Warning: this is long. Go somewhere else if that bothers you. You have been warned.
1. Why I wrote this.
I will be honest, Heimer was one of my favorite heroes for a very long time and it bothered me that his power dropped off so sharply during late game and against teams who had played against him before. But there are plenty of other heroes that people like but play badly despite them being good heroes, so how could I know that the problem was really with the character and not with how I used him? So I decided to look more closely at the character to learn more of what was really going on. Once I had done that the natural next step is to share what I found with other people. This is what I found.
2. A history of the hero.
In order to understand why a character is what they are and how to use them effectively, the first step is to learn what has made them what they are and how they have been used. This will also be important later to understand what solutions have been tried, what solutions will likely never be tried because of what has happened before, and what pitfalls to avoid in other areas. This also turned out to show itself to be important because, looking at how things change over time, it is obvious that old fears persist long beyond the point where the problems are remedied. It was actually a bit of a surprise to see how much the past of what he used to be continue to effect this hero.
Heimer began his life in the League as a horribly overpowered champ by almost any measure. He had 6 turrets that were randomly targeted but persisted beyond his death, had 120 MR and dealt full damage to towers and became more "experienced" over time causing them to deal more damage. His rockets out-ranged towers and multiple rockets could hit the same enemy if not enough additional targets were in range and targeted only champions. His grenades could damage towers and, despite being slower, had a longer blind and a shorter cool-down. And his upgrade took his already powerful turrets and made them even more dangerous instantly. Obviously he needed a nerf, though a lot of champions were apparently more powerful back then.
The developers tried to tweak him slightly by having the turrets detonate on heimer's death, making the turrets defensively weaker, and reducing the blind. They then tried to change some things around with a buff/nerf combination where they gave him a number of buffs in exchange for some slight nerfs and the major nerf/rework that nearly halved the number of rockets he could fire. Almost certainly the buffs were in part to make up for how the squishiest champion had just been made a major target due to the fact that killing him was the easiest way to remove his turrets. The next couple changes were similarly small and aimed at keeping his power under control.
Eventually these small changes were not enough and he underwent his first major remake. His number of turrets were cut in half, the abilities of "upgrade" were worked into this skill. His rockets could now hit three targets at all levels and had reduced cool-downs; but targeted minions, had lower AP ratios, and dealt less damage per hit. And "upgrade" was changed to heal the turrets and give slow shots rather than give any more permanent bonus (other than CDR).
By this point people seem to have learned how to use him effectively, as shown by what comes next. The "push" meta might have been somewhat involved too.
One patch right after the other, the nerfs start flowing. First his Turrets are made to deal half damage to towers, and then his Grenades are given the same treatment. The Grenades then have their range and damage decreased while his Turrets get a bug fix that reduces their power. Then his Turrets loose the ability to gain "experience" and become more powerful over time while having their AP ratios cut but getting 2 extra points of damage increase (probably to make it seem like less of a major nerf). Next he lost his third turret (though the remaining turrets gained 100 hp in it's place which was far less than the extra turret had), and his rockets gained an extra +.5AP ratio in exchange for dealing no damage whatsoever to towers. Literally, this all took place one patch right after the other from patch 94b through patch 99.
So let's review the things that made him overpowered before and see what is left. He now has 2 towers instead of 6 that do not gain power over time, have a bit more health per turret but much lower defensiveness overall, and deal less damage per turret. His Rockets have half the cool-down, but less range, less damage, fire off three instead of five, and now seek minions rather than hitting only champion targets. His Grenade is slightly faster, but has less range, cannot damage towers, less blind, and deals less damage. His upgrade still gives CDR, which is easy enough to max out in other ways even without it, but is otherwise only useful for healing turrets. Very literally everything that made him especially powerful or unique had been nerfed pretty much into nothing. The one exception is that he still has his signature turrets, though they are no where near what they used to be.
Sadly, despite how obvious that a well-loved character had been nerfed into oblivion, their attempts to fix him show that the fear of the pre-nerf heimer still persisted.
The next update was supposed to be seen as a "major buff" but was much less underwhelming than intended. Turrets were put on an ammo system and were given targeting programing to defend Heimer when attacked, but their health was nerfed, and the leveling order was changed to be less beneficial. Upgrade gained the ability to affect Heimer's other skills, but these changes were seen as only mildly beneficial. His next two changes were both buffs that changed his turrets through bug fixes, lowering the usage cost, and allowing it to work with Upgrade better.
There were other changes, of course, but these were the main ones. Heimerdinger was likely one of the more difficult champions to deal with, due to his unusual skills and lack of a place in a normal team composition. So now that we understand the past, let's look at how the champion is now.
