one issue with check valves at water heaters is thermal expansion, when the water is heated in the tank. Options are:
1. Just let the relief valve pop, though sometimes the relief will not pop (so messy when that happens) and sometimes it will not reset, dripping water into the bilge
and draining your water tank. Also does not meet current plumbing
codes (in the USA anyway) btw.
2. Drill a small 1/8" hole in the check valve clapper. This was the old school
way of relieving pressure back to the cold water side, while keeping the hot water in the tank. Before Oh 1998 ish this was standard practice in engineered plumbing
3. Install a thermal expansion tank between the check valve and the water heater. This is the current
"code" method for dealing with thermal expansion at water heaters. I recommend the small ones at the big box store over the more expensive marine