I recently bought a 36" Thermador gas (propane) cooktop on Craigs List. It was for sale because its ignitors would randomly spark. I replaced the control board and potentiometers behind the XLO burners. They now work fine. However, I feel the flame height and output on the center grill is very low. The flames are 3/8" tall when on HI.
What you’re describing sounds normal. The grill is 20,000 btu, which is only 5,000 btu more than the star burners, but spread out over a larger surface area. I’ve always suggested preheating the grill for about 10 minutes prior to use to get the flame spreader/lava rocks hot.
It dawned on me the number of BTUs wasn’t really that high. I must have been expecting it to be like the ones on TV cooking shows.
Many thanks,
Grant