Thermal Expansion and Dripping T&P Valves
Q: I recently replaced our 5-year-old (a builder grade) water heater with a Sears water heater. Ever since the replacement, I noticed a small occasional drip (about 1/2 gallon a week) at the T&P valve. I asked Sears about this and they said that I need an expansion tank to augment the water heater. For the past five years, my old water heater had worked without such a tank. Why do I need such a tank with this new water heater suddenly? What do you think?
A: Of course, there's also the chance the temperature/pressure relief valve on your old tank was defective. If you want to know for sure, put a water pressure gauge on the drain valve and check the pressure both when the tank is dormant and after it's been firing for awhile (you can make it fire by running the hot tap). The pressure should never go above 80 psi. If it doesn't, think about a new T&P. If it does, you probably do need the expansion tank. -- Randy (1995-2000)
Q: I have a water heater that is approximately seven years old installed in my garage. It is functioning well as far as I can tell and it appears to be in good shape (no leaks, no noise). There is what I think is a check valve installed on the top of the water heater with a pipe that runs the length of the garage to the outside. Today I noticed that there is hot water dribbling from the outlet onto my driveway. Is this just a matter of a bad valve, or am I looking at a more serious problem? Can I fix it myself with virtually no plumbing experience? Thank you in advance for any help you can give.
A: Has anything changed recently in your plumbing? Thermal expansion can cause a valve to open. It is sometimes caused when a backflow preventer or pressure reducer is installed on the cold-water line. The tank fires, the water expands and has nowhere to go, so pressure builds and forces the valve to open (it's partly what it's for). However, if nothing like that is going on, it may just be a bad valve. I suggest you have a plumber replace it. It is possible to do it yourself, but it's also possible any number of things could go wrong. -- Randy (1995-2000)
Q: I just installed a new Rheem Pro 40 gas water heater. It was a simple matter to remove the old tank and install the new tank. The problem is that the new tank, when in a heat cycle, increases water pressure in the house to 150 psi. The T&P valve then operates to relieve this excess pressure. I am on city water supply. They advised me the outside pressure to the house is about 140 psi.
I have an in-line pressure reducer set to about 45. This pressure holds until the tank heats water, then I go to 150. I can not get a satisfactory answer from city water department, any plumber or from Rheem. Do you have any thoughts or suggestions regarding this matter? Thank you in advance for any assistance.
A: What's happening is that when the tank starts up, the water heats, expands, backs into the line, hits the pressure reducer -- and has nowhere to go, so it forces the T&P valve to open to vent the pressure. Consider an expansion tank or something Watts makes called a Governor 80, which vents into the toilet tank. I'm surprised nobody could tell you anything. It's not all that rare. -- Randy (1995-2000)
Q: Can you tell me why our hot water heater stll leaks even after we replaced the pop-off valve twice? It even leaks when you shut it off at the breaker box. I shut if off to see if the leaking would stop. I figured that if it's not on, it can't heat up and that would make it stop, but it stll leaked. Any suggestions?
A: Check the water pressure in general, at a tap, and check it again when the tank starts up, with the pressure gauge on the drain valve of the water heater. The pressure may simply be too high, in which case you need a reducer, or it may be that there is a backflow preventer somewhere, so that when the tank starts up, the water heats, expands, backs into the line, hits the preventer, and forces the popoff valve to open to vent the pressure. Water pressure should be about 80 psi. If it spikes when the tank comes on, consider an expansion tank or something Watts makes called a Regulator 80, which vents into the toilet tank. -- Randy (1995-2000)
Q: We have a gas heater that's about 4 years old. About a month or so ago we adjusted the temperature to higher since our hot water was not hot enough for the cooler weather. Ever since then every couple of weeks it would release water (I assume to relieve pressure). Is this normal? This same problem happened when we first moved into the house (about a year ago) and we turned up the temperature. A plumber came in and changed the temperature and pressure gauge during that time. Should we replace the water heater?
A: One possibility is that you have a defective temperature/pressure relief valve (T&P). Another is that you have a thermal expansion problem. When a water heater fires up, the water expands. In many cases, it just backs up into the cold-water main. In others, if there is a pressure reducer or backflow preventer, then it has nowhere to go and the pressure spikes.
The T&P is set to open at 150 psi, and exists primarily to keep the tank from exploding if the control malfunctions. But 80 psi is considered the maximum generally acceptable for water heaters and piping. Anything above that can result in damage and shortened life. In no event should you replace the water heater because that won't solve your problem.
You should test the pressure (or have it tested) using a water pressure gauge. It can be screwed onto the water heater drain valve or some other hose bib. Then make the water heater fire by turning up the thermostat some more or running a hot water tap. When it starts firing, turn off the hot tap and watch the gauge. If it's thermal expansion, you should see a marked jump in pressure.
The fix for the problem is an expansion tank placed somewhere on a cold-water line (NOT on a hot line). The tank comes charged with air to 40 psi, but should be charged to match your water pressure. When the pressure goes up, the water flows into the tank. When the pressure goes down, the air forces the water back out of the tank. If the gauge shows no drastic pressure change, then have the T&P replaced. -- Randy (10/13/04)
Q: Hi, I have a problem that I hope you can help solve. I own a rental house, which was recently inspected by the city and the inspector told me that my water heater needed a Watts 220 valve installed. I think it's supposed to relieve the pressure if it gets too high by flushing the hot pressurized water into the cold line. Please send me any info on how to install it properly or direct me to a Web site that does. Thanks.
A: It sounds as if you might need a Watts 210, which shuts off the gas when the temperature gets too high. We couldn't find a 220 in the Watts catalog. That might be used in situations where there is no place to run a drain line from a temperature/pressure relief valve, the solution most commonly used. We suggest you call Watts to verify that and find out how it should be installed. Their number is 508 688 1811 -- Larry (1995-2000)
Q: Temperature/pressure relief valve on hot water heaters, would it safe to use the PVC type pipe for the pressure relief valve vs copper? Thanks Steve
A: PVC won't stand up to heat. You might be able to use CPVC if local code will permit it. Otherwise stick with copper or galvanized. -- Larry (10/05/04)Back to Tanklets