Electric Heater Woes

[Mike in TN]

I have a 40 gal. GE electric water heater that has started to blow off the pressure/temperature relief valve about once per day, depending on use. The system has a new (and apparently good) two gallon expansion tank in place on the supply, a new relief valve, new thermostats, and a pressure reducing valve on the supply line. The pressure on the supply line normally runs about 60 psi on a gauge and the pressure in the expansion tank was pre-loaded accordingly. Pressure on the system does go up to 140 psi as the water heats but the tank doesn't seem to be waterlogged. The system acts as if the expansion tank isn't even there. The temperature at the tap maxes out at the thermostats setting. Any ideas on what could be going on?

 

[Reach4]

Your expansion tank failed. Replace it.

but the tank doesn't seem to be waterlogged.

What causes you to think the tank is not waterlogged?

 

[Sponsor]

 

[Jadnashua]

Usually, if you have heat traps on the WH, they leak some, so water can get to the expansion tank. It's possible yours are sealing 'too' well. If so, that could be isolating the WH from the ET, and it can't do its job, causing the pressure to rise and opening the T&P valve.

 

[Mike in TN]

Your expansion tank failed. Replace it.

What causes you to think the tank is not waterlogged?

The expansion tank was a new installation about 12 inches from the heater. Tapping on the tank indicated the upper part of it was still hollow. I replaced it anyway and am waiting on the heater to go through a cycle to see if it corrected the problem. Thanks guys.

 

[Mike in TN]

Usually, if you have heat traps on the WH, they leak some, so water can get to the expansion tank. It's possible yours are sealing 'too' well. If so, that could be isolating the WH from the ET, and it can't do its job, causing the pressure to rise and opening the T&P valve.

While that may be true to some degree, the whole house is getting pressurized. I know this because I put the gauge on an outside hose bib on the house side of the PRV. Hopefully the new ET will take care of the issue but are there any other thoughts on what might be causing the issue in case it doesn't?

 

[Reach4]

The expansion tank was a new installation about 12 inches from the heater. Tapping on the tank indicated the upper part of it was still hollow. I replaced it anyway and am waiting on the heater to go through a cycle to see if it corrected the problem. Thanks guys.

The precharge air should ideally be between the water pressure when you trickle the water from a faucet, and 2 or 3 pounds higher. The extra compensates for possible calibration differences between your air and water pressure gauges, and the fact that in many systems the pressure gets higher in the middle of the night, when the water towers get filled higher, and there is less water being used. If the precharge air pressure is at least as high as the water pressure, a good pressure tank will be empty of water. And certainly when you removed the old tank, if it was still good, it would have been as light as the new one.

Another thing that can happen is a thermal expansion tank could be undersized. With your 40 gallon tank, that seems unlikely. Plus there is the fact that you only had problems after quite a while of no problem

http://www.watts.com/pages/support/sizing_DET.asp
http://www.amtrol.com/support/therm_res_sizing.html

 

[Mike in TN]

The precharge air should ideally be between the water pressure when you trickle the water from a faucet, and 2 or 3 pounds higher. The extra compensates for possible calibration differences between your air and water pressure gauges, and the fact that in many systems the pressure gets higher in the middle of the night, when the water towers get filled higher, and there is less water being used. If the precharge air pressure is at least as high as the water pressure, a good pressure tank will be empty of water. And certainly when you removed the old tank, if it was still good, it would have been as light as the new one.

Another thing that can happen is a thermal expansion tank could be undersized. With your 40 gallon tank, that seems unlikely. Plus there is the fact that you only had problems after quite a while of no problem

http://www.watts.com/pages/support/sizing_DET.asp
http://www.amtrol.com/support/therm_res_sizing.html

The house is a rental and whole issue arose when a leaking toilet valve was replaced and a relative family moved in (the house had been empty for a few months with no water). There was no expansion tank on the system originally and no previous issues with the heater, presumably because the leaking toilet prevented a pressure buildup. The occupants also stated that they were in the habit of all taking long showers in the morning effectively emptying the water heater which previous occupants didn't seem to do. The complete heating cycle appears to be taking about three hours during which the pressure builds to about 140 psi and then the (old and new) T&P valve activates, 10 psi below the factory specifications. The total discharge is only about a cup to a quart of liquid but the pressure remains high until the system is used after which the pressure returns and remains at normal pressure. Without a complete cold-to-hot cycle of the heater it isn't an issue. Ultimately, it appears that the two gallon ET is just too small for the system as it is currently being used in spite of it only being a forty gallon water heater. Installation of a larger tank is going to be an issue dut to

 

[Mike in TN]

Sorry guys, I got cut off. The installation of a larger ET is going to be an issue due to space limitations. My next guess would be to reduce the system input pressure and/or reduce the heater temperature setting which I know won't set well with the relatives due to the reduction of the hot water volume. I have been going over to the house (next door) a couple of times during the day to check on things and bleed off the pressure just to keep it from getting as high. It is a pain in the rear and I am afraid that the high pressure might cause some issues with the rest of the system. I am just really surprised that the two gallon ET couldn't manage the expansion from a 40 gallon tank. I have a 50 gallon electric HWH on my home system, have the same supply pressure, and have never had an issue with it. Any other ideas besides the ones mentioned?

