How to Install a Tankless Water Heater

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Ask This Old House plumbing and heating expert Richard Trethewey heads to Milwaukee to install an instantaneous water heater.

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Cost: 00

Skill Level: Expert

Tools List for Installing a Tankless Water Heater:
Pipe wrench
Hole saw

Shopping List:
1” gas line and fittings
4” PVC pipe and fittings
PVC glue
Teflon tape
¾” copper pipe and fittings
Pipe cutter

1. An installation like this that involves gas piping is best left to a licensed, professional plumber.
2. Shut off the water and the gas to the hot water tank. Drain the water out of the tank and remove the old tank.
3. The new water heater may require a larger gas line, which will need to be run from the gas meter to the water heater.
4. Using the hole saw, drill a hole on the outside of the house near the location of the water heater. Run PVC pipes for the fresh air intake and vent through that hole.
5. Connect the cold water supply line to the water heater and the hot lines out of the heater using the ¾” copper pipe.
6. An on-demand water heater requires an electrical outlet.  An electrician may need to assist in adding one near the installation location.
7. A cross-over valve can be installed at the furthest plumbing fixture to allow the built-in recirculation pump to keep hot water at the ready.
8. Turn on the cold water supply, gas supply, and power to the water heater and test.

Richard installed an RUR Super High Efficiency Plus instantaneous hot water heater, manufactured by Rinnai (

Expert assistance for this project was provided by Budiac Plumbing & Heating, Inc of Milwaukee, WIsconsin.

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Here’s how to install an electric water heater. Find step by step project details here:

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Read this if you’re not sure your water heater needs replacing.

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40 thoughts on “How to Install a Tankless Water Heater”

  1. I'm not impressed with our thankless water heater, it might be good for a ranch style home but if you a two story home with the heater in the basement it takes for ever for hot water to get to the restroom upstairs!

  2. Considering that tank was only 5 years old i think i would have just set it to like 140 and threw a mixing valve on the outlet side set to like 115 and just see how long it would last then when it breaks spend the money and go tankless lol

  3. The savings are minimal and lost to flushing the system 1 to 2 times a year depending on use if you don't have soft water. The home owner could have installed a larger hot water tank and saved the installation costs of the tank less system, but This Old House may have covered that?

  4. plumbing a lot different in America then in Australia, i did plumbing for 5 years, just installed an instantaneous the other day. Only have to run 3/4 inch for the gas line to the heater, we use copper for gas pipe, and our hot lines are 1/2 inch which is all you need (our cold main lines are 3/4 inch as well). Where most people solder bends, we just use the pipe benders, and most joints are silver brazed (while some plumbers still soft solder, but not near as common, obviously not on the gas line though). But most new houses have poly pipe except the front run from the water meter to the house, the garden taps etc. I like seeing how other countries do things different.

  5. I am not a plumber, but trust me there is nobody around here that can install, let alone maintain a system like this in my hometown. You can barely find someone qualified to set up a washing machine. So this is not for me no matter how much I'd like to have one.

  6. All I’m seeing is shit that will keep breaking and help line the pockets of the local plumbing companies and home owners dread this expensive “upgrades”.

  7. I was wondering if da home owners have 2 pay for da parts & labor of any job ya do for dem..????

  8. I got a quote for nearly $5,000 to install a tankless unit. I would probably have to install a second gas line as well since my boiler uses the current 1" gas line. I can install an indirect water heater for around $3,000 so that is going to be the best bet for me I think. The tankless looks nice, but no way will I recoup my investment on it.

  9. Tank less water heaters have been used all over Europe for coming over 20 years but are known as combination boilers as they are efficent enough to do the heating and hot water in a SMALL property like a 2bedroom house or flat at the most. The one in this video is being used for hot water only because there is another means of providing heating for that property. The size of this house with all of its pipework volume of water being tied into to from the old system would struggle. The gas lines are sized to the manufacturers instructions so no other appliance in the house would be starved if all appliances in house were on at the same time only then would the machine be able to do its job properly. A draw back he doesnt explain is that if you open more than one COLD or HOT TAP in the house the boiler would have very poor flow coming out starving appliances of water it's there major draw back hence why there not used in a property with more than one bath and shower in the house again great for small propertys with one or two people living in it any more and the plumbee should be recommending alternative means to the home owner. The re-circ system or hot water return layout he was on about is a standard plumbing set up for a system that would have a store of hot water but not for this. Poor pipework design is the reason for waiting to long for hot water as it would not of been upgraded and altered to suit this type of boiler being installed not to mention the fact they have it pumping into the cold even with a check valve is illegal in the UK and rest of europe you could cause legionellla to be present in the system. At most a well serviced one of these will last ten years there just not built to last but are priced for that across the rest of Europe they have 90 percent or more efficency rating for the energy they use and are half the size of the American type with many more controls, functions and tec as standard will do your heating also and cost less than a £1000. it's surprising that America would only be getting into this now it's seems your a bit far behind the times on this.good video tho haha

  10. The one question i have is, that recirc pump is capable of putting out over 55 psi of pressure? Most U.S. water systems pressure is about 55 psi coming into the house, so to push water into the cold lines means that the pump has to overcome that 55 psi incoming pressure.

  11. The sad thing is you don’t even need an expansion tank they are just trying to get you to waste your money on bullshit, I don’t have one and my pressure is fine

  12. Do you remove the plastic plugs that are sticking up out of the cold and hot water nipples?
    Awesome video. Thanks a lot.

  13. if your not getting hot water IT MEANS YOU NEED A HOT WATER HEATER!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

    ILUMINOTI >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>><<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<???????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????

  14. a lot of states..maybe most, require a licensed plumber and a permit to install or replace an existing water heater…they show a disclaimer at the end….

  15. This is a how to REPLACE not how to INSTALL

    Suppose you have a gas water heater you want to replace with an electric.

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