I have hot water in the cold water line. I have a tempcontrol valve Symmons (5-200) and looks like is backwards, because the cold water is going into the hot section of the valve. This can be the problem why I'm getting hot water in the cold line?

You have Cold in Hot side, and Hot in Cold side . Correct ?

Most of today's faucet manufacturers make their faucets / cartridge reversible.

Yes, that's correct. Then why I'm stilll getting hot water in my cold line? Too much pressure from my boilers?

Well, it depends. It could be one of the following of things.

1. It could be that your cold water line runs through the attic. There, water gets warm during the day. When you turn water ON, you will get couple of minutes of warm-to-hot water. We call it "Free Hot Water". Here in So.California, it even applies to buried cold water lines.

2. If you have hot water circ-pump with "comfort" valve than the valve pumps hot water into cold to assure "instant" hot water in your bathroom. Then, when you turn cold water ON, you will get this hot water out of cold faucet

3. Did you have any (any) plumbing work done at your house recently ? You could have crossed water lines...

4. You could have faulty cartridge at your faucet...

Does the Hot water eventually clears ? How long does it takes before you get Cold water again ?