Hot water from the cold tap with under sink tankless water heater

[eugstr]

Hello, I know this is a common problem, but couldn't find the solution still.
I have a detached garage converted to a shop. A water line is running from a water tank 500 ft away under gravity in 3/4 pex pipe.
The flow is strong, and I have a booster pump right at the point the pipe comes into the garage.
The sink is a commercial stainless still sink with a commercial faucet with cold and hot regulators separated.
I've installed Stiebel Eltron mini 2 tankless water heater right under the sink. Just add a T to the cold water line to the heater and connected it to the hot inlet of the faucet.
I've installed the included flow reducer as it is directed in the manual.
When I open or close the hot faucet I hear the clicks in the heater, but the water is cold.
If I open the cold water faucet the water runs hot, in fact very hot, up until the booster pump kicks in and starts adding the pressure, and at that point, the water starts running cold.
I feel like the pressure is not right for the heater, but I'm not a pro, and couldn't understand what I should do to fix this.
Any advice is appreciated.

 

[breplum]

With point of use instantaneous heaters, they will only increase a certain number of degrees at a listed flow.
I’d have to guess based on your reporting, that the flow is not low enough. True, if you have the correct aerator or included reducer, it should provide what your equipment spec promised.

 

[eugstr]

With point of use instantaneous heaters, they will only increase a certain number of degrees at a listed flow.
I’d have to guess based on your reporting, that the flow is not low enough. True, if you have the correct aerator or included reducer, it should provide what your equipment spec promised.

Thanks for the reply.
That was my thought exactly, but would this mean if I'd open the faucet just a little bit, I'd get the heater working properly?
I've tried lower the flow with the faucet but it doesn't help.
The only way I get hot water if I open cold water faucet at almost full flow. And it is hot enough I wouldn't even hold my hand under the flow very long.

 

[breplum]

Quick reply is to contact the mfr's customer service.
I know that over my years using these, that customer service knows the ins-and-outs darn well.

 

[RS]

It sounds to me like you should have installed a small tank type water heater! The horror stories I read about tankless heater make me glad I never bought one! In our current house the master bath is 75 feet from the main 50 gallon water heater, so I installed a 3 gallon tank type heater in line, in series, under the bathroom. It works great, I get hot water right away and it doesn't change temperature while showering.

 

[Twowaxhack]

Check your piping diagram again.

Make sure your water pressure stays within the specifications of the water heater.