What is a TPR Valve and How Does it Keep You Safe?

When you look at your hot water tank you may just see another appliance, but the engineering term for such a component is a pressure vessel. The definition of a pressure vessel is a container designed to hold gases or liquids at a pressure substantially different from the ambient pressure (normal atmospheric pressure). There are many codes and standards written about pressure vessels as they can be very dangerous if they don’t have the proper safety features installed. If the vessel doesn’t have the right pressure safety valve to relieve pressure when it starts to rise past the set limits, then the thing has the potential to explode like a bomb. You can see an example of this in a Mythbuster’s episode where they purposefully take a water tank to its limits (click here for the video clip).

Don’t worry, just because you have a hot water tank in your house, does not mean it is likely to explode. Most water heaters have several controls and safety features to prevent your tank from ever over-pressurizing, but I do want to stress the importance of proper temperature/pressure relief (TPR) valve installation on your hot water tank, boiler, etc. The TPR valve is a safety relief valve responsible for opening and relieving pressure when the temperature and pressure get too high in the tank.

Typical Layout of a Gas-Fired Water Heater

How does the TPR Valve Work?

TPR valves open to relieve pressure or temperature from the tank when the water in the tank exceeds the preset pressure or temperature limit. Normal tanks operate between 120 to 140 degrees Fahrenheit and are regulated by a temperature sensor. If the temperature sensor and other fail-safes go bad, and the burners are left on too long, the pressure will rise in the tank as the temperature increased due to the water wanting to expand. Most TPR valves are set to open at 150 psi or 210 degrees Fahrenheit to relieve pressure before the tank gets to its limit. Note that different tanks and components may have different requirements, so it is important you check the nameplate rating on the water heater and make sure you select a TPR valve that will actuate at or below the maximum operating pressure/temperature limits of the tank.

The TPR valve itself is spring-loaded in the shut position. The springs are set to start to give and open the valve when the pressure or temperature limit is reached, and then close again when the pressure has been properly relieved. Most likely when it is properly operating the valve will cycle on and off, versus if it is defective it will probably be leaking constantly.

A TPR Valve and Discharge Line Installed

Proper Installation of a TPR Discharge Line

As you know by now, TPR valves are vital components of keeping your water heater safe from overheating/over pressurizing. That is why it is very important that they are properly installed to ensure the safety of you and your family. The part of the TPR system that is often not properly installed is the TPR discharge piping which is responsible for guiding any water released by the valve to a safe location. Here are some important installation guidelines to check when it comes to the TPR discharge piping (Please note to also follow the manufacturer’s guidelines for your specific unit as well).

Piping Construction

The main job of the TPR discharge line is to direct the flow of any water that is released by the TPR Valve. Therefore, the material of the line must be able to withstand the pressure and temperature of the water exiting through the line. The line should be constructed of a material that is rated to handle these conditions. It is preferred that the line is constructed from metals like copper, galvanized steel, or stainless steel that are rigid and can easily handle the scalding hot water coming out. There are some special plastics that are also allowed like CPVC, polyethylene, and polypropylene that can handle those high temperatures as well. PVC and other non-approved plastics should not be used since they can easily melt.

Ensure Proper Drain Flow Without Restrictions

It is important that the flow of water out of the discharge line is never restricted as this can cause back pressure in the line and cause the valve to not release at the right pressure. The line itself should not be smaller than the outlet diameter of the valve. For instance, if the valve outlet is 3/4” diameter, then the piping should be 3/4”. The line should never be reduced downstream before it discharges as well and should be a straight shot to the floor, not using any unnecessary elbows or teeing off at any point.

In certain circumstances, the TPR valve is on top of the unit and will require an elbow for the line to go vertical. For the horizontal run, the piping should be sloped slightly away from the valve so water doesn’t backflow and puts back pressure on the valve seat. Lastly, this may seem like common sense, but do not cap, plug, or install an isolation valve on the line. This will completely block the line and make it as the valve doesn’t even really exist.

Air Gap and Safe Discharge

The last thing that we need to be concerned with for the TPR Valve and its piping is where it discharges. There is an air gap needed between where the line ends and the drain. This means the line should not be directly connected to a drain because there is a chance for back pressure affecting the valve set pressure as also some chance of back siphoning. If your water heater is not located near a floor drain or is located near a finished area, a drain pan underneath the water heater is always recommended. However, the discharge piping should point into the drain pan and not be directly connected.

Lastly, the discharge piping should terminate within 6 inches of the floor. This is to prevent scalding hot water from burning anyone in the vicinity of the tank if the valve releases. Also, you don’t want water splattering all over the place getting onto electronics, or getting walls wet.

