Gravity hot air system

Gravity hot air system

PostBy: e.alleg On: Tue Apr 03, 2007 12:26 pm

What is the problem with having a gravity forced air system? I'm talking about having a heat exchanger in the ductwork with hot water flowing through it, or just the furnace hooked to the ductwork and no blower fan. The heat should rise steadily and constantly through the ductwork, no? I hate my 1500cfm blower, I feel that it is too loud. I understand why it is needed for my propane furnace, the hot air is only temprary and needs to get blown through the house to warm it but a contant heat source should provide 24 hour warmth with no blower unless I'm missing something.
e.alleg
 
Stove/Furnace Make: EFM
Stove/Furnace Model: 520

PostBy: LsFarm On: Tue Apr 03, 2007 3:42 pm

The system really needs to be designed for gravity flow. Most fan-forced duct systems would not flow very good. It won't move as many BTUs as fan forced either.

Why don't you try disconnecting your fan, making sure there is an over temp protection still in place so you don't overheat your heat exchanger, and see how well it gravity flows??

The old 'Octopus' furnaces had huge hot air runners and even bigger cold air returns. All to move the BTUs needed without a fan. The fan provides volume that gravity doesn't.

Greg L.
Last edited by LsFarm on Tue Apr 03, 2007 5:39 pm, edited 1 time in total.
LsFarm
 
Stoker Coal Boiler: Axeman Anderson 260
Hand Fed Coal Boiler: Self-built 'Big Bertha' SS Boiler
Baseburners & Antiques: Keystone 11, Art Garland

Re: Gravity hot air system

PostBy: Yanche On: Tue Apr 03, 2007 4:07 pm

e.alleg wrote:What is the problem with having a gravity forced air system? I'm talking about having a heat exchanger in the ductwork with hot water flowing through it, or just the furnace hooked to the ductwork and no blower fan. The heat should rise steadily and constantly through the ductwork, no? I hate my 1500cfm blower, I feel that it is too loud.
As Greg says it's a problem with the design size of the duct work. The water analogy would be a fire hose with gravity flow vs. a garden hose with a pump. Both can deliver the same amount of water. Unfortunately you have a duct system designed for use with a blower. With a coal furnace or coal boiler with and air coil you will always have heat in your duct but you still need to move it with a blower. The blower speed will need to be reduced, perhaps unsubstantially. Depending on how fancy a system you want the blower could be replaced with a multi-speed blower or even a variable speed blower. Then you could select the appropriate speed based on the fuel in use. Higher speed for propane, lower speed for coal. Depending on your budget it can all be made automatic. A proportional room thermostat that varies blower speed as a function of the difference between the set point and the current room temperature. At low speed and any reasonable installation it will very quiet. As Greg says try it without the blower on. If you don't like the results AND IF it's a 220V blower AND IF you know what you are doing you could test it at half voltage temporarily, which would lower the speed.

Yanche
Yanche
 
Stoker Coal Boiler: Alternate Heating Systems S-130
Coal Size/Type: Anthracite Pea

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