Air in my hydronic heating system

tholmes

We have a hydronic heating system with three zone valves. For some reason air keeps getting into the system. The pressure tank is good and I try to maintain a pressure around 15 to 20 psi. there is an air eliminator on the system too. I bleed the air out at one of the bleeders on one on a radiators. For the last two years I ran vinegar through the boiler to remove the lime. Closed the ball valves at each zone so the vinegar stayed in the boiler. The flushed it with clean water. Anyway I am having to bleed the air every day to no avail. How does the air get into the system?

mattmia2

Is it loosing water somewhere? Does it maintain pressure if you shut off the supply to the pressure reducing valve? Fresh water has air dissolved in it and that air will come out of solution when it is heated. Could be some other leak as well.

Jamie Hall

Are you really sure you are maintaining pressure and that none of your bleeder valves are leaking? You have to maintain a pressure of at least half a pound for every foot of elevation above the boiler. Otherwise, even a slight leak at a bleeder or radiator valve stem will let air in.

Second, as @mattmia2 said, does your pressure in the system hold when the boiler is off? If not, you definitely have a leak somewhere. Might not be big, but it's there.

Zman

Are you pumping towards or away from your expansion tank?
Pictures always help...

tholmes

The Fill-trol valve on the original tank corroded and no longer worked so I add water manually. During the summer when it does not run the pressure stays constant. Pumping is towards the pressure tank. Highest radiator is 16 feet above the boiler.

EBEBRATT-Ed

Do you have a bladder type expansion tank? Or a compression tank? If you have a compression tank you can't have any automatic vents.

Try adding water to the system and get the pressure up to 25 at the boiler. Bleed with the pump off, then bleed with the pump on drain down to normal pressure

HomerJSmith

I guess this is a new problem and hasn't always been there.

Air doesn't just get into a boiler. The boiler is at a higher pressure than ambient air. If there was a hole in the sys, water would leak out.

For air to get into a sys, it must be pulled in. There has to be a lower pressure in the sys than atmospheric air. Air moves from higher pressure to lower pressure. An oversized pump or multiple pumps can pull air into the sys thru the air elimination vent, if the air vent is on the input of the pump. I don't think that's your problem?

"Pumping is towards the pressure tank." Not Good! One pumps away from an expansion tank.

Where is the air elimination valve?

How do you add water to your sys and how often do you have to add water?

tholmes

This is an American Standard boiler GPMX-7R series 1B-J1; they designed the system to pump the returning water past the expansion tank, through the boiler, through the air eliminator, through the zone valves, to the radiators, then back to the pump.

The automatic air vent is on top of the air eliminator. I installed the boiler when I built my house in 1970. The expansion tank has a bladder. I have added water to build the pressure, when hot, to 25 psi and then tried bleeding the system with the pump of and the when it was running.

Six months after installing the boiler it started liming up. I had a rep from American Standard come out and check the system but he was of no help. We have lived with the boiler noise for 48 years. 2018 I decided I would flush the boiler with vinegar. That worked for a few months then started liming up again - the snapping sound started again (the lime coats the inside of the castings, the water gets between the scale and the castings then flashes into steam making the snapping sound (like an old water heater or old tea kettle). Last fall I delimed the boiler again. The pH of the water is about 7, apparently alkaline enough to create the scale. When I filled the boiler this time I added two gallons of vinegar. Because of having to add so much water now the pH is back to about 7 again.

Jamie Hall

pH 7 really isn't the problem. The problem is the water quality itself. How hard is your water? Do you use any form of conditioner in the boiler water?

mattmia2

Does it hold pressure over time if you turn off the feeder? Liming up is usually a combination of a lot of fresh water coming in to the system and hard water.

tholmes

Water can only added to the boiler when I turn the ball valve on; otherwise the system is closed.

I will definitely check the water hardness. I did not really consider that since I haven't had lime build up around the faucet aerators or drains. That must explain the boiler liming.

hot_rod

Also consider a more aggressive cleaning of the boiler with that many years of scaling. There are Hydronics specific descaling chemicals available, they have additional components compared to just vinegar. Rhomar and Hercules are brands I have used.

Alan (California Radiant) Forbes

Liming up with a pH of 7 on a closed system? And no noticeable hardness in your domestic water? Even with having to add water, would you really get that much lime to affect heat transfer?
The boiler noise could also be coming from the water moving too slowly through the heat exchanger. Do you know that your pump is working? What model pump do you have?
If you installed a pump that is too large and because you are pumping towards the expansion tank, the pressure on the suction side could be sub-atmospheric, pulling in air from somewhere.

tholmes

The circulating pump is a B&G NRF-22 1/25 hp. The pump has been in use more than 20 years. If it didn't work we wouldn't the water going through the pipes. Plan to take a water sample to Menard's this week.

coby

If you have never read Dan Holohan's book's you need to read pumping away. You state you have been dealing with this problem since boiler was installed (48 yrs.). With no picture's it sounds like the pump is on the return and your pumping into the expansion tank. No matter what you do you are alway's going to have this problem (unless you read Dan's book or get a Qualified professional in your area). Your boiler is probably nearing the end of it's life, you should not have to be adding vinegar. Your adding too much fresh water. Boiler water that has been in a system for a while with proper PH, have no minerals left in it. They've basically been cooked out. So in saying that you need to repipe your system, if done correctly you should never have to bleed a properly installed boiler. Sorry if I sound rude, just that if it was done right the first time I promise you wouldn't be on here right now.

