Gravity Hot Water System

MRC

I recently perfomed preseason maintenance on gravity hot water system in South St. Louis City.
The home is small ranch style with just four radiators.
There is a compression tank mounted in the floor joists.
Manual water feed valve.
This maintenance included cleaning the burners and boiler sections, pilot assembly (including replacing the thermocouple) and inspecting the vent system.
My question(s) are these...

#1~What is the fill pressure for this type of system? The gauge was not showing pressure. I opened the feed water valve and allowed the system to fill to 10 psig on the gauge. After turning off the valve I went upstairs and began to bleed air from the manual air vents. With the feed valve closed, after bleeding the first radiator I had to add more water to continue bleeding the remaining radiators.

The compression tank had been drained way back in the 90's according to someone's marking it with a Sharpie.
#2~Would this type of system benefit from an air separator and Airtrol tank fitting?

My reason for posting this that the homeowner called saying that his radiators are filled with air and system is not heating very well. There was no mention of a relief valve "popping" so I am assuming the compression tank is not water logged. Or maybe it is?

#3~how can you determine the water level in a compression tank?

Any help would be greatly appreciated. The only document I have on this type of system is a design manual from a "dead man" written long ago that I believe Dan Holohan originally posted.

Mike Curran

Ironman

Well, let's see.
1. The static fill pressure is the same for this system as any other. It's based upon elevation and you want to maintain at least 5psi a the HIGHEST point in the system at all times. If it's only one story, then 12-15psi cold static fill is sufficient.
2. An air separator would probably be of little benifit with this system since whatever air enters the rads will always end up in the top of them due to the fact that the sections operate on gravity even when the system has a pump. Apparently, yours doesn't and so putting one on would be of little if any benifit. Furthermore, what air is in the boiler/piping is designed to work its way into the compression tank to maintain its air cushion. You never want to put any kind of air separator on a system with an open tank.
3. Drain the tank, bleed the rads and get the static fill to the correct pressure and the level will be correct in the tank.
4. Install an accurate gauge on the boiler if there's not one.

icesailor

Look at the temperature/pressure gauge. There is usually one side that sats Pressure or Pressure in PSI Guage pressure. On the other side of a line, it will sat "Altitude" or head in feet. Regardless of what you do, fill the system with 12 to 15# of water pressure, Vent all the radiators until there is no air in the system/radiators. Fill the boiler so that the entire system is vented from air except the expansion/compression tank. If the pressure in the system drops significantly in a day or you get a lot of air in the radiators, you might have a leak in the system and you are loosing water.

MRC

Thanks Ironman and ice sailor,
Ironman you advised to "never put an air separator on a system with an open tank". Did you mean to say never put any type of automatic vents on a system with a compression (open) tank? From what I read in Dan's books (Pumping Away and Primary /Secondary Pumping) an air separator such as the IAS and an Airtrol tank fitting is the best way to go. I understand what you said about not needing an air separator in this type of system. Would this type of system benefit from having an Airtrol tank fitting to prevent gravity circulation in the compression tank.

When draining the tank can I open the system to atmospheric from anywhere in the near boiler piping or radiator vents or should I remove another plug directly from the tank?

On your response #3 you stated that I should "drain the tank, bleed out the rads and get the static fill pressure right and the level will correct in the tank". Should the tank be filled last? If so, I have no way to isolate the tank while filling the system. Should the tank be 2/3 water? If so, how do I know when this level is reached without an Airtrol Tank Fitting?

Thanks, mike

icesailor

A system with a "compression" type tank like a 30 gallon expansion Hydro/air tank isn't considered an "Open" system. Open systems are just that. Open to the atmosphere. They depend on a tank somewhere above the top of the system to provide pressure to the system.

"Open" systems depend on air going into large radiators with the water flowing slowly through large pipes.

Paul48

Mine was an open system. The tank was in the attic, and had an over-flow pipe that ran into the main stack for the plumbing. The pipes are still there.

DanHolohan

Another consideration. Most gravity systems had the radiators connected bottom-to-bottom. The water rises to the top of the radiator and the cooler water that's in the radiator on start-up falls. When you're first filling these systems, you have to start with all the radiator vents open and fill one floor at a time. If you don't do this, the air won't vent from the radiators on start-up and you'll have to go bleed them. When you do that, the water level in the system falls and that can lead to uneven heating problems.

MRC

Thanks ice sailor, Paul48 and "Mr. Holamun",
I will admit I was confused regarding the open tank reference. I just assumed that you were talking about a compression tank and that being "open" meant it didn't have a diaphragm separating the air from water. I have read about the open tanks in attics...never seen one.
Forgive me for being repetitive but would the gravity system I am dealing with be a better system with an Airtrol Tank Fitting?
After the tank is drained and is full of air do I not need to fill this tank 2/3 full with water? How do I get the excess air from the tank and how do I know when the tank is filled to the correct level? Excuse for being dense about this.

Gordy

Yes an airtrol tank fitting is a huge benefit for a compression style expansion tank.

Air management not air removal is what you are doing once the system has been purged of air, and the compression tank set properly.

Make sure you get the right size airtrol based on tank diameter.
The airtrol will have instructions actually quite easy to do.

http://bellgossett.com/hydronic-heatingplumbing-accessories/airtrol-tank-fittings/airtrol-tank-fittings-2/

icesailor

Its my experience that, although B&G and others promoted "AirTrol type devices as something better than "home-made" pre-sliced bread at the Supermarket, in practice, the air was absorbed by the water in the tank, or ones with a vacuum breaker plug that would break the vacuum when you want to drain the full tank, were a constant source of an air leak. Sort of like a slow leak in a car tire. After time, the Ground Joint seal on the plug became worn and it leaked air. You could "tape" the threads, but the air still got bye. I had a few accounts with gauge glasses on the end of the tanks. I used to blow compressed air into the tank and get the air level to below 1/2 to 1/3 water for longer times between service. And if I filled it from scratch, there was never more than 1/3 air in the tank.

Don't worry about getting some imaginary perfect level in the tank. It won't matter. If it has one of those special tank fittings that let you break the vacuum to let the air in and the water out, its like those jumping crabs, Sit easy. They can jump as high as they need to. Just like the air leaking out of your compression tank.