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Now that it is getting colder, we have noticed that when we get home from work, our water is not that hot. It was warm at best after a four-day trip. The water heater is in an unheated basement, next to the furnace. It's not freezing by any means. After a shower or some dishes, it heats up fine and we have hot water.

It's a gas water heater. My mental image of how it works is that the fire shuts off OK when the water heats the desired temp, but doesn't come back on until new cold water is introduced. I would have guessed that it would be a normal thermostat. If it is a thermostat, how do we set the temperature that the water heater that the fire come on at?

Or, can someone explain how this works in reality, and offer advice?

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Yes, it's a thermostat, and there is usually a dial on the side of the tank, near the bottom, to adjust the temperature. More than likely, the water feels colder because it is traveling through uninsulated pipes and is losing much of its heat before it reaches the faucet. You can turn up the temperature of the tank, insulate the pipes (they make foam insulation that just slips over the pipe, just be sure to measure the diameter of your pipes so you get the right one), or both.

You should also consider flushing out any sediment from the bottom of the tank if it's been a while since you last did this. All you need to do is hook up a garden hose to the drain at the bottom of the tank (make sure it can take the warm water), run the other end to a floor drain or utility sink, and open the drain on the bottom of the heater. Make sure the water runs clear for a minute or two before closing the drain. To save on the gas, you can set the thermostat to only run the pilot light while you flush the sediment.

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    Insulating the pipes is going to be the biggest gain here. Even turning up the temperature won't help completely -- the water in the pipes is going to be the temperature of the basement, and you have to use all of that up before you can get any hot water at all. Even after that, the pipes will still be quite cold and because copper is an excellent heat conductor, they will absorb a lot of the heat from the water.
    – gregmac
    Commented Nov 15, 2011 at 5:40
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    +1, also one way to to address the problem is to use pipes as thin as possible so that less water is stored in them and so the cold water from the pipes is used up faster.
    – sharptooth
    Commented Nov 15, 2011 at 5:57
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    Couple of cautions. Be careful not to turn the temp up too high as scaulding skin can occur, 130f to 140 is usually hot enough. Reducing the diameter of the pipe is not a great idea usually. Codes in USA require min of 1/2 inch in most non-transportable residential housing, 3/4 for some longer/second floor feeds. Insulation pipes is always a good idea, but the same problem will exist after a day or two of non use. Unfortunately the only cure is to run the water a minute or two to purge cool water from the pipe and get to the hot stuff.
    – shirlock homes
    Commented Nov 15, 2011 at 10:59
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    Should have explained, reducing pipe diameter can reduce volume and pressure at the fixture, especially high volume fixtures like shower heads.
    – shirlock homes
    Commented Nov 15, 2011 at 11:07
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In addition to insulating your pipes you could install a circulating pump to keep hot water flowing through the pipes. We installed a circulating unit under the sink in our kitchen which is far from my water heater. We installed it after replumbing all the way to the kitchen with 3/4" pipe. We now have plenty of pressure in the kitchen, but without the circulator it can take a long time before hot water comes out of the tap.

Circulators can be run automatically, on a timer, or on demand (push a button). They do not have to run all the time. Ours is on a smart switch and only runs when the kitchen or bathroom lights are on. This way we circulate the water only when we are home and need it.

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There is another system I first saw on TOH. A cross-over valve and pump installed in a nearby lavatory (cabinet) hot water faucet (at the shutoff) forces cold water in the hot line into the adjacent cold line and back to the cold water input on the water heater. That increases "faucet" pressure back in the hot water line, producing hot water near the point of use very quickly. A temp sensor detects when cold water has been purged and hot water only is available for use, and causes the pump to turn off and the crossover valve to return the hot water line to normal flow.

There is no pump power consumption other than when hot water is demanded. There is no loss of fuel for heating up the cold hot-water line. There is no loss or expense of keeping hot water circulating. Energy use is strictly during demand only.

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