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Baseboard Heating

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Baseboard Heating Water Flow Question

The Home Wizard app calculates your ideal home care program to avoid problems with your Baseboard Heating, but sometimes trouble can still occur. Here are answers to questions about baseboard heating water flow question.


How does the pump in a closed hot water heating system(furnace)make the water flow?

Where does the air come from that is released by the automatic bleeder valves?


Dear Scott:

Regarding your first question, the way the circulation pump in your closed hot water baseboard heating system works is that when the thermostat in your home senses that the temperature is too low in the room for this zone of your heating system, it sends a signal to your furnace (boiler) to turn on, and also sends a signal to the circulation pump for this zone to turn on and start circulating the water heated by your furnace. This sends hot water to the baseboard radiators in the rooms that are part of the zone of your system that are controlled by the thermostat. When the thermostat then senses that the temperature in the room is high enough, it sends a signal to turn off the furnace and circulation pump for this zone. And similarly for other zones in your home.

Now if your question is specifically, how does this pump make the water flow through the pipes if the system is closed? The answer is that as the rotor inside of the pump spins around, it causes the pressure to become lower on the inlet side of the pump, and the pressure to become higher on the outlet side of the pump. This causes water to flow from the low side of the pump to the high side of the pump, which causes the water to circulate though the loop of your piping system. Further, as your furnace heats up the water in your closed heating system, the water expands, and needs somewhere to go, which is why your system will be designed to have an "expansion tank" to give this additional water a place to go.

Regarding you second question, air can come into your hot water system in several ways, such as:

1) air is dissolved in solution in the fresh water that comes into your system as make-up water to replace the water that is lost from leaks, and then when this fresh water goes through your furnace and is warmed up and then cools down, the air that is entrained in this water is released and comes out as bubbles.

2) if you have leaks in the seals of your water circulation pump, then when the pump turns on, it can suck air into your system.

3) if you have a leak in your expansion tank, it could possible be a source for air getting back into your system.

Here is a link to a webpage which gives a general description to how a baseboard hot water system works, that you might find interesting:

Hope this is helpful.

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