Domestic Hot Water Storage Tank (sibling of Hot_Water_Storage_Tank)

[jahrling]

Summary

brick:Water_Storage_Tank has subclasses for chilled, cold, hot, fire-sprinkler, and rain water, but no class for stored domestic (potable) hot water. The existing brick:Hot_Water_Storage_Tank is explicitly HVAC-scoped:

Hot_Water_Storage_Tank — "A tank designed to store hot water in an HVAC system."

Therefore, domestic hot water (DHW) can't be a child class, which is a potable medium in a separate distribution system. I'd like to add a domestic sibling under Water_Storage_Tank.

Current hierarchy (v1.4)

Storage_Tank
└── Water_Storage_Tank
      ├── Chilled_Water_Storage_Tank      "...store chilled water in HVAC systems."
      ├── Cold_Water_Storage_Tank         "...usually in a building's water supply system."
      ├── Fire_Sprinkler_Water_Storage_Tank
      ├── Hot_Water_Storage_Tank          "...store hot water in an HVAC system."   ← HVAC only
      └── Rain_Water_Storage_Tank

Proposed change

Add Domestic_Hot_Water_Storage_Tank as a sibling of Hot_Water_Storage_Tank:

Water_Storage_Tank
├── Chilled_Water_Storage_Tank
├── Cold_Water_Storage_Tank
├── Fire_Sprinkler_Water_Storage_Tank
├── Hot_Water_Storage_Tank                 (existing — HVAC closed-loop)
├── + Domestic_Hot_Water_Storage_Tank      (NEW — potable DHW)
└── Rain_Water_Storage_Tank

Proposed definition:

Domestic_Hot_Water_Storage_Tank — "A tank designed to store heated potable (domestic) hot water for distribution to plumbing fixtures and process end-uses. Distinct from Hot_Water_Storage_Tank, which stores non-potable hot water for an HVAC/hydronic heating system."

Tags (mirroring the existing siblings, adding Domestic): [Domestic, Hot, Water, Storage, Equipment].

Rationale

1. Medium and system are different. DHW is potable and lives in the domestic plumbing system; HVAC hot water is a non-potable closed hydronic loop. Brick already draws this exact distinction at the system level — brick:Domestic_Hot_Water_System vs. brick:Hot_Water_System — and at the pump level (brick:Domestic_Hot_Water_Circulator_Pump vs. brick:Hot_Water_Circulator_Pump). The storage tank is the missing piece of that parallel.
2. Naming consistency. Brick already spells out the Domestic_Hot_Water_ prefix, so Domestic_Hot_Water_Storage_Tank slots in cleanly.
3. Real modeling need. Central DHW plants commonly use dedicated storage/buffer vessels (paired with tankless, indirect, semi-instantaneous, or heat-pump heaters). Today there is no correct Brick class for that vessel — Hot_Water_Storage_Tank mis-scopes it to HVAC and Cold_Water_Storage_Tank is also incorrect.

Relationship to other work

• Complements PR Add Domestic_Water_Heater hierarchy #780 / issue Domestic Water Heater class #777 (Domestic_Water_Heater hierarchy). A Storage_Water_Heater (from Add Domestic_Water_Heater hierarchy #780) is a heating appliance that also stores; a Domestic_Hot_Water_Storage_Tank is a storage-only / buffer vessel with no integral heat source (or an external one). They are distinct and complementary — see Q1.
• This is the tank counterpart to the Q4 discussion in Domestic Water Heater class #777 about modeling storage-tank-plus-heater combinations.

Questions

• Q1 — Tank vs. Storage_Water_Heater. A combined storage water heater (Add Domestic_Water_Heater hierarchy #780) heats and stores. A separate buffer/storage tank just stores. Should a combined appliance be modeled as Storage_Water_Heater hasPart Domestic_Hot_Water_Storage_Tank, or kept as the single combined Storage_Water_Heater class (as Add Domestic_Water_Heater hierarchy #780 Q4 decided)? I don't know what's the correct setup per Brick practices.
• Q2 — System linkage. I'm assuming there's no need for a relationship to brick:Domestic_Hot_Water_System (e.g., isPartOf / hasPart). Maybe others have a different opinion.
• Q3 — Orientation (out of scope here). I don't think it's worth adding this into the class. In practice, vertical vs. horizontal storage tanks have materially different usable-volume fractions (stratification). Flagging it as a possible future property or sub-distinction across all Storage_Tank types, not DHW-specific.
• Q4 — Service water definition. I've noticed elsewhere (like LEED) that the term "service water" is used rather than "domestic water". Not looking to tackle that here... yet.

drafted with Claude Code

[jahrling]

For Q2, use includes as it pertains to the system