I have designed and installed many gas systems in my career. Gas is not to be messed with if you don't know exactly what you are doing. Protect yourself and your home. Get a pro and have him get a permit.
I recommend and have installed quite a few UV systems over the years for my customers. The one that I recommend is the Viqua VH200 9 gallon per minute unit for normal residential loads. Have never had one fail to destroy bacteria. All tested after install. Unless you have a farm or something...
Do you have a whole house attic fan? I had a situation once that when the fan was turned on it actually sucked the combustion gasses out of the boiler that was heating domestic hot water in the summer.
FWIW. I did my first pex jobs in 1992 using Wirsbo and their compression fittings. They are still in service with no problems. And Sioux Chief makes some of the best plumbing materials in the USA.
Some of those for irrigation are slow close. If they fail to close slowly you will get hammer. I suspect a bad valve. Disconnect the power one by one from the valves as they are running. See which one is the culprit.