Buying and installing a tankless hot water heater.

Know the hazards of faulty installation before trying this yourself. If done improperly there can be life threatening consequences unlike other projects for the do-it-yourselfer. I don’t recommend self installation unless you have some plumbing and heating experience.  But you can save a lot of money by doing it yourself,  just do the research.

I needed a hot water source for the guesthouse. A on-demand tankless hot water heater was a no brainier for me. A tankless water heater heats water only when the demand for hot water is made, (by turning on the faucet), this helps save energy over a traditional water heating system that continuously heats and stores water in anticipation of a hot water need. On-demand tankless hot water heaters deliver an endless supply of hot water unlike storage tank models. Since the guest house is empty more often than not I don’t want to continuously heat hot water that is not used.The other reason I went with an on-demand system is that up in the mountains we have frequent power outages. These can last for an hour or up to as long as five days. During those times I get along fine with my 5K Honda generator except for heating hot water. My electric hot water tank requires too much power for the generator. To tell you the truth I never even tried to run it for fear of burning up my generator. The LP gas tankless hot water heater draws very little electric power for flame ignition and the direct vent blower.

The first thing you need to decide is whether you want to go with electric or gas to power your unit.  If you go with electric, live in the northeast and want to serve 2 bathrooms; you would need to dedicate 2- 60 amp breakers for delivering up to 114 amps of on-demand electric power. That’s a huge amount of electric power and it stopped me right there because during power failures I want hot water. But the  cool thing about electric powered units is you can put them almost anywhere because you don’t need to vent them. That is a huge advantage. Electric tankless heaters are the most efficient because no (heat) energy goes out the vent. But in most areas of the country, because of it’s cost, electric power is the most expensive ways of producing BTU’s. You will find electric most often used in warmer climates and in additions that are a long way from a hot water tank serving a single sink and shower.

The other option is natural gas or LP gas. These models require power venting but some can be vented by natural draft through a dedicated chimney flue. They also require black cast iron or copper pipes to deliver the gas. Gas models are more likely to be used in the northeast to serve a whole house because of their maximum BTU output potential.

The biggest challenge facing most people is what manufacture to go with. Here are some of the ones I looked into; Bosh, Bradford White, Navien, Rheem, Rinnai and Takagi. Something interesting I found out when I went online to find product reviews was that you could find negative and positive stories about every brand. I started to realize that some of these posts were fraudulent- placed by people who had an agenda. You click on these reviews and end up on some strange web sites; you start to wonder about their voracity. A friend’s father who is a plumber recommended Navien, not because he has used them but from trade talk that they are the most efficient- 98%. You vent the Navien with inexpensive PVC pipe because the exhaust gases are not hot due do its high efficiency rating. The Navien, a Korean brand that has been used for a long time in that country- is pricier than most and not sold by many vendors in the USA.

A few years ago I saw a demonstration by Bosh of several models at my local plumbing supply house and I was all jazzed about getting one. Then last year when I went to buy the Bosh my plumbing supply place expert told me to go with the Rheem, which they also sell, because of it’s reliability and performance. I ultimately went with the Rheem partly because of his recommendation but mostly because of it’s industry best .26 GPM minimum flow rate. Other models are almost twice that. What that means is when you have your hot water faucet turned on just a little; the unit is still going to heat the water where other models will shut down.

Rheem, indoor, direct vent, LP gas, tankless hot water heater.


So I bought the Rheem RTG-84DV for around $900. It is an indoor, direct vent, LP Gas, 180,000BTU hot water heater, with a energy star .82 energy factor. It’s rated for 2 bathrooms and delivers 6.7 gallons per minute with a 45-degree temperature rise. The temperature of the ground water varies around the country. The colder the ground water is the more energy it takes to heat the hot water to your desired temperature. There are charts on-line that show the ground water temperature in your area. In my area it is around plus 40 degrees, so at 6.7 GPM’s the temperature coming out the faucet would be 95 degrees, something to consider if you have a high use situation. If you are drawing less water, as you usually would, the temperature would rise to what ever the thermostat is set for. A typical shower requires about 1.5-2 GPM, a bathtub requires about 2-3 GPM and a kitchen sink about 1-2 GPM.  Some people are so committed to this way of delivering hot water that in high use situations they gang tankless heaters together to achieve the desired demand temperature.



