hot water boiler with zero pressure air in pipes - how to purge/bleed air

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Meng

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I have a hot water boiler that has zero pressure, the cold water fill valve turned off, and air in the pipes (hearing periodic gurgling in the pipes and water circulates through the baseboard radiant units). There are two drains built into the piping connected to the boiler. One drain is connected to the supply zone manifold and one drain connected to the return piping near the circulation pump. When I open the cold water fill valve to raise the pressure in the system a the vent outlet on the fill valve drips a bit.

My questions are:
1) can I plug the vent on the fill valve to prevent dripping while I pressurize the system and keep it plugged, or is that a bad idea? I'm not totally clear what function that vent serves.
2) after opening the fill vale to introduce pressure to the system, which drain do I open to purge air from the system? The drain on the zone manifold or the drain connected to the return near the circulation pump?
 

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Meng

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This is a picture of the return pipes with shutoff, circulating pump, and return drain valve (located under the circ pump, but not shown in the picture).
 

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Jadnashua

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First, you should never drain your system unless it is required for some maintenance. Water from the tap has dissolved oxygen and other stuff in it that with repeated replenishment will spell havoc for the internals of your boiler, especially those parts that have iron in them. Once it has been in the system for awhile, all of that 'extra' stuff is neutralized (essentially, it rusts stuff, but once all of the oxygen is captured in the rust, it stops).

A drain plug on a valve has a little seal inside...if it leaks, it may either not be tight enough, or need a new seal.

Running the system with zero pressure will cause problems...just like in your car's cooling system, the water will expand when it heats, and without some pressure two things happen...it can boil easier at the heat exchanger and second, you'll get cavitation at the circulator which can cause excessive erosion of the impellers and the pipes around it.

You'll need a purge valve somewhere, preferably high up in the system to let the air out as you add water. It can be done with one lower, but it is much harder to extract all of the air in the system since it wants to rise to the high point. High enough pressure and flow can push most of it out, but it can take awhile. You can't get the pressure too high, or you'll open the over pressure safety valve on the boiler.

Not sure if this answers your questions.
 

Meng

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This is a picture of the boiler with the supply and return pipe locations shown
This is a picture of the return pipes with shutoff, circulating pump, and return drain valve (located under the circ pump, but not shown in the picture).
First, you should never drain your system unless it is required for some maintenance. Water from the tap has dissolved oxygen and other stuff in it that with repeated replenishment will spell havoc for the internals of your boiler, especially those parts that have iron in them. Once it has been in the system for awhile, all of that 'extra' stuff is neutralized (essentially, it rusts stuff, but once all of the oxygen is captured in the rust, it stops).

A drain plug on a valve has a little seal inside...if it leaks, it may either not be tight enough, or need a new seal.

Running the system with zero pressure will cause problems...just like in your car's cooling system, the water will expand when it heats, and without some pressure two things happen...it can boil easier at the heat exchanger and second, you'll get cavitation at the circulator which can cause excessive erosion of the impellers and the pipes around it.

You'll need a purge valve somewhere, preferably high up in the system to let the air out as you add water. It can be done with one lower, but it is much harder to extract all of the air in the system since it wants to rise to the high point. High enough pressure and flow can push most of it out, but it can take awhile. You can't get the pressure too high, or you'll open the over pressure safety valve on the boiler.

Not sure if this answers your questions.

Thanks for your response. My baseboard heaters don't have a pressure relief valve available. I checked them all. They are all solid 2/4" copper pipes without relief valves anywhere. The piping is essentially direct runs through my three zones as the baseboards connect together (e.g. Zone 1 loop looks something like this - the Zone 1 pipe leaves the boiler, runs through the joist cavity in my basement, goes up through the floor to the baseboard heater in the master bedroom, back down to the basement joist cavity, up to the baseboard heater in the guest bedroom, back down to the basement joist cavity, and back to the boiler).

From what I understand the process to remove air from my boiler and baseboard heaters is to open the cold water supply line, connect a garden hose to my drain valve, and run the drain valve hose to a 5 gallon bucket. I watch the water drain and the air bubbles emerge while watching to ensure that the pressure in the system doesn't exceed 15 PSI (or thereabouts). After a while there will be no more air bubbles, and my system will be fully purged of air. I then shut off the drain valve, get the system up to about 10-12 PSI for the appropriate working pressure, and I'm good to go.

Does that help you understand the problem I'm trying to solve? Essentially I wanted to know if it's bad to run my boiler heating system with zero PSI, which you helped me confirm, and also if it's OK for me to cap off the side vent on my cold water supply line (which drips a bit when pressure is introduced to my boiler system) so I can add water pressure to my system without dripping water on the boiler unit.

Let me know if you need additional pictures of my setup, or have any clarifying questions I can answer to help you understand what I'm dealing with.
 

Jadnashua

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There may or may not be bleed valves under the covers of your baseboards. It's hard to get all of the air out of the pipes when they go up what could be several stories before it goes down again to your drain valve. You'll have a much easier task if you can find or install a bleed valve (not the same as a pressure relief valve!) somewhere high up in your run(s). It takes a fairly forceful flow to push the air faster than the bubbles want to rise when trying to purge the air out.

THey also make air extractors for your heating pipes. One product that tends to work reasonably is a Spirovent. The less expensive and therefore more common one looks like sort of a top hat and has an internal float and gaskets in it. Once enough air has accumulated, it opens up and lets that air out, then closes until more accumulates. Both of then tend to need service or replacement if they get gummed up or clogged to regain their useful function. The Spirovent typically can be serviced, the other usually replaced when they stop working.
 

Dana

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Bleeder valves have a few different form-factors, some more obvious than others. On baseboard systems they will usually be under the end-caps of a section of baseboard:

p_SCP_237_08.jpg


AirBleedValve113DJFs.jpg


Air_Bleed_Valve_0491_DJFs.jpg



Even with air in the system you will be able to pressurize the system. For almost all 1-2 story houses 12 psi is the proper operating pressure. If it's reading very low pressure at the boiler, there may be insufficient pressure to even raise the water to the highest point on the system, and opening the bleeder valves will let air IN rather than let it out.

Air purging vents can be anywhere on the system, but work best (read "more quickly & reliably") when located at the highest elevation on the system:

Hydronic%20Heating%20System%20-%20Automatic%20Air%20Vent.jpg


Often they are located on top of an "air scoop" device near the boiler:

22982d1387050916-fitting-upgrades-additions-old-cast-iron-rad-hydronic-system-img_1565.jpg


Purging air with a hose at a system drain near the boiler is slow and not usually necessary, and may even just short-circuit flow from the filler valve through the boiler or other plumbing to the drain valve, never purging the zone plumbing.
 

Meng

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jadnashua/Dana, thanks for all your help. You both helped me better understand how my system works, and what maintenance may be required.

After reading through your responses I found that my system has a Spirovent air removal device that is located on top of the expansion tank. A few days ago I installed a 1/2" plug on the cold water supply vent to prevent the leaking/dripping, added pressure to the system so it measures 12PSI while operating, and so far I have not heard any additional "gurgling" as water circulates through my baseboards! I'm not totally confident that I solved my problem with air in the pipes, but I am totally confident that I improved the overall operation of my system. I'll keep listening and monitoring the system, and will report back if I have follow-up questions.

Seriously, thanks again for your help. It's great to bounce questions off of experts who take the time to offer advice in addition to context and "the why" which supports that advice. Very helpful!
 
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