3. The squishiest hero.
What would you call a hero with the lowest Armor, Magic Resist, and Movement Speed in the game with one of the lowest health numbers as well? Squishy just doesn't seem to encompass it, but you could also call it "heimer". His passive ability helps early game, but does almost nothing late game. The imbalance is much smaller at earlier levels, where all champions start off with the same MR and base health differs only by a max of about 150 points. In fact, his passive accounts for a similar amount of healing as a regrowth pendant at early levels. But at late levels recovering 300 health a minute is a laughable amount. Add to that the fact that he has no true escape ability except for a slow that can be unreliable and can be overdone by the abilities of many other characters. So "tank heimer" is right out. Perhaps one of his other abilities has some hidden use that balances this out.
4. A problematic passive.
I mentioned before that his level 18 regen is a joke, but this conclusion was not what I first thought. It seems useful to be able to heal a tower and 300 regen per minute over most of a group seems like a much bigger deal (a total of 1500 health per minute). Then I started looking at damage numbers of what abilities could do in the same amount of time to see how much that actually means.
This equals about the same as the harass damage from one or two characters (depending on the character), assuming that they only attack /one/ of your teammates and assuming that they have /no/ ap or bonus attack damage to power up the ability. But if they do only attack one team-mate than that teammate takes all the damage and your regen is wasted on characters that don't need it. And, if they hit even two characters with their spell over the same period of time or if they do have bonus stats than even one character can easily outdo the entire effect of your passive across an entire group of characters. And that is just with harass (like a fired voley, or piltover peacemaker, or double-up), when they are in position to really attack the results are much worse. Most attackers are only slowed in taking a tower by 1-2 seconds for each full minute you stand at a tower without leaving. But towers don't recieve critical hits, whereas characters do. The numbers I looked up were with non-bursting, normal DPS champions. So during the late game this regen literally means next to nothing.
Early game is a different matter. The regen from Heimer's passive is partway between a Rejuvenation Bead and a Regrowth Pendant on a character who excels at pokes and never needs to get near his enemy. This can be a good thing for him, since it means that killing heimer through the usual early-game method of "build damage till you can go in for the kill" doesn't usually work. This freedom from that danger helps him to be able to push and harass back more effectively than most enemies therefore giving him an extremely powerful early game. But it is also a bad thing since it heals the creeps and so pushes his lane no matter what he does. This opens Heimer up for a gank from behind even without using any other abilities (all his abilities tend to push even when you don't want them to). Laning with an team-mate, in order to let someone else benefit from his passive, has other serious problems; mainly that Heimer's skills take all the CS and push the minions so far up that your ally cannot even try for a champion kill. If you want to win with Heimer you need to push the early game advantage before you loose it, so sharing experience is just hurting your chances of success.
The biggest result of all this is that Heimer tends to have an extremely powerful solo early-game, since he can nearly ignore some harass and then simply outpace the enemy. His late game suffers for it.
How to fix it? That is a much more difficult question. With the anti-sustain push that RIOT is currently engaged in, trying to make it not be so bad late game is just not likely to happen. At the same time the early game ramifications of the ability mean that any fix to the character would require a reduction of the sustain. One idea would be to have the ability apply to everyone BUT Heimer, thereby removing the early-game sustain problem and so would open up the character to being able to be tweaked in other ways. Honestly, though, I would be surprised if the entire ability was not just scrapped for something else. Also, it really probably shouldn't continue working while Heimer is dead.
5. Those troublesome turrets.
The design concept seems simple, turrets are an immobile source of damage that reward allied players for having a battle within their range and can be countered by either having the battle at another location (thereby surrendering the ground) or by destroying the turrets. It goes along well with the themes of controlling space that was inherent in the original rocket and grenade spells, the first which would punish champions for invading his space while the second would allow him to deny space selectively beyond his immediate area. In the beginning the fact that the turrets would "level up" until they took enough damage to be killed and the fact that it would take time to build up the turrets to create a sizable defense would make the location of heimer's "turret nest" into an artificial map objective by creating a point of strength for allies and giving a reason to protect it, but negating a lot of Heimer's team contribution if it was destroyed. The risks/rewards associated with the skill were obvious; the reward was high damage and the ability to control a space (I imagine Gandalf's quote, "you shall not pass") but the risk came in the fact that having such a significant portion of his power wrapped up in these immobile devices made him much more useless elsewhere and made it so that loosing his nest would nearly turn the battle into a 4v5.
