what is the difference between the PET version and the regular version of this filter?

Had the same question - guardian website says the only difference is the 5250PT has "Pet Pure" filtration.From the company website: "The Pet Pure(tm) is an antimicrobial agent added to the filter to inhibit the growth of mold, mildew and odor-causing bacteria on the surface of the filter."The filter I'm replacing doesn't have any mold or mildew odor, so I'm going to save the $16 and get the FLT5000.

Once you replace, how do you get the filter symbol to stop flashing and beeping when turned on and off?

It may vary by model. I have AC5350B. I just replaced the filter and pressed the Timer button for 3 seconds. Under the timer button on my model, it says "HEPA RESET". This worked. It's no longer flashing.

Our filter has this same model # but it is 19 inches long not 21 as it says in the picture description.

I've found the different filters and different models to be a tough figure, so I went back to the page with the unit(s) I bought and just ordered (subscribe & save) 1 of each unit. I checked the box on my CPet filter which says it's for models in the AC5000, AC5200, and AC5300; it measured at 20". (I also have order filter size B for my "not pet pure" unit and it measured 14.5". I'd suggest contacting the company to see if those who are more familiar with everything germguardian can help you. (I had a problem with the UV light and they picked up on my review and contacted me before I could contact them. Good luck!

Does this include the hepa filter and the carbon filter? the complete set needed when it’s time time change out?

Yes, as shown in the photo of the product, you will receive the Hepa and Carbon filter combined together. If further assistance is needed please contact us at yourfriends@guardiantechnologies.com or at 866-603-5900 X213 M-F 8:30-4:30 EST.