I bought a descaler kit, which cost $150, to flush the tankless heater myself. One time use paid off the cost of the kit. The only cost later on is only the vinegar solution used to flush the tankless heater.
The instruction for descaling is clear and easy to follow. The only thing I had to figure out was which was which on isolation valves and service valves. It is a two-step flush, first use vinegar solution to flush the tankless through re-circulation of the solution for 30-45 minutes and then use clear water to flush the tankless again for 3-5 minutes. The second step was troublesome when using a bucket to collect water - we had to empty it frequently, carrying the bucket from attic to second floor bathroom.
Last year I finally decided to improve the second step flush by using a 75 ft long hose to connect the service valve outlet to the bathroom tub, eliminating the need to empty bucket frequently.
This morning I decided to flush the tankless for the 8th time, the 7th time by myself. I thought that it would take me 45 minutes to complete the job. But one misstep a the beginning got me into trouble.
Instead of following the instruction line by line, I quickly scanned the instruction, and hooked up the system by memory. Instead of re-circulation first, my hookup led to the 2nd step flush. I quickly realized the mistake and corrected it.
But the re-circulation could not be established after correction! Was it that the pump malfunctioned or that it did not powerful enough? I did a few debugging. First I checked if the pump works or not, by disconnecting the other end of the hose connected to the pump from service valve - the water could be pumped correctly. I then shut off the service valve, turn on hot water isolation valve, turn on hot water service valve, water could come out no problem. All the components were working. But when I hooked up the system again step by step per instruction, the pump was running, but there was no water coming out of the re-circulation outlet - hot water service valve.
I asked Lily to come to the attic to check my installation step by step and I reinstalled the descaler kit with her reading the instruction line-by-line. I still could not make the re-circulation happen! My tentative conclusion was that the pump malfunctioned - not enough power.
Before I quit, I did a restart. I uninstalled the kit, returned the tankless heater to operating condition. I then followed the instruction step by step, when the pump was plugged to electricity, the descaler system was working!
What happened that made the kit not working before the retsart? Another observation was that despite the shut-off of the isolation valves for both hot water and cold water , the fluid level in the bucket crept up gradually. The increase in the fluid level in the bucket should not happen because the only source of fluid was from the bucket!! Why did it happen?
Looking at the internal structure of the tankless and because of the erroneous first installation, I believe that the re-circulation is established by Siphoning. My erroneous first hookup essentially emptied the pipe in the tankless - see illustration above. When I corrected my installation, there was no water in the pipe inside the tankless any more, the pump's power by itself could not overcome the hydrostatic pressure from the height of the water in the pipe. That's why despite the pump working very hard, there was no water coming out of the re-circulation outlet.
When I restarted the process, I first refilled the pipe inside. With correct installation, the siphon could happen and the kit worked as designed.
I still could not fully explain why the fluid in the bucket continuously increase during re-circulation. It seemed very likely to be related to water stored in accessory pipes connected to the main pipe where water is heated because the instruction stated that the bucket can only be half full ( to prevent the spill over from the excess water?)
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