Bleeding air out of boiler system

[jkon]

I have a Weil-McClain boiler single zone. The water system needs to bled because it's making noises now that I turned on the heat. My boiler man showed me how to do it once but I forget some things. Should power to my burner be on or off while bleeding. And...once I connect my hose and open the valve, will water automatically replenish the system from my back flow preventer valve or do I have to trip that lever to allow water to enter the system? How do I know how much water to flow to replenish the system? Thanks.

 

[Dana]

Take a peek at this thread and report back.

This IS a pumped hot water system, and not steam, right?

Assuming yes...

Is there a pressure gauge on the system? The banging could be boil collapsing on the heat exchanger plates due to low pressure (12psi would be enough to stop that, and is more than enough for most systems.) If it doesn't have an auto-fill valve you'll have to open the valve and fill until the system hits 10-12psi, then stop. Then observe the pressure as the burner goes through a complete heating cycle. If the pressure goes to over 25psi when hot you may have to re-charge the expansion tank, which is a whole other process.

Don't drain water from the system until you're sure it's an air-bubble issue.

Pictures of the near-boiler plumbing and valves are useful, if you could post some.

 

[Sponsor]

 

[jkon]

Take a peek at this thread and report back.

This IS a pumped hot water system, and not steam, right?

Assuming yes...

Is there a pressure gauge on the system? The banging could be boil collapsing on the heat exchanger plates due to low pressure (12psi would be enough to stop that, and is more than enough for most systems.) If it doesn't have an auto-fill valve you'll have to open the valve and fill until the system hits 10-12psi, then stop. Then observe the pressure as the burner goes through a complete heating cycle. If the pressure goes to over 25psi when hot you may have to re-charge the expansion tank, which is a whole other process.

Don't drain water from the system until you're sure it's an air-bubble issue.

Pictures of the near-boiler plumbing and valves are useful, if you could post some.

 

[jkon]

Thanks...yes figured it out last evening. I am all set or am I? I always seem to have to bleed the system at the start of the heating system because of too much air in the system. Not sure if that pressure regulator valve is always working to allow water into the system automatically. My boiler man says that sometimes they stick until you manually trip the lever to get the water flowing. I know he replaced that regulator valve a few years ago because it was leaking and old. He still says even a new one gets hung up now and then. When I started to bleed the system seemed like no water was flowing from the pressure regulator valve until I tripped the lever. Another thing that bothers me and him also...my pressure guage gets up to 35psi when the burner and boiler get doing their thing. Shouldn't that gauge be showing no more than 30psi so as not to trip the system's steam pressure release safety valves? It's been that way for many many years and has never tripped the safety pressure release valves. He said my gauge may be off some. What makes me possibly believe him is when I started to bleed the system and when the pressure regulator valve was finally done automatically filling the system, the pressure gauge read 18-20psi instead of around 12psi. where I believe it should have read. What do you think? I don't want those safety release valves to go off one day when the wife is home by herself. It would freak her out big time. Like I said...it's been this way for years. Could it be my gauge is off or have I just been fortunate that nothing bad has happened? I don't want to come home one day after being gone for a couple days and find water all over the place. Thanks.

 

[Dana]

Heating systems are closed loops, and do not normally pick up air over time.

Auto-fill valves are a solution-problem- they commonly fail/leak, allowing too much water in, raising the system pressure to rise. There is usually a shut-off valve between the auto-fill and the potable plumbing. When the system is set to the right pressure, close that valve. In the event of a system leak and LOW pressure you'll hear the sizzle & bang of the micro-boil inside the boiler going macr0 LONG before it's a safety issue.

As long as there's no air in the system, turn the fill valve completely off, and bleed the system down to about 12psi when it's not firing, and no pumps running. The turn up a thermostat a few degrees to start a burn cycle, an observe the pressure. If the expansion tank is pre-charged and sized correctly it'll rise a bit, but won't go much over 20psi even when the maximum system temperature is reached.

Most boilers come outfitted with 30 psi pressure relief valves, so there could be calibration drift on either the gauge or the valve, or it's possible yours was installed with a 50 psi PRV. If the calibration on the gauge is off you may hear some unusual sizzle at the boiler while it's running, in which case it's time to buy another permanent gauge, or a cheap gauge with a hose connection to check the system pressure with at one of the drain taps.

Fresh water corrodes heating system components and it's not a good idea to do it often, but it's also good practice to flush the crud out of the system once every year or two, normally at the end of a heating season.

 

[jkon]

Heating systems are closed loops, and do not normally pick up air over time.

Auto-fill valves are a solution-problem- they commonly fail/leak, allowing too much water in, raising the system pressure to rise. There is usually a shut-off valve between the auto-fill and the potable plumbing. When the system is set to the right pressure, close that valve. In the event of a system leak and LOW pressure you'll hear the sizzle & bang of the micro-boil inside the boiler going macr0 LONG before it's a safety issue.

As long as there's no air in the system, turn the fill valve completely off, and bleed the system down to about 12psi when it's not firing, and no pumps running. The turn up a thermostat a few degrees to start a burn cycle, an observe the pressure. If the expansion tank is pre-charged and sized correctly it'll rise a bit, but won't go much over 20psi even when the maximum system temperature is reached.

Most boilers come outfitted with 30 psi pressure relief valves, so there could be calibration drift on either the gauge or the valve, or it's possible yours was installed with a 50 psi PRV. If the calibration on the gauge is off you may hear some unusual sizzle at the boiler while it's running, in which case it's time to buy another permanent gauge, or a cheap gauge with a hose connection to check the system pressure with at one of the drain taps.

Fresh water corrodes heating system components and it's not a good idea to do it often, but it's also good practice to flush the crud out of the system once every year or two, normally at the end of a heating season.

