How To Look At A House

McGarry and Madsen's home inspection blog for buyers of  

site-built, mobile/manufactured and modular homes

Why is there water in my water heater drain pan?

Sunday, July 1, 2018

We can think of six different reasons why water would be in the pan under your water heater:

  1. The TPR valve (Temperature and Pressure Relief) is leaking. It’ s the device on the top or side of the water heater with a small flip-up arm that opens when the water temperature or pressure in the tank is too high. The valve releases water to avoid a disastrous tank explosion, and is a required component in every water heater. A pipe runs from the TPR valve to within a few inches of the pan, and the valve may need to be replaced.
        A slow leak will only drip every so often, so you can feel inside the bottom of the TPR pipe with a finger to verify wetness. Sometimes the cause is that the valve was opened for testing and did not reseat properly after the test. You can flip the metal handle open for a couple of seconds and then release it again to see if the valve will reseat correctly when tried again. Be forewarned, though: it may just make the leak worse.
  2. The TPR valve has opened because because of unsafe water temperature or pressure in tank. This could be caused by a defective thermostat, or because mineral and crud buildup at the bottom of the tank of a gas water heater has caused the water to boil beneath it. Some TPR valves have piping that bypasses the pan and runs to an exterior location instead. In that case, #1 and #2 are not the cause.
  3. Leaking drain faucet at bottom of tank. This is the location where you connect a hose to drain the tank. It might have slow leak, so check the outlet with a finger for wetness.
  4. Leaking around pipe connections to water heater. Check around where each of the pipes connect to the tank: hot, cold, TPR valve, and drain faucet. If it has been leaking for a while, there will be a rust-color drip stain running down from it.
  5. Condensate dripping down the flue of a gas water heater 
  6. Tank is leaking due to deep corrosion. This is the worst-case scenario and means it is time to replace the water heater. Tanks rust from the inside-out, so there are often no early warning signs; but corrosion around the base of the tank, or a rusty wet stain under it without any drip stains down the side of the tank—like in the photo below, with the red light on the moisture meter indicating that the wood is soaked—are two signs of possible tank leakage.    Everything but #6 can usually be fixed by a brief plumber visit for $125 to $200. The only exception is when #4 is caused by galvanic corrosion, which is identified by a powdery crust around a pipe at the tank connection that looks like a lumpy, cancerous growth. If it has destroyed the threaded fitting at the tank, the tank my need replacement.

    Also, see our blog post When is a water heater drain pan required?

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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? 

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? 

Does a tankless water heater require a pressure relief valve?

What is the difference between a single element and dual element electric water heater? 

What is an FVIR water heater?

What is a heat pump water heater?

What is a dielectric union?

What's that powdery crust on the pipe connections at the water heater? 

What are the most common installation mistakes with water heater replacement?

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'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. 


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