[Bldg-sim] DHW heater efficiency calculations for baseline

Bishop, Bill bbishop at PathfinderEngineers.com
Tue Mar 11 08:53:14 PDT 2008


Ying,

I believe the correct interpretation is that there are two minimum
efficiency requirements implied by "80% Et(Q/800+110V^1/2) SL, Btu/h".
You don't multiply the two factors together.

The heater must have a minimum efficiency of 80%, and
the standby loss must be less than "(Q/800+110V^1/2)" in Btu/h.
You use these minimum values in your baseline model.

Using your heating and storage capacity numbers below,
SL = (400000/800)+(110*130^.5) = 1754 Btu/h

This is based on 70F temperature difference so
UA = (1754 Btu/h)/70F = 25.06 Btu/h-F

Use this value for your standby loss UA.

Regards,
Bill

William Bishop, EIT, LEED(r) AP | Pathfinder Engineers LLP
Mechanical Engineer

3300 Monroe Ave., Suite 306 
Rochester, NY  14618
TEL (585) 218-0730 Ext. 114
FAX (585) 218-0737 
bbishop at pathfinderengineers.com
 
www.pathfinderengineers.com

-----Original Message-----
From: bldg-sim-bounces at lists.onebuilding.org
[mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of
yizhao1 at vt.edu
Sent: Tuesday, March 04, 2008 4:42 PM
To: bldg-sim at lists.onebuilding.org
Subject: [Bldg-sim] DHW heater efficiency calculations for baseline

The DHW heater efficiency calculations for baseline are based on Table
7.8. For
Gas storage water heaters, the size >75,000 btu/h, and <4000
(Btu/h)/gal, the
baseline performance is:

80% Et(Q/800+110V^1/2) SL, Btu/h = 80%*(400000/800+100*130^1/2).
The input Q for the heater is 400,000 Btu/h, and V is 130 gal. So the SL
calculated is 3077 Btu/h.

Since the SL is defined as standby loss based on 70F temperature
difference
between stored water and ambient requirements, is it right to calculate
the
efficiency by Q-SL/Q? The result is 99.6%, which appears to be so high!

Could anyone show me if it is right or the right calculation for this?

Thanks in advance!

Ying

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