Furnace filters

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  #1  
Old 08-02-20, 08:08 AM
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Furnace filters

With a new furnace about 5 years ago, we started getting random shutdowns during the heating season. Turn off the furnace for several minutes, turn back on, and all is good for a while. The company that installed the furnace said the error code indicated too much air restriction. They said to not use the pleated air filter (FPR 5) and go to inexpensive fiberglass filters (FPR 1). That solved the issue, but it just occurred to me there could be a lot of crud getting by that's being trapped in the A-coil. What I'm wondering is if I could modify the RA plenum to take a 2" pleated filter (FPR 5) to reduce restriction. Or is restriction, restriction, regardless of filter depth? FYI, filter is now 16 x 25 x 1.

Or 1" fiberglass pre-filter, then 1" pleated filter?? Or something else?
 
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Old 08-02-20, 08:26 AM
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Think about it. What does the filter do?
I'll tell what it doesn't do. I won't filter the air you breath. That little box in your basement can't possibly filter the air you breath in a whole house. Plus you're getting all kinds of air leakage in the house itself. All houses "breath".
The filter is used to protect the motor and drive train form getting large amounts of debris (such as cat hair and bugs). Save yourself lots of money and use the fiberglass as suggested. Check it monthly and change accordingly. If you hold it up to the light and can't see through it. time to change it.
 
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Old 08-02-20, 08:39 AM
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Now see, this is what happens when an architect starts thinking about HVAC stuff LOL!!!

Do you think the fiberglass filter is enough to keep the A-coil from collecting junk and clogging? About once a year I connect a wand to my air compressor and blow from the top down. But I have no idea if that helps because I can't see the underside of the fins. And I'm scared to death to take the end plate off the coil for fear of screwing something up and ending up with a big repair bill.
 
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Old 08-02-20, 10:04 AM
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That little box in your basement can't possibly filter the air you breath in a whole house.
And to add one additional comment, think about how many minutes the unit is running each day. If it was filtering the air for your benefit it would need to run continuously!
 
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Old 08-02-20, 12:43 PM
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Just do what you're doing. Most people never do any maintenance.
 
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Old 08-03-20, 09:32 AM
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I assume you have the instructions and or manual that goes with the unit.
Start on the page where it shows the volume of air vs static pressure. This can be very telling. Then find the static pressure on the filter you are using. You may need some help on this.
I agree that the easy filter is not good choice.
With out knowing what the specs on your unit your shooting in the blind.
WWGrainger used to have a great section on air filters, you could look at the specs and compare air flow number.
I do not know if they do any more.
The filter industry some time ago decided to stop using static pressure and went to MERV. Great now we know what it will filter but not how much the air restriction is. Trust me higher MERV more restriction.

As a final thought, what changed? 5 year and this happens?
 
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Old 08-04-20, 12:02 AM
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Please give us furnace and outdoor unit model numbers so we can determine velocity and pressure drop. It'd also be good to know a large city near you, to eyeball your outdoor humidity and determine how high your CFM/Ton value should be.

We can then comment on how big a MERV 8 (FPR5) or 11 (The biggest I recommend) you need, and what length duct transitions you'll need, if any.

And before you get too excited about filter efficiency, you might want to check inside your supply ducts: Any dirt there will never be caught by the filter. The easiest way is to check behind floor diffusers or wall/ceiling registers.

Two other questions:
1. Did the contractor size the new equipment to the home, just replace in kind, or - perish the thought - go bigger? In cooling, "More Is Worse".
2. Did the contractor perform the required duct survey and recommend any improvements? It sounds like you have some duct restrictions that may be exacerbating the condition.
 
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Old 08-04-20, 04:35 AM
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I live in a suburb of Minneapolis, MN. It's been a while, but I believe the contractor did calculations based on approximate house size, but I can't say that for sure. Although I really doubt he would have done a full fledged calculation like our mechanical engineers did for commercial buildings. He made no mention of any duct issues. Here are the model numbers:
Furnace: Int'l Comfort Products F8MLV0901716A1
Outside Cond. Unit: Int'l Control Products: C2A330GKA100

Another note is that I don't have any issues at all with pleated filters in the summer. The A/c does great and is really good at cooling and taking out humidity even on the worse days. It's in the winter that the furnace will shut down randomly every 3 to 4 weeks when using a pleated filter. I turn off the switch on the furnace for several minutes, turn it back on, and all is good for 3 - 4 weeks. When using the cheap fiberglass filters there are no issues.

Give me a little bit and I'll post a picture of the plenum that contains the filter
 
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Old 08-04-20, 04:45 AM
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Here is a photo of the RA duct. the filter is in the very short horizontal portion of duct just before the furnace.

FYI, the house was built in 1957, 5 bedrooms with 3200 conditioned SF. Wall insulation is marginal (2" fiberglass), 8" blown cellulose in attic, and new clad Pella windows.
 

