I changed my thermal fuse on my Bunn grx-b because it was not heating the water. Now it's like it is overheating. What could it be

Bunn switches if the rocker style as the looks like will have two leaves on the side of the switch that will need to be squeezed in order to get it past the square hole. These can be really tight but if your sure it's at fault then just crush it to remove the pieces. The new on should snap in if the hole was not damaged.
* Since your checking the brewer you should get the manual that may be available from Bunn.com (I haven't checked for this model but they usually post the manuals)
* This seems to be a home version of a VPR commercial brewer (but nicer). So expect the same parts for the fuse, thermal fuse, thermal disk (or thermostat).
* Note: If your not familiar with Bunn brewers then make sure you look at every piece of wire as some high limit fuses are inside plastic tubing or heater braiding.
* Thermal disks look like a metal button with a ceramic cap or button attached to it. This can be a high limit (reset-able) or a thermostat. * It may have a standard thermostat with bulb but it may be a thermsistor probe with a PC board to control the brew heat.
* Final; this is the 3d question on this particular brewer. All have similar problems with heating. It's the same with VPR's. If I set them up I know it works properly. It's when someone moves them to another office that's the problem as they dump out the water prior to the move then plug it in without refilling the tank. Thermal fuse blows. There is a link below showing how to set up this brewer.
Aloha, ukeboy57
https://www.bing.com/videos/search?q=bunn+grx-b&&view=detail&mid=E6D1ADD550DA3DA5B120E6D1ADD550DA3DA5B120&&FORM=VDRVRV

This is a common question for the BTX-B.
It's a commercial brewer that needs to be pre-filled prior to turning on.. It also needs to be filled again with a full carafe for it to brew properly. I have a link below that will explain how to set up and use this brewer.
Aloha, ukeboy57
https://www.bing.com/videos/search?q=bunn+grx-b&&view=detail&mid=E6D1ADD550DA3DA5B120E6D1ADD550DA3DA5B120&&FORM=VDRVRV

If the fuse inline in the wire(s) to the heating element, simply cut off the old fuse, strip back the wires a little and insert a new thermal fuse using either but crimp terminals or screw terminal strips.

I would guess that the wire or plug in bad. It could also be the off an on switch

Could be one of several issues. MC won't hear if it has low water. Low water may be caused by scaled probe, no water supply or other flow problem. Could be that thermal fuses on element have blown and need replacing. Could be that heating element is burnt out. You will need to do some further basic diagnostics.

This is really a home style VPR brewer (but looks nicer)
if you listen to their video (link below) they mention it has a pre-heated tank of water already in the brewer. This means the brewer needs to be pre-filled with a full pot of water and pre-heated prior to using the brewer. Unplugging this unit won't be a problem UNLESS it was emptied of tank water and not refilled when powered up.
* The unit will try and heat up the tank of water but if empty then just air which overheats the unit then blows the thermal fuse. Call for service.
* The warmer is still working but it's not hot enough to brew frequently as the warmer may take up to an hour to get the tank hot enough to brew or it may not get hot at all if it's own thermal fuse blew too.
* Water leaking while the brewer just sits there? Leak in the tank. Call for service.
* water leaks only while pouring? Then your pouring to fast. Wait for the water to drop before pouring in more. If it still leaks even while pouring slowly then there is a leak in the fill tray. Call for service.
* Final; call for service to be safe.
Aloha, ukboy57
https://www.amazon.com/BUNN-Velocity-10-Cup-Coffee-Brewer/dp/B00EAQJCWW/ref=sr_1_1?ie=UTF8&qid=1516431654&sr=8-1&keywords=bunn+nhs+velocity

Okay, When going away most people empty the tank completely. When returning you must start it as if brand new out of the box. Follow the MFR's instructions for an initial fill and heating.
Some don't empty but just unplug just as you did. Places like Arizona with very low humidity can dry out or lower the tank water where the heating element is not in water. This element will overheat and trip out the thermal fuse.
**Another scenario is the empty unit was plugged in and heating while someone was going to get the water. Over heat then blows fuse.
** The "Vacation" switch is very visible so you probably have tried it already and you have already checked the outlet and breakers. Still looks like a thermal fuse problem.
** Switches do go bad as they are turned on and off daily. Possible yours was bad before but never noticed as it was never turned off till on vacation.
** There are other possible scenarios but all point to a thermal fuse, thermal limit, blown fuse, switch, Heater element, or cut wire (Mice). All are electrical problems and require a MFR tech. Contact your vendor or MFR for service.
*Have a safe New Year, Aloha! ukeboy57

Maybe limed up.Try running a deliming spring in the brew tube.Bunn p.n. 1111.0000.

If you will call 800-352-2866 we will be happy to further assist you in getting your brewer to work properly.

Sounds like the thermostat is bad.

Best to contact a local Bunn service dealer for repair