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Re: [EnergyPlus_Support] Heating and Cooling vs. U-value



Mr. Dirkes,

Thanks for your reply. You confirmed my thoughts on this issue.

But, what makes me wonder is - With better U-values, 1. I should be having lesser loads (heating/cooling) 2. I have HVAC system which is slightly more efficient than what I am comparing with (a difference of maybe 1/100 only). Nonetheless, I should have lesser energy consumption in both heating and cooling situations annually in an ideal case. But I have higher heating energy, which makes me think, my control is probably the issue as that is probably the only thing which is different and which could possibly affect the energy consumption. do you think this makes sense?

Vinay

On Tue, May 3, 2011 at 8:34 PM, James V Dirkes II, PE <jim@xxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
 

Dear Vinay,

 

There are indeed many variables!

 

Generally (probably always) more insulation in the envelope will reduce heating costs.  The cooling costs are another question. 

The big part of the question is whether most of the load comes from outdoors (sun and temperature) or indoors (lights and plug load).  There?s also an influence from the schedules and diversity of the internal loads.

The building with a lot of internal load sources may see cooling costs increase with more insulation because one avenue for the internal loads to leave the building is the walls and the greater insulation prevents this.

So, I think you need to experiment a bit with your particular project, its internal loads and schedules, and the weather file.

 

p.s., As I recall Larry Spielvogel wrote an article discussing this in the ASDHRAE Journal probably 25 years ago.  It was completely counterintuitive, but made perfect sense once he explained the logic.

Similarly, Steve Taylor and C. Hwakong Cheng wrote a recent article in the ASHRAE Journal (November 2010) about why enthalpy economizers are LESS effective than a simple dry bulb economizer.  Perfectly counter-intuitive, but the rationale is unassailable.  Check it out!

 

The Building Performance Team
James V. Dirkes II, P.E., LEED AP, BEMP
1631 Acacia Drive NW
Grand Rapids, MI 49504
616 450 8653

 

From: EnergyPlus_Support@xxxxxxxxxxxxxxx [mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx] On Behalf Of Vinay Devanathan
Sent: Tuesday, May 03, 2011 6:40 PM
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Subject: [EnergyPlus_Support] Heating and Cooling vs. U-value

 

 

Hello,

Does a better (low U-values for construction assemblies) insulated envelope result in achieving -

1. reduced cooling and heating loads (ideal)
2. reduced cooling, slightly higher heating load

Or are there too many variables to zero in on either of the above?

Hope to hear from someone.

Thank you.




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