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Old 22-05-2009, 03:30   #31
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Originally Posted by s/v Jedi View Post
... In short: the checkvalve does not prevent explosion or enable the relief valve to work. The relief valve (thermal expansion valve) works on pressure.
... The only major accidents I am aware off are caused by propane water heaters...
Prior to the widespread use of BackFlow Prevention Devices (BFD), domestic water systems were considered to be "open (to the municipal water supply, not to the atmosphere); thus the expanded hot water had someplace to go - back into the city watermain, mixing with a massive amount of cold water volume where it was easily accommodated.

When a check-valve is installed, the backflow preventer becomes an effective ‘non-return’ barrier, making the system "closed". Expanded hot water cannot find its way back to the city main, so in periods of water heater recovery when there is no hot water drawn, expanded water volume can create a pressure increase, until the heater’s safety relief valve, usually set at 150 psi, pops and hot water drains through the relief valve drain.

Water heater tank explosions are rare, thanks to the widespread requirement for, and use of, pressure and temperature (PT) relief safety valves.

But if the safety valve has been damaged, clogged, modified, or even omitted; that condition, combined with overheating, can cause a "closed system" water tank to explode, creating a BLEVE (Boiling Liquid Vapour Explosion) that releases tremendous force, and causes extreme damage. This can occur with any fuel (electric, gas, etc).

Electric Hot Water Heater Explosions:
Avon High School Hot Water Heater Explosion – Final Report - MasterPlumbers.com PlumbNews
Four hurt as water heater explodes
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Old 22-05-2009, 06:58   #32
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“Also, checkvalves tend to fail over time”
Nick.
I can assure you that BFD do work and will prevent hundreds of water heaters elements from burning out when a water supply main burst as I indicated at Post 16. BFD will also test the operation of the PT relief safety valve.
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Old 22-05-2009, 09:23   #33
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Not sure how a hot water heater will explode, water doesnt store much energy like gas pressure. However, if your house has one of those big accumulator tanks with a bladder and air chamber... they can be very dangerous! (I have been involved in water pressure forming aircraft parts for over 20 years.... using up to 15000 psi in doing so, we break parts all the time, usually at the weld... it's a non event really..)
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Old 22-05-2009, 10:26   #34
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Not sure how a hot water heater will explode ...
Google "Boiling Liquid Expanding Vapour Explosion" (BLEVE).
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Old 22-05-2009, 10:51   #35
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Point taken... but it better not be boiling or your engine is in trouble....! and if it is the relief valve should take care of that right?
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Old 22-05-2009, 13:01   #36
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Good point Gord. But we're talking boats, right? On a boat, the system is always "closed" because of the check valve in the water pressure pump (every water pressure pump has one), ie. the water can't expand back into the water tank. You do not need a check valve on the waterheater for that.

For BLEVE to occur, the compressed water must have a temperature well above boiling point, ie. more than 212F / 100C. Only then it will boil off when pressure is relieved suddenly. Two safety measures must have failed to get there. The first one is the over-pressure relief valve. The second one is the thermostat in the waterheater. None of that has anything to do with a checkvalve mounted on the waterheater.

I understand all the confusion: ashore you do need that checkvalve.

The photo's on the web of BLEVE explosions are impressive but all are for compressed flammable substances like lpg, petrol etc. The balls of fire on the photo's are the ignited liquid, which is not likely to happen with water.

ciao!
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Old 22-05-2009, 13:35   #37
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Originally Posted by JiffyLube View Post
If a person does not use the shower for some time, and the shower line is not flushed because of the possible risk of infection, are you saying that the shower can now never be used again? I think that the shower line can be flushed with hot water, with the shower head directed 'into' the head sink in a matter that will not produce mist.
Jiffy,

Yep, that would be a waste of showerheads ;-) It's best to first flush the waterlines using the faucet that is "furthest away" from the pump/water heater and only after that flush the shower head. And indeed that can be done in such a way that no mist is produced.

Chala:

Quote:
I can assure you that BFD do work and will prevent hundreds of water heaters elements from burning out when a water supply main burst as I indicated at Post 16. BFD will also test the operation of the PT relief safety valve.
I agree with you but the situation aboard our boats is different from that ashore. Your water pressure pump doesn't allow backflow.

About testing the relief valve: that works without the extra check valve too as the one in the pump prevents backflow.

Many boat installation use so much hose that the relief valve doesn't drip when heating water with the water heater. All the hoses expand a bit and the expansion is absorbed that way. The only way to test the relief valve is by closing a manual ball valve on both cold input and hot output sides of the water heater with cold water inside and then switching it on.

Also, what I said is that check-valves fail to operate correctly after some time. Some can be taken apart and cleaned but I don't think any cruiser is doing that on the water-heater. The percentage of installed check-valves on water-heaters that will not prevent backflow will be high.

cheers,
Nick.
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Old 22-05-2009, 15:50   #38
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Jedi - I installed a shut off after the check valve, so I am now double covered! As for Legionella...I wonder how bleach affects it? Quite a few cruisers bleach their water on a regular basis...
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Old 22-05-2009, 18:53   #39
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Wow exploding water heater! What kind of tubing are you folks using!? Having just experienced a failure of my tubing on the hotwater side, most likely due to excess temperature and pressure, and having to try to find replacement tubing, I'd love to know what you folks are using? The stuff i used to replace it with is rated to about 200 degrees and 150 psi. It is PVC I believe.
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Old 22-05-2009, 19:33   #40
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Strygaldwir,

Search the forum for PEX tubing and fittings. (I don't have that yet but believe it is the best option out there).

