As the heat loss goes down on warmer days we cannot change the amount of radiation in the home, but we can change the water temperature in the system. As the system water temperature goes down the heat output from the radiation reduces also. We are trying to match the heat output from the radiation to the heatloss of the home. This is performed by a control called an outdoor rest control (ODR). The mod/cons will normally include an ODR control. This will automatically sense outdoor temperature and adjust the system water temperature to a given water temperature using what we call a heating curve. The boiler will have minimum and maximum water temperatures programmed into it control. These are known as the high limit and foot temperatures. The fuel input will also change or modulate. As the supply and return water temperatures get closer to the system temperature required the gas input reduces. This will extend the boiler run time at a lower input so the boiler does not short cycle. All this combined will increase the comfort level very dramaticlly. 

Most of the mod/Con boiler heat exchangers are made of either cast aluminum or stainless steel. They hold a very small amount of water. Some are as low as about 1/2 gallon. The stainless steel is more popular than the cast aluminum but the cast aluminum has more heat transfer. The cast aluminum requires a special anti-freeze if your system requires it where the stainless steel will use standard boiler antifreeze. Since the boiler heat exchangers hold a very small amount of water and can have larger heat inputs, the proper flow is critical. Some manufacturers will offer circulators with their mod/cons and some don't. Either way they will usually have charts, which will offer the proper circulator or at least the information to choose the proper circulator.

The pressure drop through the heat exchangers and the connected piping determines flow through the boiler. Any straight pipe, elbow, tee, coil, valve etc the water passes through will create a resistance to flow. All this resistance will have to be overcome by the circulator and maintain the proper gpm flow through the boiler, system and indirect water heater if used. If the boiler is installed in a system with an indirect water heater this is where things can get a little tough. As the boiler gets larger for larger homes the flow required through the indirect may be less than the required flow through the boiler.  You could have a situation where the boiler may require a 15-gpm flow and the indirect would require a 6-gpm flow. This can change the way the boiler and indirect get piped and wired. The mod/com boilers will usually control the boiler and indirect circulators through their master controls. The indirect will be prioritized which means when the indirect water heater needs to make more hot water the heating circulators shut down until the water in the tank is hot again.

Almost all the Mod/Con boilers require primary/secondary (p/s) piping. See explanation and drawings of p/s piping. This creates a situation where flows in one part of the system do not affect other flows in the system. The boiler piping could be a primary or secondary pipe dependent on how it is piped.  The indirect could be piped from the boiler secondary piping, boiler primary piping or the primary loop pipe.