Air in system ....

Good Morning!

On Sunday I re-piped a bit of my boiler to replace an older expansion tank with a new style one and install a spirovent jr. I brought the boiler back up and started the bleeding process. I turned the boiler off from Monday morning until about 7:00 PM.

This morning I was woken up by the pipes "screeching" which I attribute to there still being air in the system. I was able to bleed the radiators again prior to going to work but there is still air.

I have a couple of questions stemming form all of this:

(1) How can I tell if the the spirovent is doing its job? Aside from one time during the initial fill-up I haven't heard it let out any air.
(2) How long should it take to get all the air out of the system that has been freshly filled?
(3) What is the correct sequence to bleed the radiators? I've been starting at the furthest one from the boiler and bleeding with the circulation pump off.

Thanks again for all the help!

Jeff

Air scrubbers do their work slowly- you won't hear them releasing air except when purging large amounts of air when filling a drained system. The amount of time it takes a vent to purge all remaining air depends, on the location of the vent, the amount of air in the system, and the duty cycle of the pump.

Thanks to gravity air tends to accumulates with altitude, so bleeding the radiators at the highest point in the system is more productive than mere plumbing-distance. If the pump has been running and the system isn't vapor-locked with a large air bubble air gets somewhat randomly distributed- will collect in local high points, but in general the high-point of the system is where the most air accumulates, making it the ideal place to install air release vents, and the most productive system-bleeding point.

Not to hijack the first question in this thread but my question is regarding the same topic, but different type of issue. I have an old gravity hydronic system in a house I just recently bought. I noticed all radiators on the second floor would only be hot from their centers down. I took this to mean there was lots of air trapped in the top of the system, which there was. So I started out by closing off the isolation valve to the expansion tank, drained the tank, left the expansion tank isolation valve closed, then went to the air trapped radiators, opened the radiator bleed valves, some air came out, had someone open the boiler feed valve (on the inlet side of the PRV) and waited... and waited... nothing. No more air came out. So eventually I put everything back to normal and gave up. Figuring something is broken (clogged PRV screen, etc...)

Today I noticed the water level in the radiator is up a little bit. It's still got a ways to go, but maybe gained about 3-4 inches in height.

What do you think could be the reason why I'm not able to get water to come out of the top floor radiators when I open the bleed valve at the radiator and open the boiler feed valve with the expansion tank closed off from the system? Something doesn't make any sense. For some further info, when I started the process my system was at 130F and 15 lbs. When I finished I was at about 130F and 5 lbs. I assume the drop in pressure is due to the fact that I let air out of the system. The 130F is because the system was on earlier in the day.

Thanks,
M

So I just re-read some literature about my PRV (and paid attention this time). It's a Thrush model 10F and features a "fast fill thumb screw" which you are supposed to turn to essentially bypass normal PRV operation and allow water to flow through freely (eliminating the need for a PRV bypass). I was not doing this before. I'm willing to bet this is what I need to do in order to get water up to the highest point. I'm a "cartoonist" by trade (engineer) so while I get all of the concepts of how to get something to work, I have almost no experience when it comes to actually doing it. I bet something like this is a no-brainer for a salty plumber.

What is the actual pressure in the closed loop of your boiler? What it should be is determined by several things, but the elevation change between the boiler and the highest point is a good part of the equation. You may not have enough pressure in the system to allow the water to rise to the high point so you can purge all of the air out.

If you turn on the fast fill someone better be watching the pressure. If not the pressure will climb above the 30# that opens the relief valve.

John

jadnashua said:

What is the actual pressure in the closed loop of your boiler? What it should be is determined by several things, but the elevation change between the boiler and the highest point is a good part of the equation. You may not have enough pressure in the system to allow the water to rise to the high point so you can purge all of the air out.

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Other than the pressure gauge at the boiler I do not know the internal pressure of the closed loop system. All I can tell you that the boiler pressure was 15lbs at 130F before I started then dropped to 5lbs after I let all the air out - at the boiler. The pressure at the top of the radiator *should* be atmosphere with the bleed valve open and all pressurized air expelled.

