The hot water radiators in two of my four zones heating my home are not getting hot even after increasing their thermostats to 80 degrees F. If there is a bubble in the circulating pipes how and from where do I bleed the system? I have a relatively new Weil-Mclean furnace (three years old). Thanks!

Do you have Old fashion standing rads? If so then bleed them with eith er a little screwdriver or key, until you get hot water from the rads... always start at the top.

Thanks for your response. I do not have old fashioned standing radiators--I have base board pipes with no apparent valves for bleeding them out. On the furnace itself, there are faucets attached to pipes for each zone. If I open the faucets for the cold zones will that bleed the system allowing the water to circulate?

Yes.. if you do it right.you have to isolate the zone 1st. There should also be a valve below the spigot.. which needs to be closed. The power should be off , so the circulator isn't running . 1st off you need to track where the fresh water is piped into. Older systems and ones installed without the circulator on the supply usually had the fresh water makeup piped to the return somewhere or directly to the top of the boiler( which is OK ) as long as its on the boiler side of the valve below the faucet.
If that's the case then you're going to have to be careful here not to overfill the boiler.. the releif valve will pop allover floor... open the fastfill valve on the water feeder or turn open valve if it has no feeder... fill till boilr pressure is slightly above 25... then with a hose attached and the valve below it closed... open the spigot and let the water flow through the isolated zone that has the issue... now keep track of the pressure while you're doing this... very important.. 1.. you don't want to pop the relief... 2nd ity won't work so well with no pressure... so it takes a careful mix of opening the valve and watching the gauge... whats happening here is we are forcing water into the boiler.. its going to flow through the zone with the hose on it and force out any air in the process... you can dso this with a cold boiler... but I find it easier when you have hot water in the boiler, then you can actually feel the return pipe getting hot because we forced all the hot water through the zone and out the "purge valve"... once the return pipe gets hot, we can assume that the bulk of the air is out. Then shut off spigot, open purge valve and also make sure pressure is back at normal range... then turn boiler on and circulate in that zone to see if return is getting hot..

Could not of explained it any better.