Repairing the Masonry Heater Firebox

Last winter was the first for my masonry heater.  While building, curing and using it I meticulously followed all the proper procedures.  I used it full out, meaning firing it as often as recommended, simply because it was an abnormally cold winter and my farmhouse is unfinished.

I knew that the mortar was cracking inside the firebox, but put off repairing it as I was occupied with running the farm. A couple of weeks ago I began to consider what should be done.  After an inspection showed the mortar in the firebox was crumbling, I called Smith-Sharpe Firebrick Supply in Minneapolis and asked an expert "What should I do?". He said I might have used a bad bag of mortar, doubtful in my opinion, but that something called "Greepatch 421" would be the best product to repoint the joints.

When building the firebox I used the refractory mortar that came with the "kit" I bought from Eric Moshier at Solid Rock Masonry. It was rated up to 2500º. The new stuff is rated at 3500º. Maybe that will help, even though my temps never got above 2000º. (That infrared heat thermometer sure is useful.)

(Click on any picture to make it bigger.)

I started repairing the wall on the right. I scraped about 3/8" out of all the existing joints with a tuckpointing trowel. The mortar, when it didn't pop right out, was surprisingly soft.

 I also removed and relaid the floor of the firebox. The grate that fits in the hole seen in the bottom of the firebox was badly damaged and in need of replacement.  The "owners manual" from Solid Rock Masonry states that the grate will need to be replaced every other year. I'm ahead of schedule.

The old grate was badly corroded and had a bow/bend in it of about 1/2".

With the sides done I relaid the floor.

I think, and a phone call to the firebrick company will clear this up, that the mortar will have to cure for at least two weeks. Meaning no fires, and then only small ones. This is unfortunate because we've already had two nights with temps below freezing. For the next few weeks it looks like temps will be 10-15 degrees below normal.