I'm fairly certain if I just use the "fast fill" thumb screw on the PRV with the radiator air bleed vents open (and the expansion tank isolation valve closed - because it's in the basement and not the attic like some old gravity systems) I will be able to get water up to the radiators on the top floor. The force of the city water should be enough to fill the system to it's highest point with the bleed valves open since we have showers and stuff on the same floor and the size of the hydronic piping is very large for a system of this size - I imagine to minimize pipe friction since there is no pump.

And yes, Tombstone, I will definitely have someone down at the boiler shutting off the boiler feed valve while I'm up at the radiators communicating via phone or walkie talkie! That would be a horrible stinky mess to have to clean up.

I'll let you know how that works.

Thanks!

I'm sorry, I meant to say John, not Tombstone in my reply. Also, I'll instruct the person engaging the fast fill thumb screw to go easy and place a bucket under the pressure relief to catch any spillage.

Unless the boiler is at the high-point of the system bleeding air at the boiler isn't usually necessary or useful. Opening up the system at a low altitude points can lower the pressure at the top of the system to below atmospheric pressure.

When you bleed the air out of the top of the system the top of the system isn't at atmospheric pressure unless you let it run until the water stops coming (which is never if you have an auto-fill valve continuously letting more water in.) City water pressure is typically several times the amount of pressure you'd actually WANT to run your heating system.

Gravity systems are either open or closed systems, relying on boiler pressure and the height of the highest radiators in the system. In an open system, as the water heats, it expands and fills the radiators on the top floor. It requires precise pressure control. If the system pressure is too high, the radiators flood out the air vents. In a closed system the boiler pressure should be around 15lbs The water will gradually rise as its heated and fill the upper radiators. Now the biggie......Tear the whole mess out and re-do EVERYTHING because you have the 2nd most inefficient system possible.

Tom Sawyer said:

Gravity systems are either open or closed systems, relying on boiler pressure and the height of the highest radiators in the system. In an open system, as the water heats, it expands and fills the radiators on the top floor. It requires precise pressure control. If the system pressure is too high, the radiators flood out the air vents. In a closed system the boiler pressure should be around 15lbs The water will gradually rise as its heated and fill the upper radiators. Now the biggie......Tear the whole mess out and re-do EVERYTHING because you have the 2nd most inefficient system possible.

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Believe me, if I had the money I would have 2 dual fuel heat pump/nat. gas furnace forced air systems - one for each floor. For now I'm stuck with what is there. I don't even have insulation in my walls so I'm going to start with that improvement first! Home is circa 1870, balloon framed construction, plaster & lathe throughout. Lots o' character.

Dana said:

Unless the boiler is at the high-point of the system bleeding air at the boiler isn't usually necessary or useful. Opening up the system at a low altitude points can lower the pressure at the top of the system to below atmospheric pressure.

When you bleed the air out of the top of the system the top of the system isn't at atmospheric pressure unless you let it run until the water stops coming (which is never if you have an auto-fill valve continuously letting more water in.) City water pressure is typically several times the amount of pressure you'd actually WANT to run your heating system.

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I agree entirely. My boiler is in the basement. I may not have been entirely clear when I wrote my first post but the drain-down I performed was for the expansion tank (also in the basement) while isolated from the rest of the system. I was never bleeding air from a low point in the system. Then, after I drained the expansion tank I then went to the high point and tried to bleed air from the two radiators up there but that's when I ran into the problem of the water not making it up to those radiators.

FWIW: A co-worker of mine who lives in Newton had a gravity/convection boiler in his 1840s antique (the boiler had been installed in the 1920s or 30s). At one point he broke it up into zones with pumps and the fuel use dropped 10-15%. About 8 years after that had modulating condensing boiler installed on that same large-piped system, and even though the mod-con is oversized for his actual loads, it cut fuel use further by about 2/3 from it's already somewhat lower consumption rate. The boiler was part of a pre-packaged system with a Honda co-generator, and between the fuel savings and the net-metered power output of the Honda (which was putting an additional ~12,000BTU/hr into the heating system when running) it paid for itself in about 3 years, despite being a ~$20-25K up front investment. ( The system was marketed under the name "Freewatt", and was bought out by ECR International, who doesn't seem to be marketing them any more.)

Bottom line, you're probably better off replacing the boiler and converting it to a condensing pumped system sooner than later, financing it with the 7-year 0-interest money deal through MassSave.

Gas, or oil, and how much fuel do you use in a typical year (to be able to estimate your peak heat load.)

