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The boiler should have a temperature gauge and a pressure If the boiler has been running and the temperature is up in the 180 degree range, adding cold water could crack the heating block. You'd need a whole new boiler.

Sometimes it takes a few tries to get all the air out. Air may be trapped in lower radiators, ans as you bleed and water cycles in, the air from lower units/rooms will work it's way up. Then you can bleed again.

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What happens if you add water while the boiler is still hot?

How do I find out if the boiler is hot or not???

The low pressure might also mean one of the old radiators/pipes might be leaking in one of the apartments.

Whatever you do, don't add water while the boiler is still hot!

Thank you all for the replies and info - will get the pressure checked out.

PS we have the old school cast iron radiators.

Yea you should always get water. Top floor will take longest to bleed since furthest from the boiler. Sounds like two things, first could be you just need a thorough bleed to get all the air out and that can take a while OR you dont have enough pressure. One boiler for multiple floors could be a big issue depending on the size of the equipment.

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Exactly. There's a pressure regulator to drop the pressure since hydronic systems don't like to run at full city pressure. Perhaps its malfunctioning.

Dcourts, that only works for hydronic fin baseboard, not the less common cast iron radiators or cast iron baseboard. A lot of old houses were retrofitted from steam to hot water leaving the old radiators.

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Yes, I'm on the top floor.

So - every time I bleed, once all the air is out, I should ALWAYS get water, regardless of whether the radiators are hot or cold. Is this correct?

Are you on the top floor? If you don't get water to come out when you bleed the radiator, then the pressure is probably too low on your boiler.

Robin.

Best bet would be to bleed the units from the boiler itself. Just did this myself earlier in winter. Attach a hose to boiler and just have some place for the water to go (it will be warm). Once you get all the air out of the house at the boiler, you are all set. There is a pressure valve that depending on your unit, may need to be replaced, not giving enough pressure to send to all the radiators.

Hope this helps.

We have hot water radiators in our small building - the boiler is located in basement, and is controlled by one thermostat in a common area. The one boiler system warms up all the radiators for all units in the building.

When I bleed my radiators, once I get all the air out (ie when I no longer hear the hissing sound), am I suppose to get water (either hot or cold) every time once all the air is out (ie shortly after the hissing sound stops)?

For my unit - sometimes when the hissing sound stops, nothing comes out. I have sometimes waited for 5-10 minutes, and nothing comes out (usually when the radiator is cold).

Is that normal? Or does that indicate the pressure is not set right on the boiler?

Also - there's usually one radiator that always have more air than the rest, and is usually colder / takes longer to warm up than the others (we have about 6 in total in our unit). What does that mean?

TIA!