Is a written agreement necessary for a rent reduction for issue?

[Daniel L.]

Hello everyone,

My rental had an issue where the heater was having issues and numerous techs couldn't fix it and ultimately the furnace had to be replaced. In the meantime the tenant wants a rent reduction for the time they were inconvenienced and we agreed on the amount. 

I have read from other forums that it might be best to get them to sign an agreement for the amount reduced for the issue. 

Is that necessary and what type of agreement exactly would there be to sign. Just wanted to see some thoughts. 

Thank you