expansion coupling for a heating systen

I don't do alot of heating work so i have not seen an expansion coupling for baseboard before. I'm not one to get really stumped by stuff, but the home owner wants something else because he has been fixing the packing on the expansion coupling for 20yrs. oh and the baseboad run is 36' long and he does not want an off set in his pipes so i know there are not many options.
I would like to know any suggestions anybody has

thanks patrick88