Looked up installation manual from Trane but no mention of needing a screen on the end for a horizontal install.
My concerns are with nature and birds or something else building a nest and blocking exhaust.
No way to predict the future! Not likely to provide a home if it is close to the ground, though
Cheers
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The way I understand it is they can be screened as long at the holes are no less than 1/4". From there, it appears to be up to you.
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'Dem holes too small 
Brian, can you provide where you are getting this info from?
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So, the way I read this, the make up air opening size would have to be calculated with this screen in mind. But, you are currently in a retrofit situation. The screen may be ok with it’s 70% and 90%, but who knows? Therefore, you default to this.
G2407.10 (304.10) Louvers and Grilles
The required size of openings for combustion, ventilation and dilution air shall be based on the net free area of each opening. Where the free area through a design of louver, grille or screen is known, it shall be used in calculating the size opening required to provide the free area specified. Where the design and free area of louvers and grilles are not known, it shall be assumed that wood louvers will have 25-percent free area and metal louvers and grilles will have 75-percent free area. Screens shall have a mesh size not smaller than 1/4 inch (6.4 mm). Nonmotorized louvers and grilles shall be fixed in the open position. Motorized louvers shall be interlocked with the appliance so that they are proven to be in the full open position prior to main burner ignition and during main burner operation. Means shall be provided to prevent the main burner from igniting if the louvers fail to open during burner start-up and to shut down the main burner if the louvers close during operation.
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That’s all good and well, but you need to remember, the screen cap (on Concentric vents) is placed over the EXHAUST VENT opening, NOT the air intake as discussed in your quoted standard!!
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Ahh, I see your point. I had my in and out reversed! LOL.
Now here is an interesting article about screen size, percentage and seasonal use.
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I just cut some pieces from stainless steel wire mesh for mine.
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I tell my clients it’s okay to put a screen or mesh over in the warm months to prevent birds from entering and ending up in the furnace, but be aware that a lot of moisture is passing in and out through the unit in the fall & winter and the screen can freeze over resulting in the furnace shutting down.
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Have you experienced any freeze over during winter months?
Don’t install the retainer screws. If it ice’s up, the back-pressure will blow the screen cap off!
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No, never in 7 years.
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Sounds like a good idea