Important Considerations When Buying a Water Heater (Tank)

In a previous article, we went over how your tank water heater actually functions. Most good water heater tanks will last about a decade on average, so having to replace a water heater is something you will most likely do as a homeowner at some point in your life. So what are some things to consider when buying a new tank? We will break down some key parameters below to hopefully allow you to make an educated decision the next time you replace your hot water tank.

1) The tank size – The size of the tank is important for hot water capacity. The bigger the tank you have the more hot water reserves you will have and the less likely you are to ever run out of hot water. A typical unit for smaller apartments and single family homes is 40 gallons even though you can go even smaller than that if you wanted. 50-gallon tanks are usually the next step up for mid-sized homes and 70-gallon tanks or even using two 40-gallon tanks (total of 80 gallons) is possible for larger homes (See Figure 1 below). Another factor to consider is how many people are living on the property. If you are living in a larger home, but are an empty nester you may be able to get away with only having a 40-gallon tank as your hot water use will be much less. If you need more guidance on tank sizing, this article a good read.

Figure 1 – Hot Water Tanks Come In Many Different Sizes

2) First Hour Rating – Maybe even more important than the tank size itself with regards to hot water capacity, is the first-hour rating of the unit. This rating is the amount of hot water a unit can provide in an hour's time when the tank starts off completely full of hot water. To give an example, if a unit is a 40-gallon tank, and has a 65-gallon first-hour rating, it can provide an additional 25 gallons of hot water over an hour's span. When you are selecting a new water heater it is important to factor the First Hour Rate into the equation. For instance, going back to the example of the empty nester, they may wish to buy a smaller tank with a high first-hour rating. This way when your kids come home to visit, you are able to handle the higher capacity, but when you are alone you are not paying to heat up a larger tank of water.

Figure 2 – A Typical Energy Guide for a Water Heater

3) Efficiency – There are a lot of different options when it comes to types of water heaters to buy. From gas to electric, conventional to heat pumps, natural draft to induced draft. We will have to get into each of these in another article but the biggest thing I want you to focus on is the overall efficiency of the unit. While you may be tempted to find the cheapest model out there, it may cost you over time in your utility bill. On almost every hot water tank you see at the store you will see a tag with a dollar amount of estimated dollars it will cost per year to run this unit. This tag is required by the Federal Trade Commission so consumers can more easily understand which units are more efficient.

While anyone can see that a unit that costs $145 a year compared to $195 a year is more desirable, there are other things you need to check (Reference Figure 2 for further detail). When they calculate the yearly cost they assume 2 things, first that the use of the unit will be average, and second that the price for the gas or electricity you pay is average. If you use your hot water less often and your local gas or electricity prices are lower, you will naturally pay a lower amount than what is listed. However, the opposite is true higher use and higher utility price will lead to more cost per year.

Use is something that is very hard to quantify without data so unless you know you rarely use hot water as the sole occupant of a property or have a family that you can field a football team with, I would recommend assuming you are an average user. For your utility rate, it is smart to check your gas or electric bill (depending on your water heater's power source) and see what the price per Therm or Kilowatt-hour is. The Energy Guide will have listed what the assumed energy output was for their calculation and also sometimes the rate they used. So for instance, if they calculated 165 therms for a gas water heater for a year and assumed $1.10 per therm for the utility rate, they would have list $181 as the yearly cost ($1.10 x 165 = $181.50). But if you look up your utility rate and see $1.20 per therm on your bill, your yearly cost will be $198 assuming average use.

So how do we use this information when purchasing a tank. If you were just concerned with saving over the life of a unit then here is a quick calculation you can run.

[Initial Cost of Tank] + 10 x [Yearly Tank Cost] = Total investment

Make sure the yearly tank cost is from your calculation using your energy bill as the different unit will look more or less desirable based off utility rate (for example, more efficient units will look more desirable when the utility rate is higher). So lets take two tanks and compare. Tank A costs $800 upfront and will cost $175 a year to run, Tank B costs $1200 upfront and will cost $1500 a year to run. Running the calculations:

Tank A: $800 + 10 x $175 = $2550 total investment

Tank B: $1200 + 10 x $150 – $2700 total investment

So as you can see, while Tank B is clearly more efficient, Tank A will save the user $150 over the life of the tank. If cost was the only factor, Tank A would be the right choice for you.

