I have a N-0631s Noritz tankless gas water heater to supply hot water to my bathroom faucets and shower. During the winter the temperature of from the shower head flucuates when the temperature is only warm. The flow rate for the unit (code:14) during this temperature is 19 to 21. Why is this flucuation hapening? I have flushed/scaled the unit this summer by pumping vinegar through the unit for half an hour with very little scale indicated in the fluid after the flushing. The unit sometimes does not fire up to provide hot water if only one faucet at the sink is turned on. The pipe distance from the tank to shower and faucets is about 12 to 15 feet. What is involved in installing a recircultion system to reduce time for hot water to the faucets?

Water Heaters - Noritz - Residential Heaters Tankless Water Heater N-0631S-LP