I picked up one of these at a boat show
. Haven't used it yet but it sure should make flushing
A/C lines easier. Your city water
test isn't conclusive since there could be hard barnacles
or a crimp in the lines. The water
would still run clear. If you put a bucket under the outflow and measure a minute of water, is it well above the amount of water in the Marine Air spec?
I think your "clue" that it happens in hot weather
may be important. A/C systems are a careful balance between cooling
water, refrigerant, and air flow. Humid air is much harder to cool (2/3's of A/C energy goes into dehumidifying not cooling)
The new units you bought are probably very different from your old ones including a new refrigerant (410A). So you have to consider almost every source of trouble.
HPF failure can be set off by airflow restrictions. The new units may need more return air and supply air than your old ones.
And it could be refrigerant. You may have a borderline undercharge or overcharge that trips the alarm
condition in hot/humid conditions.