Water heater back pressure?

[Jmcc64]

My wife works in a dental office, built around 1980. Still original plumbing for the most part. Because there are so many fixtures, the previous owner installed a master water shut off in the mechanical room and the supply is cut off there every night. When the water is shut off I notice that a LOT of water will still come from the faucet (hot and cold) and it appears that water is coming back through the cold water supply pipe to the water heater. (it's warm to the touch). In other words, the water heater is draining into the system in the absence of pressure from the street.
Is this normal? If not, what needs to be done to correct it? The heater is brand new. (about the only thing in the office.smh.)

 

[Breplum]

seemingly, will have to be tracked down through sleuthing. likely, the shutoff doesn't cover every fixture because a true shutoff kills supply. or the shutoff is defective.
and, maybe there is a mixing fixture that is bleeding back through.

 

[Sponsor]

 

[Reach4]

(it's warm to the touch).

That is the thing that confuses me. With a more common crossover at a fixture, I would not expect warm water.

In looking for a crossover, pipes that have been sitting without flow for a while will be the same temperature as the environment. If you find a supply line to the hot side that is not the same temperature as the environment, that line would be would be carrying water.

An IR thermometer could be useful at comparing temperatures. That need not be expensive.

 

[Jmcc64]

seemingly, will have to be tracked down through sleuthing. likely, the shutoff doesn't cover every fixture because a true shutoff kills supply. or the shutoff is defective.
and, maybe there is a mixing fixture that is bleeding back through.

No, I think it's a true shutoff. Previously when it was off, there was no flow at any fixture in the office. nothing. Is there enough pressure built up in the tank to push hot water back up through the cold water line(at the WH)? that's what appears to be happening.

 

[Reach4]

Is there enough pressure built up in the tank to push hot water back up through the cold water line(at the WH)?

No, at least past the first second.

 

[Jmcc64]

No, at least past the first second.

please explain

 

[Reach4]

please explain

Any tiny pressure would be released once you opened a hot faucet. If the flow continues, that would seem to indicate water crossing over at a fixture from the cold side.

 

[Jmcc64]

Any tiny pressure would be released once you opened a hot faucet. If the flow continues, that would seem to indicate water crossing over at a fixture from the cold side.

Would that explain the cold water inlet pipe to the WH getting very warm when main water valve is closed? and a faucet is opened?

 

[Reach4]

Would that explain the cold water inlet pipe to the WH getting very warm when main water valve is closed? and a faucet is opened?

Is that a hot water faucet, or cold? And if this is a single handle faucet, does it matter if the control is selecting hot or cold?

 

[Jmcc64]

Is that a hot water faucet, or cold? And if this is a single handle faucet, does it matter if the control is selecting hot or cold?

double. turn on cold and it runs, eventually slowing down, little by little. so I go to to the WH and the cold pipe is now warm

 

[Reach4]

I had misread. I had thought that you were turning off the supply to just the WH intake, and you were valving off the water to the whole office. So what I wrote was not relevant.

 

[Fitter30]

Has to be a cross connect. Probably a devise that has both hot and cold comes out mixed to ? Worked in small one dentist offices in the 70-80s not on the plumbing. Dental equipment was connected by companies that sold it and how piping and wiring was run wasn't a priority.

 

[Jmcc64]

I don't mean to be thick headed about this but I'm having trouble wrapping my head around the possible causes. See attached very crude diagram of the situation. Is it possible that the weight of the water in the heater is pushing the heated water back out the inlet line in the absence of pressure from the street? That doesn't seem possible; wouldn't the heater need to have an atmospheric opening for that to work? The previous heater had top inlet/outlets. I'm pretty sure the whole building shut off valve is working fine. It was last week. If this scenario is not possible, how would the aforementioned crossover allow it to happen?

 

[Reach4]

I suspect the valves at the filter have to do with being able to isolate or bypass the filter. So I expect your diagram is a little off at A. Bypass takes 3 valves. If you close both valves near the filter, you are going to get the building plumbing flowing back when you change cartridges. If the filter clogs, that "ancient" could be opened to provide water. I expect that valve is normally closed (off).

If you shut the water off from the city successfully, I expect flow is gravity driven. So how do you not develop a vacuum if that is the case? I can think of a couple ways, but the easy one would be if there was a vacuum relief valve installed.

Is the high-boy water heater on the same floor as the faucets involved.

 

[Jmcc64]

I suspect the valves at the filter have to do with being able to isolate or bypass the filter. So I expect your diagram is a little off at A. Bypass takes 3 valves. If you close both valves near the filter, you are going to get the building plumbing flowing back when you change cartridges. If the filter clogs, that "ancient" could be opened to provide water. I expect that valve is normally closed (off).

If you shut the water off from the city successfully, I expect flow is gravity driven. So how do you not develop a vacuum if that is the case? I can think of a couple ways, but the easy one would be if there was a vacuum relief valve installed.

Is the high-boy water heater on the same floor as the faucets involved.

yes. all same floor. I'd like to say the gate valve is never used. (Has no handle). But I'm not 100% sure. Also, it's a Low boy heater.

 

[Reach4]

yes. all same floor. I'd like to say the gate valve is never used. (Has no handle). But I'm not 100% sure. Also, it's a Low boy heater.

Low boy heater at floor level? That would limit the amount of water above the level of the faucet.

 

[John Gayewski]

No, I think it's a true shutoff. Previously when it was off, there was no flow at any fixture in the office. nothing. Is there enough pressure built up in the tank to push hot water back up through the cold water line(at the WH)? that's what appears to be happening.

No.

Is there an expansion tank in the system?

 

[John Gayewski]

With no pressure on the system and the system full the water can flow through it with symphonic action. Depending on what you open first and where a vacuum is drawn and what you open second and it's elevation you can theoretically flow all of the water out of a system or certain parts of it. I try to drain stuff this way when I can and your can get hot water crossover through any faucet.

 

[Jmcc64]

No.

Is there an expansion tank in the system?

no. no vacuum relief valve either. I think code requires one for bottom inlet WH's?? Honestly there is not enough room for an ET. This is under counter; no room above or sides.

see attached amended schematic.