Which yeast are you using, perhaps consider using something like salsafe US-05 which can iirc ferment happily down as low as 12c
whats the current gravity? if at the tail end of primary yeast activity may have simply slowed and stopped imputing heat into the FV, depending on the strain it may well be happy as larry @ 16C or lower..
You will probably get better results with an insulating wrap and leaving it another week or so in primary, or if possible shifting it to a slightly warmer spot temporally (well till you bottle/keg), ambient temps are going to increase with the advent of summer, that cool spot will be very valuable soon..
Applying heat can encourage fussil and ester production.. (no i cant spell either of em..) to levels beyond those you may appreciate, my rule of thumb guide is to ferment at the lower end of a yeasts comfort range for the cleanest ferment..
both pads and belts apply heat input to a relatively small area, some folk consider this results in hotter than optimum spots in the brew.. To avoid this you can use a larger storage box or trub to heat the fv with a small aquarium heater via the waterbath.
In all cases i personally wouldnt trust the 'built in' (present or not?) heat control of such devices and would consider using something like an stc1000 to monitor the temp and switch off when @ target.
The best option of course is a brewfridge fitted with a temp controller, tube heater, and movement fan, as that will maintain your optimum FV temp regardless of ambient conditions..
anyway thats my 2p's worth