4) Your Current Setup – Like that feeling of buying a new couch only for it to not fit up the stairs or through your doorway, it is important to make sure your new water heater fits your current set up. First and foremost, we need to make sure it will fit dimensionally speaking, so don’t upgrade from a 40 to a 5o gallon tank without measuring first. Additionally, if you want to convert from a gas-powered tank to an electric tank, you may need to hire an electrician to run a new dedicated circuit to the tank (and vice-versa with a plumber for a gas line). Unless you have a good reason, it is usually best to buy something comparable to your current setup.

There are Plastic Tanks available with Lifetime Warranties

5) Tank Construction Material – Believe it or not you can actually buy a completely plastic hot water tank versus the typical steel constructed versions. Plastic Tanks actually have a lifetime warranty and may never need to be replaced (or at least not for a long, long time) because they don’t corrode as metal tanks do. The catch is they are more expensive than metal tanks and while they don’t require replacement, they will require maintenance and replacement of certain components that are still made of metal. So if you recently bought your forever home and never want to have to physically replace your hot water tank again, plastic tanks may be a worthwhile investment. If you don’t know where you will be living a decade from now, might as well stick with a cheaper tank.

I hope you find this information helpful when you go and make your next hot water tank purchase.

How Does My Water Heater Tank Work?

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Like an offensive lineman who you don’t notice until they committing that big offsides penalty in the 4th quarter, you usually will only notice your hot water tank on the rare occasion something goes wrong. Sometimes it's because you ran out of hot water for your shower or worst-case scenario it floods your basement cause it rusted out and started to leak. Water Heaters are one of the hardest working appliances in our home and also maybe the most under-appreciated as well. It is not easy to be on call to provide luxurious hot water 247. Do you know the back a century or so ago, you would have to heat water on top of a wood stove to take a hot bath and that that bathwater was usually shared by the entire household?

So let's start showing a little more appreciation for the appliance that provides us hot water at our beck and call and dig into how your Hot Water Tank actually works. NOTE: This article solely focuses on tank water heaters, Tankless Water Heaters will be discussed in another article soon.

How your Water Heater Tank Provides 24/7 Hot Water

Image a pot of water on your stovetop with a thermometer inserted into the water. You turn on the burner underneath the pot of water and naturally the pot of water starts to heat. Now image that thermometer has a signal wire connected to the burner control, and when it hits 120 degF it shuts off the burner. With the burner off the pot of water starts to cool until it reaches 110 degF and then the thermometer sends a new signal to the burner control to turn the burners back on to heat the water back up to 120 degF. This cycle continues endlessly, 24 hours a day so the pot always has water between 110-120 degF in it.

Figure 1 – Preset Dial Controls

The example above is how your water heater tank works to supply you with hot water on demand. Whenever the tank cools from not being used or from you using the hot water inside the tank, the burner turns on (or electric coils from and electric heater) to heat the water back up to the preset amount. The dial control attached to the gas valve at the bottom of the tank (see Figure 1) is how to change what hot water temperature your unit is maintaining. It is recommended that you set your tank to be no greater than 12o degF cause above that temperature, human skin can burn rather quickly.

You are probably thinking right now that this seems terribly inefficient to keep heating water when it's not being used, but this is the price we pay for convenience. There are other water heating appliances that have solved this problem called tankless heaters, but we will get to those in another article.

The Basic Anatomy of a Hot Water Tank

As you can see from Figure 2, Hot Water to service your plumbing fixtures exits the unit at the top. When the hot water is used, cold water from the city water supply (or a private well) comes into the tank to always keep the tank full. This will naturally drop the water temperature in the tank and usually cause the water heater burners at the bottom of the tank to kick back on. When the burners kick on, you can see that it is not only them that heat the water but the flue in the middle of the tank that vents the hot exhaust gas for natural gas burning water heaters. Note for an electric-powered water heater, the burners/flue are replaced by heating coils, just like an electric stove or in some new circumstances an electric heat pump.

Figure 2 – Components of a Hot Water Tank

One other question you may have is when the tank is running and cold water is introduced to the tank, what prevents the cold water from not getting sucked into the hot water line. First, the dip tube pushes the cold water to the bottom of the tank for this very reason, so it does not mix with the exiting hot water. It also does this as the burners and flue are the hottest at the bottom and it will provide the best heat transfer rate. The other thing that keeps the cold and hot water mixing is physics. Everyone has heard, hot air rises, well the same is true for water. As the water is heated below, hotter water will naturally rise to the top.

As you can see from Figure 2, there are many other safety devices and components in your hot water tank that we will discuss in another article, but next up we will talk about important performance parameters that you need to know about hot water tanks, especially when you go to buy a new unit.