# 90.1-2007 Table 7.8 Water Heater Efficiencies

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Could anyone help me understand the calculation for the minimum efficiency of gas storage water heaters per the table above? The calculation is 80%Et (Q/800+110 SqRt V) SL, Btuh. I understand that Q is the nameplate input rate in Btuh and V is the volume. Do I use the volume of the whole system, the storage tank only or the recovery rate volume? I understand SL is the standby loss. Does SL, Btuh mean multiply the equation in parenthesis by the standby loss in btu's per hour? Thanks for the help, Paul Diglio
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Paul, I interpret as the allowable Standby Loss is: 80%Et (Q/800+110 SqRt V) Footnote a indicates SL is maximum BTU/h based on 70 degree temp difference between stored water and ambient requirements. Mark
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Hi Paul, There are two things being talked about, one is thermal efficiency and the other is standby loss. Standby loss represented as (Q/800+110 SqRt V) and thermal efficiency represented as 80%. I had the same doubt a while back. If you look at other Formulae they are represented in the same way. Like for instantaneous water heaters Energy Factor is given by 0.62-0.0019V. Btu/h is the unit for Standby Loss. Not a multiplication factor. Hope this helps. Cheers Kushagra
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I expect the natural followup is "how do I put this (SL) into eQuest?"