Air in boiler system and low pressure

[Yukoner777]

Hi guys
My boiler guy has kind of abandoned me, as he took on a bunch of commercial work. I'm hoping you can help me out with an issue that seems to crop up every fall.

I have my boiler (16 yr old Weil-Mclain Gold) in the garage, with pipes feeding through the wall to my utility room, down one level in the basement. This is a constantly circulating loop of hot water. The manifold in the basement picks off hot water from that loop as needed through a variable speed pump to feed my manifold that supplies warm water to 12 loops of radiant floor heating.

Every fall, when it's time to boot up the system, I start out with air in the system, evidenced by the sound of water moving through the boiler loop pipes. And so at the beginning of each season, I've had someone come to bleed the air out of the system. But as I stated, that person is not available now. The pressure is also very low. When it's working properly, the pressure gauge reads around 15 lbs.

I let a wee bit of air out of the expansion tank, just to check it, and there was a bit of water vapor that came out too. Does that mean that the ET is compromised? When I tap on the tank and give it a little shake, it appears that there is no water in it at all. Is that normal?

I have had issues in the past with the pressure relief valve filling the old coffee can. But currently it's dry, I guess because the pressure is so low. I've read somewhere that if the ET is not working properly, the pressure will increase when the boiler is running, then the PRV will pop open to relieve the pressure. So that means there would then be less water in the system. But I don't understand what effect that would have, nor how the air is getting in.

Any suggestions?

Here are some pics.

 

[Reach4]

When I tap on the tank and give it a little shake, it appears that there is no water in it at all. Is that normal?

I don't know the answer to your other stuff, but I know that answer is yes.

 

[Sponsor]

 

[Dana]

If it was mostly air coming out when you quick-checked the expansion tank the water was probably condensation, not a leak. A failed expansion tank would normally result in very high peak pressures, not low pressure.

The PRV should never spit water- EVER, in normal operation. If it's been "normal" to fill a coffee can it's an indication that the expansion tank isn't properly pre-charged, or that it's too small for the water volume and temperature swings of your system, or perhaps the PRV itself is defective.

Normally the expansion tank is located a foot or two before the intake side of the circulation pump(s), and very close to the system air purging device. If it isn't close to the pump, there is potential for cavitation on the pump impellers which lowers it's pumping efficiency and lifespan. In some instances the pressure on the intake side of the pump ends up being lower relative to the room air, causing air to be drawn into the system. But with a reasonable system venting scheme that air should be able to get back out.

In your bottom picture looks like you have a quick-shutoff ball valve in the "off" position next to your air vent, which would prevent it from auto-purging air from the system. Turning it back on might fix the seasonal air uptake issue (or not) but right now it looks like your vent is disabled by the valve.

A simplified ideally plumbed heating system design looks like this:

The orange triangle indicates the pumping direction. The yellow thing on top of the tank is a vent (shaped like a Spirovent model.)

I couldn't locate a primary pump in your pictures, but clearly your configuration doesn't resemble the ideal at all.

A more complex primary/secondary plumbed system with multple secondary loop pumps looks like this:

Hard to say which pumping /zoning methodology most resembles your system. You may have a recirculation loop with a thermostatic valve to limit the water temps on the radiant floors- usually the case except for suspended-tube type radiant systems.

 

[Yukoner777]

If it was mostly air coming out when you quick-checked the expansion tank the water was probably condensation, not a leak. A failed expansion tank would normally result in very high peak pressures, not low pressure.

The PRV should never spit water- EVER, in normal operation. If it's been "normal" to fill a coffee can it's an indication that the expansion tank isn't properly pre-charged, or that it's too small for the water volume and temperature swings of your system, or perhaps the PRV itself is defective.

Normally the expansion tank is located a foot or two before the intake side of the circulation pump(s), and very close to the system air purging device. If it isn't close to the pump, there is potential for cavitation on the pump impellers which lowers it's pumping efficiency and lifespan. In some instances the pressure on the intake side of the pump ends up being lower relative to the room air, causing air to be drawn into the system. But with a reasonable system venting scheme that air should be able to get back out.

