Can you install an air vent w/o draining pipe?

sunlight33

I have some air in my heating system that I need to get rid of since I installed a drain on the LLH last week. One of the baseboards on the second level happens to have a manual air vent, if I close the shutoff valves to this zone can I quickly unscrew this vent and put an auto air vent in without getting into a mess?

Grallert

You'll need to take the pressure off the system first. The higher the pressure in the system the higher the water will shoot out of that 1/8" hole.

sunlight33

Air pressure is 14.7 psi, if I lower the system pressure to below 14.7 psi will water still shoot out?

Jamie Hall

sunlight33 said:

Air pressure is 14.7 psi, if I lower the system pressure to below 14.7 psi will water still shoot out?

You are making the common error of mistaking gauge pressure -- which is measured with relation to the surrounding atmosphere -- and absolute pressure.

The absolute pressure of the atmosphere on a standard day at sea level is, as you say about 14.7 psi. The gauge on your boiler, however, reads gauge pressure -- not absolute -- and so if it reads for instance 15 psi, that is the pressure in the system over the atmospheric pressure.

Which is a long way of saying no, you need to reduce the system pressure to the point where the gauge pressure at the air vent is zero or close to it.

Now you don't have to reduce the system pressure to zero, however, at the boiler. The pressure in the baseboard will be that system pressure at the boiler, less the elevation head to the baseboard. That is a change of 1 psi for every 2.3 feet of vertical elevation. So, just for example, let's suppose that the baseboard is exactly 9.2 feet above the gauge on the boiler. 9.2 feet equates to 4.0 psi. So if you reduce the system pressure to 4 psi, the gauge pressure at the baseboard will be 0 -- and you won't get wet. Or at least not very wet...