Issue C: h_tr_em reduction may be needed to achieve correct τ for high-mass buildings

[anchapin]

Problem

The h_tr_em (exterior-to-mass conductance) is currently ~104.46 W/K for 900FF. This is combined with h_tr_ms in the effective time constant calculation, incorrectly shortening τ from ~47h to ~26h.

Current Calculation (thermal_model_core.rs:850-884)

h_tr_em is calculated based on the wall construction layers exterior to the dominant insulation:

// Formula: h_tr_em = A_opaque / R_exterior_to_mass
// Where R_exterior_to_mass = R_exterior_film + Σ(R_layers exterior to mass)

Analysis

• h_tr_em = 104.46 W/K is added to h_tr_ms = 117.41 W/K
• Combined: 221.87 W/K gives τ = 26.3h
• h_tr_ms alone: 117.41 W/K gives τ = 47.4h (correct!)

The h_tr_em calculation may be correct for the 5R1C model, but its use in the 6R2C model's effective time constant is problematic.

Hypothesis

Reducing h_tr_em (or removing it from the envelope mass time constant calculation) would increase the effective τ to ~47h, matching the high-mass building's expected thermal damping.

Investigation Needed

1. Is h_tr_em needed in the 6R2C envelope mass heat balance at all?
2. Should h_tr_em only affect the surface node (T_s), not the mass node (T_m)?
3. What should the correct 6R2C heat balance equations look like per ASHRAE 140?

References

• thermal_model_core.rs:850-884 (h_tr_em calculation)
• thermal_model_physics.rs:1743 (envelope mass heat balance)
• ASHRAE 140 6R2C model specification

[anchapin]

This issue has been addressed by PR #696 which calculates h_tr_me from physics instead of using a fixed 100.0 W/K value. The merged PR shows h_tr_me is now ~432 W/K.