What is the sequence of operation for a boiler?

expert EXPERT April 26, 2007

There are several possible operation sequences. We'll talk about a few of the more common ones in these two scenarios:

1. a system where the water in the boiler is kept hot all winter
2. a system where the boiler is allowed to cool when there is no demand for heat

Start: The house is comfortable. There is no call for heat. The water in the boiler is kept at anywhere between 100F and 200F. The burner comes on and off as necessary to keep the water in the boiler at this temperature. A primary control (operating control) is used to keep the boiler heated within the desired range.

The circulator is at rest. A flow control valve is normally used to prevent convection flow of the heated water through the system, since this might overheat the house.

When the house cools a. the thermostat calls for heat b. the circulator is activated c. hot water flows through the pipes to the radiators d. the rooms are heated by the hot radiators e. the boiler sees cold water when the circulating pump comes on f. the water temperature drops below the cut-in setting on the primary control g. the burner is activated h. the thermostat is satisfied i. the circulator shuts off j. the burner keeps firing until the water in the boiler reaches the cutout temperature of the primary control k. the burner shuts off l. the burner cycles on and off as needed to keep the water hot

2. The boiler cools when the system is at rest We'll look at three different possibilities. a. The circulator runs continuously In this system, the circulator runs continuously throughout the heating season. There is no flow control valve.

When the system is at rest, the water in the boiler is cool (perhaps as low as room temperature) and the burner is off.

1. The thermostat calls for heat.
2. The burner comes on.
3. The water in the boiler is heated.
4. The circulator carries the hot water through the pipes to the radiators.
5. The radiators heat the rooms.
6. The thermostat is satisfied.
7. The burner turns off.
8. The water cools.
9. The pump continues to run.

b. The circulator is on while the burner is on. This system is typical with copper-tube boilers, for example. These boilers can't afford to have the burner on when the circulator is not. These boilers do not typically have a flow control valve.

When the system is at rest, the water is cool (perhaps at room temperature), the circulator pump is off and the burner is off.

1. The thermostat calls for heat.
2. The burner and pump both come on.
3. Water in the boiler heats up.
4. The heated water is carried through the pipes to the radiators.
5. The radiators heat the rooms.
6. The thermostat is satisfied.
7. The burner turns off and the pump turns off (Note: sometimes the pump runs for a couple of minutes after the burner shuts off).
8. The water in the boiler cools.

c. The circulator comes on at a set temperature. In this system, there is typically no flow control valve. At rest, the water is cool, the pump is off and the burner is off.

1. The thermostat calls for heat.
2. The burner comes on.
3. The water in the boiler heats up.
4. When the water in the boiler hits the cut-in temperature for the pump (e.g., 110F), the circulator comes on.
5. Hot water is carried through the pipes and radiators.
6. The radiators heat the rooms.
7. The thermostat is satisfied.
8. The burner shuts off.
9. The pump runs until the temperature drops below the pump cut-in temperature (e.g., 110F).
10. The pump stops.
11. The water in the boiler cools.

There are many variations. Zoned systems can have different operating sequences. Large systems and some radiant systems don't use room thermostats to control the heat. These use outdoor temperature sensors to adjust the temperature of the water circulating through the system. On these systems, the boiler water may always be kept hot. A water blender may be used to mix cold water in with the hot water to deliver a predetermined temperature of water through the distribution system.