HOWTO: Install a Tankless Hot Water Heater

Unlimited Hot Water

When I bought my latest house I was already planning on some upgrades to various things. One of these was the hot water heater. The water heater that came with the house was old and would start to run rust-colored water if the faucet was left on long enough. I'd been hearing about the new "tankless" style heaters so I went gathering information about them. Turns out they did what I wanted, using much less energy in their off state since they only heat water as it flows, there are no large reservoir tanks to worry about keeping warmed.

I installed my heater after finding rust and silt in the bath tub when trying to take a bath. I performed a second installation with a friend after she saw how nice the tankless heater worked, and her hot water heater just plain stopped. This article is the result of the lessons learned during both of those installations.


When I went looking for information on installing a tankless hot water heater most places told me simply to find a "qualified professional" and have them do it. Well, I'm a do-it-yourselfer so that wouldn't cut it. I found very little information on installing a tankless heater, thus this guide was born. The following items are the things I wish someone had told me ahead of time.

  1. Sizing: From what I have found, you determine which tankless hot water heater you will buy simply by deciding how many active faucets you want to be able to use at one time (each instance of you using hot water is known as an "application". So a 2-application water heater would be good for, say, running the shower and the dishwasher at the same time.) Since I am able to plan my days well enough and I have a smaller house with only two people in it, I figured a single application tankless would fit my needs. This also cut the cost of the heater in half, and brought it to within a close range of comparable tank models. The heater I ended up with (based on local availability) was the Bosch Aquastar 125B from Lowes. Home Depot did stock the Aquastar 250B (a two-application model at twice the price) at one of their stores, but they couldn't confirm stock. Since I already had a natural gas heater in the basement, I naturally went with the natural gas version. There are also LP (liquid propane) and electric models available from the various tankless hot water heater manufacturers.
  2. Placement: A normal tank hot water heater sits in the middle of the floor. A tankless model hangs on the wall (by two small hooks). This makes a huge difference in the amount of space you need, but it also means you need an open part of the wall to hang it from. My heater isn't particularly big at 30"H x 18"W x 10"D (with the cover on), but if you have a hot water heater stacked in a closet with a furnace you may not have available space to hang it. It is also light enough for one person to unpack and hang by themselves. My tankless heater (and I believe most others) is rated to hang on a finished wall, but I placed it on a couple of horizontal pieces of 2x4 against the cinder-block basement wall so I really had no concerns about excess heat. The heater I purchased also included a handy paper template I could tack on the wall to determine height and spacing.
  3. Plumbing (Water): If you decide to do this yourself you will likely need to relocate your water pipes. Since most hot water heaters sit in the middle of the floor the cold and hot lines are pre-run to the inlet and outlet of the tank. I ended up cutting a small section of my existing pipes off and running the new pipes back to the wall then down to the bottom of the tankless heater (the location of the new heater's inlet and outlet pipes.) To do this I just purchased a couple of 90 degree elbows and a few sticks of copper pipe. The pre-soldered elbows make this job a lot easier to do, especially if you will be soldering your new pipes up against a finished wall.
  4. Plumbing (Fuel): My hot water heaters use natural gas (while yours may use electricity or liquid propane.) As such the feed pipe for the existing heater needed to be extended to the new mounting point on the wall. This was accomplished by changing the horizontal run of "black pipe" from 6" to 24". I could have re-routed the pipe along the ceiling and dropped it closer but I was trying to make it as simple as possible (and having a vertical pipe didn't impact the amount of room the heater was using much, it's right next to my boiler in the utility room anyway.) One thing to keep in mind if you are using natural gas is that the kit should include a gas regulator that mounts vertically. So before mounting your new tankless heater on the wall put the regulator on the gas inlet and try to hang the heater high enough that you only need to change your existing horizontal run. I learned that lesson the hard way and the second installation went much smoother for it.
  5. Exhaust: By far the biggest trouble I had was doing the exhaust on the new tankless hot water heater. Basically the old heater had 3" ducting and the new one required 5" ducting (or 6" ducting at high altitude, which Denver qualifies as). The existing 3" duct needed to be removed, which was easy enough, but finding the right duct work and getting it shoe-horned into the existing exhaust was not easy. My boiler sits right next to the hot water heater and I was finally able to find a Y pipe that would let me splice in the new exhaust piping. A friend has also heard you need double-walled ducting for the entire system, though I only used double-walled into the existing ductwork. I may upgrade later, but for now it seems to be working well. On the second installation I performed we mounted the heater directly under the existing duct, so the only upgrade that was needed was a straight piece of double-walled exhaust ducting.
  6. Missing Brass: One thing I did find annoying was that in the kit there was a purge valve (useful for draining the heater), but all of the brass components required to get the valve installed were not included in the box. In the photo below you will see what I ended up purchasing, the parts include:
    • 3-way adaptor.
    • (2) brass reducers between the 3-way and the outlet threads (shown as the steel-braided cable.) The outlet is 1/2" and the brass 3-way had a 3/4" inlet. My copper piping was also 3/4", so I needed a second reducer on the left side to get the threads to the right size.
    • Male-Male 3/4" pipe adaptor. This was due to my poor planning. If I'd been thinking ahead I would have soldered a male end on the pipe you see on the left of the photo.
    • Garden hose adaptor. This sits on the drain side of the purge valve, letting me attach a hose to it so I can drain out the trap in the floor of the basement, rather than letting the water just run all over.

