Hot water heat was designed for many years to heat a house without pumps. In fact, without electricity, they burned coal, which was fed by hand. The boiler was placed in the basement and the water circulated up through house by gravity, the cast iron radiators and the large pipe size allowed the water to flow easily. The first pumps were called booster pumps and were introduced to boost the flow in these gravity systems to make them more dependable. These were soon adapted by the plumbers to heat homes that did not have the elevation to make a gravity system work. The early pumps did not have the insulation in the motors to handle heat. The boilers had very poor controls on them. The supply side often  produced steam, so the pump had to be put on the return side to protect it.
   The early pumps were very low head and the plumbing system had no resistance in it, but as baseboards were introduced the pump head had to be increased to make them heat properly.  Boilers today have very good fail safe controls, and excessive water temperatures are a thing of the past.

   Today's heating systems often have high head requirements built into them, such as radiant tubing, zone valves, or mixing valves. The new cartridge or sealed pumps are very well suited to these higher head requirements. However they do present different problems to the designer or installer.                     

PUMPS:    Air wants to go up in water. When you try to pump water down with air in it, the air is constantly trying to go back up the pipe. When the air reaches the pump impeller it causes the pump to cavitate, which slows down the water flow. If, it is only small bubbles they will bounce in the pump causing noise and the overheating of the pump. Larger amounts of air will completely stall the water flow, causing the pump to overheat and burn out.
   THIS CAUSES MOST EARLY PUMP FAILURES;   When the pump moves water upward, the air moves freely through it, and the heat from the motor is carried away by the water.

   Here is a very misunderstood fitting, that is seldom used properly. The problem that a dielectric union is suppose to solve is a dissimilar metal problem between steel and copper, such as a steel water tank and copper pipe.
   Dielectric unions in a hot water heating system are nothing but trouble and usually leak in a short time. The antifreeze, boiler additives or acid water conditions cause the gasket to deteriorate.
   There are better ways to handle this problem:   
   FIRST REMEMBER THAT COPPER HAS NO PROBLEM WITH BRASS OR CAST IRON, IF YOU PUT A BRASS OR CAST IRON FITTING BETWEEN THE STEEL AND THE COPPER THE PROBLEM IS SOLVED.
   EVEN ON A HOT WATER TANK, A BRASS NIPPLE OUT OF THE TANK WOULD BE THE BEST CHOICE.

RUST OR RED WATER IN A BOILER:   Rust in the tubing is a sign that there is air getting into the system either because of a leak or tubing without a oxy barrier. Cast iron or steel boilers are very susceptible to damage if there is any air in the water. I have seen boilers completely destroyed by the use of Poly-B or HDPE tubing, the rust will completely fill the boiler up with rust. The best solution is to simply replace the boiler with a unit that can handle the oxygen, flush the system and start over.