In your bottom picture looks like you have a quick-shutoff ball valve in the "off" position next to your air vent, which would prevent it from auto-purging air from the system. Turning it back on might fix the seasonal air uptake issue (or not) but right now it looks like your vent is disabled by the valve.

A simplified ideally plumbed heating system design looks like this:

The orange triangle indicates the pumping direction. The yellow thing on top of the tank is a vent (shaped like a Spirovent model.)

I couldn't locate a primary pump in your pictures, but clearly your configuration doesn't resemble the ideal at all.

A more complex primary/secondary plumbed system with multple secondary loop pumps looks like this:

Hard to say which pumping /zoning methodology most resembles your system. You may have a recirculation loop with a thermostatic valve to limit the water temps on the radiant floors- usually the case except for suspended-tube type radiant systems.

I just checked the ET pressure (but the boiler is still warm) with a working tire pressure gauge and it's only 6 lbs. I just read a recent post on ETs. So I was thinking of shutting off the isolation valve for the ET, unscrewing the tank enough to release some water and relieve any pressure, then check the pressure again. And then if it still reads low, charge it to 15 lbs. Would that be a logical first step?

 

[Yukoner777]

If it was mostly air coming out when you quick-checked the expansion tank the water was probably condensation, not a leak. A failed expansion tank would normally result in very high peak pressures, not low pressure.

The PRV should never spit water- EVER, in normal operation. If it's been "normal" to fill a coffee can it's an indication that the expansion tank isn't properly pre-charged, or that it's too small for the water volume and temperature swings of your system, or perhaps the PRV itself is defective.

Normally the expansion tank is located a foot or two before the intake side of the circulation pump(s), and very close to the system air purging device. If it isn't close to the pump, there is potential for cavitation on the pump impellers which lowers it's pumping efficiency and lifespan. In some instances the pressure on the intake side of the pump ends up being lower relative to the room air, causing air to be drawn into the system. But with a reasonable system venting scheme that air should be able to get back out.

In your bottom picture looks like you have a quick-shutoff ball valve in the "off" position next to your air vent, which would prevent it from auto-purging air from the system. Turning it back on might fix the seasonal air uptake issue (or not) but right now it looks like your vent is disabled by the valve.

A simplified ideally plumbed heating system design looks like this:

The orange triangle indicates the pumping direction. The yellow thing on top of the tank is a vent (shaped like a Spirovent model.)

I couldn't locate a primary pump in your pictures, but clearly your configuration doesn't resemble the ideal at all.

A more complex primary/secondary plumbed system with multple secondary loop pumps looks like this:

Hard to say which pumping /zoning methodology most resembles your system. You may have a recirculation loop with a thermostatic valve to limit the water temps on the radiant floors- usually the case except for suspended-tube type radiant systems.

My pump is not even in the same room as the boiler; it's in the basement mounted under the manifold (black pump).

The ET sits on the end of a pipe which comes directly out of the boiler - it looks like it's attached where it was meant to be (?).

The sound of air in the waterlines is noisiest as the boiler temperature cools off to the lowest temp (about 148 degrees). After the boiler cycles, the temp is up to about 170 degrees and there's very little air noise in the waterlines. But the reading on the pressure gauge is still the same (about 7 psi).

 

[Dana]

At 7 psi what you may be hearing is pump impeller cavitation or a bit of flash-boil sizzle on the heat exchanger plates. The U-turn comprised of a couple of ells and a 3" stub just ahead of ball valve at the primary pump is a recipe for cavitation at low pressure. The tee and short stub just ahead of the secondary pump isn't doing it any favors either. Longer straight stubs (at least 10 pipe diameters) allow the turbulence induced by the turns to damp out a bit.

Add water to the system so that it's at least 12 psi @ 148F!

The expansion tank port on the top of Weil McLain boilers is dedicated for a CLOSED type expansion tank only. With diaphram/bladder types (like yours) on p.19 in figure 9 the manual indicates locating it at the intake side of the boiler, on either the intake side of the primary pump pumping toward the boiler, or on return side boiler itself with the primary pump located at output on the top of the boiler, pumping away.