Dana said:

FWIW: A co-worker of mine who lives in Newton had a gravity/convection boiler in his 1840s antique (the boiler had been installed in the 1920s or 30s). At one point he broke it up into zones with pumps and the fuel use dropped 10-15%. About 8 years after that had modulating condensing boiler installed on that same large-piped system, and even though the mod-con is oversized for his actual loads, it cut fuel use further by about 2/3 from it's already somewhat lower consumption rate. The boiler was part of a pre-packaged system with a Honda co-generator, and between the fuel savings and the net-metered power output of the Honda (which was putting an additional ~12,000BTU/hr into the heating system when running) it paid for itself in about 3 years, despite being a ~$20-25K up front investment. ( The system was marketed under the name "Freewatt", and was bought out by ECR International, who doesn't seem to be marketing them any more.)

Bottom line, you're probably better off replacing the boiler and converting it to a condensing pumped system sooner than later, financing it with the 7-year 0-interest money deal through MassSave.

Gas, or oil, and how much fuel do you use in a typical year (to be able to estimate your peak heat load.)

Click to expand...

Just as an update, I was able to successfully bleed the air out of my system by using that thumb screw on the PRV. It was a cinch after I figured that part out. The pressure relief valve didn't end up dumping any water either (I placed a bucket under it just in case). The radiators on the top floor throw some serious heat now. So I *should* be all set with my heating system for this season.

The only reason I would want to go with forced air is because the radiators are so physically large that they take up a good portion of our entryway and living room. Other than that I have no issue with hydronic heating systems. While I don't have the money to put in that Cadillac of a system, I would seriously consider going with a high efficiency natural gas boiler replacement over a forced air system if that's all we had money for. Our existing boiler is an old PennCo, natural gas fired, 160Mbtuh and is likely sized that way because the exterior walls have zero insulation in them and, at one point, single pane windows. I have no idea how much our heating bills are going to be this year. We just moved in on Sept 1. Whatever it is, I'm sure it will be cheaper than our old apartment with an oil furnace and windows so leaky you could stick 3 quarters between the gap in the sashes.

Regarding MassSave, I think that's a great program. However, I think it's being abused by those intermediary companies like Conservation Services Group. This weekend I had the extreme pleasure of having one of their employees swing by to evaluate my home. Like I've already mentioned, I'm an engineer - a former MEP engineer who specialized in HVAC for some time. This guy didn't know anything about what he was doing. He was basically hired to show up and fill out a boilerplate form on his computer which printed out a report at the end.

The best part was when there was such a serious issue with one of his CO readings in the basement that he didn't feel the need to tell me about it, or the fact that he called national grid to come check it out. As he's leaving my house after everything is done a random guy rings my doorbell, I answer it and it's a national grid employee who practically races to check the levels in the basement. He goes "everything's fine, why am I here?" I go "I don't know. I didn't even know you were called!" I told the Ngrid guy that I saw the guy from Conservation Services Group stick a probe into my boiler exhaust flue and take a reading. The guy must have thought the elevated CO levels in the flue were concerning enough to call Ngrid even though the ambient CO levels in the basement was 0 ppm with the boiler operating. All I really wanted was the paperwork to get the MassSave rebate on installing insulation. I'm praying the contractors on the MassSave approved list aren't of the same quality as this guy who showed up.

The MassSave contractors aren't vetted by the state for competence, only licensing type issues to avoid scammers from stepping in. MassSave doesn't recommend- it's up to YOU to find the insulation contractor. You can pick/vet your own contractor- if they're a legitimate company they'll already be on the "approved installer" list, (unlike an unlicensed unbonded guy who just bought a used insulation blower on Craigslist and put out an ad.)

Yes, it's true the the the inspectors they send out aren't building scientists or engineers, and if you know what you want them to sign off on it's possible to steer them. Most can see primarily the dead-obvious low hanging fruit (and only sometimes a little of the rest), but if you show them the part you're interested in they usually "get it". There are a lot of newbies in the home energy inspection biz. They didn't used to require an energy-inspector visit to be able to collect the subsidy, but I guess there were people abusing the looser system. At least sending sending someone on-site can keep them from sending checks to out & out fraud situations, or pay to air seal & insulate someone's NEW construction (like an addition to their 1980s McMansion) rather than the intended target: Retrofitting sub-code existing buildings.