How do I repair cracks in my fireplace?

[hpcbmw]

My house was built in 1975 and has a mortar and rock fireplace with a heatilator installed (metal insert with piping and fans). Had my chimney cleaned today and the guy pointed out that there were gaps between the fireplace and metal heatilator insert. The gaps are almost all the way around the fireplace. Gap is mostly an inch or less, but in some spots a few inches. You can see in my picture where my finger is pointing to the biggest gap. Where I'm pointing is a gap with just insulation. I'm also attaching a pic of the fireplace itself.

Whats the best way to repair this? I see refractory cement at Ace hardware, but it says only for cracks less than 1/4 inch.

I've got a ton of wood from some trees that were dropped and split, so plan to be burning lots of wood this winter (but considering I'm in Northern California, I'm only dealing with temps about 32 degrees, so not that much wood compared to you guys in the cold parts of the world). I'm planning on buying and installing a wood stove insert, but not til next year or the year after due to finances.

Thanks for any help.

 

[hpcbmw]

Anyone? Anyone? Bueller? Anyone?

 

[ccmac]

Love the Bueller comment. Well I am no pro but hopefully a few will chime in. Personally, I'd mix Portland Cement (1 part) with masons sand (2 parts) and fill those gaps. I built my shower floor with this stuff and it is hard and does not seem to crack. Can get both these at Lowes.

 

[hpcbmw]

After some research and calling Rutland products, I think I'm going to try Rutland castable refractory cement. I'm concerned regular cement won't take the heat.

 

[bholler]

I'd mix Portland Cement (1 part) with masons sand (2 parts) and fill those gaps. I built my shower floor with this stuff and it is hard and does not seem to crack. Can get both these at Lowes.

No you do not want to use straight portland at all it is to hard and will crack and fall out. If using mortar you need a mortar mix which is a combination of portland and lime I would recommend type s for that at a mix of 3 sand to 1 cement with an acrylic binder. But honestly what we use is chamber parging material like chamber tech it stands up to the heat and is more flexible.