Water Heaters - Low hot water flow at faucet with tankless heater

Does anyone have an idea of why the hot water flow is so low with our tankless heater? The water flow is normal the instant the hot valve is turned on, then 2 seconds later the flow reduces by half when the hot water starts to flow.

There does not appear to be kinks in the lines around the tankless heater (the installer used braided steel lines in the heater closet).

Also, we need to use half hot/half cold water when starting the water flow, then always reduce the cold water to keep a consistent warm water mix, even during a 5 minute shower?

We just bought this house with the tankless heater already installed. The unit is supposedly less than a year old.

Any help is greatly appreciated!

My first question is what is the quality of your water? Is it soft or hard and if hard just "how" hard? Second question is what brand and model of water heater is installed? There are quality differences among the various brands.

Tankless water heaters and hard water do not play well together. They WILL build up mineral deposits in the heating pipes and that will reduce the flow rate through the heater and also reduce the capacity of the heater to raise the temperature to the desired level. Tankless water heaters REQUIRE periodic descaling and the frequency is determinate on the water quality.

Did the plumber (or previous homeowner) leave the operation and instruction manual for the unit? If not, you may be able to access the manual via the manufacturer's website.

It is also possible that your unit has an inlet screen that is partially plugged. The instruction manual would describe how to clean this if it is fitted.