6 gallon hot water heater only delivers 4 gallons

Suburbans have a bent interior pick up tube(mounted on the interior hot exit fitting) that KEEPS Hot water flowing without getting the diluted cold water intake. IF that tube is knocked loose, the cold water coming in the bottom as it mixes with the hot water will dilute to normal hot water that would exit at the top. Things that can dislodge this tube
1. Inserting a screwdriver into that hole to clean out or check for mineral or debris
2. Inserting a threaded fitting too far into that hole(very hard to do this)
3. Inserting a pressurized clean out wand/tool into the anode rod hole to powerflush the inside of the water heater.
I have only seen 3 or 4 of these over 41 years. It is NOT a servicable part as they do not sell that part. Once inside the tank it will just sit there.

NOW, if you are POSITIVE you do not have a Water Heater Bypass malfunction, then this pick up tube would be the likely problem. Doug

180 degree water can cause almost instant burns. No way would I have one.

OP has a Suburban with a FIXED temp t-stat (130*F +/-)

If your is heating to 180*F (which I doubt) then it must be either an Atwood using a Mixing Valve with a factory FIXED t-stat of 160*F and Mixing Valve controls output to 130*F

OR yur normal T-stat has failed and the High Temp T-stat is controlling but it should trip all heating cycles when it opens and have to be reset for any further heating
Suburban...High Temp T-stat FIXED at 170*F and has MANUAL Reset
Atwood.....ECO (Energy Cut OFF/High Temp) FIXED at 180*F and tirggers Red FAULT Light/has to be Reset after temp drops below 150*F


Atwood EXT models do heat to higher nominal temp so that a 6 gal TANK will provide the equivilent of 9 gals of 130*F water and 10 gal tank provides equivilent 16 gals of 130*F water.
(water heated to higher temp in tank then output controlled by mixing valve)


So what model of Water Heater and how are you measuring 180*F water temp????
It isn't like the OPs.....apples to oranges




MOD'S EDIT: Deleted quote from deleted ignorant political post.

You will not get 6 gallons of hot water. The cold water coming into the water heater cools the existing water that is already hot gradually turning it to warm water. And the recover rate of the water heater is not fast enough to overcome that. And the colder the incoming water is, the faster it will cool that existing hot water giving you even less hot water.

Seriously?
This is funny.

6 YO water heater, if it's never been flushed, could have sediment build up in it. You need to replace a leaking pressure valve. And as others have said, are you measuring 6 gals of hot water only to know? That's hard to do given that the water in a water heater is not finite. As you draw hot water it's replaced by cold water so measuring 6 gals of hot only is pretty hard. And the incoming water temp will vary from CG to CG and season to season. Heating water that's 55 degrees will be harder than 70. What may be happening is that it can't heat the water as fast as it used to. I would approach in this order - Make sure your outside shower is off, replace the leaking valve, flush it. If that does not fix it, replace the rod.

You are saying that you measured 4 gallons of hot water before it gets cold?

That sounds normal to me...you are putting in cold water as you take water out...so now you have just put in 4 gallons of cold water mixed with 2 gallons of hot.

x2 and replace that safety valve home depot. question wheres that drip going? if its getting to your wood it,ll rot in no time.

Yep, how are you measuring?

Of course, if you take a shower and use 4 gal, 4 of the 6 gal are replaced with cold water over the course of 5-10min...that will mix with the final 2 gal and it won't have time to reheat.

How are you measuring 4 gallons of hot water?

Check that your winterizing bypass valves are not partially bypassed. Also checked the outside shower and make sure the hot and cold knobs are off.

After that I would drain and flush the tank as it might be full of sediment and insulating the burner.