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How does the circulator on a boiler work?

How does the circulator on a boiler work?

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pump (circulator)

on a hot water heating system has the same function as the fan on a forced air heating system. We are pushing the heat transfer medium (water) from the boiler out to the rooms we want to heat.

Just like the fan on a forced air system, the pump pushes the heated water out to the system and pulls the cool water back. The pump does not create large pressures and is relatively small. The pump motor is often 1/12 horsepower. It works more like a paddle wheel moving the water slowly through the system, than a fire pump trying to build up pressure.

The pump is typically located on the return piping adjacent to the boiler. Some feel that the pumps work better if they pump away from the boiler. This means putting the pump on the supply piping. Either location is acceptable.

Pumps are usually beside the boiler. They shouldn't be at a low point in the piping because sediment collects here. They shouldn't be at a high point because air bubbles collect here. Air in a pump can damage it.

The pump is driven by an electric motor and there is often a mechanical coupling between the motor and the pump. The pump is an


and does not shut off the flow of water when the pump is idle.

If the pump does not operate, some flow of water can be expected through the system by convection.

The motor should be beside the pump rather than above or below it. The shaft of the motor and pump are designed to be horizontal. If the shaft is vertical, the bearings will fail quickly.

Conventional pumps require lubrication. The electric motor typically has two lubrication ports and the coupler has one. These pumps have been around since the 1930s. Their performance is excellent as long as they are lubricated. If neglected they will seize.

Amore modern and less expensive pump is a

permanently lubricated

pump and motor. The sealed bearings (actually bushings) in the coupler and motor do not require regular maintenance. However, some maintain that their life is shorter than the older type.

Some pumps are

water lubricated

. The pump and motor are integral in one housing. The water in the heating system lubricates the assembly. The motor is sealed so that it can be immersed in the water. These are common on new systems because they are maintenance-free and are inexpensive. Many modern package boilers are sold with an integral pump, sized correctly for the boiler.

The pump may be activated by

the thermostat

electric power being on to the boiler. (In this case, the pump runs continuously through the heating season.)

pump control (aquastat) that senses the water temperature leaving the boiler

Copper tube boilers have to have the pump operating before the burner can come on safely. These very thin heat exchangers need a constant supply of cold water to keep them from overheating.

Many people feel that the best type of pump control is having the pump come on only when the boiler is up to temperature. (This only applies if the heat exchanger can take the heat without water moving through it.) They feel this will minimize condensation in the boiler. When a boiler is warming up, the exhaust gases are exposed to a cold heat exchanger and cold water on the other side. The exhaust gases may cool to their dew point (about 125F) before they escape up through the chimney.

Turning the pump on and cycling cold water through the heat exchanger makes it harder for the boiler to heat up and increases the risk of condensation. Leaving the water in the boiler while it's heating should help reduce condensation. Systems that use an aquastat to control the pump usually do not have the pump come on until the boiler water temperature is 110F to 140F.

When the boiler shuts off, some pumps shut off immediately. This leaves heat in the heat exchanger that finds its way up the chimney. It's more energy-efficient to keep the pump running until the heat exchanger has cooled. We may as well capture as much of the heat as we can.

If the pump runs continuously throughout the heating season, the condensation problem exists, but the efficiency problem on shutdown does not.

Pumps are sometimes used to control various zones in heating systems. Individual thermostats for various parts of the house can activate their own pumps to move water through that part of the house. Where zone valves are used, one of several zone valves can be opened and the main pump can be activated at the same time, to heat a single zone.

A pump does not provide a positive shutoff to water movement when the pump is at rest. In a zoned system, this can lead to overheating of a zone that's not calling for heat. A flow control valve is often put into the system to prevent water flowing through a zone when the pump is off. When the pump is activated, there is enough force to open the flow control valve. When the pump shuts off, the flow control valve closes, and there will be no gravity or convective flow through that zone.

The flow control valve also prevents reverse flow through an idle branch, which is possible on some systems, when another zone is working.

The flow control valve is also often used whenever the boiler is kept hot on standby, even if there is only one zone. Without it, the system would operate as a gravity (convective) system when there is no call for heat. The house may overheat.
by Manage My Life
April 26th, 2007
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