Water heater T&P valve drain lines

[jcm55]

Hi all,

I'm getting ready to install a solar water heating system. It's a pretty typical install with a 120 gallon solar storage tank in line with my existing gas fired water heater. That way the gas heater will kick in when there's not enough sun.

The 120 gallon storage tank will go right next to the existing heater. The temp & pressure relief valve on that heater has a drain line that goes through a concrete wall to the outside. Can I tie the t&p relief valve on the new storage tank into this existing drain line, or do I need to run a separate one? I'm trying to avoid drilling another hole through the concrete foundation wall which is a real pain.

 

[Bob NH]

It is not clear to me that you need a T&P valve on an unfired tank.

It is also not clear that a T&P valve will protect the tank. Most T&P valves relieve at 150 psi. The pressure relief setting of the T&P would have to be lower than the operating pressure of the storage tank.

You need a pressure relief valve if the tank pressure capability is less than the pressure relief or capability of the source pressurizing the tank.

 

[Sponsor]

 

[jcm55]

The storage tank is essentially a 120 gallon single-element electric heater with two extra ports for the solar collector inlet and return. The tank will be at the same pressure as the rest of the plumbing.

You could use this tank in a single-tank solar setup with the electric element as your backup heater for when there's not enough sun. I already have a gas heater for backup though, so I will leave the electric element disconnected. I think code requires the T&P valve on this tank because of the electric element -- even if it's disconnected.

The question remains though: can I tie the two T&P valves into a single drain line?

 

[jcm55]

Here's a little diagram to make things clearer:

http://solarexpert.com/dhwpic.html