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Water heater sizing rules of thumb

asked 2014-11-06 13:53:30 -0500

updated 2015-02-11 19:03:41 -0500

When using the WaterHeater:Sizing E+ component, there are multiple design mode options to choose. I plan on using this component for two situations: PeakDraw design mode for a water heater in a commercial building without solar hot water collectors, and PerSolarCollectorArea for a storage tank in a commercial building with solar hot water collectors. In an idf file, the component looks like this:

        WaterHeater:Sizing,
        DHW Water Heater,          !- WaterHeater Name
        PeakDraw,                !- Design Mode
       *,                       !- Time Storage Can Meet Peak Draw {hr}
       *,                       !- Time for Tank Recovery {hr}
       *,                       !- Nominal Tank Volume for Autosizing Plant Connections {m3}
        ,                        !- Number of Bedrooms
        ,                        !- Number of Bathrooms
        ,                        !- Storage Capacity per Person {m3/Person}
        ,                        !- Recovery Capacity per Person {m3/hr-person}
        ,                        !- Storage Capacity per Floor Area {m3/m2}
        ,                        !- Recovery Capacity per Floor Area {m3/hr-m2}
        ,                        !- Number of Units
        ,                        !- Storage Capacity per Unit {m3}
        ,                        !- Recovery Capacity PerUnit {m3/hr}
       *,                       !- Storage Capacity per Collector Area {m3/m2}
       *;                       !- Height Aspect Ratio

Does anyone have experience using this component or know any good rules of thumb for the inputs with asterisks in front? Any of the inputs dealing with storage per person, floor area, and unit would also be helpful info to know.

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answered 2014-11-07 11:17:42 -0500

updated 2014-11-07 13:03:10 -0500

Aaron, this question relates to actual, real-life, sizing rather that energy modeling.

DHW sizing is both an Art and a Science. Figuring out the expected domestic water load "accurately" is obviously the biggest challenge. Load diversity is the wild assumption for new buildings.

(Side note: for existing buildings, metering is an excellent idea to get the actual load profile of the facility... My experience with that is that any other theoretical methods tend to grossly oversize the DHW systems for Multifamily buildings at least... Hunter curves/Fixture-unit methods are worse than the ASHRAE method but ASHRAE oversizes by 30+% already)

First, there are many options you can choose for your system setup (instantaneous, direct, indirect, etc.). Once you have settled on a system type, there are basically an infinity of combination of heater capacity (recovery rate) and storage that will satisfy the assumed load you previously calculated.

To quote the 2011 ASHRAE Handbook - HVAC Applications, Chapter 50 - Service Water Heating:

To serve a hot-water load adequately, the needs of both the peak energy withdrawal rate and total integrated energy delivery for end uses must be met. Meeting these needs can be done either by providing a heating rate large enough to meet the peak energy withdrawal rate of the system (and modulating that heating input for smaller loads), or by providing a lower heating rate combined with storage (from which the peak rates can be satisfied).

I suggest you take a close look at this chapter 50. Table 7 basically tells you that for an office building, the maximum hourly demand is 0.4 gal/person, the maximum daily is 2.0 gal/person, and the average daily is 1.0 gal/person. If it's an office building, you're lucky because it's way simpler than residential (and a much lesser usage)

Knowing whether you'll have a recirculation loop or not is important as well.

To quote an engineer who had a lot more wisdom than me: There is no right way to size a DHW system, only better or worse ways to approaching the task. Only the engineer with the greatest understanding of the system and who is making the least amount of assumptions will provide the best system. Experience is key.

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answered 2014-11-10 10:00:18 -0500

updated 2014-11-10 11:54:12 -0500

Aaron, this is for residential water heaters and you are probably aware of it, but for others who come across this question, the ASHRAE HVAC Applications Handbook provides a table of minimum residential water heater tank volume and input capacity based on number of bedrooms and bathrooms.

From Table 8 in the Building America House Simulation Protocols: image description

This is based on the minimum capacity permitted by the U.S. Department of Housing and Urban Development and the Federal Housing Administration, and is probably a good rule of thumb for residential tank water heaters (heat pump water heaters may need to be sized larger or set to a higher set point for equivalent hot water delivery).

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Asked: 2014-11-06 13:53:30 -0500

Seen: 2,636 times

Last updated: Nov 10 '14