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JoeLee
12-30-2013, 01:30 PM
I'm wondering if anyone out there knows how long hot water tanks have had pressure relief valves on them.
I can't ever remember seeing one that didn't have a safety valve on it. I'm pretty sure it's a law that they do. My reason for asking is last week I walked down into my aunts basement and there was bit of a flood. I noticed that water was dripping out of a small relief valve that was mounted on the out put (hot) line of the tank. I know this thing has been there ever since I can remember, as it's part of the original plumbing (iron pipe) but I never paid much attention to it. The safety valve on the tank was fine, there is no need for two safety valves on the tank so I removed the leaky one and put a plug in it's place.

The only thing I can think of is in the old days when hot water tanks were first introduced to houses they didn't have safety valves on them and it was up to the installer or plumber to put one in the line. The tank has probably been replaced sever times over the last 70 years or so and the lazy plumbers probably just left it there not wanting to go further back into the original plumbing.
Any one have any thoughts???

JL.....................

jep24601
12-30-2013, 01:35 PM
Sounds logical to me. You ought to post your location though as plumbing practices have varied throughout the world.

Ed P
12-30-2013, 01:38 PM
The TV show "Ask This Old House" showed what happens when someone puts a plug in the outlet of a relief valve on an electric water heater. Not only was the house totally destroyed, but it damaged the neighbors house too! They found the tank, three blocks over! Must of frightened someone when it landed.

Ed P

SteveF
12-30-2013, 04:46 PM
The TV show "Ask This Old House" showed what happens when someone puts a pug in the outlet of a relief valve on an electric water heater. Not only was the house totally destroyed, but it damaged the neighbors house too! They found the tank, three blocks over! Must of frightened someone when it landed.

Ed P

My brother has a pug he would love to get rid of because that dog is such a pain in the ass and ugly as hell. I will show him your post and maybe it will give him some ideas. :D

Steve

Cuttings
12-30-2013, 05:37 PM
A couple of thoughts.
Now that most houses have pressure reducing valves in the inlet water line from the street it is essential to have a pressure relief valve.
The pressure reducing valve also acts like a check valve and will not let water go back into the inlet line, thus if the hot water tank was full of cold water then heated without using any water from the house side of the system
it could build a lot of pressure do to the expansion from heating.
Also I believe the pressure relief valve on the hot water tank is also a high temperature relief valve to let some water out if the thermostat were to malfunction and continue heating the water.

Guido
12-30-2013, 06:00 PM
'This Old House'?? Was it not 'Mythbusters', put together by those two clowns in San Francisco'

Never thought to ask the question outloud, in front of people with real life experience, but if a standard everyday hot water tank is filled with water, sealed off on any/all inlets/outlets and now heated with either a gas burner or an electric element, what happens???????????

Mythbusters 'demonstrated' a complete disaster. I'm saying rigged.

--G

Daveb
12-30-2013, 06:41 PM
Gents, it seems your water tanks are made of the same stuff that cars and fuse panels (as depicted by Hollywood) are.
Do your water heaters raise the temperature beyond boiling point in a sealed tank? Are you talking about a steam explosion?
Dave

ironmonger
12-30-2013, 07:21 PM
Very old systems did not have a temperature and pressure relief valve, only required a pressure relief. The pressure relief you saw was likely very old, and when the heater was replaced the installer put in a code required temperature and pressure relief valve and left the old one. If it wasn't leaking at the time there was no need to remove it.

If the tank was completly filled with water, and the heater elements were powered up the temperature would continue to rise and the water would continue to expand until the pressure burst the tank. At that point the water flashes into stream and the fun begins...

paul

jdunmyer
12-30-2013, 07:40 PM
I see no problem, you removed and old, leaky pressure relief valve, but you left the tank-mounted T/P valve. That's the ONLY safety valve in most modern installations, so you didn't create any danger. I do agree with the necessity for the T/P valve, as a water heater CAN explode if the heat source doesn't shut off when it should.

Bob Fisher
12-30-2013, 08:14 PM
I would think that IF the tank is completely filled and no water is allowed in or out, that as the temp increases and the water expands, the tank would rupture and relieve the pressure. That should happen Long before the temp is high enough to flash to steam. Bob.

jep24601
12-30-2013, 08:19 PM
Gents, it seems your water tanks are made of the same stuff that cars and fuse panels (as depicted by Hollywood) are.
Do your water heaters raise the temperature beyond boiling point in a sealed tank? Are you talking about a steam explosion?
Dave
They are indeed made of steel - that's so that they will rust out every few years and you have to keep buying new ones so the tank makers make more money - unlike in the UK where you have (or used to have) the intelligent idea of using copper for the tank.

JoeLee
12-30-2013, 08:50 PM
Yes, I did remove the leaky relief valve that was in the hot line. I made sure the one on the tank was working and I flushed it, it's not clogged or stuck. The one on the line had no flip lever, it was just a small brass valve with a flat plastic top. I saw no need to put another such valve in the line.

