My home was constructed in 1996 and has a CH 200 amp main panel with all available slots filled. In the past several years I have added service to my garage (20 amp, light load) which involved replacing a single breaker with a dual. I also replaced our 240v electric cook top with a gas unit which should have lighttened the total household load considerably.

I want to install a Watts Premier tankless water heater which requires 4 - 30 amp circuits. I plan to do this through the installation of a sub-panel. Since my main panel is full I plan to remove the two existing double 40 amp breakers (80 amps total) used for the dual air conditioner units in the main panel with larger breakers to feed the sub-panel and then move the air conditioner breakers to the sub-panel. This would mean a 200 amp sub-panel, is this possible or I am over loading the main feed? Can I feed the sub-panel some other way without installing new breakers in the main?

Thanks

Really need to confirm your 200 amp service can handle the load of this water heater. According to the specs I found, the max amp draw will be 116 amps, and this must be calculated at 100 %.

Your air conditioner will also need to be assumed at running full load, and with the water heater.

If you provide,
Total square footage of livable space,
List of all major electric appliances.
Total KW or amp load of AC

I can do the service sizing calculations. My intuition is your service cannot handle this added load.