Ventuiri effect on waterheater

[MasterPlumbersKeymaster]

I have a 40 gal elc. Wh located in a basement. The main water supply line from the meter comes in about half way up the wh and is a 1 in PVC this pipe turns up and just above the WH there is a Tee to 3/4 in flex pipe to the Wh. The 1 in pipe extends upward 2 ft. then feeds all the cold water to the house. I thank that when there is major usage of cold water I am pulling hot water out of the Wh. Is this possible/

[fourth yearParticipant]

No. The only way to pull water out of the water heater would be to let air into the system to break the vacuum. But since it is a pressure system there would not be a vacuum in the first place, since water would flow out of any open faucet rather than letting air into the system.

[rymojoParticipant]

What is happening here, if you are getting a “slug” of hot water out of the cold water side of one or more fixtures in the house is probably this: When the heater is filled with cold or mostly cold water and begins a heating cycle, the water, as it expands, backs up into the cold water inlet piping, probably towards the meter. This places a “slug” of heated water in the cold water piping system and it will be carried along by the flow of cold water to a cold outlet by the first usage of cold water following a heating cycle. Test this hypothesis so: After a large usage of hot water( a couple of showers or loads of clothes), warn everyone in the house not to use any water for an hour. Feel the cold water piping going to the heater. As the heater recovers (heats up) you should be able to feel the cold inlet piping begin to heat up further and further away from the heater. If it gets hot upstream of the tee on the PVC inlet main, I am right. (Remember that PVC loses its strength rapidly at elevated temperatures and that if the inlet main temps are exceeding 110F an alternative method of handling this expansion should be addressed.

[SylvanLMPParticipant]

Yes, it IS HIGHLY POSSIBLE. There is a condition called cavitation like when installing a water meter too close to the inlet piping of the house main where under certain conditions even when no water is being used the meter will show movement.

For example when installing water mains too close to the city main water then actually pull out of the home VENTUI effect, impossible to FREEZE these mains as water is constantly moving,compounding this problem is HEAT always go to cold (absence of heat in refrigeration terms)

So yes it is possible to DRAW water out of this tank quite easily.

Now about PVC YES it can handle water up to 140 Degrees “DEPENDING on pressure” according to the thermoplastic piping system specifications guide BUT hey who bothers to read codes as it takes time to LEARN to do it properly huh?

What you can do to make sure you don’t have any back feed as we call it in the big city

“stratification of hot water molecules into the cold water piping”

is to install a 27″ heat sink.

What we big city plumbers have found is hot water will seek to move to the colder water BUT it cannot go DOWN more then 27″

So to be sure you can drop a loop to 30″ on your cold water supply to your tank. Like a big U shape.

(I just may write a future article on the proper way to install a water heater).

By putting in this loop you cannot get stratification Or the venturi effect.

Could you imagine all that CRAP PVC failing all over the country everytime a dishwasher drains USING HOT WATER. Hey how about when folks let the “hot water” run down a PVC traped sink does it disform or melt?

Oh well Hey Have a great Holiday and consider all your options like calling in a Licensed Master Plumber and double check he is licensed and NOT a helper wanna be for your protection.

[Harold KestenholzParticipant]

Consider yet another possibility. Air enters a system with the water as air is always part of water (about 5% by volume.) As bubbles form when water warms a bit, small air pockets form in several parts of the hot water side of the system. When the water pressure drops in the cold water side while cold water taps are open, the pressurized bubbles expand as the water from the water heater/hot water side flows back into the cold water inlet (even through a gravity loop.)

Isn’t this fun? Three out of four possibilities to choose from. (And a ball check and expansion tank to install.)

On the first day of Christmas, my true love gave to …

[SylvanLMPParticipant]

NO WAY, NO HOW Do I like using a “check valve” on pressure vessel as it is not really a great thing to do.

Considering the fact a pressure vessel has only ONE SAFETY device and ONE operating controller unlike a boiler that has back up systems why take a chance of an explosion?

An expansion tank can take up for the expansion of the heated water when the incoming main pressure increases to the point the T&P discharges and some means of protection is warranted.

The problem today is the use of back flow preventers placed all over the place and thus not allowing for the normal pressure build up of these vessels to move harmlessly into the system.

It always made me curious as to why the ASME allowed the T&P to be rated to a pressure of 150 PSI knowing most plumbing fittings are rated (WHEN NEW) to 125 PSI what is wrong with this picture?

The heat sinks do work and will not restrict pressure release of the tank.

To solely depend on a T&P valve which cost about $4 to protect life and property and also knowing these devices are installed and forgotten until the tank has to be replaced is not what I would consider prudent plumbing.

