“C. Eckler is correct in her review. I had to replace our Taco SR-504 EXP unit because it burned out at the 120vac input terminals. The replacement for the EXP unit (the one in the shown photo) is part number SR506-4. The SR506-4 is different in appearance and is completely solid state, and the relays are not replaceable. The unit is similar in size and all the internal connections are the same. Our original unit was only 5 years old and was defective from the start as we experienced trouble with it during the time we used it. We had a repair made to it at the 120VAC input terminals. We cannot state if the replacement unit is of better quality because it was just installed 10/18/15. A photo of the failed unit is attached. We were in a bind and paid 318.00 for the replacement SR506-4 unit from a local heating/plumbing supply store.”
