Modulating Ramp strategy and how it reacts to setting a max modulating?

Dave Carpentier

So a mod boiler fires up at 100% burn and then eventually start ramping back down the burn as it neared the setpoint. (I dont have one yet, but it seems to me thats how it goes).
If you were to cap the max burn at 70% or such, would the overall efficiency be reduced or improved ??

(yes, presume that the 70% is adequate output for the task .. like maybe your boiler was spec'd a little too large)

EdTheHeaterMan

The way that you worded this discussion comment, It sounds like you already have an oversized boiler and you should end your query with: "Asking For A Friend". LOL


Hypothetically Speaking... That is what a Modulating boiler does by design. Unless you set back or have the boiler OFF for some extended time during a cold weather event, once the boiler gets close to the setpoint, it will modulate based on the sensor inputs from the outdoor temperature, supply water temperature sensor, the return water temperature and the room thermostat input. With a properly designed control system, the boiler will never go above the Hypothetical 70% firing rate. Likewise the burner should never shut off during the season until the mild weather creates a heat loss that is less than the minimum turn down output of the boiler. At that outdoor temperature and above, the burner will cycle based on the thermostat input as an ON/OFF boiler of the lowest (minimum capacity) input boiler.


Since operating at 100% will be a very rare occurrence and only happen when an abnormal event occurs, I do not believe there would be a measurable difference in energy usage if you restricted the operation to 70% max input compared to 100% input.


This opinion has been formed based on absolutely no research, no clinical studies were conducted, and no test cases have been sited… This was formed using the SOMPWAG method.

That stands for Seat Of My Pants Wild A$$ Guess method.

Click on the spoiler for more information about SOMPWAG

JDHW

Dave,
I think the efficiency will improve with reduced power. Think of a radiator: for a given power output the water temperature can be reduced the larger the radiator. So for a boiler the same logic applies. If the boiler is oversized and you reduce the output by modulating down: then hot side, with the flames, will be cooler and extract more energy from the hot gasses and reduce the flue temperature. This is more efficient.

John

Dave Carpentier

(@Ed - Although I do indeed have an oversized conventional oil boiler now, the question is more for the modcon gas that we intend to install for next winter.)

The burn has always had me curious since a few postings (hotrod ?) of a graphic that shows an efficiency increase as the burner decreases, for a given unit/scenario.

Other than the upfront cost of the unit, why not just install a larger than required size boiler and then just cap it down to the required output via the max burner% ? If this was done though, I just dont know what the effect would be on the software's modulating strategy during normal cycles.

Afaik, ODR etc only affect the setpoint strategy, and not the burner rate/strategy directly ?

hot_rod

The same would apply, the oversized boiler hx has more surface area, so more condensation is possible

Those slides I showed were from a Coffee with Caleffi that Jody a Viessmann trainer presented.
As I recall in the presentation he showed the example of an oversized HX under low fire I’ll see if that is still on the You Tube channel to view

but the oversized boiler will not have the lowest turndown, most are 10-1 So a bit of a trade-off
In multi zoned systems its nice to have the turn down close the lowest load, for long boiler run cycles

EdTheHeaterMan

Dave Carpentier said:

(@Ed - Although I do indeed have an oversized conventional oil boiler now, the question is more for the modcon gas that we intend to install for next winter.)

The burn has always had me curious since a few postings (hotrod ?) of a graphic that shows an efficiency increase as the burner decreases, for a given unit/scenario.

Other than the upfront cost of the unit, why not just install a larger than required size boiler and then just cap it down to the required output via the max burner% ? If this was done though, I just dont know what the effect would be on the software's modulating strategy during normal cycles.

Afaik, ODR etc only affect the setpoint strategy, and not the burner rate/strategy directly ?

By using an oversized boiler, the minimum input will be higher... so that point in the fringe season where the lowest input is above the actual demand will of course be higher. For example: A 120,000 BTU boiler with a 10% turndown will be a 12,000 BTU boiler at minimum input, so operating at a 45°F outdoor ambient may be enough for the burner to keep operating at 12,000 and modulate higher as the temperature gets cooler over night. That same condition with a 199,000 BTU boiler with a 10% turndown will have a minimum of 20,000 BTU. That boiler may need to cycle off and on at the LOW flame until the outdoor ambient gets down to 38°F. More OFF/ON cycles that are slightly less efficient that the smaller boiler's 12,000 constant operation at that same outside temp.
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