We recently had our hall bath renovated. It's been almost 8 months and we cannot get hot water in the shower. We replaced Moen faucet cartridge,replaced Moen balancing spool, bypassed piping leading to shower, all to no avail. All the other faucets in the entire home work fine.Oil fired boiler with tankless domestic hot water coil. Home is 27 years old. Pulling the handle to turn on water brings us very hot water turning warm then just warm but not enough to shower within 15 seconds after turning on. What could be the problem and how can it be solved. Original contractor said recommended acid washing domestic hot water coils, but it doesn't make sense since there is adequate amount of hot water in the rest of the house. Also the water quality in our area is very good, soft and no other related problems from neighbors. Looking for any advice.

Thank you