Hi,

I recently installed electric tankless water heater and geothermal unit, if I have the kitchen sink running and turn the shower on, the water flow really decrease, hot water remains, is there anyway to increase the flow so when I have the shower on the water flow for other unit stay the same? the tankless is suppose to supply hot water for multiple unit at once and it does work, but not enough water if everything runs the same time.

You need to give us more facts.  Is the home plumbed with pex or copper?  In other words, does each plumbing fixture have its own dedicated line or do all of the fixtures share the same line?  If the line is shared, then the line may be too small or partially blocked.  How long has the plumbing lines been in place?  If it is an old home and the line is shared, then there may be an accumulation in the line that blocks the flow or the water pressure may have dropped.

Its copper, recently we installed hague watermax system to treat our well water, each  plumbing fixture share the same line. the plumbing lines been in place probably around 20 years as the age of the house. the water guy told me that the new clean water will get rid of the scale build up in the pipe and should increase the flow and eventually clean out all the pipe, is that true?