Last edited by Bruce H; 08-04-20 at 05:25 AM.
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Old 08-04-20, 05:29 AM
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Follow what your installer says. They know what they're doing. Don't make a mountain out of a mole hill.
I'm not a HVAC engineer nor do I live in a laboratory. With all due respect to FERD42 and DRANGD, you say the system works well and heats and cools sufficiently. Then use the fiberglass filters and let it go.

The smart money goes to the people who have their heating A/C system checked about every two to three years. Mechanical things do wear out and do require maintenance.
Have your coils in fact been filled with heavy debris?
Are you energy bills reasonable?
If it ain't broke, don't fix it.
 
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Old 08-04-20, 05:47 AM
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Norm201, the problem is that it is sort of broke in the winter using a pleated filter. The topside of the A-coil is clean as a whistle, just as I would expect, but no idea what the bottom side looks like because I can't see it without taking apart the SA ducting that encloses the A-coil. I have been blowing out the coil from the top down hoping it does something...can't hurt. Bottom line is that I think I can modify the the RA duct myself to accept a 2" filter if that's something that will actually help reduce restriction. But if there's lots of money to get a contractor involved, not sure I want to do that, being on a very limited fixed income.

MERV8 is what I would upgrade to and is what's commonly available in all the big box stores.
 
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Old 08-04-20, 06:03 AM
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But Bruce, you said the fiberglass filters resolve the problem of shutting down. And that's what the installers recommended. If your heating or A/C is not diminished, why would you be concerned. And if the coil is filled with debris that would be a problem just not with your system but many others. And I have never heard of it as being a typical problem. Maybe your system is unique but I'm trying to save you time and money. You seem to want to look for trouble.
I use the cheapest filters I can buy. In the winter I need to change them about once every two months. In the summer I can go the whole season and the filter is nearly clean as when new.
Try this suggestion...start off with the cheapest filters and then each month (or week) move up one level of filtration until you find the one that does not shut down the system.
 
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Old 08-04-20, 06:35 AM
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LOL, here's the problem Norm201: I worked for about 45 years as an architect, although it was mostly commercial rather than residential architecture. I know that furnace should work with a better pleated filter and it drives me crazy that it doesn't. I can only imagine how much money I could have saved over the years if I were not an architect, especially when my hobby is remodeling my homes!! It's a hazard of the profession. If you know any architects with a lot of years of experience, they will probably say the same thing.

And BTW, I really do appreciate your comments may well leave well enough alone as you suggest.
 
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Old 08-04-20, 06:40 AM
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Yea, I understand. Sometimes we so involved with things that interest us that it's hard not to tinker with it. Keep us posted if things change.
 
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Old 08-04-20, 08:53 AM
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They said to not use the pleated air filter (FPR 5) and go to inexpensive fiberglass filters (FPR 1). That solved the issue
Ok, IMO way more comment than needed, the solution was provided, the OP confined it worked, so why is there more discussion!
 
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Old 08-06-20, 01:19 AM
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Couldn't find a lot on that furnace, but it looks like an 80% two stage with 90,000 input at high fire. Is that correct?

If it is two stage, please check the furnace nameplate and tell us the high and low OUTPUTS and Temperature Rises. If it's single stage, just output and temperature rise.

That this only happens in the winter is a clue. And don't bother answering the question about the duct survey, the image says it all.
 
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Old 08-06-20, 03:19 AM
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Just in case the image is hard to read:
Input @ high fire: 88,000
High output: 71,000
Low output: 47,000
High temp rise: 40-70
Low temp rise: 30-60

Thank you SO much for looking at this ferd42, I really appreciate it!!
 
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Old 08-07-20, 02:32 PM
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Now I need the air delivery chart from your Installation Instructions. It might look like this old Trane chart. I think your problem has to do with (High Speed???) heating airflow being higher than cooling airflow, but what do I know? We'll see.

There appears to be a very unsafe condition at the furnace return: The duct joints don't appear to be sealed, which could negatively pressurize the space containing the furnace. International Code Council considers this hazardous when a 90+ furnace (AKA "Sealed Combustion") is provided, so it's extremely unsafe with an 80%. I raised this issue on another forum, and the contractor responsible for the work said he taped the joints on the INSIDE of the duct (AS IF)!

Please give me the size of the return "drop" to the furnace while you're at it, might as well check that too. That's a high resistance elbow at the base of the drop, BTW - Lot of turbulence created by the square inside corner.
 
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Old 08-07-20, 04:14 PM
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No, the duct joints are not taped. But there is a 6" dia. insulated flex duct for outside air that terminates next to the furnace in a J; doesn't that resolve the negative pressure issue? The furnace is in a 12' x 26' unfinished room, with a 6" x 22" trunk running the length of the room. I could tape only 2 of 4 sides of the trunk. I assume it would be the UL listed foil tape? I assume you're talking about more than just the RA drop you can see in the picture??