In deciding on going with a gas model you need to consider if a practical location is possible. You want the hot water heater to be reasonably close to the delivery point and still be able to vent it outdoors properly. You want the vent pipe length to be as short as possible, with as few bends as possible. My vent situation only required six feet of vent pipe and one elbow but I know they run them over 30 feet. You can’t terminate your vent outside next to a window that opens or a patio where people would be sitting, or near any animals for that matter. Carbon Dioxide comes out of that vent pipe and it can be an odorless killer.

Concentric Vent

This Rheem unit has a concentric vent- in other words a double walled pipe. The 3” inside pipe is stainless steel for gas discharge and the sheet metal outer 5” pipe is for air intake to the gas burner. Some models have two pipes at the top, one for the exhaust and one for air supply, which means you have to run two pipes the whole way and cut two smaller holes through each wall penetration. The concentric vent feature was also a big reason I went with the Rheem unit.

Fuel supply lines to the tankless hot water heater are another thing to consider. In my case LP gas is the fuel and the outdoor storage tank is near the heater location. Pipe size may need to be increased for long runs because these units need to have an adequate gas flow rate to heat the water properly during high demand situations. I went with ¾ inch black pipe. The only other thing a do-it yourselfer can use is copper. I do not have good success with flaring copper tubing. Black pipe is no cakewalk but I think it is the best way to go.

Whatever model you buy I would recommend going online to the manufactures website, download a PDF of the instillation manual and read it over before you buy it. I would do this even if I wasn’t going to install the unit myself. This way you will know what you are getting into.

Some Installation Highlights:

Every manufacture has their own installation requirements but I will just speak about some universal issues.  Most but not all direct vent tankless water heaters require stainless steel vent pipe, Rheem seems to partner with Metal Fab Inc.  All vent pipe types are a big deal, you need to get it right and install it according to the manufactures recommendation. Go to to find out more details.  

Running your concentric vent lines and gas lines.


Think hard about your vent pipe path before you cut any holes. First thing I did was mount the unit to the basement wall but in a temporary way.   You may need to move it slightly when you connect the vent pipe. I soon realized I had made a mistake in calculating my pipe length because the concrete basement wall was 10″ wide not the same as the above 2×6 wall of 5 1/2″.  So I needed to order more  vent pipe. When I placed the order the local plumbing supply company wanted to charge me a shipping fee of like $25 because they do not stock the vent pipe, it is special order. It worked out for the better in the end because I decided to buy a 3″ to 10″ adjustable pipe which made my very tight, under stairwell connection easy. There was no head room to fit the pipes together. So first, I just put on my top elbow piece and then my 3′ floor penetrating tank connection piece. Then I was able to put the adjustable piece in the middle to make a perfect fit. Otherwise I would of had to go down in the basement, take the unit off the wall, fit it onto the vent pipe while holding it’s weight and then push the unit up into the vent pipe.  The adjustable piece saved me. Sweet!


outside vent termination

Outside vent termination.


The vent outside wall terminator needs to have metal flashing flat against the siding with chalk around it. In cold weather condensation tends to form around the termination and the flashing protects your house siding. The gases can discolor your wall also. Vinyl siding needs a special spacer to prevent the plastic from melting. The discharge gases from my Rheem model are not hot. Outside the house you can grab hold of the inner pipe and it is only warm. Put your hand in the vent gas stream and it does not burn. I have not checked the vent pipe temperature after long high demand use. You want to follow the manufactures recommendation.

The other very important thing you need to be aware of is acidic flu gas condensate. This stuff is corrosive and needs to be drained away. Most manufactures have a catch ring where the vent pipe attaches to the top of the unit. This catch ring collects the condensate as it drips down the vertical pipe wall. The horizontal vent pipe portion of your run needs to pitch down so the condensate flows outside the house and doesn’t lay in the pipe, corroding it.

condensate drain nipple

Condensate drain nipple.

The inside catch ring has a nipple on it where you attach a high temperature silicone tubing suitable for use with acidic condensate. Preferably clear tubing with a water filled trap loop to stop gases from escaping out this tube with the condensate. The tube should drain somewhere that you local code allows. Your manual will explain it all to you. This whole condensate thing made me think twice about going with a tankless hot water heater– but I got over it.

