Suggest installing the items immediately and giving notice for the rent increase to cover it.
There is no mention of the tenant being disagreeable with a rent increase.
I would advise the tenant that the property was offered at a low rent without a heater. Adding the heater would cost $15 p.w. extra rent. Let me know what you'd like to do. Get the tenant on a lease or he'll might leave with the warm weather.
Cheers.
thecrest