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domestic water heater part load efficiency (E+)

asked 2019-01-28 13:45:13 -0500

updated 2019-01-28 13:50:23 -0500

I just (inadvertantly) used a part load factor curve for my gas-fired WaterHeater:Mixed which produced VERY low efficiency. That got me thinking about what sort of curve is appropriate for a water heater....

I have a sense and some technical references for boiler part-load efficiency, but a water heater is a different piece of equipment in design and control. Can any of you point me toward a better understanding of water heater part load efficiency?

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answered 2019-01-31 11:40:52 -0500

Water heaters have different efficiency rating conditions depending on the equipment size. Generally, cycling and other loses are factored into the efficiency for small water heaters, and calculated separately for large water heaters. See ASHRAE 90.1 Table 7.8 Performance Requirements for Water Heating Equipment. More detail on this is in ASHRAE 90.1 and the water heater section in the PNNL report Enhancements to ASHRAE Standard 90.1 Prototype Building Models.

As most gas water heaters are storage water heaters that operate by cycling, they shouldn't have a part-load curve. The are on or off. If you do have a curve, set it to one.

In the instance where you don't have a storage tank and you want to model a gas-fired instanenous water heater at a fine time resolution, the operation will be similar to a gas boiler. However, you'd need to explictly model water draws (rather than an average draw), and would want to include the ramp up/down conditions in the water heater in your part-load curve.

Pipe loses are a separate factor which matter quite a bit in service hot water distribution systems. You can model these explictly with pipe loss, or adjust the thermal loss from the water heater storage tank to account for pipe losses.

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Very Helpful, Matt. I hadn't gone down the thought path far enough to think about instantaneous water heaters, but that's something my client has mentioned as a possibility for the project. Thanks for that. I guess I'm inclined to think that defining a storage-type water heater as on/off is reasonable. Nonetheless, there are flue losses for the atmospheric combustion types which (ideally) should be modeled in some fashion. I guess I won't worry about it for now. Thanks for taking time to respond.

Jim Dirkes gravatar image Jim Dirkes  ( 2019-01-31 14:52:38 -0500 )edit

Flue losses for gas storage tank hot water heater off-cycle operation is defined in the 'Off Cycle Parasitic Fuel Consumption Rate' field. These are included if you make a model from one of the DOE prototype buildings. If you want to match to a standard, you could run the 'create baseline building' measure on your model and then copy back the 'Off Cycle Parasitic Fuel Consumption Rate' values into your original model.

mdahlhausen gravatar image mdahlhausen  ( 2019-01-31 17:16:54 -0500 )edit

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Asked: 2019-01-28 13:45:13 -0500

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Last updated: Jan 31 '19