JL.....................
I see no problem, you removed and old, leaky pressure relief valve, but you left the tank-mounted T/P valve. That's the ONLY safety valve in most modern installations, so you didn't create any danger. I do agree with the necessity for the T/P valve, as a water heater CAN explode if the heat source doesn't shut off when it should.

gvasale
12-30-2013, 09:50 PM
Copper lined tanks are not common in the USA as far as I know. I've seen two in my liferime so far, 64 years. But then, I'm not a plummer either. The copper lining was not very thick. I cut them up for scrap, one a few months ago, the other probably 2 decades ago.

Many tanks are "stone lined" whatever that means. Now there are fiberglass lined tanks. I've not seen stainless steel.

JoeLee
12-30-2013, 09:58 PM
Fiberglass lined?? Do you mean fiberglass insulated??? How would fiberglass hold up to a flame???

JL.......................
Copper lined tanks are not common in the USA as far as I know. I've seen two in my liferime so far, 64 years. But then, I'm not a plummer either. The copper lining was not very thick. I cut them up for scrap, one a few months ago, the other probably 2 decades ago.

Many tanks are "stone lined" whatever that means. Now there are fiberglass lined tanks. I've not seen stainless steel.

jep24601
12-30-2013, 10:23 PM
Fiberglass lined?? Do you mean fiberglass insulated??? How would fiberglass hold up to a flame???.

I think it's resin impregnated fiberglass on the inside for corrosion protection.

US practice does not use storage tanks so entire domestic plumbing is under more pressure than with an open storage tank system. With an open storage tank for house pressure on the hot side (and most of the cold) you don't need a pressure relief valve because the open storage tank provides your pressure relief.

ironmonger
12-30-2013, 11:53 PM
There was a line of fiberglass tanked electric water heaters. Sounded like a good idea except:
http://www.thetankatwaterheaterrescue.com/forums/forum3/2845.html

The 'glass lined' tanks, for both gas and electric heaters, are porcelain lined, but the only thing keeping them from rusting out is the anode rod. A few heat cool cycles and the glass lining is compromised.

People didn't like the cost of the steel tank heaters, so the chance of selling them a water heater with a stainless steel was remote, never heard of one anyway. They might be out there though...

In our area with water hardness around 20+ grains I get about 10-12 years out of a water heater. No point buying the longer warranty, I just buy scratch and dents when I can. If I changed the anodes out every 5 years or so the tanks would last a lot longer... just lazy.

paul

J Tiers
12-31-2013, 12:03 AM
I would think that IF the tank is completely filled and no water is allowed in or out, that as the temp increases and the water expands, the tank would rupture and relieve the pressure. That should happen Long before the temp is high enough to flash to steam. Bob.

Should do, but then, boiling point is higher under pressure..... So when a hole develops, the pressure is released, and steam can be generated as the pressure drops.

What can happen is that the tank turns into a "water rocket", blasting off as it ejects water from the hole. Was very common with old vertical type boilers before certification.... the fire end tube sheet would develop a hole or crack, and off they went. They could travel for blocks.

boslab
12-31-2013, 07:23 AM
I think 2 PRVs are required in the UK, one on the tank, one on a pipe venting outside, the outside one blows first, if that fails the fusible tank one backs it up
Boilers have 2 also
Mark

gvasale
12-31-2013, 08:13 AM
For those able & interested, Rheem has the fiberglass heater of which I mentioned. Seen at the Home D. Orange Box store. It is of course, electric.

vincemulhollon
12-31-2013, 08:31 AM
Now that most houses have pressure reducing valves in the inlet water line from the street

Really? That must fail a lot and when it does, it must be a fabulous disaster, pop all the pipes in all the walls like overcooked bratwursts, completely ruined insulation and wallboard, then the mold sets in, etc. Total house loss, from one boring clogged open valve.

I'm not saying its impossible, people do a lot of stupid things with houses, I live in one with a partial flat roof, but still, geeze.

I've never lived in a house with a valve like that, so if a old fashioned tank failed "on" it would merely fill with steam at the same pressure as the supply water. I suspect given the poor conductivity of steam an electric element would quickly burn out in steam, but a gas one might overheat and crack the firetube, although then its not going to be an impressive boiling water explosion but a very boring low pressure steam leak. Followed by a most impressive flood and that probably followed by a natgas leak or electricity electrocution. Isn't obsolete technology like giant water tanks fun?

I've had tankless for more than a decade now and giant tanks as an idea are thankfully dying and going away, so its all a moot point. Never gonna have an old fashioned tank again.

loose nut
12-31-2013, 10:01 AM
Tanks may have to have relief valves but after 10 or so years how many of them still work right.

wmgeorge
12-31-2013, 10:09 AM
Really? That must fail a lot and when it does, it must be a fabulous disaster, pop all the pipes in all the walls like overcooked bratwursts, completely ruined insulation and wallboard, then the mold sets in, etc. Total house loss, from one boring clogged open valve.

Isn't obsolete technology like giant water tanks fun?

I've had tankless for more than a decade now and giant tanks as an idea are thankfully dying and going away, so its all a moot point. Never gonna have an old fashioned tank again.

