I don't want my WH T&P to vent in the room, but code says it has to?

[Arizona CJ]

In my area, more often than not, the t&p discharge line has to go up in order to exit the house. Forget the air gap, forget gravity. Just put a drain cock of some sort on the line to bleed off any water that might end up in the line for whatever reason. The discharge terminates 6" above the dirt once you exit the house, as Terry indicated. You worry too much. It's all been worked out.

I probably do worry too much.

What got me worried was the inability of the city code desk to indicate whether it was or wasn't okay, coupled with item #2 of the code (detailed upthread) that seems to prohibit it, plus some commercial home inspector check sheets objecting to it.

The existing 3/4 T&P copper line slopes downhill, all the way to the termination. (I checked with a level to be certain). Part of it is a quarter inch per foot, the rest is an inch per foot. This may sound weird as it's in a basement, but it's a walkout basement (the home is on a mountainside, and the line goes to the downhill side.) It's also free of corrosion. I could put in a drain cock, but I'm unsure that'd be okay under the code, plus with it all being downhill (every inch, right from the relief valve) I can't imagine how water could stay in it. I do test the relief valve once a year (which flushes the line out too).

 

[LLigetfa]

I can't imagine how water could stay in it. I do test the relief valve once a year (which flushes the line out too).

Unless air can leak in around the T&P stem, then the line might not drain (similar to holding your thumb on the top of a drinking straw). I think that is one reason for the air gap, the other reason being possible contamination. Let's not debate possible contamination as it is a far-fetched notion that contaminated water could ever get across the T&P in normal use.

 

[Sponsor]

 

[Arizona CJ]

Update: I haven't been able to get an answer out of the city yet (they don't seem to know their own code) but, I had an idea: check out some new homes. My reasoning was that if they are approving an exterior-vent no-air-gap T&P line for new construction, surely it's okay to us an identical per-existing arrangement. And bingo, I found several. These had to get permits, so the city did okay it.

And, contra this, a commercial home inspector said he'd definitely fail such an arrangement because that's what his checksheets say. But, I don't much care, it's the city permit I'm worried about, and what I found today seems to put the issue to rest: they're approving it in new construction.

 

[Phog]

Here's another issue; the T&P valves I'm seeing release at 212 degrees. The boiling point of water here is about 199 degrees (due to altitude). So, if the T&P opens, you're going to get a release of over 8000 BTU just to get the water down to boiling, plus every gallon that comes out from a 75 gallon tank will release (and that doesn't count the BTU of the hot water that comes out too). That';s a lot of energy and steam to blast into a small room (for comparison, that's often human-occupied, yet code says it's okay to discharge into a bucket or drain beside the water heater. I consider that madness.

Roughly, one BTU equates to the energy released by burning a single wood phosphorous match. So, who wants to be in a tiny room with 8000 of those going off in seconds, plus a thousand more for every gallon of water that comes out?

No way, no how, am I going to allow that thing to discharge inside the room (where there is no drain an no way to add one). Outside the building has gotta be safer.

Late to the discussion -- just wanted to touch on this one point for a second. I don't think you should worry about the energy release as you have outlined.

While the boiling point of water under atmospheric conditions at your elevation is 199F, remember that the tank is at your service water pressure, around 40-50psi higher than atmosphere -- and this means a boiling point very roughly in the 260-270F range. So, under no circumstances will the water inside your heater be anywhere close to boiling.

And, if the T&P releases some 212F water out of the tank, it will be replaced by incoming cold water, which will drop the tank temperature back below the T&P opening threshold & close the valve. You would never get the entire hot contents of the tank coming out all at once & flashing to steam as in your 8000 BTU scenario. You will instead get little burps every so often as the water heats up, releases a little bit, drops back down, and so on.

Finally, although under temperature-induced relief at 212F the water coming out of a T&P will start out above your AZ ambient boiling point, the exact same amount of energy will be released into the room as would be released at sea level. The difference in elevation just means that it will come out in the form of a trickle of steaming hot water, instead of a trickle of hot liquid only.

Long story short, no I would not want to put my hand under the relief line while the T&P is activating, but the only way it's going to harm someone even a little is if they are standing in flip-flips with their feet directly underneath. For T&P you don't worry so much about harm to people from the water coming out, you worry about harm to property from water damage. Because you didn't notice it was slowly leaking for a long time.

 

[Wraujr]

Option that would meet air-gap. Replace 3/4" copper from outside back to water heater with 1 1-1/2 PVC sloping appropriately. When it reaches water heater continue with PVC vertical up to about 2 ft from T&P. Then use 3/4" copper from T&P extending it about one foot into the PVC. Since ID of PVC is more than OD of 3/4 copper you will have your air gap. No way volume on release will overwhelm the PVC. Think like its a washing machine standpipe. Air gap in this case is not something mechanical, etc.