[Equest-users] my math on DWH efficiency?

[Omar Salaymeh]

I typically use RMI’s EMIT tool for these calcs. http://www.rmi.org/rmi/ModelingTools

From: equest-users-bounces at lists.onebuilding.org [mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of Nick Caton
Sent: July-06-11 4:20 PM
To: James Hansen; equest-users at lists.onebuilding.org
Subject: Re: [Equest-users] my math on DWH efficiency?

Hmm… I read this quickly and it seemed off so I double checked:
TANK-UA
The overall heat loss coefficient (UA) of the heater. The heat loss is calculated as this value times the temperature difference between the water in the heater and the environmental temperature.

You got the 90.1 calc right:  SL = 20 + 35*sqrt(240) = 562

Rephrasing the DOE2 help entry copied above:  SL = UA * delta-T

Set:  562 = UA * 70

Then: UA = 562/70 = 8.03

As I understand it, TANK-UA rolls together the properties of both the insulation and surface area, so given TANK-UA you shouldn’t need to work in the tank volume or area to find the standby loss for a given delta-T.  That’s my take – hope it helps =)!

~Nick


[cid:image001.jpg at 01CC3BF9.E95E5230]

NICK CATON, P.E.
SENIOR ENGINEER

Smith & Boucher Engineers
25501 west valley parkway, suite 200
olathe, ks 66061
direct 913.344.0036
fax 913.345.0617
www.smithboucher.com

From: equest-users-bounces at lists.onebuilding.org [mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of James Hansen
Sent: Wednesday, July 06, 2011 10:31 AM
To: equest-users at lists.onebuilding.org
Subject: [Equest-users] my math on DWH efficiency?

I had a question about calculating baseline 90.1 efficiency for electric DWHs (tank type).

Based on Table 7.8, my efficiency needs to be SL = 20 + 35*sqrt(V) at a 70 degree delta T between water and ambient, where SL is the standby loss in btuh, and V is the volume of the tank.

If, in my proposed model, I have a 240 gallon tank (meaning my baseline has the same volume), it would appear that the maximum standby loss at a 70 degree delta should be 562 btuh [20 + 35 * sqrt(240)]

Since eQuest calculates the heat loss by multiplying the UA (thermal conductivity of storage tank) by the tank, it would appear that my UA for this particular example would be 562 / (70*240), or 0.0334.

That way, at a 70 degree delta, the heat loss (SL) would be 0.0334 Btu/hr-gal-F * 240 gallons * 70 F = 562 btuh

Does that sound about right?

Thanks to anyone that has a quick chance to review!

GHT Limited
James Hansen, PE, LEED AP
Senior Associate
1010 N. Glebe Rd, Suite 200
Arlington, VA  22201-4749
703-338-5754 (Cell)
703-243-1200 (Office)
703-276-1376 (Fax)
www.ghtltd.com<http://www.ghtltd.com/>


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