How fast does corrosive water eat metals?

[TreeFiter]

A customer claims that their heater needed to be replaced because the heat exchanger went bad. The installer claimed that it was due to improper chemistry. I personally handle this pool, and the chemistry has been maintained within the acceptable ranges since I started caring for this pool. I'm willing to accept that the chemistry was not perfect at all times, but it certainly wasn't too far off.

This customer has been a pain in the butt from the beginning, and is just one of those people that makes you feel like you need a shower after talking to them. In the past, there was a problem with his DE filter. The pool builder told him that the grids broke because someone vacuumed with the filter on backwash. This of course is ridiculous. What good would a pool filter be that didn't allow for vacuuming out algae. When this happened, I contacted the manufacturer, and they confirmed that vacuuming on backwash was perfectly acceptable. So I suspect that this is a case of a crappy pool builder trying to blame everything on the company maintaining the pool to get out of having to warranty their installations.

So how quickly will corrosive conditions eat through a heat exchanger? Just how corrosive would the water need to be to do it in only a few years? It seems to me that the pH would have to be pretty low all the time to eat away enough copper to cause a failure in only a few years.

 

[JamesW]

You should never vacuum on backwash because that sends debris inside the grids and can ruin them.

Corrosion can be tricky. In addition to pH, there are other factors. What details can you provide about chemistry, equipment, maintenance, etc?

 

[Richard320]

Maintain your chemicals correctly

 

[TreeFiter]

JamesW, I have to disagree with you about vaccing on backwash. For that matter, so would the tech support representative at the manufacturer of the filter in question (Pentair). The only thing that might be an issue is that there is no DE coating the grids, and fine particles could clog up the fabric. This is rather unlikely since the flow is directed out the waste line. Also if this were the case, backwashing itself would be just as risky.

As for the water chemistry. I don't have exact numbers, as we are talking about a span of several years. This is a vinyl liner pool, DE Filter, Intellichlor SWG, all Pentair equipment, which means it comes with the Pentair Warranty. The pool builder holds customers hostage by only guaranteeing their plumbing if the customer uses them to open and close the pool. So the pool is opened and closed by the pool builder, but the company I work for is hired to do weekly maintenance. Typically in my pools, I maintain pH on the high side, 7.6-7.8 with Total Alkalinity somewhere between 80 and 100. I keep my CYA around 60 in salt water pools with salt concentration right around 3000ppm. In vinyl liner pools I don't worry too much about calcium hardness, but they are typically above 100, many of them above 200. The pool is kept at roughly 84 degrees. I've been maintaining pools this way for over 10 years, so if my methods were conducive to dissolving heat exchangers, I suspect I'd see problems more often.

A few things that came to mind as I thought about this situation:
Wouldn't the salt cell be damaged by corrosive water along with the heater?
If the water has been dissolving significant amounts of copper, shouldn't I see signs of that, such as staining?

- - - Updated - - -

Maintain your chemicals correctly

I remember seeing this thread a while back. This situation makes sense. With a pH of 5.0 for an extended period of time, it seems pretty obvious there is a problem. I never let my pH go lower than 7.0 without correcting it. If anything, it tends to run too high.

 

[JamesW]

When the valve is on backwash, the water goes inside the grids and then out to waste. I think that what you're intending to do is vacuum to waste. Multiports have a waste setting, which can be used for vacuuming to waste.

If you're regularly vacuuming to waste, I would recommend getting a pump that you can use for that purpose.

People usually vacuum to waste when there is a lot of debris, like algae etc that they don't want to clog up the filter. Vacuuming while on backwash would send all of the debris, like algae etc inside the grids. That defeats the purpose of vacuuming to waste.

I'm quite sure that if you contacted any manufacturer, they would not agree that vacuuming while on backwash is a good idea. While an employee might have said that, they were wrong. What a single employee might say does not necessarily mean that it's the position of the manufacturer. I'm sure that if you emailed Pentair with the question, they would respond that you should not vacuum while on backwash.

The heater issue is more difficult and complex. There are many factors involved in determining why a heat exchanger failed. Some possible causes.

1) Stray currents causing loss of electrons from the copper.
2) Excessive flow rates.
3) Turbulence in the exchanger
4) In some cases, cavitation.
5) Condensation.
6) Pool chemistry in general
7) Freeze damage
8) Chemical exposure from localized sources, such as from a nearby feeder allowing excessive chlorine attack the copper.

I would suggest removing the exchanger for analysis including contacting Pentair for their opinion.

 

[chem geek]

Unless the water chemistry wasn't maintained well, in particular the pH, usually the most common corrosion for copper heat exchangers is from the outside where condensation that may be acidic from escaped flue gases is corrosive. This happens the most if one heats in shorter bursts in cooler weather. It also depends on the type of heater where ironically the less efficient heaters had less of a problem because they would tend to heat steam away through the exhaust flue, but efficient gas heaters have much cooler waste gas output and humid air is more likely to condense. Some designs try to handle that.

So the first question is whether the corrosion was from the outside-in or from the inside-out. The second question is whether the pool was a salt pool -- did it have higher salt levels. The third question is whether this customer used Trichlor pucks in their skimmer or did they have an in-line chlorinator that perhaps did not have a working check valve (so concentrated chlorine flowed back into the heater when the pump was off).

 

[TreeFiter]

When the valve is on backwash, the water goes inside the grids and then out to waste. I think that what you're intending to do is vacuum to waste. Multiports have a waste setting, which can be used for vacuuming to waste.

If you're regularly vacuuming to waste, I would recommend getting a pump that you can use for that purpose.

