The main internal unit is the heat exchanger that can not be replaced, at least I certainly have never heard of replacing one, never. Questions to answer. How old is the unit? What is the expected lifetime of the fireplace? How much to replace new? Prices can vary state to state and country to country generally for labour, product prices are fairly similar unless in the Yukon, Alaska and Hawaii etc. Price a new unit. My rule of thumb is if the repair cost is 50% of the replacement cost and the lifetime of the appliance is past it's half way point I replace it. It is easier today to make that call as most products made seem to be disposable. I have gone through 3 fridges in the last 20 years while my elderly parents still are using the Viking refrigerator they purchased in,ready, 1978. Harvest gold is OK for Mom. She complained just this week their 7 year old microwave is on it's way out. The first one lasted 25 years. I reminded them they paid over $1000.00 for it back then. Before I would replace I would hire a reputable HVAC tech that is well versed in NG fireplaces to inspect the heat exchanger, I'm somewhat surprised the tech. Didn't comment on the heat exchanger while he was checking the gas valve. Hope this helps.

I wish someone would answer you, I was going to post something similar myself. We have a contract on a home and I had the chimney inspected today (realtor says I'm the only person in her entire career to have a chimney inspection, but you'll see why) Inspector told me the flue tiles needed to be replaced and the to reline a chimney was about $4K-$5K or could have the tiles that needed to be repaired which iwas about half that price - he wasn't positive on pricing as he does not do this type of work. He actually called me back and the low end pricing from the company gave him a guesstimate of $1500-$2500 depending on what I decided to actually have done. Originally, this was a wood burning chimney and now is gas. I know there was an issue with the damper not opening correctly and was having the owners clean it out before the inspection. Due to the issue above, he didn't do the cleaning - not until we decided what to do about the flue issue. Anyhow, I'm not putting out any more money on the home, it would be the owners who'd have to repair. My realtor called me back a little while later and said the listing agent spoke to the owners and came up with what they think would be a solution - a vent free gas fireplace. Since I'm a Floridian who's never had a fireplace in her life, I know nothing about any of this. I'm trying to find out if what they are proposing, how it differs from the gas fireplace that's there already. Will what they put in look the same as what's there now? They claim it's more energy efficient. I'm glad you are asking the questions you did above because I wouldn't have known to ask them, so I really hope someone knowledgable will answer. In the meantime, I was given the name of a guy who does gas fireplaces for a living. Will try to reach him and if I can , ask those questions and let you know what I may be able to find. If I go with the ventless, I'm still having them clean out the damper. With it not opening all the way, that means cool air is still coming in, and in summer, cool are is going out. Please let me know if you came across any info. THANKS!!!

One question that may help you find what you need: Is this a pre-fab unit, free-standing, or an insert? There may be a model number located in the firebox - if you put your head in the firebox, turn so you're facing out, and look up toward the top of the firebox there may be a number located there.

Before you buy the 60" TV, check to see how close it can be to the firebox. Looks like you have a gas fireplace which can really pump out the heat, if it's like mine. Is the cabinet to the left built-in, or free-standing? How wide is the frame above the mantel now, from end to end outside measurement? Is it 60+ inches?