Calling All Plumbers

[Raspy]

Transit,

There are a couple of ways that air can get into a closed loop system. One is if an auto air vent is located on the suction side of the pump. Then if the system static pressure is lower than the pump head pressure, you will have a partial vacuum at the vent and air will come in through it. Another way is if the system is "spitting". In this case the system pressure at the pressure relief valve is getting higher than its set point. This happens through the accumulation of pump head pressure, system static pressure and heat expansion. Then the relief valve leaks enough to lower the pressure while the system is running and hot. When the system turns off and cools the pressure lowers enough to allow the fill valve to let in more water. With this water comes air that accumulates and becomes a problem over time.

Don't put a vent on the suction side of the pump. Even back along the piping is bad for this. And don't have one mounted high up on the second or third floor unless you can close it off securely.

One post mentioned cavitation as a source of air and that is not correct. Cavitation does not create air. It's just turbulence in the impeller because the pressure is too low for the impeller design.

Always consider the "zero pressure point" when considering system design. This is where the expansion tank tees into the system. If it is on the pressure side of the pump, the suction side pressure will drop when the pump is running. If it's on the suction side of the pump, the outlet side of the pump will go up when the pump runs. This affects the pressure relief, the fill system, the venting and the size of the bubbles at various points, at startup. It also affects the boiler by changing the boiling point of the water in it.

As far as expansion tank rusting is concerned, ANY system that uses non-metalic tubing, such as PEX and barrier PEX, will have oxygen in it to a certain degree. Not air, but molecular oxygen. This oxygen will cause rust at the steel or iron parts in the system. This rusting process will cause expansion tank failure eventually. A micro bubble air vent, like the one shown in a previous photo, is useless for molecular oxygen and will not prevent expansion tank failure. My experience is that this type of vent is a waste of money and should be left out. A simple float type auto vent can cost about $5.00 and do every bit as good a job.

A good, non toxic preservative should always be used in systems with non metallic tubing and any iron or steel parts. And with it, iron pumps, iron flanges, steel tanks, iron boilers, iron air scoops and iron primary piping are all fine.

Oh, and by the way. A hydronics contractor is not a plumber and a plumber is not a hydronics contractor. I'm a hydronics guy, but I know very little about the fine job plumbers do in their field. Mostly, I've found that they know very little about hydronics. Separate fields that use different equipment for different purposes, that just happen to both be concerned with liquid flow in pipes.

 

[rScotty]

Transit,
There are a couple of ways that air can get into a closed loop system. One is if an auto air vent is located on the suction side of the pump.....
When the system turns off and cools the pressure lowers enough to allow the fill valve to let in more water. With this water comes air that accumulates and becomes a problem over time......
One post mentioned cavitation as a source of air and that is not correct. Cavitation does not create air. It's just turbulence in the impeller because the pressure is too low for the impeller design......
As far as expansion tank rusting is concerned, ANY system that uses non-metalic tubing, such as PEX and barrier PEX, will have oxygen in it to a certain degree.....
My experience is that this type of vent is a waste of money and should be left out. A simple float type auto vent can cost about $5.00 and do every bit as good a job......
A good, non toxic preservative should always be used in systems with non metallic tubing and any iron or steel parts. And with it, iron pumps, iron flanges, steel tanks, iron boilers, iron air scoops and iron primary piping are all fine.....

Now that's a really great posting! Thanks for the info. But now I'm full of questions! BTW, I've taken the liberty of snipping the quoted message to just the things I've got questions on. Agree on the hydronics/plumbing thing in general - but they aren't always separate. Locally our best hydronics guy is a plumber as well. Just a naturally gifted guy. And then there are some who shouldn't be doing either one.

Ok, to the questions....Is it common in the hydronics industry for a system with autovents and fill valves to be considered a "closed system"? If so, my previous postings might be misconstrued, as my system is more "closed" than that. In particular I don't have any type of fill valve because of not wanting to have an entry point for an unknown amount of gases and minerals into my system.

That's a nice shortcut way of saying that "cavitation" doesn't create air, it just forms low pressure bubbles which collapse back into the fluid stream as soon as they get away from the impeller. At least that's how I say it, but your definition is more elegant. Those bubbles can be noisy and damaging while they are forming and collapsing. What I can't figure out is that since the previous poster who mentioned cavitation in the first place was getting the cavitation before he replaced his air valve and not afterward then what does that mean?

There's also a type of Pex made with the inner and outer layers separated by a heavy metal shield....almost like a thin metallic pipe itself. Does that kind of Pex stop molecular oxygen any better than plain Pex or barrier Pex? And what is it about metal pipe that stops molecular oxygen anyway?

Can you elaborate a bit on the spirovent versus the float vent design? I have very little experience with hydronics and confess that the way the spirovent in my system works does bothers me a little bit. Apparently it is doing its job well, but how can it vent at all without eventually leaking pressure? The pressure in my system has gone down a little in the last five years....but not much.

