So here’s my plan to backwash my water-to-air heat pump to remove hardness deposits. Install a hose bid in the EW and LW pipes. Pump white vinegar from an elevated position into the HP LW pipe, through the HP coil, out the EW hose bid and back to the plastic bucket. Allow pump to run for 2 hours or so – to be determined by the effluent color. Close EW hose bid, open EW valve a little a fill bucket several times to rinse the coil from LW hose bid directly. PS I am planning on having a professional do an acid wash every 5 years if needed.

Might work. other options I've read of is ice machine cleaner or Lime Away. I can't say how the heat exchanger will hold up against any of the three

Congratulations!! You are a graduate of the diy common sense academy. Set up will work fine. Trick is to find the least caustic so;ution that is still effective at cleaning the coil and stay cost effective.
Eric Sackett
weberwelldrilling.com

I wonder if there would be any warranty issues with WaterFurnace.

PS The best improvements I made after the installation I believe were a.) a spin down filter on the incoming water line b.) an inline water flow gauge that can be bypassed and c.) a pair of inexpensive temperature gauges on the EWL and LWL.

Does any one know anything about this product that is set up for cleaning Tankless HWT's ?

www.flowaide.com

Another reason I dislike tankless HWTs

Yes, a product geared toward descaling a water heater should work for heat exchanger

Eric hit the nail on the head - trick is to periodically, no more often than necessary, descale the heat exchanger with a solution of minimum strength to get the job done. Any cleaning process will somewhat impact the heat exchanger - too strong an acid will clean it but also dissolve it.

A worst case scenario has loop water or cleaning solution getting into the refrigerant loop, ruining the compressor. That should not happen if caught in time since the early stages of a leak to the refrigerant side will result in refrigerant leaving the system since its side is under higher pressure, at least until all the liquid refrigerant boils away.

It is worth noting that swimming pool heat exchangers are often made with Titanium to ward off damage from Chlorine.

NuCalgon Ice maker cleaner. Nickle safe is what I have been using.

Duly noted