Hearthstone Heritage won't get above 350 degrees

[Socratic Monologue]

Also -- check the draft. It is easy (10 minutes), and cheap (~$30). Dwyer manometer, drill a hole in the flue, insert brake line with tube attached to manometer.

The stove requires a pretty high draft (0.06 - 0.10) -- likely you're not getting that with the improperly sized chimney, though you simply won't know until you test it.

 

[Woody Stover]

After all I've dealt with trying to get this thing going, I might just sell it. I know they are capable of getting much hotter though. I'd really like to know why I can't get the heat up on it.

I'm now thinking that maybe this particular model doesn't draft well when rear-vented.
Part of the problem is undoubtedly the leaky, un-insulated house. Tighten that up, and you'll be better off, both winter and summer.
If you're gonna go to an 8" outlet stove, Bucks are made in Spruce Pine, NC, and there should be plenty of used ones available to you, either cat or non-cat. I had a model 91 at my MIL's on a 21' liner, and the draft was plenty strong. Bucks don't rear-vent, but a couple might fit into your fireplace, and they can be run as an insert, or free-standing.
You can probably swap stoves without much if any cash outlay..
Why did you end up moving the Heritage? Moved it to the room you spent more time in?
What kind of sq. footage are you trying to heat? Is it open and easy for heat to move around, or is it a chopped-up layout with high ceilings?

insulated 8" liner. Put it to work!
Go get a big stove that isn't plated with insulation! I'd put a BK King...put a damper inline, 40' is a whole lot of draw

You guys that don't like soapstone keep spouting the same line, "soapstone is insulation," but I don't think so. Stone merely takes longer to heat up because it stores more heat than steel, pound for pound, and a stone firebox is thicker than steel so it stores yet more heat. My stove has some double-thickness stone..like 1.25" with both layers. Once you get it loaded with heat though, it blasts it out as fast as it's loaded from the inside of the box. And the additional time to get the stone hot from a cold box isn't all that much longer, compared to the steel non-cat stove I've run recently. They're all gonna take at least some time to get up to temp.
Your BK is actually more insulated than other stoves when you consider the firebrick and extra "heat shields" they put inside the box. Peak heat output isn't impressive compared to other stoves, according to a few different users. The "insulation" would explain the anemic high-output EPA numbers for the BKs. The soapstone stoves you call "insulated," trounce them at high end. Even the little Ws Fireview, at less than half the firebox size, thrashes the King.
No, the OP's problems are not because he has a small soapstone stove per se.

 

[Mech e]

You guys that don't like soapstone keep spouting the same line, "soapstone is insulation," but I don't think so. Stone merely takes longer to heat up because it stores more heat than steel, pound for pound, and a stone firebox is thicker than steel so it stores yet more heat. My stove has some double-thickness stone..like 1.25" with both layers.

Thermal Conductivity Values

Soapstone 6.4 W/mK (Watts/meter Kelvin)
Steel (A36) 50 W/mK
Cast Iron 52 W/mK

The added thickness of a soapstone stove also increases the thermal resistance, further decreasing the ability of the stove to transfer heat to the surroundings. Relative to steel and cast iron, soapstone is an insulator.

 

[Highbeam]

It sure is pretty though!

 

[Woody Stover]

Thermal Conductivity Values
Soapstone 6.4 W/mK (Watts/meter Kelvin)
Steel (A36) 50 W/mK
Cast Iron 52 W/mK
The added thickness of a soapstone stove also increases the thermal resistance, further decreasing the ability of the stove to transfer heat to the surroundings. Relative to steel and cast iron, soapstone is an insulator.

OK. I was wrong, soapstone is an insulator to some degree, but that would appear to be a moot point since a soapstone stove like the Woodstock PH has the ability to deliver one of the highest EPA BTU/hr. output rates. There are clearly other thermodynamic factors at play. As I said, once the stone is up to temp, soapstone stoves can crank out the heat.
Mr. Flotsam' implication that the the OP's problems with heat production are due to the soapstone box, and that if he gets a BK King (best at slow, steady heat production) his problem will be solved, is missing the mark IMO. There are stoves that will throw more heat than the Heritage, but until the OP solves serious heat loss issues with his house, it may be an uphill fight. He lives in a mild climate..shouldn't be that hard to heat, even with the stove he has. But we are lacking a lot of details of the area he's trying to heat; He may well need a higher-output stove, even if he addresses the heat loss issues.

It sure is pretty though!

Yeah, you coulda tore down the Heritage and slapped that stone on the Princess Fiona.

 

[Mech e]

OK. I was wrong, soapstone is an insulator to some degree, but that would appear to be a moot point since a soapstone stove like the Woodstock PH has the ability to deliver one of the highest EPA BTU/hr. output rates. There are clearly other thermodynamic factors at play. As I said, once the stone is up to temp, soapstone stoves can crank out the heat.

To be fair Woody, the soapstone does have a specific heat (ability to store thermal energy) about twice that of steel and iron. This would not necessarily increase heat output, but would lengthen the heat output cycle compared to a metal stove.

But now that I really think about it, most of the internal surfaces of my steel stove are covered with fire brick. Fire brick has an even worse thermal conductivity value than that of soapstone (actually, a magnitude worse, about .47 W/mK). It may be that the top of a steel stove transfers more heat than a soapstone stove, but I think the back, sides, and bottom surfaces are transferring much less.

So, you are correct. There are definitely other thermodynamic factors at play.