Alan (California Radiant) Forbes

Ditto @coby

As far as the operation of your pump, if it's not working, the hot water can be circulating by gravity; sometimes called phantom flow or ghost flow. I'm not saying this is happening necessarily, just grasping at straws trying to figure this out.
And if the pump is not working, you will get kettling as the flow of water is not fast enough going through the heat exchanger to pick up all the heat being generated.

tholmes

Have attached a photo of the system in the area of the pump. I put a screwdriver on the pump - is running.

Alan (California Radiant) Forbes

Read this:

https://heatinghelp.com/systems-help-center/the-point-of-no-pressure-change/

It's still hard for me to understand, but I take it as a given and it's helped me to understand systems better.

hot_rod

If there is any sort of valve on the pump discharge, you can close it down slowly to be sure there is actually flow happening.

tholmes

American Standard designed the system to have the pressure tank on the discharge side of the pump - that was 1970 - ten years after Holohan's book. Maybe their engineers didn't read his book. For some reason I always thought that the pump should be between the boiler and the zone valves so it wouldn't have push the water through the boiler.

When I drain the air it is at a spot by two radiators which are parallel and ten feet apart. I hear some bubbles enter one radiator, travel about twenty-four feet in about eight seconds and then exit where I am eliminating the air - roughly 3 feet a second. Also, when a zone valve closes you can really hear the water stop flowing as it is being restricted.

Alan (California Radiant) Forbes

We all know that pumps were installed - like yours - on the inlet of the boiler because they were sold as a package and it was an easy way to mount them for shipping. They were also installed there because the water was cooler being on the return and the pumps would last longer.
In 1960, I don't think Mr. Holohan was writing books yet. As a teenager, he was probably chasing girls. At least, I was.

SuperTech

The point of no pressure change is a real thing. Just try to think like water and maybe you can understand it....

If you saw the difference that pumping away from the expansion tank and air separator makes you would easily be able to see why that's the proper way to pipe a hydronic heating system.

tholmes

For these small systems, 1/25 hp, B&G says it doesn't matter which side of the pump the expansion tank is on.

"One small, low pump head systems, such as those that use a series 100 or SLC Bell & Gossett pump, it may not be necessary to pump away from the boiler and tank because the pump energy is not enough to affect system pressures very much. It certainly does no harm to put a system together correctly and prevent problems. In general, systems that require pumps with 1/3 H.P. motors or more should definitely be installed pumping away from the boiler and compression tank."

https://www.industrialcontrolsonline.com/training/online/how-avoid-problems-your-hydronic-system-pumps

I am not sure about this article when they say: "A 10’ piece of 3/4” copper tube will expand 7/16 of an inch at a 100oF temperature rise! " Actually it is less than an 1/8" inch (0.0000094x10x12x100)

SuperTech

Maybe it's not 100 percent necessary, but it's 100 percent the best way to do things to avoid air problems and failed expansion tanks. Who uses those pumps anymore? Seems like it might be outdated information.

hot_rod

tholmes said:

For these small systems, 1/25 hp, B&G says it doesn't matter which side of the pump the expansion tank is on.

"One small, low pump head systems, such as those that use a series 100 or SLC Bell & Gossett pump, it may not be necessary to pump away from the boiler and tank because the pump energy is not enough to affect system pressures very much. It certainly does no harm to put a system together correctly and prevent problems. In general, systems that require pumps with 1/3 H.P. motors or more should definitely be installed pumping away from the boiler and compression tank."

https://www.industrialcontrolsonline.com/training/online/how-avoid-problems-your-hydronic-system-pumps

I am not sure about this article when they say: "A 10’ piece of 3/4” copper tube will expand 7/16 of an inch at a 100oF temperature rise! " Actually it is less than an 1/8" inch (0.0000094x10x12x100)

Really no reason both the expansion vessel and circulator could not be pumping in to the return. The key is the relationship, circulator pumps away from tank connection regardless where on the boiler it is located.

BDR529

Could be as simple as turning down the high limit.

Don't try to reinvent the wheel.

Alan (California Radiant) Forbes

I don't think we're any closer to solving Mr. Holmes' air problem.
Can you send us some more pictures? I'd like to see where that pipe goes, the one that comes off the tee above the expansion tank.
I'd also like to see the supply piping coming off the top of the boiler.

tholmes

The branch side of the tee is the input for the water supply (manual feed). The water exiting the boiler passes through the eliminator to the manifold that feeds the zone valve

Alan (California Radiant) Forbes

That automatic air vent on top of the air eliminator looks like it's been leaking at some point. I would replace it with a new one (Caleffi).
Maybe it's allowed air into the system, maybe not, but it's all I got now.

tholmes

If the internal system pressure is greater than atmospheric pressure not sure how it can let air in?

Right now the vent hole is plugged shut, that happened a few weeks ago, likely from sludge, as a result of the vinegar I had added when I filled the system.

The only thing I can think of as to why the air keeps getting in is that it is not getting in. It is coming from gas being generated by the vinegar I added to the system to keep the pH on the acidic side. Maybe at the end of the heating season I should completely flush the system and add chemicals to remove the calcium and magnesium. Water is considered hard when the pH level is above 8.5. (I still have to check the hardness level)