Gas fired condensing boiler and baseboard heating

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Old 10-18-15, 08:24 PM
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Gas fired condensing boiler and baseboard heating

Hey all,

With a condensing boiler that uses hot water to heat baseboard, is it better for the target temperature to reach set point at a faster rate (100% input) or reach target temperature at a slower rate (100% input at ignition then modulate down to 1% as the temperature reaches set point).

At the faster rate for example, the set temperature will reach 180 degrees faster and input will modulate until thermostat is satisfied. - Faster to heat the house

At the slower rate, the set temperature will reach 180 dregrees if thermostat is still calling for heat. - Slower to heat the house.

What is the most efficient way to heat a house by hot water baseboard given the scenario above? I'm just trying to understand and see what is the best way to keep my condensing boiler running efficiently.
 
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Old 10-19-15, 05:19 AM
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Boiler temp

Hi- no expert here, but my understanding is that the efficiency of a condensing boiler comes from keeping the temps below the condensing temp, which is I think in the 130 degree range. So the idea would be to keep water temps lower for as long as you can. Maybe someone with more expertise will chime in-
Steve
 
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Old 10-19-15, 06:23 AM
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Steve's reply is more or less correct. The savings on a mod/con are all about CONDENSING, and to do that the boiler return water has to be below 135F (appx) Once you go above that temp, the efficiency is back to that of a conventional boiler.

In general, with a mod/con boiler, extreme night and away setbacks (more than say 3-5 degrees) won't save you anything. It's better to maintain close to setback to avoid having the boiler have to 'boost' to get to set point.

Your boiler probably has a 'boost' function. Read up on that. What it does is raise the boiler target temp if the room setpoint is not reached in a certain amount of time. In other words, it will automatically do what you are suggesting if need be.

Let's say you want to raise the house from 58 to 70.

If you've got BOOST set for 10 degrees after 10 minutes, the boiler will raise the target water temperature 10 degrees for every 10 minutes that there's a continuing heat call.
 
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Old 10-19-15, 12:22 PM
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So if the boiler need to stay below 135F, how will the house get heated? Seems like it would take a considerable amount of time.

Yes, my boiler has the boost setting.
 
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Old 10-19-15, 12:37 PM
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In a perfect world, a mod/con would be paired with heat emitters (baseboards, etc) that can output the needed BTU to heat the home at the lower water temperature.

For example, you have a box with a heat loss of 10K BTUH.

In order to maintain heat in that box, you install baseboard and pump hot water through it.

AT 180F water temperature, typical baseboard is rated at around 550 BTUH/FT.

You would have to install 18.2 feet of baseboard and pump 180F water through it to replace the heat loss and maintain temperature.

That same baseboard might have an output of let's say 300 BTUH/FT with 135F water.

So 10,000 / 300 = 33.33 feet of baseboard.

You would have the same net result, temperature would be maintained, with either scenario.

Simply throwing a mod/con boiler into a setting that was designed for 180F water (as the vast majority are) is not going to get you all the savings that could theoretically be had. You will gain SOME savings because most of the heating season you can run the cooler water because it's not as cold outside and the heat loss isn't as high.

That's one thing you will never hear a boiler salesman tell you!
 
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