Tankless water heater - Auto off after 20 minutes

[the-butterfry]

We need a PHP function that will account for tankless water heater cutoff:

Tankless water heaters by law have a 20 minute duty cycle. They’re required to cut off gas after 20 minutes, but the electronics are still on and won’t re-ignite unless you switch them off to reset. We need to build build a counter into the website/db so that after 18 minutes a tankless water heater will shut off for 1 cron_min cycle. Here’s my idea in the /cron_files/cron_min.php at the very end.

If heater = 0 (on) and type = tankless (I have already inserted a column in db, and chooser in the add/update relay section for this)
Then increment column time_on by 1
Read column time_on, if < 18 then set heater = 1 (off)
Set colum time_on back to 0

Here's what the table looks like