I am not a plumber and have never played one on TV, but I did commercial HVAC work for about 25 years. Seldom will you find a PRV (pressure reducing valve) in the building water supply line, what you will find is a backflow preventer required now by Code in many locations. These valves have cleanable filter screens and internal check valves and in commercial buildings are required to be tested on a regular schedule. Modern plumbing installed to Code will usually not fail do to overpressure. Your dishwasher, washing machine and etc. will however.
Boilers have PRV's installed to limit the amount of pressure in the system, they also have relief valves in that water line feeding the boiler and of course the boiler relief valve. They are also required to have backflow preventers.

dave5605
12-31-2013, 01:28 PM
I would hazard to guess that pressure reducing valves and back flow preventers are as common as your municipal water supply is in moving forward with enhancements. I live in a 300 house development where the houses are about 14 years old and on the Raleigh municipal water system. Back in 1998 when the houses were built they had back flow preventers so you couldn't contaminate the municipal water system.

We also have adjustable pressure regulators on the line where it comes into the house because the municipal supply is probably in the 80-90 psi range and the regulator is at around 50 psi.

Lastly we have about a 5 gallon air bladder tank mounted right on the hot water output of the HW tank to compensate for water expansion as the water in the tank/system heats up.

We also have your everyday HW tank relief valve plumbed right into the side of the HW tank.

Pretty sure all 4 are code requirements.

To answer Daveb's question, no our tanks don't heat the water to boiling. On electric tanks there are adjustable surface sensors you can set to some reasonable temperature. Typically 120F or lower. How many you have depends on how many heating elements. Most tanks have 2. Real small tanks have one. Typically the elements are wired so only one can be on at any given time.

If the tank is heated with natural gas or propane then there is a dip tube in the side of the tank near the bottom that senses water temperature and controls the burner. Again, a typical setting might be 120F.

I don't think any of the mechanisms allow setting above about 160F.

Daveb
12-31-2013, 02:49 PM
Thanks for the info. It seems your domestic water is all mains fed. The older stuff over here uses a 50 gallon storage tank in the loft, there's a gravity feed from this to a hot water tank (50 Gallon copper cylinder) with coil from boiler and electric immersion heater. The hot water tank is vented to atmosphere by a vent pipe teed into the hot water take off, this drops any overflow back into the cold water tank. Drinking water only comes direct off the mains.
Newer systems use a combination boiler which runs the central heating system and provides hot water on demand, no hot water tank necessary with these but no back up if the boiler fails. Most new central heading system are sealed (closed circuit) and have expansion vessels and pressure relief valves. Pressure reducing valves work only when the water is flowing, they don't stop the pressure building up once the flow stops.
Dave

lakeside53
12-31-2013, 02:59 PM
If your pressure reducing valve is working correctly it will stop pressure from building up on the downstream side under no flow circumstances. If it's faulty due to a bad seal or dirty seat, then no.

There are "through hole" type pressure reducing units like those used on low pressure irrigation systems, but these reduce pressure by volume restriction and by inducing back pressure. None of these can be used in a situation where flow isn't continuous, such as a house/building.

My house has a diaphragm pressure regulator - pegged at 60psi, flow or not; Inlet is 93psi. If it allowed the 93 to pass, it would be pressuring my relief tank and everything else - bad news. The commercial buildings I deal with have 135-150psi in... no way that's allowed past the regulator valve.

I just replaced the seal in my Dad's pressure reducing valve in NZ (ha, vacation... son, can you fix xxxxx?). A tiny valve seat leak was raising downstream pressure in his tank so the hot side pressure relief valve was releasing water (dribbles) into the exit pipe and his garden. Their house is 12 years old - has a inlet pressure reducer, cold side pressure relief, expansion tank, hot side pressure relief AND hot side mixer to keep domestic hot water temp below 125F. And all they have is a hot water heater - no boiler. Way too complex... nanny state codes. Their old house (1960) had a cold water attic tank that fed the hot water cylinder. The HW cylinder had a vertical pipe that vented to the roof. If the thermostat stuck, there would be a geyser of boiling waters/team straight up from the roof. Saw it often in the city as kid.

JoeLee
12-31-2013, 04:07 PM
That's why you are supposed to check them yearly.

JL................
Tanks may have to have relief valves but after 10 or so years how many of them still work right.

Carm
12-31-2013, 04:13 PM
We also have your everyday HW tank relief valve plumbed right into the side of the HW tank.









Yep, code requirement, and even if not, the safest place for the T&PR valve is preferably mounted in the top of the tank, or very near the top on the side.
Years past, some would install the T&PR in the outlet piping. To meet code, the thermal expansion sensor (usually an extended plastic tab attached to the valve seat) must project INTO the tank.
People with acidic well water should replace the TP&R every couple years.

wmgeorge
12-31-2013, 07:33 PM
You will find very few PRV's installed in the flat land areas of the prairies, as here in Iowa. Most of our problems with water pressure deals with not enough. Any PRV will fail with time and should have a relief valve installed downstream like I noted for boiler piping above.