  7. Time: Keep in mind while you are doing this that you will not have hot water in your house (and if you don't have a separate shut-off valve for the hot water heater you will need to turn your mains off so you may not have running water at all.) Budget enough time for this. It took me about 5 hours on a Saturday for my first install (partially due to having to run to Home Depot and Lowes a couple times), the second install was about 3.5 hours.


I am very pleased with my home upgrade to a tankless hot water heater. As you can see in the photos below the space savings were enormous. My utility bill (for natural gas) has dropped by almost a third. I can shower for as long as I want without ever worrying about running out of hot water (or getting rust and sediments out of it.) Another advantage is that the service life on this tankless hot water heater is 20 years. Most of the new hot water heaters I looked at charge a big premium to have a service life of even 12 years, so I will be enjoying my trouble-free hot water for even longer. The installation was challenging, mostly due to my lack of advance knowledge and planning, but it certainly is within the reach of a competent DIY'er. After reading this you have any questions or comments, go ahead and contact me and I will do my best to answer them.



Frank Johnson says:

bob says: i have heard that tankless tend to plogg up regularly. do you know anything about this?

James Mullett says: Great site! It's good to see other do-it-yourselfers posting on their website. Where did you find that 5" double walled stainless steel vent pipe after all?

melvin says: is the tankless water heater as good as they say

Steve says: I believe the y pipe install is a no-no. Aren't you afraid of exhaust gas backing into the other appliance?

jim says: nice job! i've been looking into tankless also. what about electric? did you have a plug near by? also, any problem going fron 3/4" coper pipe to 1/2". is there enough water flow thru the 1/2" to keep the heater running? thanks for the info...jim

Curtis says: Jim, so far the water flow seems fine, I haven't had a problem with using the reducer. As for electricity, the unit I am using has a pilot light, no electricity needed! There is another Bosch unit that has a small turbine inside, it uses the water flow to trigger an ignitor and start the gas (if you have say a remote cabin where you don't want a pilot light running.)

Linda says: Thanks Mark, I'm not sure I understand all that but I'll tell my contractor and see if he can do that. I thought getting a whole house water heater was the way to go now I'm not so sure. Might just go back to a regular tank water heater. This is getting expensive.

Samuel says: I just wanted to know how much energy you think you save a month on your gas bill. I cant wait to install one myself! Thanks for the tips!

Tom says: Am I missing something here? the unit is vented through the existing vent line. As long as stainless steel venting was used, it looks fine as this is not an outdoor unit. It pulls air from inside the room for combustion and vents through the vent on the top, which looks like it is connected.

Tom says: Stainless steel venting only! venting into an existing line that is not stainless will rust out very quickly and you will be venting Carbon Monoxide into the wall space or room.

williams says: I am thinking of installing a gas tankless - outside - any thoughts?

Doug says: Where does your intake air come from? Most of these heaters have a concentric vent system meaning that the exaust vent and the intake are one inside the other. I hope you didn't cover the intake with the vent pipe, because then you would be drawing your intake air through your exaust vent.

Doug says: After a little research on Bosch heaters I feel you have a very dangerous situation here. You should not use this unit until you get a professional to correctly duct this. You are currently drawing your intake air from the exaust vent. You sould do some research on how power venting works, and maybe read, and follow the instructions. This is an unsafe instalation that should not have been posted on the internet. Please remove before someone gets hurt or killed.

phase says: I won't comment on the venting, since I don't know much about the heater you installed. But you can achieve long life from an inexpensive storage-type water heater by paying attention to the anode rod and replacing it periodically (the 12-year heaters have 2 anode rods instead of one). Check out for information, but don't buy anything from them. One can find a replacement anode rod for $15.

O'Malley says: This product is power vented and is dangerously vented into an existing exhaust pipe. Tankless water heaters are power vented and are ALWAYS vented directly outside by a single pipe specifically designed to be sealed for the high pressure of the exhaust expelled from this type of heater. Install a carbon monoxide detector in the basement test immediately.

kent says: Have you had any problems with your vent. I'm looking to install a tankless and the vent is going to cost more than the system. i was wondering if using the existing vent had caused you any problems.

David says: The Y pipe is in fact a no-no for exhaust on tankless units. The exhaust from tankless units is under positive pressure and must be vented separately to the outside. This is often an issue since the unit it replaces rarely is vented this way. There is the option of buying an outside mounted unit for those wanting a larger unit. With the reduction in venting cost the outside unit becomes more cost effective. Another issue with larger units is they usually require 3/4 inch gas lines where 1/2 is common with tank units.

vine says: Also, you are never allowed to reduce the opening of the relief valve even with the hose adapter you used. Nor should a hose ever be connected. This is a safety device. It's only job is to open under dangerous pressure or temperature. Not to be drained by. You could have piped a tee in before it and put in a hose bibb (faucet) to drain it from. The 1/2" nipple before the relief valve is a no no also it should be 3/4" all the way. Just thought I'd add some FYI.

