<p>I have a 15x15 cm heating resistor from my current printer (printing area: 12x12 cm).</p>

<p>I would like to switch to a glass bed and to rework my printer to increase the printing area to 20 cm (22x22 cm glass plate).</p>

<p>Would it be possible to use the old heating resistor placed only in the centre? this way I would have a smaller heated bed for ABS and a bigger one for PLA.</p>

<p>Would the glass crack due to non uniform heating? This is because glass has a conductivity of less than 5 W/mK, therefore the hot area will stay hot and basically never really spread the heat to the surrounding area. So the frame will be cold and the center hot, causing stresses.</p>

<p>Related: <a href="https://engineering.stackexchange.com/questions/31842/how-much-an-unevenly-heated-glass-plate-bows">https://engineering.stackexchange.com/questions/31842/how-much-an-unevenly-heated-glass-plate-bows</a></p>

The glass will be taking up the heat to slowly cover its full area.
That means two things:
it will not crack as there is no thermal shock
it will put more pressure on the heater as there will be bigger heat absorption, so in an edge case, you could end-up with not getting a required temperature on the glass surface and/or the heating process will be very slow.
In that case, I personally will try that out and then if I have an issue with getting proper temperature, then I will buy a new heater.
A comment regarding heat expansion:
my glass is about 20mm longer than the bed and it is warm during the printing process. The 'breaking' stress is connected with temperature shock (an immediate temp change), not a relatively slow heating process. However, borosilicate glass also known as Pyrex (TM) expands very little. Much less than most metals. This is why it doesn't break when exposed to sudden hot and cold changes. So in short words - check tea lighter under your tea-pot and see how te heat is comming out from the centre
source