ESP8266 COP HeatMeter

A low-cost Coefficient-Of-Performance meter for an air-to-water heat pump, built around an ESP8266 NodeMCU v2 with an SSD1306 OLED (Ideaspark "NodeMCU+OLED" board, roughly $7 assembled).

The meter measures hydronic-loop flow and cross-exchanger temperatures, pulls the heat pump's electrical consumption from existing MQTT sources, derives instantaneous and cumulative COP, and publishes everything under the HPEM/ MQTT topic tree for Node-RED / InfluxDB / Grafana consumption.

More discussion about the project and the heat-pump system: https://www.reddit.com/r/DIYHeatPumps/comments/189mlyy/diy_airtowater_cop_measurement/

Repository layout

• ESP_HeatMeter/ — current working firmware. Also serves as a local snapshot of the code running (or staged to run) on the device.
  • ESP_HeatMeter.ino — active sketch
  • ESP_HeatMeter (v2).not_ino — earlier experimental revision, kept alongside for reference
  • arduino_secrets.h — not tracked (in .gitignore); contains Wi-Fi SSID/password and MQTT user/pass. Create your own from the #define skeleton in the sketch.
• sketch14_012424_sCOP/ — prior generation sketch, left in place for history / diffing.
• old_setting_up_sketches/ — incremental learning sketches from bring-up; ignored in .gitignore.
• setup_notes_8266-ssd1306_OLED.txt, pinout_setup_...odt — pin-out notes and wiring reference for the Ideaspark NodeMCU+OLED board.

Hardware

• ESP8266 NodeMCU v2 + SSD1306 128×64 OLED (u8g2 library, I2C)
• Hall-effect turbine flow meter on the hydronic loop (interrupt-driven pulse counter, calibrated in LPM)
• DS18B20 1-Wire temperature probes — up to ~12 channels, labelled for process use:
  • EWT, LWT — entering / leaving water temps across the refrigerant heat exchanger (used for heat-output calculation)
  • LIQ, VAP — refrigerant liquid / vapor line temps
  • RET_Bd, RET_HW, RET_Bsmt, RET_N, RET_S — zone return-water probes around the house
  • LAT, EAT — leaving / entering air temps on the duct coil
• OLED splash shows the firmware version string (currently v041827)

Computed quantities

• heatQkW — instantaneous heat output = flow × ΔT(EWT − LWT) × ρ·cp
• heatEWh — running heat energy, integrated from heatQkW
• elecPkW — instantaneous electrical input (see "Dual power source" below)
• elecEWh — running electrical energy (mixed integration / metered-delta, see below)
• COP — instantaneous heatQkW / elecPkW, gated by a minimum flow threshold (minFlowLPM = 2.0) to suppress noise when the pump is off
• cCOP — cumulative COP over the current run
• sCOP — seasonal COP, accumulated across runs
• heat_sEQkWh, elec_sEkWh — seasonal heat and electrical energy totals

MQTT interface

Published under HPEM/:

heatQkW  heatEWh  COP  cCOP  sCOP  heat_sEQkWh  elec_sEkWh
elecPkW  elecEWh  ePwrSrc  flowLPM
EWT  LWT  VAP  LIQ  deltaT
RET_Bd  RET_HW  RET_Bsmt  RET_N  RET_S
LAT  EAT
debug    (per-iteration diagnostic trace — see below)

Subscribed:

gridvue/power/sensor/ashpgrid/state   — grid-side ASHP watts (ESPHome gridvue)
emporia/ashp_og/power                 — off-grid ASHP watts (Emporia Vue via HA)
emporia/ashp_og/energy_today          — off-grid cumulative kWh for the day

Dual electrical power source

The heat pump can run either from the grid or from the off-grid battery/inverter system depending on a transfer switch. Two upstream monitors each see only their own side:

• Grid feed: an Emporia Vue-2 monitor flashed with ESPHome publishes a ~5-second gridvue/power/sensor/ashpgrid/state reading in watts.
• Off-grid feed: a separate Emporia Vue (cloud-integrated via Home Assistant) exposes circuit-level power and daily-energy sensors, republished to MQTT by HA automations.

The sketch arbitrates per-message: whichever source reports the larger power is treated as "active" (ePwrSource = 1 grid / 2 off-grid / 0 idle, threshold 5 W). Only one side carries current at a time in practice, so the larger value is the true consumption.

Energy accounting by source

• On-grid: the ESPHome feed updates at ~5 s cadence, so elecEWh is integrated from elecPkW × Δt.
• Off-grid: HA's Emporia Vue integration publishes energy_today (cumulative kWh) at a coarser cadence. elecEWh is advanced by the metered delta between consecutive energy_today messages, and elecPkW is derived from Δenergy / Δt rather than integrated.

This hybrid approach avoids double-counting and keeps daily totals aligned with whichever meter actually saw the current.

Emporia-cloud glitch guard (v041827)

sensor.ashp_og_energy_today is a total_increasing counter fed from polled cloud data. The cloud occasionally reports an oversized cumulative jump that self-corrects on the next poll — e.g. on 2026-04-18 at 14:08 EDT the counter stepped +205 Wh in 60 s while the averaged power held at ~1960 W, producing a bogus 12.3 kW "spike" on the derived power. The 20 kW sanity cap that previously guarded derivedPkW was too loose to catch this.

The sketch now cross-checks derivedPkW against ogW (the averaged power from emporia/ashp_og/power): when ogW > 100 W and derivedPkW > 2 × ogW, the delta is declared a glitch, and ogDeltaWh is reconstructed as ogW × Δt. Both the published elecPkW and the elecEWh accumulator are fed the reconstructed value, so neither the power plot nor the daily-energy total inherits the artifact. The HPEM/debug topic emits dPkW, ogW, and a GLITCH marker when the guard fires, so these events are visible after the fact without needing serial capture.

Debug topic

HPEM/debug publishes a short trace on every electrical-side iteration. Two formats:

• On-grid integration: GR add=<Wh-added> PkW=<kW> dt=<ms> EWh=<total>
• Off-grid consume: OG dWh=<delta-Wh> EWh=<total> dtOg=<ms> dPkW=<derived kW> ogW=<avg-W>[ GLITCH]

These are intentionally noisy while the dual-power accounting is still being validated in the field. They'll be removed in a future flash once the numbers hold steady.

Known limitations

• IRAM headroom: the build sits at 93 % IRAM with "Basic SSL ciphers" selected. Any new IRAM_ATTR / ISR code risks the link overflowing. Flash RAM and main RAM have plenty of room.
• OTA is unreliable: at 93 % IRAM the Wi-Fi / SSL / OTA stacks coexist only marginally. Prefer USB flashing. The Arduino IDE's SSL-cipher dropdown does not persist between sessions, so double-check the "Basic SSL ciphers" selection before every compile.

Ecosystem

Data flows into the household IOTstack (Mosquitto → Node-RED → InfluxDB → Grafana). The "Heat Pump" and "Heat Pump cCOP sCOP" Grafana dashboards plot these topics directly; the Garage Dash plots HPEM_elecPkW as the ASHP_kw series alongside grid / off-grid totals.