Yesterday we installed a 3 liter instant water heater which uses 3 KW. After the install, hot water is coming through the tap, albeit very slowly. The cold water is coming fast. Our geyser was installed by an electrician who told us that instant geysers do not require pressure. How can I get the hot water to flow quickly?
30°C temp raise (10°C to 40°C) for 1L/min requires about 2,1kW of power. So with a 3kW POU you can't expect a big flow. An 'adequate' shower consists in about 6L/min. These are the numbers. Especially if main water is cold, instant POU are not enough, better to install a under-sink tanked heater (10L), it delivers much better flow even at higher temperatures.
Electric 'point-of-use' hot water heaters have a water flow rate much less than the cold water line. The behaviour you describe may be normal operation for this unit.
Is this unit being used for a shower or to heat water for the kitchen sink faucet for dish washing?
From this unit one could get a small amount of very hot water on demand and by mixing with cold get a low volume stream of barely warm water which would last several minutes, until the 3 L of very hot water in the mini tank was used up, then the water would go cold and one would have to stop the flow for maybe 10 or 15 min until the water in the tank got hot again. It seems that this unit is what is called in the US a 'mini tank'; the water in the tank will be very hot (~>60 C), but will not have a high flow rate.