I have a Bunn NHB coffee maker, after the coffee brews the warmer plate is cold and does not keep coffee hot, light switch is on but warmer plate is cold, Is it replaceable or should I send the whole coffee maker back to Bunn for repair???

The manual for this coffee maker can be found at this link:
http://manualheaven.com/assets/bunn-nhbx-b-contemporary-coffee-maker-manual.pdf
While the manual does not contain repair instructions, it does have some troubleshooting tips to follow if your coffee is not staying hot.

Did you check to see if the switch on the side is in the proper position? The "l" should be down and the "O" should be up in the air with orange showing. If this is the case, then please get the code date and model located on the bottom of your brewer and contact customer service for further help. 800-352-2866 is the toll free #.

I work for BUNN. Please call our customer service team at 800-352-2866 so they can talk thru what's going on and solve this for you.

Bunn makes a spacer that goes on top of the burner to put space between the burner and the coffee pot, or you can make your own out of a coat hanger. The only other way to fix this is to replace the warmer plate.

it has a reserv tank equal to 2 pots which are hot all the time
the power switch is only for the warmer plate
you get out about what you pour in,

It sounds like you have a pour over model and the problem is the tank isn't full. Turn the machine off (or unplug it). Put a pot on the warmer like you are going to brew coffee. Pour clean cold water into the top until it starts coming out the sprayhead (above where the coffee grounds go). Now plug it in and turn on. Let it heat up then you can brew.

The way your machine works is that there is always about 2-1/2 pots of water in the tank. The water you pour in to make coffee is actually pushing the hot water in the tank out. So, if you don't fill the tank before using it, you may be hurting the heating element since it isn't totally submerged in water.

Sounds like something is miswired from the factory. Note that the tank heater switch on most home brewer models also acts as the "master" power switch. If the tank heater switch is not on, the warmer switch will not work, either - this is normal. Under normal conditions, the tank heater (or vacation switch) should always remain on, except if you are leaving on vacation.

I had the same issue, poured in 8 and out came 6. Here's what I did to clean and fix.
1) Unplug the NHB
2) Remove accessories like the pot and basket
3) Bring NHB over to the sink
4) Snap off the entire top cover (not the swing lid) by starting at the front and lifting upward (the top cover is about 1/4" thick). The back of the top cover will pivot a bit and then pop off exposing the pour reservoir. Remove the flow stopper inside the pour reservoir by lifting it straight up.
5) Place the top cover and flow stopper aside.
6) Remove the brew diffuser (located underneath the top where the brew basket goes. Twist counterclockwise gently but forcefully enough to snap it past the lock tabs which hold it in place. You'll know that it you've done this correctly when it just falls out.
7) Still over the sink, tip the NHB in a fashion where the back side is facing upward and the brew side is facing down into the sink. You'll be holding the NHB horizontally by the top and bottom of the unit. This will allow any water in the tank to pour out the brew spout. Begin to tip the unit as if tipping upside down as this will allow any remaining water to drain out. If this does not work the brew spout hose may be clogged and you can just tip the unit entirely upside down to empty it from the fill hole in the pour reservoir.
8) Return the NHB to the upright position in the sink.
9) Using a scotch brite sponge and some baking soda, clean the inside of the pour reservoir and rinse.
10) Using the tool Bunn provided, clean out the brew spout hose which is located under the brew hat. This is the little hole where the water comes out. If you don't have the tool Bunn provided you could use something like a plastic tie-wrap. Don't use anything metal as the hose you're cleaning is made of plastic tube and could become punctured. Clean this hose for just a minute or two to ensure that there are no significant obstructions.
11) With the NHB upright in the sink, pour a slow stream of hot water from the faucet, about two cups, directly into the fill hole located in the pour reservoir.
12) Tip the unit horizontally, as in step 7, and place the tip of your finger over the pour reservoir fill hole covering it completely.
13) Shake the unit back and forth horizontally so the water in the tank splashes around and frees anything which may be built up in the tank. Some water may splash out of the brew spout hose.
14) Tip the unit as in step 7 to drain the tank water out.
15) Fill a 16 ounce glass with hot water and 2tbsp baking soda, mix and pour into the pour reservoir fill hole.
16) Repeat steps 13 and 14 once.
17) Repeat steps 11 through 14 three times.
18) Clean the pour reservoir cover, swing lid and flow stopper.
19) Put the flow stopper back in place.
20) Attach the top cover.
21) Clean the exterior of the NHB.
22) With the boil and warmer switches off, plug the NHB into the electrical outlet.
23) Turn the boil and warmer plate switches on.
24) Fill the NHB with one pot of cold water and close the swing lid. Wait one minute.
25) Fill the NHB with another pot of cold water. Place the pot on the warmer plate and close the swing lid. If water does not yet come out, fill with one more pot of water.
26) Allow water to drain from brew spout until it stops draining.
27) Empty pot.
28) Fill pot to 10 cup line and pour into the NHB. You should find that the NHB now delivers 10 cups back to the pot. Empty the pot.
29) Clean brew spout diffuser and replace back in the brew hat.
30) Clean the pot, pot lid and brew basket.
31) Wait ten minutes for the water in the tank to boil.
32) Brew your coffee as you normally would.

