.

Excellent post! If you take the time to learn how to take the readings and buy either a Dwyer Magnehelic or a digital version you can learn a lot about your system and its issues. In New England where everyone has a basement and the furnace is located there, the biggest issue I saw was large external static on the return drops. Working back up the return, frequently I found that the static would come into line in the return main. A larger return drop quieted the system down and brought the pressure into line.

 

[SGKent]

yes. If the return is lengthy with several bends, it isn't zero resistance. Let's pretend the filter above, whatever size it is, is used on a 3 ton system with a return like you describe.

The system is designed for .5" water column resistance plus .1" allowed for the filter. In your first description, the return in the basement is small, although it probably seems fine to the average owner. Let's say it has .18" wc resistance. Add to that the .4" that is designed into the air handler itself. So without any filter the fan sees .58 " wc resistance. (.18" + .4" = .58) Now we throw in the resistance across the filter. On the filter shown, let's pretend it is a 3 ton system. At 400 CFM typical per ton air moved that would be 1200 CFM that a 3 ton system wants to move. That affects both cooling and heating. Too little air and the heat exchangers would get too hot and crack maybe. The AC coil might freeze up in summer. So when we look at the filter specs for that size filter, whatever it is because each filter sold has numbers printed on it for how it performs, at 1110 CFM, which is close to 1200, the resistance is at the max allowable for that filter to even be used. That number is .50".* So if we add that to the .58 above, the system would be seeing 1.08" of water column. The fan motor is not happy, the heat exchangers are too hot and in summer the system ices up. Maybe the control board burns up because the current to the fan passes thru it. The only way this person is going to be able to use a high MERV filter without eventual damage is to improve the return duct (like you suggested) and increase the size of the filter area by 3 or 4 times whatever this filter size represents.

I think most consumers just want the question answered, "which filter is best for me?" To answer that someone has to take readings on their system to insure it is working at its optimal design range with that filter.

*the .50" that the filter manufacture places as a limit is not the same as the .50 the system is designed for. It is a coincidence that the number on the filter is the same. Best guess is that above .50" across the filter it bends in and collapses, or just particulates just punch thru.