We are not liable for interruption or malfunction in service.

Will you have money put away for system malfunctions?  

I wouldn't charge anything for the lawn because (a) it doesn't sound like your watering system was 100% functional when they moved in, and (b) it doesn't sound like you stressed the importance of reporting malfunctions so you could repair the system.

Either the washer or dryer can be expected to malfunction given the very heavy use cleaning bedsheets and towels after every check-out.

That is what we did to avoid this problem since all ice makers eventually malfunction.

Also check  the t&p valve either on the side of the water heater or on the top of it and I have had them malfunction and when they do they stick open and just leak water outside the home so you wouldn't see any water inside anywhere. 

The water was shut off from the pool, the heater malfunctioned and kept burning.

After brushing up my leases and thinking more about I am wondering if this puts me liable if there were to be a CO leak and it were due to detector malfunction from the residents.  

It's oh so convenient that the outside unit 'undercharging' you is the malfunctioning one and not the meter inside overcharging you.Did you ask them to check for a leak on their side of the meter?  

I did not see a statutory definition of "normal wear and tear" under Florida laws after only a very brief search, however the legal definition in Texas is:"Normal wear and tear" means deterioration that results from the intended use of a dwelling, breakage or malfunction due to age or deteriorated condition, but the term does not include deterioration that results from negligence, carelessness, accident, or abuse of the premises, equipment, or chattels by the tenant, by a member of the tenant's household, or by a guest or invitee of the tenant (Tex.