We are replacing the boiler in our house and need to rent the electrical approx. 40 L water heater to plug in. I will appreciate any suggestion on where we can rent it out.
Check some of your local rental places. I doubt you'll find anything on the list, there's not much of a call to rent a boiler. It wouldn't be cost effective to purchase one just to have it sit around for years but, it's worth a try.
You're probably going to have to shower at the neighbors or rent a hotel room if you want hot water during the installation. The only portable type rental HW units that I've seen are sized for natural disaster type aid not individual homeowner use.
Get a short term membership at a health club and work out swim shower and go home.