We have two upstairs baths and lately, neither are getting hot water. The water pressure is fine, but the temperature will only get warm. Both showers have a separate hot and cold handle as do the sinks. The strange thing is that sometimes the water will be hot, but most times not. Our house was built in 1994 and has copper piping. Water heater is the original one that came with the house in '94. Is this a water heater issue (bad bypass valve?) or a pipe issue?

My guess is that you have an electric water heater, could be bad thermistat, or burnt element. My first question should be is the water heater gas, or electric?

My guess is that you have an electric water heater, could be bad thermistat, or burnt element. my first question should be is the water heater gas, or electric?

It's a natural gas water heater.

If all other down stairs fixtures still get hot water then I'd say your heater is fine but you have some junk in your valves. Tell us about the rest of the house, good hot water? Draw water from the drain valve on the bottom side of the heater, is it really hot?

If all other down stairs fixtures still get hot water then I'd say your heater is fine but you have some junk in your valves. Tell us about the rest of the house, good hot water? Draw water from the drain valve on the bottom side of the heater, is it really hot?

Water pressure in first floor sinks is good as is the hot water temperature.
I opened the drain valve on the water heater and the water is hot but it's also dirty/rusty looking. When you say valves, do you mean the hot/cold valves in the showers and sinks? Last year, I replaced the entire valve assembly in one of our upstairs showers and now the water in that shower is only warm.

Are all the fixtures on the second floor affected or is it just one tub/shower valve? Two? How about the lav faucets? If it is just the tub valves then it could be as simple as a limiter on the valve or some blockage in the line.

are all the fixtures on the second floor affected or is it just one tub/shower valve? Two? How about the lav faucets? If it is just the tub valves then it could be as simple as a limiter on the valve or some blockage in the line.

Right now it's just the showers only. The hot water at all the sinks appears to be OK. If it is a blockage, how do I clear it?

I would start by flushing that water heater. If you are sure how to properly flush just let us know. Close the main and remove both the hot and cold shower valves, cartrdiges if you have them. Cover the shower valve body with a tightly held towel and have someone open the main for about a full minute to flush all debris. Clean the valve or cartrdige and reinstall. You should be good. Even a brand new valve can clog in a minute if there is junk in the heater

I would strat by flushing that water heater. If you are sure how to properly flush just let us know. Close the main and remove both the hot and cold shower valves, cartrdiges if you have them. Cover the shower valve body with a tightly held towel and have someone open the main for about a full minute to flush all debris. Clean the valve or cartrdige and reinstall. You should be good. Even a brand new valve can clog in a minute if there is junk in the heater

Thanks, I'll do that. Also, I have not flushed the water heater, how do you do that? Is it different than draining it?

Yes, draining is very static. Attach a garden hose to the drain valve at the bottom of the tank. Run the hose to a floor drain, sump pump pit or outside. Open the drain valve without closing the main supply to the tank. This will cause a power flush by stirring up all the debris and flushing out the hose. Let the tank flush for about a minute after you see clear water and no particles coming out. I sometimes drain onto a white towel to help these old eyes see the color of the water and the particles.

Today I replaced the water heater and now we have hot water in the entire house. Replaced our old 50 gal with a new 60 gal GE with a pilotless ignition. Very nice, the family will be happy when they get home. Turns out the install wasn't as difficult as expected. My brother in law and I did it in about 2.5 hrs.

Now... another issue had come up. The vent on the new heater is 4" and our old vent pipe system and B-vent is 3". I called the manufacturer (Rheem) and they said I can't use a reducer (4"->3"). Anything short of replacing the entire vent system, is there anything I can do?

Nothing you can do to meet code and Rheem's warranty. How far do you have to add new 4" pipe, thats what needs to be done. Based on what you tols us "Right now it's just the showers only. The hot water at all the sinks appears to be OK. If it is a blockage, how do I clear it?" I would not have taken the route of a new heater.

From the top of the heater to the base of the roof vent is approximately six feet. The roof vent is also a 3". Would I still be violating code if I put a reducer at the base of the roof vent or do I need to change that out as well to a 4"?

Any reducer anywhere is a bad idea. Rheem engineers sized the flane for a 4" pipe, not safe or wise to reduce anywhere. The flame draws a particular volume of combustion and its exhaust can not be reduced