Thanks for that
But if I have a double Mat I have the option to stick 1 or 2 fvs on
Might go for the eBay large Mat with the thermostat included
I am also thinking of building a insulated box to put them in
I am sure someone said not to put my fv directly on the heat pad and to put something in between them
A couple of things for you to think about:
A heater provides heat only - it doesn't provide any temperature regulation other than - perhaps - at a very crude and probably inaccurate level. Some other form of temperature control is desirable to hold temperature where you wish it to be.
A double mat, with two FVs on it, will not necessarily keep both FVs at the same temp. One may be closer to a radiator/draught/cold window etc than the other. And beers that are fermenting vigorously will generate some heat of their own.
You may only want to ferment one beer at a time sometimes, wasting heat & money with a double mat.
If you do go for a temperature controller, if you only have one mat for two FVs, you then have to decide which of the two FVs you're going to place your temp probe/sensor on/in.
Ideally, but at more expense, IMO you'd be better off using two mats for full flexibility, plus two temp controllers. Inkbirds, perhaps? (I have an old, Mk I Grainfather controller spare, which I now use as general temp controller for fermenting and sparge water heating)