Long HDMI Cables Don't Work?

Are the HDMI cables good enough quality to retain signal? Sometimes a bad quality HDMI signal can weaken signal. Also, you say that the screen flickers so this is an HDMI cable issue. Try plugging the cable into a different HDMI input on your TV first and post back. 

They have really good quality HDMI cables on amazon.

https://www.vanco1.com/product/certified-premium-high-speed-hdmi-cables-with-ethernet

I am running these at 20 feet no problems for 4k. Not too bad on the price.

You just need a better cable. Lots of cheap cables don't work properly with 1080-p, althouggh I would have thought 20ft should not be an issue. https://www.bluejeanscable.com/articles/how-long-can-hdmi-run.htm

Higher price does not always equal better cables, and neither does lower price mean poor cables, but you should check that they're certified for what you need them to do.

I have traditionally had very good luck with buying cables from https://monoprice.com 

Whichever cable you decide on be sure and hook it up to the equipment and make sure it works before running in wall.

Sorry. One other thing. Leave old cable in place until ready to run the new one. You may be able to tie the new one to the old one and slowly and carefully snake it right behind the old one.

He does not say it's going in the wall.

My mistake. I just thought since it is above a fireplace and He has a 20 ft HDMI cable. I just thought the cables would run behind.

I have a discreet surface mount chase not technically in or behind the wall but the great point to be made from some of my research is the UL listed cable or CL3 I believe would be the most appropriate product for a buried in wall cable.  Also for a certified cable, the multitude of amazon vendors word their cable very carefully either to convey that the cable is certified to the highest standard or to make you think it is when it's not.  There are plenty of slight variations and in the end I concluded that even the slightest of wording deviation almost always turned out to be an un-certified cable.   Key wording to look for is 'Certified Ultra High Speed'.  Those are also the cables where you will see the trademarked silver label in the lead photo.  It has a QR code and hologram.  Also, it's much harder to find anything over 16 ft certified on Amazon.  After 30 min of searching the massive selection, I finally found 2 choices with Prime shipping.  Coming today...

Yes, did you read the blue jeans article?  For their cheaper cables, 15ft is about the max certified distance.

I did.  Thanks.  Some of that information is dated, referencing max lengths for older HDMI standards but still some good info.  So my new 23 ft Maxonar cable was $59 but is legit certified, thicker heavy duty, and is working great.  Case closed.  Thanks all.

Good deal