 

[Jadnashua]

Double-check your system with this calculator

http://www.watts.com/pages/support/sizing_DET.asp

it will tell you the size of the ET you need for your system. Note, some deep wells can have frigid temperatures, but public supplies in your area are probably a lot more temperate, but you do need to know the inlet and outlet temperatures to adequately size the tank.

 

[Mike in TN]

Double-check your system with this calculator

http://www.watts.com/pages/support/sizing_DET.asp

it will tell you the size of the ET you need for your system. Note, some deep wells can have frigid temperatures, but public supplies in your area are probably a lot more temperate, but you do need to know the inlet and outlet temperatures to adequately size the tank.

I did check the page mentioned and, according to it I should have plenty of capacity for the conditions. The supply is public. The pressure rises over about a five hour period when the system is unused and isn't an issue overnight because most of the hot water use is in the morning.

 

[Jadnashua]

That slow of a pressure rise would not be characteristic of expansion from the WH...a tank would likely reheat in less than an hour or so after you stop using hot water. That would imply maybe a failing pressure reduction valve that is letting the water pressure creep up.

 

[Master Plumber Mark]

It sounds like a bad prv valve to me/....
also you might want to consider putting a 5 gallon expansion tank on this one
I have never been a fan of the wimpy little 2 gallon tanks.. they just dont cut it

 

[Mike in TN]

It sounds like a bad prv valve to me/....
also you might want to consider putting a 5 gallon expansion tank on this one
I have never been a fan of the wimpy little 2 gallon tanks.. they just dont cut it

A failing PRV is certainly a possibility but wouldn't it be unusual for a municipal (rural) supply to have pressure high enough to set off the T&P valve? Since the bib where I apply the pressure gauge is after the PRV I really can't say what the raw supply pressure is running, however, wouldn't it be expected that the pressure would climb overnight even though the heater was not being drained during the evening? Also, I had the heater off overnight recently and there was no increase in the gauge pressure during that period. If it was just the pressure leaking past the PRV and the raw supply pressure was causing the problem then the pressure on the system should climb overnight shouldn't it? After the expansion raises the house pressure above the supply pressure then the supply pressure shouldn't matter unless the supply is high enough to set off the T&P. The other possibility that comes to mind is that the PRV is leaking and the rise in the supply pressure is affecting the ET's ability to accept the expansion. The problem with that possibility is that the house pressure stays about 60 psi on the gauge until at least two to three hours into non-use of the system and then climbs after that. Could it be that there is a bad heating element that is causing a slow heating cycle? After a slow increase in the supply pressure due to the a leaking PRV the water continues to heat, exceeding the ET capacity. After the supply pressure is reached, the continued heating causes the increased pressure, maybe?

 

[Mike in TN]

A failing PRV is certainly a possibility but wouldn't it be unusual for a municipal (rural) supply to have pressure high enough to set off the T&P valve? Since the bib where I apply the pressure gauge is after the PRV I really can't say what the raw supply pressure is running, however, wouldn't it be expected that the pressure would climb overnight even though the heater was not being drained during the evening? Also, I had the heater off overnight recently and there was no increase in the gauge pressure during that period. If it was just the pressure leaking past the PRV and the raw supply pressure was causing the problem then the pressure on the system should climb overnight shouldn't it? After the expansion raises the house pressure above the supply pressure then the supply pressure shouldn't matter unless the supply is high enough to set off the T&P. The other possibility that comes to mind is that the PRV is leaking and the rise in the supply pressure is affecting the ET's ability to accept the expansion. The problem with that possibility is that the house pressure stays about 60 psi on the gauge until at least two to three hours into non-use of the system and then climbs after that. Could it be that there is a bad heating element that is causing a slow heating cycle? After a slow increase in the supply pressure due to the a leaking PRV the water continues to heat, exceeding the ET capacity. After the supply pressure is reached, the continued heating causes the increased pressure, maybe?

I replaced the PRV and that was definitely the problem. The delay in pressure rise was simply the ET doing it's job. Many thanks with all of the help. The supply pressure (checked at a neighbor's outside unregulated bib) maxed out the gauge by the way.