 

[jkon]

Thanks Dana. All my PRVs are 30psi. The reason I thought to bleed the system was that I heard some gurgling and knocking in my forced hot water baseboard heating when I first turned on the heat for the season. It went on for about a week until I had some time to bleed the system last evening. Just called my burner/boiler guy who is coming out tomorrow morning. When I told him I heard some knocking, he thinks that is a sign that the system had not enough water in it which makes him think there is a leak. I don't see any water anywhere although the in line tightening nut on the pressure regulator was leaking for me a while back until I tightened the nut. Wonder if that along with the sticking valve in the pressure regulator was enough to drain the system down??

 

[Dana]

There isn't a pressure regulator on the system. Are you talking about the auto-fill valve?

There can be small slow leaks between fire side and water side at the boiler plates that don't show up as a puddle on the floor anywhere, since it's sending that water up the flue in the form of water vapor. If it leaks fast enough it might show up as a puddle under the boiler, which can also go undetected if it's slow enough. Most hydronic heating systems have vents to automatically purge air bubbles while it's running, so as long as you have it sufficiently air-free that the water can flow it should pretty much self-purge over the first 20-50 hours of running, and any gurgling should subside.

Set the system pressure manually, then turn the full shut-off valve closed to isolate the system, independently of whether the auto-fill valve is seeping, then monitor the system pressure over then coming days/weeks. It should stay pretty much in the same range. If it's dropping over time, there's a leak somewhere.

Knocking & banging is a sign of low pressure, as larger bubbles form on the water side of the heat exchanger plates, then collapse as the flow pulls them away toward the distribution plumbing. It's usually most audible right at the boiler.

 

[jkon]

Yes I am talking about the auto fill but isn't that sort of a regulator because aren't they normally set to automatically only allow around 12-15 psi into the system? Yes my water leak was coming from the in-line tightening nut on the auto-fill. It leaked (dripped) for a good two weeks before I tightened it to stop the drip. But that nut was on the town water supply side of the auto fill so I guess that shouldn't have mattered.

Well I know for sure that gauge definitely reads much higher than that during the heating cycle. When I got done bleeding the system last evening the gauge read about 18-20psi. Went upstairs and set my hall thermostat for 65 degrees. Went back down in basement and turned on the system. By the time my house hit 65 degrees and the system reached the end of the heating cycle that gauge was reading 36psi. All my safety pressure release valves are 30 psi. Something is just not right.

Wonder if my expansion tank is bad...full of water? It's all original stuff. Bought the house new back in 1990.

 

[Jadnashua]

It's fairly simple to check the ET to see if it's in good shape. First thing is to tap on it. If it feels heavy and thuds, it's probably shot. It should mostly be full of air, not water. If it sounds okay, it's a bit harder to check the prefill pressure since that must be done with no water pressure on the wet side, which would mean depressurizing the system. When everything is setup properly, the pressure should remain fairly stable during and before a heating cycle.

While the auto-fill valves are often adjustable, the pressure safety device normally is not. It should have a label on it. If your pressure got to 36psi (and the gauge is accurate!), and the relief valve is good, it should have let out some water to keep the pressure at 30psi. Those are an emergency, safety device, and should only operate when there is a problem in the system. Now, it's possible you have one designed for higher pressure, but 30psi is fairly common...higher is potentially needed on multi-story buildings, higher than a typical home.

 

[Dana]

After 25 years of use it's pretty common for the pre-charged pressure level on the expansion tank to have changed. Air leaks out of the air-side of the diaphragm slowly over time. There's tire-type air valve on the air-charge end of the thing (you can check the system pressure with a tire pressure gauge, if the thing is working correctly), which can be used to adjust the charge.

To set the air -charge level you have to start by relieving the pressure on the water side- hopefully there is an isolating valve to make this easy. With the isolating valve closed, unscrew the tank until some water dribbles out fairly easily, then pump up the air charge side to about pressure you intend to run the system. (12-15psi is fine). Then re-tighten the tank, and open the isolating valve. If this hasn't been done since 1990, odds are pretty good it's off. (Ever had a tire valve that didn't leak a molecule for 25 years?)

When the pre-charge is too low it runs out of expansion capacity quickly as the system temp rises, causing higher than desired system pressures.

If the diaphragm is leaking, you'll get water out of the air valve when you push the center stem- the thing is toast, and needs to be replaced. They come indifferent sizes, and the needed sized is determined by the total volume of water in the system, and the difference in high & low temperatures.

 

[jkon]

After 25 years of use it's pretty common for the pre-charged pressure level on the expansion tank to have changed. Air leaks out of the air-side of the diaphragm slowly over time. There's tire-type air valve on the air-charge end of the thing (you can check the system pressure with a tire pressure gauge, if the thing is working correctly), which can be used to adjust the charge.

To set the air -charge level you have to start by relieving the pressure on the water side- hopefully there is an isolating valve to make this easy. With the isolating valve closed, unscrew the tank until some water dribbles out fairly easily, then pump up the air charge side to about pressure you intend to run the system. (12-15psi is fine). Then re-tighten the tank, and open the isolating valve. If this hasn't been done since 1990, odds are pretty good it's off. (Ever had a tire valve that didn't leak a molecule for 25 years?)

When the pre-charge is too low it runs out of expansion capacity quickly as the system temp rises, causing higher than desired system pressures.

If the diaphragm is leaking, you'll get water out of the air valve when you push the center stem- the thing is toast, and needs to be replaced. They come indifferent sizes, and the needed sized is determined by the total volume of water in the system, and the difference in high & low temperatures.

 

[jkon]

Yepper....It was a bad expansion tank. It was full of water. My boiler guy put a new one in and system runs good now.