Mounting the vent directly on a tee where the water flow makes a turn is a bit bogus too, since it's at a point of high turbulence. Ideally the vent too has at least 10 diameters straight pipe ahead of a tee with a short stand-pipe perpendicular to the flow, with the vent at the top of the stand pipe. Fatter is even better, for lower velocity (= lower turbulence) which allows bubbles to migrate upward into the stand pipe with the vent. The way yours is configured looks like turbulence might even get the air valve on the vent to spit water or fail early. It's less than ideal, the vent can't work at all with the small valve in the off-position, as seen in your picture. The thumb-handle should be parallel with the pipe.

Why your boiler guy installed the pumps, tank, & vent this way is a mystery, but it violates just about every rule of thumb, as well as the manual. He may be installing boilers, but it's not clear that he ever took the class (or even reads the manuals.)

In your very first picture the ball valve between the auto-fill valve a potable supply is in the "off" position, which prevents the auto-fill from automatically maintaining a minimum system pressure. That's fine if the system never loses water (as happens when it spits water out of the pressure relief), but not so fine as it is currently being operated.

Start with pre-charging the tank to ~12-15 psi. You have a ball valve to allow isolating that stub, so turn it off, then unscrew the tank enough to let the water out (or take it off completely), then pump up the tank to at least 12 psi, and use soapy water to check for leaks a the air valve before re-installing it. Then re-open the tank-isolating ball valve, and be sure to open the small valve at the vent.

Before running the system, open the ball valve ahead of the auto-fill so it can bring more water in. Keep an eye on the pressure gauge- be sure it reaches at least 12 psi, but doesn't go over 15 psi.

Then fire it up and watch the pressure gauge as it goes through a burn cycle, noting the peak pressure & temperature. If the pressure relief spits water at pressures well under 30psi it probably needs to be replaced. If the system pressure exceeds 25psi right after pre-charging the tank, the tank may be undersized for the water volume & temperature swings of the system.

 

[Yukoner777]

At 7 psi what you may be hearing is pump impeller cavitation or a bit of flash-boil sizzle on the heat exchanger plates. The U-turn comprised of a couple of ells and a 3" stub just ahead of ball valve at the primary pump is a recipe for cavitation at low pressure. The tee and short stub just ahead of the secondary pump isn't doing it any favors either. Longer straight stubs (at least 10 pipe diameters) allow the turbulence induced by the turns to damp out a bit.

Add water to the system so that it's at least 12 psi @ 148F!

The expansion tank port on the top of Weil McLain boilers is dedicated for a CLOSED type expansion tank only. With diaphram/bladder types (like yours) on p.19 in figure 9 the manual indicates locating it at the intake side of the boiler, on either the intake side of the primary pump pumping toward the boiler, or on return side boiler itself with the primary pump located at output on the top of the boiler, pumping away.

Mounting the vent directly on a tee where the water flow makes a turn is a bit bogus too, since it's at a point of high turbulence. Ideally the vent too has at least 10 diameters straight pipe ahead of a tee with a short stand-pipe perpendicular to the flow, with the vent at the top of the stand pipe. Fatter is even better, for lower velocity (= lower turbulence) which allows bubbles to migrate upward into the stand pipe with the vent. The way yours is configured looks like turbulence might even get the air valve on the vent to spit water or fail early. It's less than ideal, the vent can't work at all with the small valve in the off-position, as seen in your picture. The thumb-handle should be parallel with the pipe.

Why your boiler guy installed the pumps, tank, & vent this way is a mystery, but it violates just about every rule of thumb, as well as the manual. He may be installing boilers, but it's not clear that he ever took the class (or even reads the manuals.)

In your very first picture the ball valve between the auto-fill valve a potable supply is in the "off" position, which prevents the auto-fill from automatically maintaining a minimum system pressure. That's fine if the system never loses water (as happens when it spits water out of the pressure relief), but not so fine as it is currently being operated.

Start with pre-charging the tank to ~12-15 psi. You have a ball valve to allow isolating that stub, so turn it off, then unscrew the tank enough to let the water out (or take it off completely), then pump up the tank to at least 12 psi, and use soapy water to check for leaks a the air valve before re-installing it. Then re-open the tank-isolating ball valve, and be sure to open the small valve at the vent.

