Last year I noticed the walls/sheetrock above two of my baseboard heaters had dark patches in a couple of areas that were hot to the touch. Assuming they were old and worn out, I replaced them with new heaters, matching the wattage and voltage. The walls continue to get fairly warm in those areas, not as hot as before but still enough to draw concern. They are mounted 2" above the floor, no curtains, d****s or furniture are located near the heaters. The remaining 4 heaters in the house work great, as expected. Any ideas? Thanks in advance.