Gas Insert at 7,500 feet elevation?

[pfwag]

Bought a 1965 vintage house in Colorado at 7,500 elevation. It had a fireplace upstairs and down, each with their own flue in a common chimney. Last year put in a 72,000 BTU Lopi Republic 1750i wood burning insert in the bottom fireplace along with stainless steel chimney liner. Love it! This year I want to put in a ~22,000 BTU gas insert upstairs in the living room fireplace.

I understand a ventless gas insert can't be used above 4,500 feet. Is that correct?

While I could vent up the existing flue (a B-Vent?), is there any reason I can't punch a hole through the back of the fireplace and put in a direct vent which could also pull in outside combustion air? If so, wouldn't that be more efficient than a B-Vent that uses inside air for the combustion?

Recommendations on a direct vent gas insert?

If I do use the existing flue, would it be prudent to also put in a stainless steel flue liner for a 22,000 BTU gas fireplace?

 

[Tech Guru]

Many manufacturers make what are called Direct Vent inserts. They'll actually use two liners going to a specially designed cap that brings in combustion air down one line, and allows flue gases to exhaust up the other. Saves knocking holes in the masonry and makes use of the pre-existing chimney cavity. This keeps that efficiency you were worried about. Not as many companies make the B Vented inserts these days, but some still do.
Vent Free is sketchy at anything above 4,500 feet of elevation, though you'll find some would argue that they can be sketchy at any elevation. Regardless, their pilot assemblies are incorporated oxygen depletion sensors, and when you put in in thinner air, those systems react overly quickly and results in erratic function and need higher frequency cleanings.

I would suggest you look at the DV Inserts from Companies like Enviro, Valor, Travis, & Regency etc for a good selection.