Temp needed for water to circulate in gravity system?


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Old 01-26-07, 05:23 PM
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Temp needed for water to circulate in gravity system?

Hi again, I have another question. What temperature must water be for it to begin circulating through the hot water heating system via gravity? I have a circulating pump, but I don't want to aquastat for the pump set higher than the temp needed for gravity to begin moving the water. Any thoughts?
 
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Old 01-26-07, 06:26 PM
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Water temp

If you have a circulator, you probably also have a flow control valve. The combination of the two (unless you manually open the valve) prevents gravity flow regardless of temperature.
 
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Old 01-26-07, 08:18 PM
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There are so many factors... if you pictured your heating systems flow like a ferris wheel, the smoother the path up and the smoother the path down the better. Assuming you have one circ and no flow check valves like Grady mentioned, any up down flow away from that shape would cause thermal traps. In the absence or any mechanical traps or thermal traps, theoretically it should take a very small amount of heat to start a flow. Mind you, that flow would be very slow. The flow depends on the amount of heat being applied. How much flow you consider circulation would be another factor yet...

Why are you asking and how is your system piped?
 
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Old 01-26-07, 09:40 PM
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Reason for asking

Thanks Grady and Who for the responses. I was asking because, as I posted about two weeks ago, my boiler was not heating the water to the temp at which the aquastat for the circulator was set (160). So I guessed that the hot water had been getting to the radiators via gravity, since the circulator was not turning on. But I have turned the circulator aquastat down to 130, so the hot water circulates much quicker now, thereby heating my house faster.

With temps dropping here in Philadelphia (20s-30s), I have noticed that my boiler is running much more often. The radiators are warm, but not warm enough. I had considered turning up the circulator stat to 145 or 150 so the radiators will get hotter, thereby keeping my house warmer for longer periods of time. I was thinking that the higher stat setting would cause gravity to circulate the hot water before the circulator were to turn on. But it sounds like this would not be possible because of the flow control valve.

Any thoughts?
 
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Old 01-27-07, 07:41 AM
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Originally Posted by atphilly
The radiators are warm, but not warm enough.
Define "not warm enough" .

Is the room thermostat being satisfied ? The rads are only going to get hot enough to satisfy the thermostat, once the t'stat stops calling for heat, the boiler will shut down.
 
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Old 01-27-07, 01:59 PM
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Cool

Gravity systems were designed with large headers and returns (2" or larger) it was a very slow and difficult to control operation. The gravity flow central warm air system was another breakthrough of the same era. This consisted of a warm furnace in the basement & cutting large holes into the floor to allow the warm air to rise & circulate. Later on they did it with large ducts connected to a massively oversized heating plant.

3/4" copper pipes with lot of turns and 90's will not allow enough gravity flow to satisfy the needs of a home.
 
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Old 01-28-07, 11:48 AM
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Radiators not warm enough

Originally Posted by NJ Trooper
Define "not warm enough" .

Is the room thermostat being satisfied ?
Yes, the room t-stat is being satisfied, in that the desired set temp is being reached and maintained. However, the boiler seems to be running more often than it might need to because I had the circ pump stat set at a temp that would not allow the rads to become warmer and stay warmer longer. Since I bumped up the circ stat to 130 (from 120), the rads are becoming warmer, causing the boiler to run less often.
 
 

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