Level 40. Heat Blast. THIS is the point in the game when you should start getting a visceral feel for your heat level. Heat management is the hallmark of a good Shieldtech and will determine whether you hold aggro or your group wipes again and again.
Single target ability priority:
Rail Shot > Rocket Punch > Combust > Explosive Dart > Heat Blast > Unload > Flame Burst > Rapid Shots
Heat Blast comes after Explosive Dart because ED is actually very high damage and aggro. Also from a QoL perspective, having HB further down on my priority list means that I will be less likely to accidentally use it at a time I cannot benefit from its heat dissipation. After some comments about channeled abilities when I posted level 20 tanking advice, I decided to just include them in the main priority list. I feel that on average I get approximately 2/3 of the channel off and this is the estimate I used when making this list. If your target is incapacitated and you can get a full duration Unload off, prioritize it above ED. Flamethrower is the same damage as Unload but costs 50% more heat. I really would not use Flamethrower in a one target scenario.
AoE ability priority:
Death From Above > Flamethrower > Combust > Explosive Dart > Flame Sweep > Heat Blast > Rapid Shots
Obviously, feel free to single target at your discretion. This list is only for times when all of your DPS are AoEing as well. NEVER OPEN WITH FLAMETHROWER!!! Flamethrower does not tick instantly, it ticks after one second. That may not sound like a lot but it is PLENTY of time for mobs to walk out of range of the channel. Always use Flame Sweep or DFA first and make sure you have control of the mobs before you use Flamethrower. And remember, AIM. Get used to Flamethrower's range because if you don't hit a mob with it, guess what? You get no aggro on that mob. Yeah, THAT one. The one over there eating your healer's face.
Positioning: Awareness is key to tanking. Grapple and Jet Charge should be used to move mobs wherever you want them. Use them to open on targets, peel straying mobs off of group members, and gather mobs for easy AoE tanking. Positioning is the tank's job, no one else's. Don't worry about where the DPS or healers are throwing down AoEs, just get the mobs where you feel they should be and let the other group members readjust to that. If a DPS misses an AoE it means the fight will take roughly 3 seconds longer. If you miss an AoE it could mean aggro loss and a wipe.
Heat management: Unload will break even on heat for a full channel. It's 15 heat cost is offset by the three seconds of passive heat dissipation you'll get during it. Expect and anticipate this. Flamethrower costs 25 and dissipates 15 over the duration, so expect to end up with about 10 more heat than you started with after a full channel. Unless you have a very good reason not to, always skip down the list and use the first ability you can that won't bring you over 40 or under 16 heat. Rail Shot, Rocket Punch, and DFA are exempt from this rule because they are just so damn strong.
You'll notice I added a recommended minimum heat level there. 16 is the about the amount you dissipate in one GCD if Shield Vents procs. It is roughly the line between quickslots 1 and 2. QUICKSLOT 2 IS NOW YOUR HOME. LIVE THERE.
Always remember that being a Shieldtech is all about maximizing heat usage over a fight. You never want to fritter away heat dissipation unless you have to. Use Heat Blast, Rapid Shots, and Flame Burst to control your heat level and stay in the target zone. But don't stress out over losing some heat dissipation; we're only human. Lastly, HAVE FUN! Tanking can be stressful business and even feel like work on occasion. A happy, relaxed tank is a good tank.