Manufacturers call it an “isolator valve kit” or “service valve kit,” and plumbers often just say “flush kit.” It’s a set of four valves, two each on the hot and cold water pipes below a tankless water heater, that enables you or a service person to flush out any mineral scale buildup inside. The bottom two valves disconnect it from the house plumbing system and the top two valves open the pipes to threaded front hose connections for recirculating vinegar through the water heater to dissolve the scale buildup.

    “It’s real easy to tell if you have one,” according to Jimmy at J W Freeman Plumbing. “Look for four valves right below the water heater.” They are usually color-coded red and blue for the hot and cold water sides of the system, like in the photo above. Some manufacturers include the valve kit with the water heater, while others sell it separately. As far as we know, they are not required by the building code.

    Having a service valve kit is important because you cannot flush the system without one and, if you have a problem with your water heater after several years and call the manufacturer for warranty service, they won’t honor the warranty unless the system has been flushed. Flushing the system annually is necessary if you have very hard water, but can be done less often otherwise.

    The valves are the first part of what you need to flush the system. A large bucket, hoses, and a sump pump are also necessary to circulate several gallons of vinegar through the water heater for about an hour to clean it out. You can buy the other items yourself, but plumbers have the additional equipment for a flush if you call one for service—except that they can’t do it without a service valve kit in place. An example of a tankless gas water heater without a kit installed is shown below.

    If the bottom of your new tankless water heater looks like this, you can expect to have problems after three or four years that neither the manufacturer’s or builder’s warranties will address.

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Here’s links to a collection of more blog posts about WATER HEATERS:

• Are water heaters required to be raised off the floor? 

• Can a Temperature and Pressure Relief (TPR or T&P) valve be mounted to discharge horizontally? 

• Where are gas water heaters not allowed to be installed?  

• Is the minimum size water heater inlet pipe 1/2" or 3/4" according to the building code?

• Can you use a light switch for a water heater disconnect? 

• How can I tell if a water heater is HUD-approved for mobile/manufactured homes?

• Can you wire a 240-volt water heater with 120 volts? 

• Is it alright to have a shut-off valve on both the hot and cold water pipes at a water heater?  

• What is the minimum clearance to doors and windows for an outdoor tankless gas water heater? 

• What is required clearance for access and working space in front of an electric water heater? 

• Why is the water heater older than the house? 

• Does a water heater need a shut-off valve? 

• When was a gas water heater first required to be elevated 18 inches above a garage floor?  

• Can the temperature and pressure relief (TPR) valve discharge pipe for a mobile/manufactured home water heater terminate under the home? 

• What is the purpose of a thermostatic mixing valve above a water heater? 

• Does a tankless gas water heater still work with no electricity during a power outage?  

•  Why do water heaters have a sacrificial anode? 

• What is the difference between a manufactured/mobile home water heater and a regular water heater? 

• Where do I find the water heater in a mobile home?

• Does a tankless water heater in an attic require a drain pan? 

• Does an electric water heater require a disconnect? 

• Is a catch pan and drain piping required for a replacement water heater?

• Why is my water heater making strange (rumbling, gurgling, knocking or banging) noises? 

• What can I do to make my water heater last longer?

• How can I determine the age of a water heater if the serial number is missing or decoding it is impossible? 

• How does a hydronic heating system work?

• What is the difference between a regular water heater and a direct vent water heater? 

• What is the difference between a regular water heater and a power vent water heater?

• What is backdrafting at a gas water heater? 

• How do I determine if a water heater is gas or electric?

• What does it mean when a gas appliance (water heater, furnace, or range) has been "red tagged"?  

• What's the valve with the flip-up handle on the water heater for?

• Why is an older water heater an insurance problem?

   Visit our WATER HEATERS page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.