What causes a boiler to modulate its burner?

Old 02-22-13, 07:49 PM
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What causes a boiler to modulate its burner?

My guess would be an outdoor reset. If so, what determines the amount of modulation? How does the modulation match up with the house heat loss at outside temperatures that are above the design temperature, such as 30 degrees? What is a “heat curve” that I have seen referred to in other posts? Is this some kind of factory simulated heat requirement that a boiler tries to match? An inquiring mind wants to know. Thanks.
Old 02-22-13, 08:19 PM
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My background is with commercial and industrial sizes boilers, not residential, but in all cases the boiler's burner is modulated under control of the desired output temperature. The burner is "throttled back" to a point where it can continue burning and not exceed the "setpoint" temperature. Outside temperature reset does NOT directly modulate the burner but only "resets" the desired output temperature setpoint.

The "heat curve" is a chart defining how much heat must be added to the structure to maintain a predetermined inside temperature under varying outside temperatures.
Old 02-22-13, 08:50 PM
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Basically the outdoor reset tells the boiler to maintain a certain temp according to the outdoor temps . Like in this graph.

Mod con boilers usually have a 5 to 1 turn down ration. So say a 75k boiler burner will run at 75,60,45,30, or 15k btu to maintain the corosponding temp.

So when its 60f out the boiler will only produce 90f water to heat the home, and burn at 15 or 30k btu. This saves energy.

Now at -10 and below the boiler will run at max temp to heat the home. 180f typically. Btu will be close to max the boiler puts out. Now depending where you live or how much heat element you have these curves can be adjusted for maximum comfort and savings. ( Not all models but you can just adust the high limit I believe)

Ideally you want the coolest water temp possible on the coldest day of the year to heat your home and work back from there.

Old 02-23-13, 05:19 AM
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I dont have a mod/con, so Im curious..
At whatever setpoint temp , does a modulating boiler operate like a cold-start (going right down to ambient if no demand) ?

Also, I thought I read on here that there is some situations where you loose the advantage of modulation ? (during the coldest part of the season ?)
Old 02-23-13, 07:53 AM
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Yes, mod/con is cold start.

some situations where you loose the advantage of modulation ?
Not the advantage of MODULATION, but rather the advantage of CONDENSATION.

After all is said and done, the REAL efficiency gain comes from condensation, which enables the recovery of the latent heat that usually goes up the chimney.

Without condensing, the efficiency limitation is 'around' 85% maybe a point or two higher.

By condensing the flue gases it is possible to extract some or most of the latent heat that is trapped in the water vapor component of the flue gases.

For a condensing boiler to condense, the RETURN WATER has to be maintained BELOW THE DEW POINT of the flue gases. This is generally accepted as appx 135°F (gas fired systems).

In order for the return water to be maintained at or below this temperature, there must be sufficient radiation installed in the home to meet or exceed the heat loss of the home at the outdoor design temperature with return water temperature that does not exceed the dew point of the flue gases.

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