The RA drop down from the trunk line to the filter is 10" x 25".

Here is the chart, I hope it's what you are looking for.

 
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Old 08-07-20, 09:46 PM
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That is what I was looking for. Your cooling airflow (2-1/2 tons) is 875 CFM, low heat is 965 CFM and high heat is 1,210 CFM. So heating airflow, hence filter PD is always higher than cooling airflow and filter PD. I think you have the ability to reduce heating flows by 10%, which would make cooling and low heat essentially the same. Then your problem only occurs in high heat. Might be you don't need high heat, and I'd think a good tech could disconnect a stat wire so the furnace fires at low heat only. Sorry I can't provide further info on these changes, beyond my K&E.

As for the filter, the PD should be less than 0.10". So the filter would have to be 1 @ 16x25x1 MERV 1 and 2 @ 16x25x2 MERV 8 and 11.Those are guesstimates based on COOLING airflow. Avoid the $$$ high efficiency filters sold at the home stores - like the plague they are.

The outdoor air duct is a combustion air inlet, and no it does not solve the negative pressure problem. You need to seal every accessible return joint in the room, and then think hard about how to access the remaining joints. UL180 foil tape or UL duct sealant (AKA "mastic"), and there's now a "Mastic Tape" that you have to scrape off if you want to disassemble a duct, similar to Hardcast Foil-Grip 1402. Pricey, but you do get what you pay for.

25x10 is a good duct size, oversized if anything - BUT a duct can't be too big, the air doesn't get lost in there. And the oversize somewhat mitigates the restrictive fitting at the base of the drop.
 
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Old 08-07-20, 09:51 PM
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And read the furnace Instructions some more, they usually spec the minimum furnace filter size. I'd be interested to hear what you find.
 
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Old 08-08-20, 05:16 AM
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First things first. Thank you again for all the time and effort you have put into this!! Greatly appreciated.

So to get a higher MERV rating, you're saying a 16 x 25 x 2 Filter is what's necessary?

Very interesting on what mfg. says: (1) 16" x 25" filter; 1" thick cleanable or 4-5/16" thick media-type. So media-type is the replaceable kind I'm currently using?? I assume this what they mean by a cleanable filter? Expensive up front, but I save on replacing filters and I get a higher FPR https://www.homedepot.com/p/Web-16-i...5FPR/100004206

Spent a lot of time looking at sealing the RA trunk line. Without taking down finish ceilings and the RA trunk line, no way to seal all 4 sides, can just get to 2. Any options like undercutting the room door or a register between the furnace room and the adjacent finished space?

Random question, what's the deal with FPR vs. MERV? What looks to be the same filter at Lowes says MERV 8
 

Last edited by Bruce H; 08-08-20 at 05:48 AM.
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Old 08-08-20, 05:37 AM
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Avoid the $$$ high efficiency filters sold at the home stores - like the plague they are.
So after all is said and done thsi is the conclusion as originally suggested.
 
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Old 08-08-20, 08:08 AM
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No clue about FPR, first time I've heard of it. TWO MERV 8 to maintain less than 0.10": 32x25 total area. Wasted breath discussing MERV if your supply ducts have debris, of course. You're probably sticking with a furnace filter.

I'd say seal what you can on the return side, especially near the furnace. That Code requirement predates your home. So much for your expert HVAC contractor, and shame on him for the old school thinking - "Oversize the furnace to be safe for cooling airflow" - that got you to the rare case where required heating airflow is 50% more than cooling airflow, and you have to jury rig it to get it to run. Not saying all contractors are bad: There are many highly trained, skilled contractors out there, may their tribe increase.

Best of luck!

 
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Old 08-09-20, 09:46 PM
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Hey Norm!

"Follow what your installer says. They know what they're doing." You still think so?

It always amazes me when lay people say things like this, with no clue as to a contractor's competence other than that they arrived on time, smiled, were courteous, didn't get the white carpet dirty and treated them nice.
 
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Old 08-10-20, 05:12 AM
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"Follow what your installer says. They know what they're doing." You still think so?
Kind of like the guy who is 100 or 1000 miles away can tell from just a few words or pics and be an expert to give advice without ever being there or seeing exactly what's going on. Verses those who installed and actually worked on it. And actually gave advice that in fact worked. See post #15. Enough said!
 

Last edited by Norm201; 08-10-20 at 05:41 AM.
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Old 08-10-20, 01:16 PM
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I agree: I am a "guy who is 100 or 1000 miles away can tell from just a few words or pics and be an expert to give advice without ever being there". Thanks!
 
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Old 08-10-20, 06:16 PM
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I think we've thoroughly beaten this thread to death. Does it need to be closed ?
 
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Old 08-11-20, 05:56 PM
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Yes, but not before I ask Norm to please excuse my "snarkiness". Thanks all!
 
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