# Re: [EnergyPlus_Support] boiler:simple

```Chaoqin:

My apologies that it took so long to pinpoint the problem.  Two
things are going on here.

First, the boiler is greatly oversized, because you have thermostat
setback active on the design day and the sizing parameters are
defaulting to an averaging window of 1.0 (see SIZING PARAMETERS)
object.  This means that the boiler is being sized to recover
completely from setback in one zone time step (15 minutes in your
run).

Second, the boiler "min part load ratio" is set to 0.15.  Since the
boiler is so oversized, the load on the boiler is generally well
below 0.15 times the boiler capacity.  No matter how small the load
gets, the boiler fuel input will always be at least 0.15 times the
full-load fuel input.  This is why you are seeing such a low
efficiency when you compare the loop demand with the fuel input.

I would recommend two changes:

a)  Either add a special thermostat setpoint profile without setback
for design day sizing calculations or add a SIZING PARAMETERS object
and set the averaging window to 4 or higher.

b)  If you want the boiler to always run a 0.9 efficiency, then you
will need to set the min part load ratio to 0.0.

Mike

On 31 May 2003, at 1:14, zhaicq98 wrote:

> Hi,
>
> The cooling and heating of my building is provided by a fan coil unit
> in each zone and the cooling plant consists of a electric chiller
> while the heating plant consists of a boiler. When I tried to output
> the energy consumption of this system, I got something confusing.
>
> The theoretical efficiency of my boiler is 0.9 and the fuel use
> coefficents C1, C2 and C3 is 1, 0 and 0 respectively. As I understand
> from the user manual, this kind of input should be modeled as a
> constant efficiency boiler.
>
> I output the FANCOILHEATINGCOIL:Total Water Heating Coil Energy for
> each fan coil unit and sum them up. I also output the Heating:Gas. I
> expect the latter is the former divided by 0.9, which is my boiler
> efficiency, but it is not. In my output, the latter is the former
> divided by 0.34. I would like to know what is going on here.
>
> Thanks,
> Chaoqin
>
>
>
>
>
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