A version of my article originally appeared in the Oct/Nov 2003 edition of the Bay Creek Bulletin newsletter.

Tanks Away!

by Jim Winkle, updated 13-Feb-2022

Many Bay Creek neighbors care deeply about the environment and may be interested in my experience with a new gas tankless water heater. The AquaStar 125FX from Bosch Systems has no tank, but rather heats water instantly when needed.

If you use hot water just a small percentage of the day, a tankless water heater could save energy and money. A traditional water heater uses energy to keep water hot all day, but a tankless heater only heats water on demand, consuming zero energy the rest of the day, and cutting energy consumption about 30%. The usage charge on our last gas bill dropped to only $3.25 (4 therms), less than half of what is was in a previous year.

Tankless heaters score higher environmentally in other ways. Whereas tanked heaters end up in the landfill, tankless heaters are made of recyclable (metal) parts. They are also repairable, and manufactures claim they last at least 20 years (much longer than tanks); Bosch warranties the heat exchanger on our model for 15 years. 

Tankless water heaters complement solar hot water nicely, since they cost nothing to operate when they're not in use. Our model doesn't support solar hot water, and I somewhat regret not purchasing a model that does, just to leave the option open.

On the downside, tankless heaters initially cost more (in 2003, $1000 to $1500 vs. $400 to $1000, installed), though they more than pay for themselves by using less energy and lasting longer. The higher price includes the common one-time expense of relocating the heater, $500, so the next one will pay for itself even faster. 

A tankless water heater could require some changes in your hot water consumption habits. 

Smaller heaters like our model can only supply enough hot water to supply one major thing at a time. This hasn't been an issue at all for our family of three, but if you have a larger family and need to have several things running at once, you'd need a bigger one.

Also, since our unit has no pilot (it uses electronic ignition to save energy), it takes about four additional seconds -- beyond the normal delay of moving water from the heater to the faucet -- for the unit to begin to heat the water. So when you want to wash your hands with warm water, you'll have to wait the extra seconds before the hot water arrives. If someone comes along five minutes later to wash, they'll get the leftover hot water in the pipe, and then cooler water from when the heater was turning on again. So guess what? We gave up and just wash with cold water. Models with a pilot probably don't have this problem, but consume a bit more gas.

Our tankless water heater uses just four therms per month

You can partially combat this problem by mounting the heater close to where you use hot water the most; ours is in the basement just below our kitchen sink. This is a nice feature... they mount pretty much anywhere, freeing up basement space. Because they typically end up on the wall, make sure your plumber leaves a little space from the wall so you can insulate around the pipes - Wisconsin walls obviously get cold in the winter. Insulating between the wall and heater is something I wish our installer had done.

Plumbing raises another downside: there were no plumbers in Madison certified to install or service our tankless heater in 2003! Ours came from Oconomowoc. In 2006, I have heard that Pertzborn is comfortable with tankless. MG&E knows of other plumbers who now do tankless.

You can adjust the 125FX output temperature from 95 to 140 degrees. We set ours at 110 degrees so that running hot water alone gives us a perfect temperature in the shower. No more fiddling with the cold knob to mix in just the right amount of cold water! 

110 degrees might have been a little cool for dishes and laundry, but fortunately our model gets hotter when more water is called for, so it's not much of a problem. More expensive models have a remote control for the temperature, so you can turn it up temporarily. Ours has a manual control, and we rarely change it (we raise it somewhat in the winter). 

For the few folks who have a super water-conserving showerhead, you may need to set it to at least 1.2 gallons per minute (standard water-conserving showerheads flow at 2.5 gallons per minute). Why? Because the heater requires around 1 gallon per minute to turn on. Update: I heard recently (2007) that the minimum flow rate is significantly lower, so this may no longer be an issue with newer models. Check the data sheet for your unit.


Opinions vary about the effect of Madison's hard water on these heaters. Some say hard water is bad for them, and you shouldn't use them without a softener. I think this was true in the past when they were designed differently. Others say that since they are constantly being flushed out by water going through them, they actually do better than tanked heaters where the sediment falls to the bottom, lowering the heater's efficiency and eventually cracking the bottom. I dismantled our unit when it was 4 years old and found hardly any hard water deposits on the heat exchanger (left photo is inlet; right photo is outlet). The fact that we run ours on the cool side (just warm enough for a shower) probably greatly helps with this. I soaked the heat exchanger with vinegar which de-limed it somewhat. I haven't done any maintenance since then, but keep intending to check out the heat exchanger again someday.


It was a little tricky working through some of the problems since we bought it at Home Depot, Everything Water in Oconomowoc installed it, and Bosch manufactures and supports it. In the end, everything worked out pretty well.

That's my mixed review of tankless water heaters. I wanted to write a glowing article about them, but there are obviously issues with tankless heaters. Overall though, we're willing to put up with a little inconvenience since we're happy that they use less energy, reduce greenhouse gasses, are mostly recyclable (and repairable), and take up less space in the basement. The tankless system is not a panacea, but overall we like this heater.

https://www.bosch-climate.us/tankless-water-heaters/ is the website of the manufacturer.

One other positive point about the 125FX is that it was sold through 2011!  This means parts will be readily available for some time, and in fact seem to be even in 2020.

Thanks for reading; please let me know if it was helpful!  I'm also happy to answer any questions or show you our tankless heater.