People usually vacuum to waste when there is a lot of debris, like algae etc that they don't want to clog up the filter. Vacuuming while on backwash would send all of the debris, like algae etc inside the grids. That defeats the purpose of vacuuming to waste.

I'm quite sure that if you contacted any manufacturer, they would not agree that vacuuming while on backwash is a good idea. While an employee might have said that, they were wrong. What a single employee might say does not necessarily mean that it's the position of the manufacturer. I'm sure that if you emailed Pentair with the question, they would respond that you should not vacuum while on backwash.

The heater issue is more difficult and complex. There are many factors involved in determining why a heat exchanger failed. Some possible causes.

1) Stray currents causing loss of electrons from the copper.
2) Excessive flow rates.
3) Turbulence in the exchanger
4) In some cases, cavitation.
5) Condensation.
6) Pool chemistry in general
7) Freeze damage
8) Chemical exposure from localized sources, such as from a nearby feeder allowing excessive chlorine attack the copper.

I would suggest removing the exchanger for analysis including contacting Pentair for their opinion.

JamesW, with regard to the DE filter, you are right about where the water is directed, but keep in mind that a DE filter gets its ability to filter from the DE. The grids are only there to hold the DE in place. The water containing fine debris and algae will pass through the grids, because during a backwash, there is no DE on them. Larger particles are caught by the pump basket. There may be a small amount of particles small enough to get past the pump basket, but too large to fit through the grids that could begin to clog up the grids, but would also be pushed back to the pool when the system is returned to normal operation. The damage in question was mechanical. There were broken grids. This shouldn't happen due to backwashing. The installer claimed that the "debris" smashed the grids. As I already mentioned, the "debris" had to be smaller than the holes in the pump basket. It just doesn't make sense. I honestly believe this was a situation in which the installer was trying to avoid taking responsibility for problems with their installation. And doubtful as you may be, I did contact Pentair about this particular issue, and I discussed in detail how the filter worked, and if it made any sense. The pentair tech support rep agreed with me.

I appreciate the list of possible causes for the damage to the heater. They all make perfect sense to me. I suspect it could have been freeze damage, where the exchanger was stretched out and made weak, but didn't quite burst. It fits my suspicion that the installer has a tendency to try to blame me whenever their equipment has a problem. Unfortunately, we were only told about the heater problem after it had been replaced, and never even had a chance to look at it.

 

[swimcmp]

Tree I think whoever you talked to at Pentair misunderstood what you were saying. A de filter on backwash would put your debris into the inside of the grid, then when you go back to filter it would push that debris out of the grid back into the pool. That accomplishes nothing. If you are vacuuming on backwash that is a no-no. Look at your owners manual I do believe it says not to use the filter in that manner. Vacuuming to waste or rinse would be the correct manner. We had a customer that failed to maintain water chemistry many many years ago and in less than 45 days destroyed a copper heat exchanger.

 

[TreeFiter]

Tree I think whoever you talked to at Pentair misunderstood what you were saying. A de filter on backwash would put your debris into the inside of the grid, then when you go back to filter it would push that debris out of the grid back into the pool. That accomplishes nothing. If you are vacuuming on backwash that is a no-no. Look at your owners manual I do believe it says not to use the filter in that manner. Vacuuming to waste or rinse would be the correct manner. We had a customer that failed to maintain water chemistry many many years ago and in less than 45 days destroyed a copper heat exchanger.

It is possible that the owner's manual says not to vac on backwash, but it isn't because it will result in catastrophic failure of the grids. Yes, whatever is picked up while vacuuming will come into the grids, but the grids themselves are not intended to prevent fine particles and algae from passing through. That is the purpose of the DE. Yes some of the larger particles vacuumed will be blown back into the pool, but the majority of what you would be vacuuming to waste will pass through the grids.

How could a manufacturer justify a product that is only useful if a pool never has algae issues? How could a pool builder/installer justify selling a filter that does not allow for vacuuming out algae?

When I spoke with the Pentair Tech Support Rep, he did say that it wasn't recommended, but the only harm would be that there is a chance of clogging up the grid fabric.

 

[gilbee]

It is possible that the owner's manual says not to vac on backwash, but it isn't because it will result in catastrophic failure of the grids. Yes, whatever is picked up while vacuuming will come into the grids, but the grids themselves are not intended to prevent fine particles and algae from passing through. That is the purpose of the DE. Yes some of the larger particles vacuumed will be blown back into the pool, but the majority of what you would be vacuuming to waste will pass through the grids.

How could a manufacturer justify a product that is only useful if a pool never has algae issues? How could a pool builder/installer justify selling a filter that does not allow for vacuuming out algae?

When I spoke with the Pentair Tech Support Rep, he did say that it wasn't recommended, but the only harm would be that there is a chance of clogging up the grid fabric.

I don't understand what the hang up is here...why would you even WANT to vacuum to backwash instead of to waste? If you wanted to incorporate a backwash into it, it would make more sense to vacuum to filter, then backwash after (so contaminates stay on the same side of the grid), but that just puts things into the filter media that could otherwise just be shot out the waste pipe without going there in the first place.

 

[JoyfulNoise]

TreeFiter,

Not trying to pile on but you should never vacuum on backwash. If you open up a DE filter after back washing it, most of the grids are still covered with DE. Back washing, even for long periods of time, only washes off maybe, maybe 50% of the DE. That is why when you recharge the DE, you only use about half the amount of DE as you would use after a full tear down and cleaning.

So, if half your grids are still coated, you can easily clog the filter's mesh with debris that then may not be removable once the filter is put back into normal operation.

Sorry, but that Pentair tech you spoke to was dead wrong. You can only vacuum to waste when you have the proper multiport valve, not when it's the standard push/pull valve.

As for the corrosion issues, others have answered that better than me.


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