Water Heaters - Gas heater has short supply of hot

OK I'm a plumber so I know the basics. 50 gal gas hot water heater. Has only about 5 minutes of hot water when used. Thermostat dial is way up too. Dip tube is intact. I found no cross connections anywhere in the house that might be blending hot water with the cold. The pilot stays lit, the furnace next door to it works fine. BRAND NEW heater too. Just installed last week. The mystery is that the last heater did the exact same thing, which is why the homeowner replaced it. Very odd. The 3/4 gas meter upsizes to 1" before branching back down to 3/4 which supplies the furnace and the HWH. There is a bunch of fireplaces, stovetops and all the stuff seems to be scabbed in so I don't know if the gas system is properly sized. Although the pilot never blew out so I don't think it is a gas supply problem.

A faulty thermostat is my leading theory but the fact that the last one had the exact same problem worries me. Any ideas?

I would be curious to know how many gallons are being pulled from the tank in the 5 minutes of use. It is possible to use all the hot water while filling a large bathtub and using a faucet without a flow restrictor.

Certainly not 50 gallons running a couple of 2 gpm showerheads and the lav sink when I tested it.

You will not get 50 gallons of hot water on a fast draw. You'll get about two thirds of the tank capacity due to mixing of incoming cold water with the hot water already in the tank. In your case you should expect no more than 33 gallons of hot water.

You said you have two shower heads each using 2 gpm, and 1 lavatory (probably 1 gpm, maybe less). So, if you are pulling a total of 5 gallons per minute for 5 minutes, you get something like 25 gallons. It appears you have 8 gallons of hot water missing. On the other hand, you may not. You would first have to measure the hot water actually being consumed. The gpm rating of a shower head is usually at 50 psi. Many places have water pressure substantially higher than that. Use a 5 gallon bucket and time how long it takes to fill it at each shower. From there you can get a better idea of the actual amount of water consumed.

I think you may need a second or bigger water heater.

When you say 5 minutes, does it start hot and gradually go to warm and then lukewarm, or do you get hot and then it goes cold?

Is there a recirculating system which could have a bad or mis-installed check valve?

Water goes from piping hot down to lukewarm where it stays for a long time. No recirc on the system, and I looked carefully for any other source of cross connection that might blend hot with the cold but I found nothing.

I havent bucket tested the amount of water coming out of the heater to see when the temperature starts to drop but it, maybe I will suggest the client do that. Still I think it is happening way too soon. I would almost swear it was an electric heater with a bad bottom element but she's a gas cooker.

The yellow energy consumption sticker on the tank says that it uses the least amount of energy that they rate it for. So does that jive with what your saying, the fact that it is an energy miser means that it doesn't reheat as fast?

Would a bad thermostat on the heater affect it this way? I have to admit I don't have any experience changing that part of a gas heater. A primer on such a thing would also help if anyone knows of one.

OK, more info. the client did a bucket test and he is only getting 18 gal of hot water before it goes lukewarm. There isnt a heat trap nipple on this thing. They put a branch tee after the shutoff to supply the laundry room that is only a couple of inches above the inlet. Could that be causing that much heat loss?

Hmmmm. The branch tee could not cause that kind of heat loss. If you get a chance, you may want to stop by and supervise the test yourself. It does sound like cold may be mixing with the hot. The added laundry room may be suspect. Did the problem exist before it was added? Washing machines can allow mixing of hot and cold water even when the machine is set for cold only. I would also feel the water heater oulet pipe when the shower turns luke-warm.

I wish I could be more helpful.
Good luck with this.

Are you saying that the washing machine may be blending the hot and cold even when it is not running? I haven't seen that one before. Also the bucket test was performed from a utility sink in the laundry room so I tend to believe the accuracy of the bucket test. This thing is still pending. I was planning to return and put heat trap nipples on the heater, I guess that would be a start. If the problem is with the washing machine, what is the fix? Is it a defective part that causes this?

Hello: Paulypfunk

Couple of other factors to consider and explore as possible causes.

Not likely an undersized gas supply houseline nor gas meter. (Nat Gas) But a one inch houseline indicates a small square footage house. Around 1200 square feet or less. Or at least it should be.

Also indicates one shower in one bathroom. Thus house with one bath and the discussion thus far indicates one shower. As does the gas houseline. No other facts known based upon the discussion thus far. No mentioning of the condition or any other leakage, etc problems existing at any other faucet, etc.

No hot water leaks? Not likely or water bill would be excessive as would fuel consumption. Therefore, other possible causes need to be explored. Dwelling on the same possible causes gets us nowhere.

So my suggestion, based upon the discussion and facts thus far, would be to replace the shower faucet. Reasoning: Same problem existed prior and no other household changes made between old tank and new tank. Hot water expansion of and in the shower faucet fixture may resolve the problem.

"My Two Cents"