SylvanLMP Certified Low pressure boiler and unfired pressure vessel inspector ASME section IV

[Harold KestenholzParticipant]

Sylvan, you are right about that. One wonders why anyone would install a check valve, or other devices like a pressure reducing valve or backflow preventer on a plumbing system without installing an expansion tank on the ‘pressure vessel’ side of it. After all, when you put a backflow preventer on a plumbing system, therefore isolating it from the water mains, you are making the entire system a pressure vessel. To do so without giving the water someplace to expand, you are just asking for excessive pressure in that system.

Although there is some air in the incoming water (about 5% by volume) and the water will expand about 3% through heating it from 50 to 120F, this is not enough safety. After heating water, some of the bubbles leave the water. This can be evidenced by the spurting and release of air upon opening some taps. So the whole ‘pressurized vessel’ of the water system bottled up by a backflow preventer can then cause an excessive pressure as the heated water expands into the cold water side as well.

So every time you install a backflow preventer, double check, or pressure reducing valve separating the domestic system from the main you are creating a possible problem that the T&P valve has to handle alone, unless you add an expansion valve.

Adding another check before the water heater after already causing an isolation with a backflow preventer simply isolates the problem expansion to the hot water side, which should have an expansion space to prevent lifting the T&P as the water is heated.

This is, after all, the purpose of an expansion tank in a modern hot water system, which is always a sealed system isolated from the cold water system by a pressure reducing valve, check valve, or backflow preventer, unless it is totally isolated by hose fill.

[SylvanLMPParticipant]

Harold Kestenholz wrote on 23 December 2000 at 07:44 PM:
Sylvan, you are right about that. One wonders why anyone would install a check valve, or other devices like a pressure reducing valve or backflow preventer on a plumbing system without installing an expansion tank on the ‘pressure vessel’ side of it Answer. Cause they are DUMMIES who never bothered to READ codes or bothered to ask WHY..they see some stumblebum do it and the apprentice thinks its right.

After all, when you put a backflow preventer on a plumbing system, therefore isolating it from the water mains, you are making the entire system a pressure vessel.

Including the fact that the T&P is rated to discharge @ 150 PSI and the normal rating of “plumbing” fittings is rated at 125 PSI whats wrong with this picture?

To do so without giving the water someplace to expand, you are just asking for excessive pressure in that system.
Although there is some air in the incoming water (about 5% by volume) and the water will expand about 3% through heating it from 50 to 120F, this is not enough safety.

Exactly.. Also as the temperature increases so does the pressure.
The problem is most piping guys THINK like “plumbers” and should try to learn the basics of heating to get an idea of whats happening in the hot water sections of “plumbing” A whole new science awaits them.

After heating water, some of the bubbles leave the water. This can be evidenced by the spurting and release of air upon opening some taps.

BUT these “taps” are well above the sensor of the T&P valve and thus the pressure can rise without this T&P even knowing it PLUS it can FLASH into steam if the temperature is high enough. It does happen unfortunately and there is no valid reason it should occur

So the whole ‘pressurized vessel’ of the water system bottled up by a backflow preventer can then cause an excessive pressure as the heated water expands into the cold water side as well. , YES

How many folks actually test or inspect the combination temperature pressure valves as directed by ASME?

INSTALL and forget is the name of this game.
At least boiler guys have common sense and common decency to test safety and relief valves monthly if not more often during the heating season. But then again MOST licensed Heating professionals CAN READ and comprehend gas fitting and heating codes.

So every time you install a backflow preventer, double check, or pressure reducing valve separating the domestic system from the main you are creating a possible problem that the T&P valve has to handle alone, unless you add an expansion valve.

Ever see a “plumber” bother to install a compound gauge to know exactly what is going on in the system?

As a stationary engineer and heating guy I learned a long time ago to know what my delta is especially dealing with pressure and temperature relationships, most stumble bums could careless about thermo stress to pressure vessels and piping

Adding another check before the water heater after already causing an isolation with a backflow preventer simply isolates the problem expansion to the hot water side, which should have an expansion space to prevent lifting the T&P as the water is heated.

This is, after all, the purpose of an expansion tank in a modern hot water system, which is always a sealed system isolated from the cold water system by a pressure reducing valve, check valve, or backflow preventer, unless it is totally isolated by hose fill.

Harold you ht the nail on the head the aprenticeship training seems to be lacking in several areas.

The plumbing publications I have read lately deal with how to price a plumbing job and commissions BUT very little written about codes or WHY we have to install devices for vacuum and pressure protection.
Harold always a pleasure talking to a real professional.. Thank you again and have a GREAT healthy holiday.. Sylvan

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