Chimney / Fireplace Question

The other week I had a fire going in my family room fireplace. Shortly after starting the fire, I noticed the smoke was backing up in the chimney - and the whole downstairs was quickly flooded with smoke. This is the second time in a year this has happened. After the 1st time, we suspected it might be a negative pressure situation, so I have been sure to crack a nearby window every time I start a fire ... guess that doesn't help.
I decided to have a CSIA certified chimeny sweep come out to clean the chimney and inspect the chimney for safety. He offered to install two caps on my chimney (on the fireplace and furnace crowns), which I had him do. He though the smoke backup was probably caused by blowing winds, and said the caps may help that problem. He gauged the clearance and height of the chimney, and said it was up to code and that probably wasn't the cause of the smoke backup.
Now here's the real issue: During the course of the inspection, he noted that the crown on the chimney was crumbling (which I knew about already). He also noted that there were gaps between the chimney flue and the firebox, where the box narrows into the chimney. The gaps are not holes, but it's clear there is some masonry which is either rough, or crumbling.
The chimney sweep (also a fireman for a local FD) made it sound as if this was a potentially severe problem, and recommended that I not use the fireplace until getting it fixed. Of course, he had a solution ready to go: to use some product called "Smoketite" to seal the gaps. Cost $500.
I hate getting inspections and repairs from the same people, as the potential for abuse and fraud is too great. Does anyone know if this truly is a problem or, better yet, if the repair he's suggesting is worth the $500. Being the holidays, cash is very tight ... but I really enjoy my fires this time of year.
Any insight appreciated !
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It can happen for a couple of reason. One is the temperature differential. If the fire is just starting, the damper is open, the cold air in the chimney wants to come down as it is heavier than the air below it. Once the fire gets going and produces a lot of heat, the air flow can be reversed. If you feel a cold down draft, try lighting some loosely crumpled newspaper paper just to get a hot air flow going up. The paper gives a lot of heat quickly.

He is correct. I've often had back puffs in windy weather and the cops do help.

Yes, it is. I can't see it so I don't know the severity but fire can geet into cracks and cause overheating of the surrounding area.

I'm not familiar with the Smoketite so I cannot comment. You have to rely on the reputation of the inspector in this case. Has he been around a long time? Locals use him a lot? If he is on the local FD, you can probably ask around and find out if he is OK to trust.
As for the "worth $500", I know of people that had chimney fires. They would have paid that much gladly to prevent the fire. Is this particular job worth that much? I have no idea what is involved.
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And hold the burning paper as close as possible to the TOP of the firebox until it starts to draw up.
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My parents have a fireplace with a masonry chimney which is on an exterior wall. We used to have to crumple a news paper, and hold it up into the chimney, light it, and then let the news paper burn a couple seconds to get the draft going. I felt like the Statue of Liberty, but it did work.
--

Christopher A. Young
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Lighting a piece of newspaper near the flue is a good idea. Also, if you have central forced air heating make sure that the fan is not running when you light the fire - the intake may be causing some draw down the chimney.

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