the cost feasibility would be questioinable
How much do they want to charge for 6 tankless
water heaters????? That has got to be expensive..
where do you live in the USA????
In our area, I would advise someone to simply
RENT a couple of commercial gas heaters from
the local gas company.....
when they break down ...its the gas companies
service departments responsibility
to make reapirs to them ASAP...