Many people see Heimerdinger as a mid lane ap champ, but I see him better at playing supporting role than soloing mid. First of all, when people see a Heimerdinger, they expect him to go ap and never think you will be supporting bot. Secondly, lots of people see Heimerdinger as a useless champ sometimes even when he is kinda fed. With this build, you can still be useful just staying alive in a teamfight.

For summoner spells, I prefer :

Teleport makes you gank or help better. This is always useful no matter you use it to defend a tower or to gank/help your teammate.

CV- A support's spell and it should always be considered

Exhaust also makes you gank or help better. Combine this with your grenade gives you a much higher chance to stun someone.

Others:

Flash - Best spell in most case for any champions. Flash is useful to do lots of thing. But with this build, you don't need flash at all.

Ignite - Ask someone else on your team to pick this. Remember you are a support now.

Promote - This is going to be removed soon =[

-Pros
* Doesn't need to do much in teamfight to be useful with this build
* Usually you are able to control bushes well in lane
* Free wards start from level one
* Far range harassment to your opponents
* Only champion that has stun and blind at the same time
* Protect dragon well

-Cons
* Need a very good ad carry partner
* Very slow grenade
* Can't walk alone at anytime
* Low damage compare to ap Heimerdinger

For runes, it doesn't really matter what you want as long as it fits your playing style. I prefer to have so early ap, so I can harass with his "w". You are expected to have low damage output in mid to late game or even early game with this build.

Like any other support champions, mid and late game items very depends on what situation you are facing.

Core

: Heart of Gold Mercury's Treads Aegis of the Legion

Oracle's Elixir-Get rid of enemy's ward. Take this all the time.

For boots I pick Mercury's Treads for a better chance to survive in teamfight.

Think of every "area of effect" items:

Must take Aegis of the Legion as good support does. It makes you more tanky as well as your teammate.

Get zeke's herald when your team is full of ad champs.

Get Abyssal Mask and Will of the Ancients in most cases especially when your team rely on ap dmg more than ad.

Get Soul Shroud in most cases because it gives your whole team cooldown reduction and mana regen which allows your team to stay on the lane longer. You can definitely choose to buy this early.

These items are additional buffs for your team, so it is very important to stay alive to maintain these buff for your team.

1. Don't take any cs ( least as possible)
2. Release your rockets whenever your enemies are within the range
3. Don't spam your stun, save it for kills or escape
4. Turrents can always be your wards (not no.1 choice)

Remember you need to level up your turret to 3 in order to have 2 turrets at the same time.

Ways to use your turrets:
A.You can set up both turrets on the lane
B.You can set one of them as a ward, one of them on lane
C.You can set both two in the jungle as a ward
Although Turrets are great tools to guard the bushes, I recommend using the turret as wards before your turret reach level 3 to prevent early ganks. After turrets are level 3, it is good to have one turret on the lane to make pushes and use the othe turret to create sights in jungle.

Basically you just try to find cover during a teamfight while trying to stun or at least blind enemy's carries

. Remember to set up turrets with ulti on for the slow and spam your rockets whenever it is available. This is very important to stay alive and always stick with someone.