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# Water Heater Efficiency per Table 7.8 of ASHRAE 90.1-2010

Hello,

I'm currently working with two (600,000 Btuh, 130 Gallons, 97 EF) boilers for a dormitory building. From my interpretation of Table 7.8 the equation I'm suppose to use is 80%Et(Q/800 +110sqrt(V)) SL, Btuh. From I've what I've read, I'm suppose to disregard the 80% Et and that leaves me for the remainder of the equation (600000/800 + 110sqrt(130)) to get 2004 Btuh as my stand by loss for my baseline water heaters. A few questions I had were:

1. Should I leave this number in the service water heating section as what I calculated as for the SL or do I need to convert this to an EF ?

2. Is there a way to convert to this to an efficiency (EF)?

In previous projects my co-worker worked on they always left as a % EF on the MEP calculator and used that along with the HW usage to calculate an energy usage in MBH/hr for each water heater. They then plugged that energy usage into Trace 700 for the base utilities for the water heaters.

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ASHRAE90.1-2010 specifies two requirements for Gas stoarge water heaters in TABLE 7.8. One is Et = 80%. The other is SL = Q/800 +110sqrt(V) [Btu/h]. Your calculation of SL is correct, but you should not disregard Et. Apart from SL, Et needs to be input somewhere in the simulation tool you use.

Energy Factor (EF) is an indocator of a water heater's overall energy efficiency. Please refer to the old post: Water Heating Equipment Performance Requirements per ASHRAE 90.1-2007. I'm not familiar with Trace 700, but there should be no building energy simulation tool with an input filed for EF.

You don't need to convert SL to EF, but you may need to convert SL according to the simulation tool you use. For example, SL can be input as Off Cycle/On Cycle Loss Coefficient to Ambient Temperature in EnergyPlus. The unit of this input field is [Btu/h°F] (or [W/K]). SL in ASHRAE 90.1-2010 is based on 70°F (38.9°C) temperature difference between stored water and ambient. If SL of your Baseline case is 2004[Btu/h], Off Cycle/On Cycle Loss Coefficient to Ambient Temperature should be 2004[Btu/h] / 70[°F] = 28.63[Btu/h°F].

You seem to be trying to use the same heating capacity and tank volume as the Proposed case for the Baseline case, but please note that they are not necessarily the same. They are more often different. The service water heating system in the Baseline case shall be sized according to ASHRAE90.1-2010 Section 7.4.1. There is no single sizing calculation. Even ASHRAE Handbook—HVAC Applications provides various sizing options.

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