I have a hot water heating system it has two zones one side is ok. The other side doesn't heat well?

bleed the air from the second zone before you try anything else. Chances are it is airlocked and the water can't circulate. Turn the power off to the boiler, close off all valves going to the known working zone to prevent air from being pushed into that zone. Close the valve that feeds the 2nd zone, then bleed the air from the system, insert the end of the hose (if possible) into a bucket of water. This will allow you to see when the bubbles stop.

Now, open the feed valve for the bad zone and close the return valve and bleed it the same way. When the bubbles stop, open all valves again, turn the power back on to the boiler. Check to see that all registers are heating, turn the thermostat up. If they don't I would guess that you have a bad zone valve or thermostat.

The split doesn't matter, the system is closed, check to see that your bolier pressure is somewhere around 15 - 20 psi. If the other zone is heating and your pressure is good, the pump should be too. Without actually seeing your system and its setup, this is my best guess.

I have read your question a couple of times.. It can't be the pump, because you are getting heat in another zone. The pump just circulates the water. The boiler is a pressurized vessel so the amount of lift doesn't come into play.. Look where you zones are, each should have a zone motor, a little metal box about 2 1/2 inches by 5 inches, open the cover see if the lever on the bottom is to the right. If it is slowly push it to the left,, that is open..I had this problem last winter in one of the buildings and it turned out to be the zone motor.. Then are easy to repalce but you do have to drain the bolier down a bit, then pump it back up to desired pressure.. I hope this helps

I assume that all valves in the system are open. Have you checked for air in the line that does not seem to be heating? With the type of system you describe, it would appear the a single pump is driving both circuits. The level circuit will have the least flow resistance and hence take more of the flow. If there is air in the upper circuit this will make the problem worse. Consider installing a second circulating pump in the upper line.

I am not sure you have a circulator problem as much as a piping problem. Generally speaking, 3/4" copper tubing will carry only about 40,000 BTU's of heat, which equals about 66-68 feet of radiation typically. If you have more than that, and it is not on a larger size line (1" carries about 80,000 BTU), then that could certainly be the issue. The way the piping sounds like it goes up & down quite a bit, perhaps some air trapped in the system, or even system pressure too low.

Lots of options,

good luck

Steve