I was told with my first HTT that warranties were voided if you put the screens on but does not seem to be the case any more. I have the screens on everything, but not home-made ones, I bought the ones mades specifically for my appliances to ensure a good fit and to be sure I do not impede air flow. Where we live, they are a must over all openings where propane is used because mud daubers will build their nests in a matter of days when I am parked. I've even had mud daubers build a nest on one of my A/C fan fins. Tried to get a screen for the A/C on my new HTT but no luck. Just need to make sure I run the A/C every so often so they don't have time to get busy up there.