Linda says: My contractor installed my tankless it's works great but only in the master bathroom sink. When I try to get hot water any where else it doesn't work. I have to turn on the water in the master bath sink first to get the heater to start and then turn on the water where I actually want to use it. Does anyone know a solution to this problem?

mark says: Linda if you install an on demand circulation pump under your master bath sink you can circulate water in aclosed loop back to the tankless and this will acitivate it. They are designed to circulate water back throught the cold water line until the internal thermostat senses the increase in temp at the hot line then it shuts off. It also can be remotely operated with a remote or button at each sink or shower in the house.

Rich says: Thanks for the site/info. I'm considering a tankless and will also do a self install. If I may and no offense: the solder joints shown are overheated. These may fail due to lack of capillary action (solder adhesion). I'm not a pro, but have done OSHA approved installations in the course of my employs. That being said, if the copper discolors, as in the above photo, it's an indication of overheating. Pre fit extremely well, test for ease of alignment. Clean, flux and assemble (sub assemble if needed to allow for heating in tight spaces). Don't over apply flux... it will seep into the fitting, bringing solder with it and cause an obstruction inside of the pipe. Too much flux can also cause pre-mature pipe failure due to the flux acids settling along the pipe. (unless you flush out the system with an anti corrosive which should not be needed in home systems) Apply heat only to the middle of any fitting (never to the ends). Use the upper most part of the middle of the flame for the heat source you use. Heat around the entire fitting, not just on one side. Keep the heat even. Keep your solder ready to apply, quickly. Once it starts to flow, apply and remove heat. Have a wet rag at the ready (not sopping wet, but wet). After soldering, wipe the joint to remove excess solder, excess flux and cool the joint. Solder time is under two minutes, depending on joint/pipe size/access. Do not overheat or attempt to "add" solder to the lip of a fitting. THAT is NOT where the adhesion factor is. It is internal (hence the need for a proper pre-fit) [I've had installs where, too tired, I've missed soldering a joint but due to proper pre-fit it passed an initial low pressure pre-check] I'd also recommend that you install some pipe clamps to prevent vibration/pipe movement (which will accelerate the fitting failure). Make these part of your pre-fit also. If it's too tight or you have to 'bend' or use a bit of 'persuasion', don't do it, correct it. Again, no offense, just an observation and perhaps some advice for others who may visit your site.

mw says: the reference to ss pipe is news to me.i purchased std galvanized double walled. why not ,been used for years on hw heaters.

Lee J.Guerrero says: Quick question if I have a Boiler system already installed ventilation is out the wall already how far should I need to be from the ventilation Hot water tankless duck out in Colorado code's are different here ...hummmmm half of the water heating bill huh ?

Sophia Castella says: I am getting more points to joint my heating condishner properly, and really its great post for more information. centralized heating Dee Why

Sophia Castella says: Northern beaches provides many services of best equipment of heating to make your life releasable. ducted heaters Dee Why

Sophia Castella says: Northern beaches provides many services of best equipment of heating to make your life releasable.

Water Heater Guy says: Read the comments others have made. This water heater installation was done very poorly by someone who doesn't know what he is doing. Bad solder joints, improper relief valve install, a very dangerous vent (should be on its own duct, not shared). This installation will probably kill someone if it already hasn't done so. Get a permit, a pro and an inspection. And for tankless heaters they don't save as people think they do. They don't last as long. They need more BTUs to heat the water quicker. They require more maintenance. They cost more to install (over the life of the heater you won't recoup enough savings in gas to cover the extra cost). Wastes water since you need to have full water flow for the hot water heater to kick on. Buy a good quality tank heater and insulate it and the hot water line and you will be very happy.

Rod says: Ok I hooked up a Marley tankless hot water heater Propane.this second year now but it has beed cold.Now the poor design in the unit allows cold air to come down the vent it freezes the top tube elbow and causes a leak which would have flooded my Basement if I didnt check. SO WHAT I THOUGHT WAS THE GREATEST COOL THING REALLY IS NOT.I called the Marley company about the ten year Warrantee what a joke the told me its not covered because it froze and I should have drained it as tge manual said,I said I will never drain it its in a house with a heated basement and used all year long.So cold air from the vent freezes the unit un real poor design crap just lucky I was home it lasted two years not ten and their sales person should have never sold me this unit if it was going to freeze up in Upstate NY.

Bosch Aquastar owner says: I have a different Aquastar model and I don't know enough info to be sure about his model, but mine is not power vented and not high efficiency. The intake air is not coming from outside and he likely did not install Type B venting over the intake. Also, these heaters don't need Stainless Type B because they have hot exhaust temps and don't condense.

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