Unauthorized Maintenance on Water Heater - Bill the Tenant?

[Garry C.]

Apparently there was a 'hiccup' with the gas the other day, and my tenants needed to relight the stove, fireplace, and hot water heater. They told me that on the heater they removed the door with the sticker that said 'Do NOT Remove' and tried to relight it. But this didn't work and it looks as though some of the wires are melted.

I sent a guy out and he can't find parts to fix it, says it would be $600 to fix, and it's a 14 year old tank so I should probably just spend the $1650 for a new one. OK, price is a little high, but not too bad. The real issue is, should I hold the tenants liable? He says that this damage is almost certainly brand new and done by the tenants in an attempt to relight. Pictures are coming soon...

I think the obvious answer is 'Yes, bill the tenants for the new hot water heater'. But, how? Bill them full price of a new water heater with next month's invoice? Offer to spread payments over the next three months to be nice? Tell them to get out and find new renters, since they breached the lease?
The lease is fairly general, it simply states that they should not do anything other than routine maintenance like light bulbs, furnace filter, etc. Perhaps this could be considered routine under those terms.