I was going to put an inline heater on my filter as I want a backup and liked the idea of the inline out of sight heater. Hydor have two units, 200W and 300W which are a little larger than I need for a 37 gallon tank.
I currently have a 150W Aqueon Pro (black plastic case, no glass) that I like as it hides beside a vertical piece of driftwood so it doesn't stand out. It also has a bi-colour LED that is green for power and red for heating. I always know that it is plugged in.
This would become my backup heater.
The issue arises that a 150W heater is not enough to maintain the water temperature without adequate water circulation, at least in winter when the ambient room temperature can be 65F or lower.
Apparently I need the filter on to circulate the water for the heater to do it's job, it's own convection is not enough. Oversizing the heater is a good idea for those in colder climates or in homes that the temperature may vary throughout the day. Even air conditioning blowing in the vicinity of the tank could cause some problems.
My conundrum is that I was going to consider the intank a backup to the inline and in another thread I even said that an inline shouldn't be the sole source of heat as with the filter off, the tank has no heat. I might suppose that my intank would have still maintained a somewhat lower temperature but I don't think I can really rely on it as a full backup. Even during water changes in high 60F temperatures I am surprised how fast the water loses temperature... yesterday I did a 10% change and left the heater plugged in and still lost a few degrees.
So I suppose that I am technically still relying on my filter to provide heat throughout the tank, even if indirectly, and my backup is not truly a backup afterall.
It's never as simple as it seems.