Plumbing, Heating, and Air Conditioning Service and Repair
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Boise, Nampa and Caldwell's Water Heater Expansion Tank Pro's

Water Heater Expansion Tanks

Do you have a Small round tank up above your water heater in your home and have often wondered what it's purpose is?  The purpose of that tank is pretty simple, this tank is for hot water expansion.  As your water heater heats the cold water up the water wants to expansion, new water meters that the cities are putting in typically have check valves in them now that prevent that expansion from going back out in the street.  The same can be said if you have a commercial building with a back flow preventer in it.  Once there is a check valve or back flow preventer on the incoming water line there it is a requirement by code that we install an expansion tank.  This does two things.  One, it absorbs expansion as the water heats since there is no where else for it to go unless you have a faucet open and two, it prevents the T&P valve on the water heater from dripping and wearing out before it's lifespan due to that being the only place for the expansion to go.  As these water heater expansion tanks get older they can wear out due to the rubber bladder that is inside of them.  If the bladder breaks or the tank looses it's air charge it will not work at all.  If you see your T&P (temperature and pressure relief valve) leaking then you may need a water heater expansion tank installed or your existing water heater expansion tank replaced.  Whatever is causing it to drip, it needs checked out, that valve is a safety valve that keeps the water heater from potentially exploding or injuring your family and property.  If you have questions about whether your water heater needs an expansion tank just give Drake Mechanical a call and we will schedule one of our plumbers in Boise to come out and answer all your questions and get you the information and pricing you need to make a decision.  

Water heater expansion tank installation in Boise

Water Heater Expansion Tank Products That We Regularly Stock and Install


Therm-X-Trol ST-5

The ST-5 is a water heater expansion tank designed water heaters up to 40 gallons.

Therm-X-Trol ST-12 

The St-12 is a water heater expansion tank designed for water heaters up to 75 gallon.

Therm-X-Trol ST-25

The ST-25 is a water heater expansion tank designed for water heaters up to 120 gallon.

Water Heaters That Require an Expansion Tank

  • Tank type water heaters typically will require an expansion tank.  Most areas of the Treasure Valley require by code the installation of an expansion tank on any tank type water heater installation or change out.
  • Hybrid water heaters are also not considered tankless and in most areas require an expansion tank by code.
  • Tankless water heaters by code do not require an expansion tank but there are a couple municipalities like Meridian who will require an expansion tank in order to get a final inspection.
  • Mobile Home water heaters if there is a check valve in the water meter or on the incoming water supply to the mobile home.

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Water Heater Expansion Tank Installation Locations:

  • Boise
  • Meridian
  • Eagle
  • Kuna
  • Star
  • Middleton
  • Marsing
  • Mountain Home
  • Nyssa
  • Ontario
  • Idaho City
  • Garden Valley
  • Notus
  • Parma


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24/7/365 Emergency Service For All of Your Plumbing, Drain Cleaning, Water Heater, and Heating and Air Conditioning Needs!

Boise: 208-362-6200
Meridian: 208-887-1878
Nampa: 208-468-9919
Caldwell: 208-468-9919

Drake Mechanical
5551 W. Gowen Rd.
Boise, Idaho, 83709

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*Financing options are available upon request and qualification