I have a very good gas hot water heater in my home. Unfortunately, it is located too far away from my master bathroom and the water has to run a long time to get hot. Can I install an auxiliary inline water heater to supplement the existing heater?

I had the same problem, and about 12-15 years ago I found the Laing Auto-Circ (http://www.homedepot.com/p/Autocirc-Undersink-Instant-Hot-Water-Circulating-System-ACT-E1/100037011) ACT-303-BTW pump. Still runs fine with no maintenance.

You mount this under the sink at the furthest point from your water heater. It pumps water from the hot line into the cold line (so no water is wasted) and runs until the water is hot enough to trigger the thermostat. This thing runs nearly silently; mine is under the master bath vanity which is about 8 feet from my bed and I barely hear it when it is running. I can't recommend it highly enough.

There is a downside--albeit a minor one. I installed this under MY sink in our double vanity and now instead of running water for a while to make it hot, I have to run it a while to make it COLD. My wife's side does not have this situation. As you can see from the illustration, there is semi-hot water in the cold water line of the faucet that is hooked up and it has to flush out in order to get good and cold.

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It has a timer that you can set to have it run only between certain hours, but I never know when I will be using it, so I set it to run all the time. There may be a small hit to my electric bill, but I think it is negligible (and worth it nonetheless!).

I see there is a new style of this now. Mine is the old style and if you can find that one you might be better off. Seems that manufacturers tend to cheapen their products over time. I can't say in this particular case. You may not even be able to find the old style now. Be sure to read the reviews at the link in my first line to see the good/bad. My experience was good, but mine is over 10 years old.
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You will need access to power, and I poked a hole inside my wall and ran a romex up to the outlet topside of the vanity. I added a new surface-mount outlet below for this pump and my electric razor charger. Very clean, nothing shows on the outside.

You will also need 2-4 new supply lines for your water. You might be able to use the existing lines (thus 2 new), but I took the opportunity to use 4 new braided stainless-steel lines while I was in there.

Hope this helps!
Wally

P.S. I unplug this when we go on vacation, and if I forget to plug it back in upon our return my wife soon reminds me she's missing the hot water.

P.P.S. Here's another possible solution (http://www.hotwaterlobster.com/) that sounds intriguing. I have not tried this myself, but it sounds easier/cheaper.

Good advice above ! Yes you could install in-line hot water heater but the circ-pump is a lot better solution...

Milo

Depending on physical layout you may be able to install a recirculation line with pump from close to the master bath back to the water heater.

I have one. I have a two story house with basement. When the pump quit years ago I remove it. I temporarily reconnected the line while I looked for pump. Found out that the system worked due to convection. Best of all worlds.

I have install a Watts instant hot water recirculating system in 2005 works great, open the PDF below for more information, good luck.

John