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Re: cast iron radiators or slant fin baseboard

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Posted by HeatPro on August 03, 2005 at 09:22:12:

In Reply to: cast iron radiators or slant fin baseboard posted by john on August 02, 2005 at 22:54:25:

As 100 percent of the heat in the water comes out of the baseboard and the definition of efficiency is the ratio of energy invested to the energy that comes out, they are both equally efficient. Once we get by the usual understanding that the public doesn't have the slightest idea of what efficiency means and they usually mean 'effective,' further information becomes useful.

They both are equally effective depending upon how they are controlled. Cast iron first absorbs heat, then gives it off, so it has a different effect than lightweight copper-tube baseboard. The 'charm' of cast-iron baseboard is its heat-delaying capacity. When a thermostat demands heat, many control designs then turn on the circulator to move hot water from the boiler. Hot water heats copper-tube baseboard quickly, cast-iron slower. When the thermostat is satisfied, that control strategy stops the circulator and copper-tube baseboard gives off its heat rapidly, meanwhile, the heated cast-iron gives off heat for several minutes as it has stored some. Cast-iron, in that strategy, acts more like the steam you are used to.

It is also possible to control the boiler water temperature with an outdoor reset that lets the water only get as hot as needed. Then, the circulator can be left on all the time, a heavy boiler acts as a heat sink storing some heat, and the cast-iron and copper baseboard act similarly because the boiler heat passes from the copper more continually, like the cast-iron. The new sales pitches might include sophisticated comfort strategies that utilize the 'no cold cycle' possibility of baseboard, which would counter the sales of on-off 'wind-chill' ducted systems. For now, the industry is still aiming for the 'dumb-down' and 'common denominator' approaches.

This difference in cycling rate of cast and copper-aluminum fin baseboard is the reason installers are told not to mix the two in the same zone circuit.

One reason for confusion between 'efficient' and 'effective' in the public eye is because the marketing pitches have been aimed at efficiency over the past 50 years. Marketers will have to learn more tricks in the future, as most boilers sold are over 85% efficient in the GAMA ratings, and offering 10% more in a 95% efficienct boiler is not much money-saving over the standard unit. Swapping out a 50-year-old gas boiler usually means a 50 percent savings.

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