17? thats low. The instructions for my kit says fermentation will slow below 20 (as expected) and will stop at 15C.With regards to temperature my current batch of Mild is going quite happily at 14°C there are limits of course to how low you can go and it will likely take a bit longer but it’s quite possible to ferment beers at 17°C. Also as fermentation is exothermic it will typically be a couple of degrees above atmospheric in the fermentor.
I'm hoping if the heated belt can hold the temperature at about 19ish, then the yeast will raise it by another 1 or 2C which will be more comfortable at about 21C which is within the preferred temp range. 22C will be smack in the middle of the preferred range but may be a challenge with the weather getting cooler.