Before running the system, open the ball valve ahead of the auto-fill so it can bring more water in. Keep an eye on the pressure gauge- be sure it reaches at least 12 psi, but doesn't go over 15 psi.

Then fire it up and watch the pressure gauge as it goes through a burn cycle, noting the peak pressure & temperature. If the pressure relief spits water at pressures well under 30psi it probably needs to be replaced. If the system pressure exceeds 25psi right after pre-charging the tank, the tank may be undersized for the water volume & temperature swings of the system.

Thanks for all the great info, Dana. When I was having the house built, I didn't have a lot of options for heating guys to install the system, since almost all were busy with regular contractors and didn't have time for an independent like myself. The fellow I got was well established and nearing retirement age. Just goes to show (which I've seen before) that years of experience does not necessarily make a better tradesperson. Couldn't even follow the freaking instructions!

Okay, so I shut off the isolation valve and removed the ET. No water released. With the tank removed, I tried to pump air into the air valve, but air continually escapes through the bottom threaded pipe of the ET. So, I assume the ET is toast. I'll pick up a new one tomorrow, then carry on with the rest of your instructions.

Would there be any advantage to installing a length of pipe to make the air bleeder valve higher? Because you're right, it did emit water (from the turbulence at that bend, I guess), also evidenced from the years of corrosion on the outside of that big pipe. If so, the easiest way I can see would be to add a length of small diameter pipe the same size as the air vent. To use a larger diameter vertical pipe extension would require removing the corroded reducer on top of the large tee and I would guess that would be next to impossible to remove.

When it comes to bringing more water into the system, I'd prefer to use distilled water, rather than tap water coming from the DHW tank supply. Should I pump distilled water in through the fill valve at the bottom right of the boiler? Or is the amount of water needed going to be insignificant enough to not bother with that?

Thanks again for your help.

Ross

 

[Dana]

Adding a short stub to the air vent (6" would be enough) may help keep it from spitting water, and allow it to perform it's function despite the sub-optmial location at a bend. (It's pretty common to find them mounted that way, and it'll still do at least something.) If it continues to spit water after installing a stub the vent may be due for replacement.

Your assessment is correct- the fact that the tank can't be pre-charged is an indication that it's toast. But since there wasn't any water in it means it's still behaving as a "closed" type expansion tank. If correctly sized for the water volume and temperature swing of the system it would still perform that function if keeping it right where it is. A blown/leaky diaphragm would be a problem if mounted where tanks of that type are usually mounted, but not when mounted to the closed tank port.

I'm not sure what the advantage is of using distilled water in the system vs. tap water. It's a closed system, and the amount of dissolved minerals that would form lime deposits or chemically react with anything in the are finite (and quite small.) Scale & lime build up in boiler systems only happens if the water is constantly being replaced at a fairly high rate rather than once every 1-10 years.

 

[Yukoner777]

I just watched a short video demonstration which clarified for me the necessity of locating the air vent away from turbulence.

Since winter is upon us here in Yukon, I'll make do with installing the 6" stub for the air vent, but next summer I'll get the air vent moved left about a foot, to the middle of that horizontal pipe.

I removed the ET to check it on Friday and it turns out none of the places that would carry these are open on Saturday. Luckily, we're having a couple of days of mild weather and the house is super-insulated, so we'll be okay with the heat turned off until I can get a tank on Monday.

Good to know I don't need to use distilled water. That'll make it a lot easier to fill the system.

 

[Yukoner777]

I just watched a short video demonstration which clarified for me the necessity of locating the air vent away from turbulence.

Since winter is upon us here in Yukon, I'll make do with installing the 6" stub for the air vent, but next summer I'll get the air vent moved left about a foot, to the middle of that horizontal pipe.

I removed the ET to check it on Friday and it turns out none of the places that would carry these are open on Saturday. Luckily, we're having a couple of days of mild weather and the house is super-insulated, so we'll be okay with the heat turned off until I can get a tank on Monday.

Good to know I don't need to use distilled water. That'll make it a lot easier to fill the system.

