I bought a 2400 square foot cedar house last year with a 27000 Regular Buck free standing stove in the basement. The house was built in 1976, and I assume the wood burner is about as old. I have been using it 24/7 this winter to supplement the oil furnace with a lot of success. I recently noticed cracks in the fire brick. After investigating, it started crumbling. What do you recommend I replace this with? The shell seems to be in great shape. Thanks...