Buying the part yourself and installing it can save a lot of money! Search for a great deal online and you're more than likely going to find it. It's always good practice to check out the prices for a spare part for your Kidde HD135F from several different places. That way you can be sure you're getting the best deal. There are usually more than one company producing the Kidde hardwired inter-connectable 120-volt auxiliary heat alarm part that you're looking for, especially aftermarket-type spare parts. Find the right part and fix it right!