Applying pipe dope to the threads.

Black gas pipe can be a bear. You can buy it at box stores, hardware or a plumbing supply house. I found the box stores had more of a variety of pipe lengths because plumbers, who buy at plumbing supply stores, thread their own pipe to the exact length. You can never use regular white Teflon tape because it can shred and clog appliances. Buy a high quality thread compound or pipe dope approved for gas use. Spread the dope liberally on the male threads. I use the non-hardening kind.


18 inch pipe wrench

18 inch pipe wrench

How tight is tight enough? If you have ever assembled a complicated piping system,  get it all done and find out there is a leak in the middle forcing you to take it all apart, there is a tendency to want to over tighten. Some people say go two turns past hand tight with a pipe wrench. I find you can’t go this way because some threads may have a bung stopping you from hand tightening them good. The way I do it is after hand tightening use a 18″ pipe wrench. When the pipe taper feels like it is getting tight I go one more turn for proper alignment and stop. You know you are going to tight when the pipe starts talking to you by making a ping sound- that’s the threads getting stressed. Try to avoid backing the fitting up for alignment reasons as this may cause a leak. On used pipe you might want to add a piece of thread to help clog up the joint.

using soap to check for leaks

Using soap to check for leaks.

Pressure test your work before you go to far along because a leak requires you to dismantle multiple fittings as you make turns. I hook my pipe up to an air compressor at about 30lbs of pressure. You can use a bicycle pump also. Turn the valve off to your appliance because to high of a pressure could cause a leak. Then you take a soapy sponge and soap up your connections. You want to see bubbles but not bubbles that grow and pop. The bubbles should just sit there. A leak is pretty obvious as the bubbles grow. Make sure you have enough light on the joint and maybe your reading glasses on to get a good view. In hard to view areas a small mirror is helpful.






Tahnkless water heater details.

#1. You need to install a temperature and pressure relief valve. A type 40XL Watts T&P relief valve or equivalent is recommended by most manufactures.

Watts temperature and pressure relief valve.




The end of the temperature sensor should not extend into the flow of hot water so a nipple is needed to prevent that.








tank drain tube

Vinegar flush tube.

#2. A drain valve is provided with the Rheem model to allow cleaning the unit with vinegar. Over time depending are your water hardness the unit may build up scale disrupting it’s operation. The Rheem unit has a digital remote temperature control that will give you a read out when it needs to be cleaned. You put a gallon of white vinegar in a bucket with a circulating pump, (i.e. aquarium pump), and circulate the vinegar through the grey cold water service valve (above #3) for a certain amount of time which I have not needed to do yet so I can’t talk about it.





backflow preventer valve



#3. A back flow prevention valve on the cold water side was not required in my installation manual but I was reading elsewhere that it was a good idea.

1/4 turn gas shut off valve#4. Gas shut off valve supplied by the manufacturer.  Extreme care must be taken when tightening any connections to the  unit itself. You do not want to crack anything.

sediment trap#5. Sediment trap to allow any particles inside the gas pipe to fall harmlessly down instead of traveling on to block an appliance orifice.





gast shut off valve#6. LP gas supply shut off valve. Required before the flexible supply line.

Installed tankless hot water heater.


The cold water shutoff valve is not shown in this photo but it is 2 feet away on my water distribution manifold.









SharkBite 1/2inch x 3/4 inch T fitting .

SharkBite 1/2"x3/4" T fitting for PEX.


I have just started to use PEX and have been very happy with it so far. SharkBite push fittings are terrific. You just cut the PEX evenly and push the PEX pipe and fitting together. Presto! It makes a water tight seal. They come apart easily in most situations except when there is no room to pull the pipe or fitting away from each other. Then you may have to cut the PEX shorter. I would not use them in an enclosed wall but everywhere else they are great. I know in the beginning they recommended home runs for all the lines when using PEX. That doesn’t seem to be the case now, the buzz out there is so far so good with the SharkBite fittings.

sharkBite supply valves for 1/2" PEX

SharkBite supply valves for 1/2" PEX. AWESOME!



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