When I tried to fill the system with water, it poured out of the Backflow Preventer vent (like full blast). I did happen to have another backflow preventer on hand so I put the new one on and no more water coming out of the BP vent. Yay!

I installed a 6" extension for the air vent and installed the new expansion tank.

I turned on the water supply and brought the pressure up to 12 psi. The air vent doesn't seem to be doing anything yet and when I press on the valve, only water comes out. Maybe that will change once the water (and air) in the lines is circulating.

Time to turn it on and see what happens!

 

[Yukoner777]

When I tried to fill the system with water, it poured out of the Backflow Preventer vent (like full blast). I did happen to have another backflow preventer on hand so I put the new one on and no more water coming out of the BP vent. Yay!

I installed a 6" extension for the air vent and installed the new expansion tank.

I turned on the water supply and brought the pressure up to 12 psi. The air vent doesn't seem to be doing anything yet and when I press on the valve, only water comes out. Maybe that will change once the water (and air) in the lines is circulating.

Time to turn it on and see what happens!

So when I fired up the boiler, I had to keep releasing an ounce or so of water through the PRV, as the pressure kept creeping up toward 20 psi. On the second cycle, the pressure was a little low, so I added a bit to bring it up to 15 psi. It seems to be holding steady at 15 psi for the second cycle. For the third cycle also.

 

[Yukoner777]

Thanks for all your great advice, Dana. I couldn't have done it without you!

Cheers
Ross

 

[Dana]

An operating pressure of 20-25 psi when the boiler is hot is just fine, but if it's over 25 psi it's an indication that either the tank is too small or the system was overfilled. When no zones have been calling for heat for some time and the radiation is cool, and the boiler is near it's minimum temperature you still want the system pressure to be at least 10 psi.

If it's much below that pump cavitation becomes more pronounced, and flash-boil sizzle-pop-bang on the boiler's heat exchanger plates can occur as it first fires up after a long off cycle. The flash boil has to be really bad before it would cause any damage, but if you can hear it sizzling even a little bit the heat transfer efficiency is suffering quite a hit.

If the system vent is operating properly it'll release the air without manual intervention. Having it spit water when manually opened is normal. Having it spit water without being manually opened would be a problem.

 

[Yukoner777]

An operating pressure of 20-25 psi when the boiler is hot is just fine, but if it's over 25 psi it's an indication that either the tank is too small or the system was overfilled. When no zones have been calling for heat for some time and the radiation is cool, and the boiler is near it's minimum temperature you still want the system pressure to be at least 10 psi.

If it's much below that pump cavitation becomes more pronounced, and flash-boil sizzle-pop-bang on the boiler's heat exchanger plates can occur as it first fires up after a long off cycle. The flash boil has to be really bad before it would cause any damage, but if you can hear it sizzling even a little bit the heat transfer efficiency is suffering quite a hit.

If the system vent is operating properly it'll release the air without manual intervention. Having it spit water when manually opened is normal. Having it spit water without being manually opened would be a problem.

The only time I hear a sizzling sound is right after the boiler shuts off at the end of a cycle. It's quiet and only lasts for about 30 seconds. I'm not hearing any noise in the waterlines now - good sign. And the air vent is not bleeding water - another good sign.

The pressure climbs to 19 psi by the time the boiler finishes its cycle.

 

[Dana]

A peak of 19 psi sounds perfect. What is the pressure at it's lowest, when the boiler is at it's low-limit temperature?

It's good that it's not sizzling popping or banging at the beginning of a purge, an indication that your system pressure is probably adequate. I'm not sure what the post-burn sizzle is- there is probably still some air in the system that would hopefully self-purge despite the sub-optimal vent configuration. That symptom may abate over time.

 

[Yukoner777]

A peak of 19 psi sounds perfect. What is the pressure at it's lowest, when the boiler is at it's low-limit temperature?

It's good that it's not sizzling popping or banging at the beginning of a purge, an indication that your system pressure is probably adequate. I'm not sure what the post-burn sizzle is- there is probably still some air in the system that would hopefully self-purge despite the sub-optimal vent configuration. That symptom may abate over time.

The low-temp pressure is about 13 psi. The system has been working beautifully since doing the work. Thanks so much for your help, Dana.