Turrets have great utility, being useful as makeshift wards that can attack and they can continue to function and farm when Heimer is not around. Ironically this built in functionality seems to be what has caused problems. In reading up on the character Heimerdinger has been called the "club-master" because he would find a spot and then set up turrets as "bouncers" to keep others out and create a near impregnable defense. The idea that this behavior is something that Riot wants to discourage seems to fit with most of the changes to this skill. The ammo system means that players are not stuck having to sit in an area waiting for the ability's cool-down if they want both turrets, the reduction of their defenses and numbers mean that the turrets are easier to destroy in order to bypass them, and the removal of the damage bonus over time (as they attack) means that there is no penalty for removing the turrets and replacing them somewhere else. Unfortunately in removing these aspects the risks and rewards associated with the skill were also removed. The turrets no longer create a place of much greater power for allies, but rather are a minor annoyance for any vigilant enemy. The reduction of power, both offensive and defensive, ends up reducing Heimer's overall power without anything to keep him competitive. The addition of mobility, for a character who is too slow and squishy to ever safely be alone, is no good trade for the ability to defend oneself, even if that self defense comes at the cost of other versatility.
That said, it would be a lie to say that the turrets are completely useless. They have the capability to add a lot to a fight simply due to their presence. The continual damage can nearly add the same effect as half a character while being far more expendable than a champion. This is partly because they give only 12g on death compared to a character's death reward averaging at 300g. They are easy for many players to miss and can completely change the fight whenever they are ignored. They also allow the character to virtually "be in two places at once" so that they can go shop or help their teammates when not actually being present. The healing and slowing effect of "Upgrade" can mean a nasty surprise for opponents who walk up to one to kill it or try to run past it thinking that it can be ignored.
The advantages stop as soon as the opponent gets smart enough to not ignore the things. The gold bounty means that killing them is not only useful to force Heimer into mana starvation and to destroy the lion's share of his damage, but it also makes the opponent stronger for the extra farm every time heimer uses it. This means that using the turrets at all can work against you and actually aid your opponents. The fact that they only prioritize champions when they are attacking heimer or when his ulti first activates means that opponents can walk up to them to take them out. Of course, most of the ranged champions don't need to do this since they can simply out-range the things. Currently the turrets do not continue firing at things sitting near their max range, so this further reduces their effectiveness. This means that just under a quarter of the champions can attack from outside range without fear at level one with just their basic attacks, and the number gets a lot higher when you include those with a spell that has or gives them longer range. Even caster minions can out-range Heimer's turrets on occasion, that is literally how bad the situation is.
The range becomes less of an issue late game, however, as the squishyness becomes the turrets bigger problem. Turrets final defenses at level 18 is 655 health with 49MR and 99Ar. While this is very good for early game and requires several hits for most characters to destroy, during late game this defense is completely used up by one or two pokes or by AoE spam that is not even targeted at it. In a game where critical hits above 1700 damage have been recorded, that 655 health only lasts between one or two auto-attacks at best. This defensive problem not only makes them easy to bypass but removes their offensive value as well, since they cannot deal damage when they are destroyed. In fact, they might as well not exist or be placed since doing so in most occasions will only mean a gold bounty for the enemy when you or they are destroyed (yes, the gold bounty for the turrets is paid when heimer himself is killed). This circumstance, either the inability to use or the destruction after use of his turrets, means that the character's contribution is reduced by the entire value of his turrets.
Speaking of damage, while the damage can build up over time when other characters are not paying attention, the amount of damage it does is fairly low and scales badly into late game. This is mainly due to low AP ratios and the inability to benefit from Heimer's magic penetration or on-hit effects, but this is additionally made worse by the pseudo-auto-attack way that the turrets deal damage which makes them susceptible to things like Frozen Heart and Randomen's Omen. In effect, the turrets receive the negative effects of the counter-play aimed at both AP characters and auto-attackers without receiving the benefit of anything aimed at softening that effect. This problem is not immediately noticeable and most people simply do not realize that it is a damage problem because in early mid-game the turrets seem to deal a good amount of damage, thanks to Heimer being extremely fed. By early mid-game it is not uncommon for Heimerdinger to have nearly twice the minion kills of anyone else in the game, therefore the extra items can create a temporary imbalance. This imbalance corrects itself as other characters begin to catch up, as Heimer fills his item slots, and as diminishing returns begin to set in.
How to fix it? Something needs to be done about the damage that will at least need some ratio increases and likely allowing some on-hit effects (like spell penetration). The problems with them not attacking seems to be due to a programing glitch where there activation range (when they start to attack) is less than their total range, though this can be bypassed when Heimer is targeted by another character for an attack. The easiest way to keep them from sitting and dying to things within their range is to add "attack those who attack me" into their targeting programming (which would also have the added effect of not players face palm when the things die stupidly). This does not eliminate the need for longer range, it only means that they will use the range that they do have. The need for the towers to not immediately die is a more difficult problem, since it could easily lead back toward the old issue of the club master's wall and since it needs to work not only end-game but also early and mid game. For that problem there is no easy solution, and it may turn out that with better damage and range this issue resolves itself, but there are plenty of ideas that others have suggested if something still needs done. Also, stop giving the turret's bounty on Heimer's death, that is just insult to injury. If RIOT really wants to make Heimer players happy they could also add in one of those cool timers to show when he will be getting another turret added to his ammo stock.