What are my options to plumb a T&P valve to the main building drain, including providing an air gap and P trap. Particularly, what air gap devices are available?

What I would do in this case is to install a pan and divert a very small amount of water to the pan with a water sensor or add a flow switch in the drain line.

Then you can use any number of devices to shut off the water such as water cop. That way you do not have to dump continuous amount of water down the drain at city pressure.
There are devices which will shut off the pilot in a gas water heater too.

Air gaps can just be a 3/4 copper pipe above a reducing coupling. (expansion in this case).

kdgUSA...

Let me see if I'm clear on what you are asking... ok?

You want to pipe the 3/4" T&P discharge tubing over to the main building drain... then you want to install a ptrap off the building drain and you plan on installing the pipe so it has an airgap between the T&P discharge tubing and the ptrap... right?

That T&P discharge tubing cannot be reduced in any way.

You will also need a TRAP PRIMER to keep the trap wetted because the T&P valve is not supposed to discharge on a regular basis and without the trap primer the trap will dry out and allow sewer gasses to enter home, etc... very unhealthy!

Check out this link for more info. On trap primer:

http://en.wikipedia.org/wiki/Trap_primer

And there are no air gap devices for this situation... air gap devices are for dishwashers. You just want to stay above the ptrap standpipe about 1.5" with the T&P discharge tube and the trap primer tubing so plan on installing a 2" ptrap with a 3"x2" reducing coupling at the top of the standpipe to increase area for discharge. The point of the air gap is simply to keep sewage from contaminating the tubing should a blockage in drain pipe occur.

The ptrap should at a minimum also have an air admittance valve (AAV) also called a mechanical vent so the ptrap will not siphon out when major fixture discharges past p-trap... unless the ptrap will be a floor drain in floor... then won't need the vent at all... ;)

Let me know if I missed this one or if need more info. On the trap primer, etc.

MARK

Agree with Mark...

Additionally, T and P valve can be drained just about anywhere to the outside. 99.99% of times it doesn't leak - only if there is pressure or temperature problem. Just install 3/4"discharge pipe to the exterior and terminate it 6" above the ground.

What are my options to plumb a T&P valve to the main building drain, including providing an air gap and P trap. Particularly, what air gap devices are available?

Why go through all that hassle? We simply pipe the T & P Valve outside. Good luck, Tom