GPM vs Minimum FPS To Keep Air In Solution

D107

A heating zone's design heat loss is 10Kbtu which requires only 1gpm through 3/4" copper pipe but that would be a velocity of only .73 feet per second, far below what is said to the required minimum of 2fps to keep any air in solution. On a shoulder season day, zone heat loss could be only 3000Kbtu which would mean far less than 1gpm could cover it. But do you overpump to meet the fps requirement? OR if you properly remove most of the air, can you then pump at the best speed to cover the heat loss? And then how do you factor in the larger near-boiler piping, and old 1" gravity risers into the equation?

EBEBRATT-Ed

Basically if the pipe is too large air can get trapped at the high points and with the velocity being low the water cant move the air back to the air vents or air separator.

Just make provisions to be able to vent the high points. Once you get the air out it should be ok. Use a good microbubble air separator

Air will be absorbed into and out of the water as the water temperature changes and you will probably have to bleed everything a few times.

Pretend you are an air bubble inside the pipe where will you get trapped? Put vents there

Ironman

If it's an old gravity system with CI rad's, air is going to the top of them no matter what you do. As Ed said, just expect to have to manually vent them more than one time.

The rad's are the greatest air separator you could have.

D107

Yes two zones are ci rads. We have caleffi auto vents on them--so much easier to deal with than the old keyed vents. Also have a caleffi discal air separator. But given what's been said about air coming in and out with water temp change, do you design pump speed to cover heat loss (1 gpm for 10kbtu loss) or for 2 fps?

Jean-David Beyer

Basically if the pipe is too large air can get trapped at the high points and with the velocity being low the water cant move the air back to the air vents or air separator.

Just make provisions to be able to vent the high points. Once you get the air out it should be ok. Use a good microbubble air separator

When I had my heating contractor replace the two 3-foot long baseboard units with two 14 foot long baseboard units, they put in no air bleed of any kind. I asked why not. He said they were not needed with my system. And it turns out that is correct. Even though the baseboard are upstairs, and higher than the indirect and the radiant slab.

When they first put the system in, they did have valve downstairs where the boiler is, and they ran water in and out, and when the bubbles no longer came out, they said they were done. Now most of that pipe is either 1/2 inch or 3/4 inch copper. And all in series with a Taco 007 circulator. About 110 feet of pipe, with lots of 90 degree elbows. It does have a fancy Taco microbubble air eliminator. I could hear air bubbles in there for a couple of months until the air in the high spots dissolved into the water and was removed by the microbubble air eliminator. But it did.

D107

Thanks for all the answers; just that my original question is not fully answered. Given good air separator, proper purging, venting etc. do you set the circulator (most easily done with the new variable speed settings of ECMs) to cover the heat loss or do you try to ensure the fps velocity that entrains the air? It could be the difference between 1gpm (for 10K btu) and 3gpm, which would cover the velocity but possibly overpump.

Ironman

The heat loss at the design delta T.

hot_rod

ideally both, adequate gpm to maintain 2-5 fps for air entrainment and to provide required heat transfer
With properly applied variable speed smart pumps you should be able to accurately match flow rate to required and changing loads

modern micro bubble air eliminators include low velocity zones and media to scrub air across a wide flow range