As I once had an Intex 15' donut I'm vaguely familiar with what you can and cannot do. I agree: Lose the heater.

If you have a 10'x4' patch you can put a flexible solar heat panel on, or one of those new "Solar domes" (with the coils in a spiral) you can get VERY effective heating for your little pool, without killing your electric bill, EVEN USING YOUR LITTLE FILTER! No joke! I had the even smaller filter they put on it 14 years ago and I successfully ran the 10x4 panel with--got the water up to 105 deg one day! You'll save the cost of the panel and external hoses in electricity savings.

Plus, you can wash out and reuse the filters several times before you must discard them.