After 11 years, my 50gal gas water heater started leaking. I shopped around for alternatives, and decided that tankless was too expensive. I was at Lowe's to buy a new tank for $338, when I saw the Bosch 125B on clearance for $369. It's normally $499, with a list price of $599. So I bought the 125B instead.

My only concern is that the 125B might be a bit undersized for my house. We are three adults, but I think it unlikely we'll want to run more than one shower at a time. This is clearly not the unit for a busy household full of kids.

I am an experienced do-it-yourselfer. Even so, it was a lot of work to install this unit. None of this is the fault of Bosch. It's just that you can't simply swap a tank for a tankless.

The first step was to turn off the water and gas to the old unit, cut the water lines, unscrew the gas line, drain the tank, and then cart if out the back door. Tomorrow it goes into the SUV and over to the recycling center. If I had bought a new tank, I'd simply cart it in, hook up the gas line, solder on flex water lines, and I'd be done. Oh, if the new tank wasn't the same height as the old, I'd also have to adjust the vent stack.

I decided to mount my new tankless on the wall behind where the old tank was, right under the chimney flue. The Bosch 125B is very easy to mount on a wall. It weighs less than 50lbs and mounts on two hooks. You simply screw a board to the wall studs, then screw the supplied hooks into the board. I added a piece of 1/2 inch cement board behind it for fire safety, but that's not required by the instructions.

While a tank water heater calls for 40,000 btu/hr of gas, this small tankless needs 120,000btu/hr. This is because the tank can take a whole hour to heat up 40gal of water, but the tankless must heat 3gal per minute. That means the gas pipe must be larger.

WARNING: If you do not feed a tankless water heater enough gas, it will work, but cannot give you full heat. Since my old water heater was fed by a 3/8 inch copper tube, I needed to run a new gas line. This involved turning off the gas to the house, removing the old line back to the furnace, then adding a new line (3/4 inch black pipe) about 10 feet to where I mounted the Bosch 125B. I was lucky that I had 1-1/4 inch black pipe up to the furnace. That's enough to feed both the furnace and the 125B at the same time. Otherwise I would have needed to run new gas pipe from the meter.

Next problem is the exhaust. The old tank only needed a 3" vent pipe. The 125B needs 5 inches. Because of where I mounted it, I only needed to run 10" of pipe, and then merge with the furnace vent into the chimney pipe. Even so, I had trouble finding the correct pipe.

If you're buying a new tankless water heater, you need to make sure you can supply the correct gas line and venting before you buy. Some units have other requirements. This unit cannot be co-vented with a power-vented furnace (which might blow air back into it.) DIY installers should download and read the manual before you buy.

Third problem is the water pipes. Since this is in a basement, the pipes come down from the ceiling to where the old tank was. I had to move these pipes over to the wall, then add some flex copper tubing with screw-on ends, plus a bushing to reduce to the 1/2 inch pipe the 125B wants. I also had to add a pipe from the hot water to the popoff valve, and attach that to the relief pipe. While I was at it, I added a shutoff valve in the hot water line, and a faucet for hot water in the basement. This involved cutting the copper pipes to length and soldering connections with a torch. Easy work, but time consuming.

Finally, you have to test all your pipes for leaks. There's no electric hookup for the 125B, because it has a standing pilot. I would have preferred their older unit that used two D-cell batteries, or the 125HX that uses a water turbine for power. But I bought what was on sale, so I get a standing pilot. At least it's a small one.

After turning the water and gas back on, and leak testing (had to redo one copper joint) I lit the pilot. (Had to wait the first time, to get the air out of the new line.) With the pilot lit, I turned on a hot water tap. After a few seconds delay, the 125B fired up, and I had hot water. VERY hot water. Had to adjust the temperature setting on the 125B down.

Reading the manual, and reading about the 125B on the web, I see that unlike a water tank, this unit needs maintenance. There are some plastic parts that need to be replaced every 3-5 years. Bosch sells a kit for this for $18. I'm going to order one now so I have it when needed.

In the end, I'm very satisfied with the job. It took a lot more work than swapping a tank. It took a lot more time, and several trips to several hardware stores to get all the right sized connectors. But I took my first hot shower with it this morning, and it works great. I like the idea of not burning gas all the time to keep a tank of water hot. And I like the idea of saving money on my gas bill. I'll also get a tax rebate. But I probably would not have switched to tankless if I had to pay full price, or had to buy the larger unit.



Recommended: Yes