I presume you have a 24v supply. If so then loosing 10v would leave you with about 18v at the heater when dumping.
If you have a modest rated 12v heater that will stand 50% overload it will be ok but that is a lot of overload and I would question whether the heater will last long.
Do you want the heat? if so you are wasting quite a lot of your available heating in cable loss. If it is just convenience and some heat is going to help then fine but I would think carefully about overloading a 12v heater.
Not enough details to help much.
Flux