I also had hot water hoses fail. They were the ones like for an exhaust, with steel spiral. Before they burst, they formed a bulge and I'm sure it was caused by their old age.

cheers,
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Old 22-05-2009, 19:39   #41
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Jedi - I installed a shut off after the check valve, so I am now double covered! As for Legionella...I wonder how bleach affects it? Quite a few cruisers bleach their water on a regular basis...
Chris,

I think that it depends on concentration but have no clue. However, the water supply in Holland and the US is bleached and still people die so it must be more resistant to it than other bacteria. Also, bleach evaporates out of standing water and I don't think that stainless steel components like tanks & water heaters like that...

cheers,
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Old 23-05-2009, 02:55   #42
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Quote:
Originally Posted by s/v Jedi View Post
... Two safety measures must have failed to get there. The first one is the over-pressure relief valve. The second one is the thermostat in the waterheater. None of that has anything to do with a checkvalve mounted on the waterheater.

I understand all the confusion: ashore you do need that checkvalve...
The check valve (or backflow preventor) prevents the expanding water from backing into the municipal water supply, or the boat’s storage tank, thereby relieving pressure.

In both cases, the check valve has complicated the situation when/if both the PT relief valve and thermostatic control fail.
I see no need for the check valve, at the HWT, on a boat.
A backflow preventer, and pressure reducer, are required at the deck fill, if the boat is permanently connected to a shoreside water supply.
Eventually, mine was connected to city water, as Maggie was in the habit of draining the water tank and burning up electric HWT elements, when moored shoreside.


Water heaters do fail explosively, especially larger tanks; which are generally specified to be ASME pressure vessels. The smaller tanks, we use on boats, will be much less susceptible to BLEVEs.

See the previously-linked (post #31) reports:
Avon High School Hot Water Heater Explosion – Final Report - MasterPlumbers.com PlumbNews
Four hurt as water heater explodes

and:
Water Heater Blast!
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Old 23-05-2009, 03:32   #43
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...I wonder how bleach affects it? Quite a few cruisers bleach their water on a regular basis...
Not adequately.

As with all microbiological organisms, Legionella bacteria require nutrients and optimum water quality to proliferate. While water temperature is an important factor in bacteria growth, other conditions must also exist. These include the presence of nutrients, sediment and other micro-organisms (particularly protozoa amoeba and/or algae) in the water. These "dirty" systems often provide favourable growth conditions of all types of bacteria, including Legionella. Our “normal” practices, that maintain the aesthetic quality of our drinking water, should mitigate against these factors.

Hyperchlorination of potable water has been used to mitigate Legionella colonization; but is not a practical solution for most applications (including ours). “Normal” chlorination is not an effective method of disinfection against Legionella.

See the US EPA Advisory:
http://www.epa.gov/waterscience/crit...gionellaha.pdf

Marine Domestic Hot Water Systems (my inexpert: advice)
• Domestic hot water systems should be operated and maintained at minimum temperatures above 55º C (131 ?F). See my post #22).
• Regularly flush hot/warm water pipe lines (e.g. showers, taps) to prevent stagnant water. Eliminate “dead end” hot water legs.
• Regularly clean shower heads and faucet aerators.
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Old 23-05-2009, 06:16   #44
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Gord:

I absolutely believe the pressure in a hot water can make a water heater explode! My thought was, for that to happen the pressure inside the system is going to find the weakest point and cause that to fail. The implication being that if there is no valve that is keeping the pressure from escaping to the hose, then the hose has to be more pressure resistant than the tank. In homes and industrial settings they use I think at a minimum schedule 40 PVC pipe that has a high minimum burst pressure of around 1400 psi (see PVC Pipes - Pressure Ratings). Indeed, most of the systems I have seen have used copper pipe on the inside hotwater systems. So, that led me to the question of what kind of tubing are folks using inside their boats? My tubing burst at a tee junction about 2 feet from the tank. Since it is routed in it own conduit, the hot water started coming out under the galley sink! Scared my wife silly! I know I have an operating pressure releif valve on the tank, and it is rated at 150 psi, so the implication is the tubing burst at a pressure less than that.
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Old 23-05-2009, 06:43   #45
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Imagine was built with a unique hot water application to the shower. She was built with no shore power, and no hot water heater. The builder meant her to be on the hook, and self suffecient for energy.

Under tha vanity is a 5gl tank that can be filled from the faucet on the vanity. The faucet will swing far enough to reach a funnel sitting over the shower tank. With the faucet you fill the shower tank, and if you want a warm to hot shower you use the tea kettle to add hot water. It's not the prettiest solution, but it's not ugly either. It's simplicity is what I admire.........i2f

P.S.

For you mono guys. That toothbrush holder stay put in all weather....hehehehehehehe
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