The only issue I experienced during this process was with the warmer plate switch. It appeared that the light in the switch blew although the warmer plate still warmed normally. Shortly after the water in the tank started boiling the warmer switch light started operating again.

Try connecting two elements in series. See if this keeps it hot enough. (One switch controlling both burners.) See if another brand of burner will fit in the hole. Check to see if the wattage is lower.

I had the same issue, poured in 8 and out came 6. Here's what I did to clean and fix.
1) Unplug the NHB
2) Remove accessories like the pot and basket
3) Bring NHB over to the sink
4) Snap off the entire top cover (not the swing lid) by starting at the front and lifting upward (the top cover is about 1/4" thick). The back of the top cover will pivot a bit and then pop off exposing the pour reservoir. Remove the flow stopper inside the pour reservoir by lifting it straight up.
5) Place the top cover and flow stopper aside.
6) Remove the brew diffuser (located underneath the top where the brew basket goes. Twist counterclockwise gently but forcefully enough to snap it past the lock tabs which hold it in place. You'll know that it you've done this correctly when it just falls out.
7) Still over the sink, tip the NHB in a fashion where the back side is facing upward and the brew side is facing down into the sink. You'll be holding the NHB horizontally by the top and bottom of the unit. This will allow any water in the tank to pour out the brew spout. Begin to tip the unit as if tipping upside down as this will allow any remaining water to drain out. If this does not work the brew spout hose may be clogged and you can just tip the unit entirely upside down to empty it from the fill hole in the pour reservoir.
8) Return the NHB to the upright position in the sink.
9) Using a scotch brite sponge and some baking soda, clean the inside of the pour reservoir and rinse.
10) Using the tool Bunn provided, clean out the brew spout hose which is located under the brew hat. This is the little hole where the water comes out. If you don't have the tool Bunn provided you could use something like a plastic tie-wrap. Don't use anything metal as the hose you're cleaning is made of plastic tube and could become punctured. Clean this hose for just a minute or two to ensure that there are no significant obstructions.
11) With the NHB upright in the sink, pour a slow stream of hot water from the faucet, about two cups, directly into the fill hole located in the pour reservoir.
12) Tip the unit horizontally, as in step 7, and place the tip of your finger over the pour reservoir fill hole covering it completely.
13) Shake the unit back and forth horizontally so the water in the tank splashes around and frees anything which may be built up in the tank. Some water may splash out of the brew spout hose.
14) Tip the unit as in step 7 to drain the tank water out.
15) Fill a 16 ounce glass with hot water and 2tbsp baking soda, mix and pour into the pour reservoir fill hole.
16) Repeat steps 13 and 14 once.
17) Repeat steps 11 through 14 three times.
18) Clean the pour reservoir cover, swing lid and flow stopper.
19) Put the flow stopper back in place.
20) Attach the top cover.
21) Clean the exterior of the NHB.
22) With the boil and warmer switches off, plug the NHB into the electrical outlet.
23) Turn the boil and warmer plate switches on.
24) Fill the NHB with one pot of cold water and close the swing lid. Wait one minute.
25) Fill the NHB with another pot of cold water. Place the pot on the warmer plate and close the swing lid. If water does not yet come out, fill with one more pot of water.
26) Allow water to drain from brew spout until it stops draining.
27) Empty pot.
28) Fill pot to 10 cup line and pour into the NHB. You should find that the NHB now delivers 10 cups back to the pot. Empty the pot.
29) Clean brew spout diffuser and replace back in the brew hat.
30) Clean the pot, pot lid and brew basket.
31) Wait ten minutes for the water in the tank to boil.
32) Brew your coffee as you normally would.

The only issue I experienced during this process was with the warmer plate switch. It appeared that the light in the switch blew although the warmer plate still warmed normally. Shortly after the water in the tank started boiling the warmer switch light started operating again.