[EnergyPlus_Support] Re: Residential heater sizing and cycling question

["Troy Harvey" <taharvey@xxxxxxxxxxxxxxxx>]

Mike thanks for the reply.

I'm not real familiar with the HVAC options, I normally use purchased air.
But I've look through the docs, and I'm looking for clarification.

 

What I'm hoping to be able to simulate is the effect of over-sizing
residential forced air heaters, and the effect of variable BTU/air volume
vs. Const. BTU/air volume, and their impact on heater efficiency (due to
heater cycling effect on combustion efficiency, burner/flue reheat, etc).
That is I'm hoping that, if I oversize, I should see more cycling, and less
efficiency. Is this feasible in E+? 

 

 

As Don and Rich have explained, the HVAC system timestep does vary, and you
can 
report results at the timestep level, but this is not relevant to what you
are 
trying to model. Each HVAC timestep in EnergyPlus is a quasi-steady-state 
simulation. For the gas heating coil cycling loss is modeled by looking at
the 
quasi-steady part load ratio and making an adjustment to the efficiency
through 
the "Part load fraction correlation (function of part load ratio)" curve in 
COIL:Gas:Heating. 

For example, if the load on the furnace from 9:00am to 9:30 am is 50% of 
capacity, it doesn't matter if EnergyPlus simulates two 15-minute HVAC 
timesteps, or 30 1-minute HVAC timesteps. Each timestep will evaluate the 
efficiency at 50% part-load. There may be minor differences in results when 
the timestep shifts down, but you should not need to be concerned with the 
length of timestep. But, you do need to make sure the Part load fraction 
correlation curve accurately represents the type of equipment you are
studying.

Mike

On 4 Apr 2007 at 10:30, taharvey2001 wrote:

> I'd like to be able to model a standard constant volume residential
> forced air heater in EnergyPlus and see the impact of Sizing on
> efficiency and heater cycling. 
> 
> I haven't been able to figure out how to model the heater
> appropriately in E+ or and make sure the heater has sufficiently small
> timestep resolution so the heater can cycle, and the difference in
> efficiency can be seen. 
> 
> As I understand, the HVAC system timestep is decoupled from the 15
> minute building timestep, allowing it to free-run with an adaptive
> timestep (down to seconds if need be) - is this true, and is there
> anything that need to be done to enable this behavior? Is there anyway
> to see the high-res HVAC transients in the output?
> 
> What is the best way to get a const. volume, const BTU heater to model
> residential forced air systems?
> 
> thanks,
> 
> troy
 

 



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