I am sure the cold ones are on the same zone as the hot ones because I turned the other 2 zones off while I was bleeding the cold unit, and eventually, the cold unit got hot. Additionally, even when all 3 zones are heating, these 2 baseboards still stay cold. I'm at a complete loss; you may be correct about a blockage. I hope that's not it.

All units are on the same level - this is a 1-story home.

Careful of that expansion tank. If it does not have a bladder, it is supposed to have air in it.

Are you sure that you have a simple loop in that zone with no (mono flow) T's or bypass thermostats?

ionized - sorry if I wasn't clear. There are two valves on the expansion tank. The "main" valve where you regulate the tank's air pressure, and then a smaller knob, similar to a car tire stem valve cover, that you turn very slightly so that any free air in the lines gets purged out. It is the smaller valve that is not releasing any air from the pipes (which is a good thing.) The "main" valve shows the tank itself has a PSI of 20.

I do not know for sure that there are no mono flow T's. I'm fairly certain there are no bypass thermostats, but I'm not sure where I'd look for one.

Spent some more time tonight draining water from the non-functioning baseboard to make sure there wasn't air or some other obstruction blocking flow after about 3 gallons bled out, the water started to get hot again, both baseboards got warm and were emitting heat at about 98*. As soon as I closed the bleed valve, the baseboards cooled back down.

I'm thinking Snidely may be correct, that there's a blockage in the line that leads from the baseboard back to the boiler. Any ideas how I might check that?

Do the baseboards have shut off valves? Have you tried shutting off the baseboards on zone 3, except the two that are cold?

Sounds like the water is following the path of least resistance, and not flowing to the two baseboards. I had this problem, and used the shut off valves (partially open) to balance the system.

Geoffrey - they do not have shutoff valves. That would be a great recommendation if they did!

There are lots of boiler/hydronic experts at the following fora. This is a great forum but there is a lot more activity in these areas and some sharp people posting:

http://www.terrylove.com/forums/index.php and http://forum.heatinghelp.com

If you do a web search for "monoflow T" you will find a much better explanation that I can field. The mechanical bypass t-stats that I am familiar with would be right on the radiators themselves and you'd need to have a bypass pipe with them as well. Electrical valves could be elsewhere. All of this kind of stuff puts you outside of the "simple loop" concept. have you been able to view and evaluate the whole of the pipes that run your system to be sure what you have?

You bled the other radiators in the problem zone? Still sounds like there is air and it is preventing water from flowing to those two baseboards. Turn the boiler fill valve on, and try bleeding all the radiators in that zone. Don't forget to turn the fill value off when you're done.

OK, I will give this a try.

Do you suggest I open all of the baseboard heaters with bleed valves in this zone at the same time or one by one?

I would start at the baseboard closest to the boiler. Don't bleed a long time, just make sure there's no air - the air should bleed out right away. Then go to the next and the next. Don't forget to close the boiler fill valve when you're done.

If no difference try this: shut off the zone, turn the other zones on, and then bleed the baseboards - this way it will bleed the air out of the return instead of the supply.

Is the boiler in the basement and the baseboard above, on the 1st floor? Or everything on one level?

Everything is on one level.

I have bled and re-bled every individual unit and also performed a full purge at the boiler itself.

Just a quick question to help me clarify. When you say close the boiler fill valve what do you mean? Mine is set up so that water is automatically added when it is needed. If you are talking about moving the lever that is typically on the pressure reducer valve, then I don't have one of those levers. My pressure reducer valve does not have the option to temporarily increase the water pressure. I am wondering if this is the reason the major flush did not work? Perhaps there was inadequate pressure to dislodge any air bubbles?

Jay, here was my situation many years ago. I purchased a 'state of the art home' - built in 1950. It has hydronic hot water heat in the ceilings. Yeah the ceilings are about 2" thick, and weigh tons. Anyway, in order to remodel I had to open the system. There are no vents to purge air. Of my 4 zones, the lower level zone (same level as the boiler) would not heat. Tried purging air at the boiler.

A really old plumber fixed it: He had a pump with enough force (more than 15 psi) to purge the lower zone. Pump drew water from trash can, forcefully pumped into zone, water returned to boiler - trash can. After a couple of minutes it was working!