My hot water heater has recently begun pushing hot water up the cold supply line when there is not water actively flowing into the tank. Nothing has been changed, including the temp, on the heater. I installed this heater three years ago and have not had any issues with it. For the last couple months, our hot water has been depleted faster than normal. Then, a few weeks ago while changing my water filter, I noticed a problem, or at least what I perceive to be a problem. I shut off the main water supply, drained the lines and removed the filter. The same method I have used when changing the filter countless times. While I was placing the new filter in the "cup", hot water began pouring out of where the filter attaches and out of the tap which I had left the cold side open to drain the line. This heater was installed three years ago and nothing like this has ever happened. Now when there is no water being used, I can feel the inlet pipe get warm. Over time it becomes warm for quite a distance from the heater and very hot right at the connection to the heater. The manufacturer assures me that this is normal and that there is nothing that can be done about it. Does anyone have any suggestions?

If there is a fixture in your home that mixes hot/cold water, say a single handle faucet, a washingmachine etc. the valve is malfunctioning. most common are older tub shower valves that are a cartridge type. However flow or use is required for this problem to present itself. Even pure cold use can cause this if you have a bad mixing valve. Thermal expansion alone can push hot water back into the city mains under no flow conditions if your incoming water pressure fluctuates often. Check your water meter by marking it and checking for needle movement after 1 hour (overnight is better)of non use. Or your pressure gauge if you are on wellwater. If you have excessive thermal expansion, you will need to install a thermal expansion tank on the cold inlet to your water heater and possibly a check valve down stream from the expansion tank. Therm. exp. is often caused by revisions installed in your water providers supply system. Causing your meter to house system to become "closed" ie, check valves. First check for flow under no use conditions. It can be a unseen leak and faulty mixing valve, otherwise, therm exp. is simply pushing towards a weak or leaking portion of your water system. Note it is common for a w/htr to back siphon when the the cold line gets opened. I know it's alot, hope it helps

Thanks for the reply and the suggestions. I recently finished my basement, including a new bathroom, and relocated my water softener and laundry to the other end of the house. I did the plumbing myself, so I double checked every line to make sure nothing was crossed. I also recently replaced the leaky faucets, so I know those are no longer an issue. The only thing I have not done, is replace the tub/ shower valve in my main bath. This is old and has been causing problems with water temp, but has not caused this problem with my water heater in the three years since I had the heater installed. Also, the only time the hot water is going up the inlet is in a no flow condition. When there is water running anywhere in the house, the inlet is cold, as it should be. If it is thermal expansion, why would it have just now shown up, rather than doing this the entire three years since the heater was installed? The temp has not been changed on the unit since it was initially set. We also noticed a drop in the length of time we have hot water, and that has only been the last couple months.

Check the elements on the water heater. There are two, one on top and one on the bottom. Only one should be heating at a time. Most times it will be the bottom one. If for some reason the upper element is coming on with the bottom, OR if only the top one is working, you could be expanding water in the tank more than normal. Also if one element is not working as it should, that would explain running out of hot water prematurely. Also, be aware that if you put any kind of check valve on the supply side of the water heater, the expansion in the tank will cause the safety valve to "pop off" relieving the pressure and consequently the water onto your floor. Hopefully it has an operating pop off valve, because the alternative is a catastrophic rupture somewhere.

If only one element is firing, then the reduced capacity is very noticeable. If your old tub shower valve is an old style Moen, that is your culprit! Note: We are talking about something that has just begun to happen. Something has changed in your system. Period. You either now have a closed system or a crossed connection, ie, mixing valve. Do you have any kind of recirc system? Trust me, things just don't happen. Something has changed in three years. Besides back siphoning your water heater by turning your water off and opening the cold line via your water filter, what fixture dumps hot water when it should be cold? If you are simply concerned about the inlet to the water heater being hot, that is normal thermal expansion. A fifty gallon water heater develops almost one gallon of capacity during no flow thermal expansion. That gallon has to go somewhere. It will always push back against the cold supply because it is typically larger than your hot water system. Have you checked your meter or well water pressure gauge?

Sorry, I guess I forgot to mention it is a gas heater. I added plumbing when I finished my basement, but I double checked all the connections and there is not a single fixture in the house that dumps hot water when the cold is turned on. The only other possibility would be the city plumbing. They just began operation of a new water treatment plant, and that is the only change that I am aware of in the outside system. We also have no recirc system installed.

if nothing is dumping hot water from the cold side, are we talking simple back siphon when you change your filter? Turning the water off to your w/htr before changing your filter will deal with this. Or better, installing a valve after and before your filter housing. Reduced hot water capacity for a gas w/htr is usually a dip tube problem...shortened by breakage. How far is your hot water expanding back into your cold system? more than 5 feet?

A few items can be causing this.
Askaplumber covered most of them.
1. leakage back through a valve such as a shower or tub.
2. Missing back check valves within the top of the water heater.
3. Dip tube within the hot water heater missing or broke off. (this will cause the water to come out cool very shortly after the water is run on the hot side)
4. hot and cold water lines backwards on heater. Very common mistake.
5. Extream loss of water pressure which would allow the hot water to siphon into cold. But when that would happen, you would not have any flow out of faucets.

Thats it. Gas or electical hot water heaters does not matter. My guess it is the dip tube is either missing on the cold water side, or hot water heater is installed with cold on hot water outlet side of unit.

I'd really be interested in reading some follow up on what this was at the end of the day - if you're stil around, please let us know what happened and what the problem was. thanks, -Fishfool @ The Reef Tank

I have a natural gas water heater (40 gal)that is 15 years old. It is in a vacation home on Cape Cod which I keep heated at low temps. I want to replace it. It gets little or no use in the winter and the pilot light is kept on. I want to reduce my energy expense.

I was thinking tankless, but concerned about installation, venting, and cost.

Most of my energy would be saved if I can turn off the new tank in winter and turn it on easily only when I visit. Is there a feature on new, but traditional tanks where I can turn it on without having to light the pilot light on my hands and knees and avoid that hassle? What natural gas heaters are most reliable and efficient? Clueless as you can tell!