Repair and Maintenance: Ac Unit Repair Cost

My question involves landlord-tenant law in the State of: Texas

The Landlord is using the Lease Agreement form that's used for Texas Association of Realtors. In that agreement template, article 18 mentioned that the Landlord has to pay the entire cost to repair the heating, AC system and heating water. In article 26 'Special Provision', it mentioned that Tenant has to pay $50 for each repair call.

I had rent another townhouse earlier, the lease agreement is also prepared using the form of Texas Association of Realtors. In Article 26, they put clearly that I don't have to pay for the cost of repairmen AC and heating unit.

The AC unit doesn't work properly and it's not because of my negligence. The Landlord is asking me to make a co-pay for the cost of repairing the AC unit. He claimed that he will pay everything when he changes the whole system, he'll not pay everything for the repairmen cost.

I have no issue to pay $50 for him this time but I would like to ask is it a law requirement or typical practice in Texas that the Landlord will pay in full for the cost of repairment the AC unit ?

Thank you,

Repair and Maintenance: Ac Unit Repair Cost