And what is a good non-toxic preservative to add to a propylene glycol closed system?
thanks, rScotty
Tractorwise, it's an old JD530, a newish Kubota M59, and a couple of US Yanmars. Hydraulics is just plumbing too...

 

[Raspy]

rScotty,

Thanks for your kind words! I'm glad you've found a guy that can do both plumbing and hydronics

I'll see if I can answer your questions.

The cavitation thing. None of the kind of pumps used in hydronics will pump air. They are all centrifugal pumps. They will pump water that has air mixed in it to a certain degree. But they get noisy whenever there is air in the water. In fact they will tell you exactly what they are doing simply by the sound they are making. In other words, they have their own language and can be quite articulate. Grundfos pumps, in particular, have a very high performance impeller that is the least tolerant of air. They are also sensitive to system pressure and will cavitate at very low system pressures. So, you can get a similar sound because of air in the system, or because of low system pressure under some circumstances, but it's not the same condition. To prevent air problems at the pump its nice to have the pump pumping up instead of down so that the air will naturally try to move on through.

Consider "closed loop" to mean any system that either has no fill system or just one fill point to maintain pressure. Like a circle of pipe with a pump in line is closed and circulates. Then to that circle add a tee with a fill regulator. The regulator is also a check valve and no fluid can flow back out of the loop, but the loop can fill and circulate. Closed loop. Now we can add additives if we want and we can prevent air from entering, we don't build up sediment like house plumbing might and the little bit of initial oxygen that came in with the original water gets used up and no further rusting can happen. This means we can use iron parts without having them rust. Of course, as I mentioned before, that only holds true for metallic pipe or tubing.

More later.

 

[Raspy]

rScotty,

The PEX you mentioned with the metal inside is designed, as far as I know, to prevent oxygen permeation. That doesn't mean it's a good product. Maybe it is, but conventional barrier PEX is the industry standard, has proven to be better than other tube types and is guaranteed for 25 years. I have been using it since it came out and have never had any problems with it. Good Stuff. Its only weaknesses are high temperature (beyond normal hydronic temps), high chlorine concentrations and UV.

Oxygen permeation is a process where oxygen molecules can actually pass through the tube wall into the system. This process has nothing to do with the system pressure. Polyethylene is very susceptible to this and metals are not. Once the oxygen is in the system it causes rust on ferrous parts. Barrier PEX is a product that attempts to eliminate this problem, but it is not 100% affective. Consequently, you either can use all potable approved materials in your closed loop system or you can add a product like Formula 10 preservative and then us iron parts. It's an advantage to go the additive route because the cost of the equipment is less, the pumps are less likely to get stuck after a seasonal shut down and even the copper and brass look better over time. Also, many boilers have steel or iron parts inside and would not be compatible with non metallic piping. In other words, you have lower cost and more options. Some older radiant systems used steel tubing in their slabs. Corrosion is the worst enemy of those and leads to their failure. I have been keeping many of those old systems going for many years with the correct additives and pressure management tactics.

The Spirovent is a vent that has the entire system flow pass through it. The flow goes through a chamber full of pall rings, or little metal circles designed to break up the flow and catch air. This air then can accumulate and enter an upper float chamber. The upper chamber has a float that floats on water, but not air and controls a valve at the top that is open to the outside. So, as the air bubble grows larger the float drops and the system pressure causes the air to escape out the top. When the water level rises, the float stops the hole. The system catches air and vents it out. An auto vent just has the float chamber and not the pall ring chamber. it I mounted on top of a tee somewhere that might need venting.

The thing is, we're talking about a closed loop system here. So, once the air is gone that's it, the venting is complete forever. Spirovents were sold, in the beginning, as a way to get out micro bubbles that were thought to be the cause of oxygen problems in systems. Once it was understood that it was molecular oxygen, the need for them evaporated. And they always add a little restriction to the system. And they are expensive. I never use them. It's better to install a couple of purge valves to get the system going in the beginning, and these are needed anyway, then let an auto vent or two handle the rest over the next day or two of operation. Then the caps can be closed if you like and that's it. Venting can be quite simple and logical once you see the interaction of water and air, what pumps do to the mix and how to use velocity, momentum and gravity to your advantage.

Pressure do eventually drop in these systems. Even when they are not, apparently, leaking. This can be caused by a very slow evaporation through the air vent, mechanical pump seals, or from the expansion tank air pressure slowly dropping. I'd say if you have seen a small drop in five years, you are normal. Even gauges fail and indicate the wrong pressure sometimes. Throw on a test gauge, let some water out (if you have an auto fill system) and check your expansion tank pressure. Make sure the fill system responds correctly and then holds the right pressure without creeping up over time.

You asked "what is a good non-toxic preservative to add to a propylene glycol closed system?" I don't understand this question because propylene glycol itself, is an excellent preservative and antifreeze. It's non toxic and lasts for many years. It won't stop minor leaks, but otherwise it's fine. You really only need it though, if you are concerned about freezing. It also allows you to tie a fill system into a potable system. But if freezing is your concern and you get a hidden leak, the fill system could run long enough to dilute your mix and cause a freeze up.

Hope this wasn't too long winded!