Why Bother?#
My boiler room has a solar thermal system with a Resol DeltaSol controller running the circulation pump. It does its job — but it’s a black box. I can’t see temperatures, pump status, or energy production from anywhere except a tiny LCD on the wall.
The electric backup heater is worse — a mechanical thermostat with no scheduling, no target temperature, and no safety beyond a basic overheat cutoff.
I wanted full visibility into the solar thermal system and smart control of the electric heater. An ESP32 handles the monitoring — reading the Resol, measuring pressure and climate, driving the local display. Home Assistant handles the control — scheduling the heater, enforcing safety, sending notifications.
The Hardware#
| Component | Role |
|---|---|
| ESP32 DevKit | Runs ESPHome, talks to everything |
| Resol DeltaSol SLL | Solar thermal differential thermostat |
| VBus optocoupler | Reads Resol’s VBus protocol via UART |
| DHT22 | Boiler closet temperature + humidity |
| Pressure transducer (0-100 PSI) | Hydraulic system pressure |
| SSD1306 OLED (128x64) | Local status display |
| Shelly Pro 4PM | Switches the heater via a power contactor (see below) |
Heater Switching: Shelly + Power Contactor#
The Shelly Pro 4PM handles up to 16A per channel, but the boiler’s heating element draws close to that limit. Running a smart relay at its rated maximum continuously is asking for trouble. Instead, the Shelly drives a Hager ESC225 power contactor (25A, 2NO) — the Shelly switches the contactor coil (a few watts), and the contactor handles the actual heater current safely.
Reading the Resol DeltaSol via VBus#
The Resol uses a proprietary single-wire protocol called VBus. An optocoupler circuit (817A with a 3K3 input resistor and 4K7 pull-down) isolates the bus and feeds data to the ESP32’s UART.
uart:
id: resol
rx_pin: 4
baud_rate: 9600
vbus:
uart_id: resolThe DeltaSol sends two frame types. The main frame (source 0x2271) carries solar collector temperature, boiler temperature, pump speed, operating hours, and error codes. The heat meter frame (source 0x2272) carries instantaneous thermal power and cumulative energy — perfect for Home Assistant’s Energy Dashboard.
The solar collector temperature needs signed decoding since it can go negative on winter nights:
lambda: |-
unsigned lsb = x[4];
unsigned msb = x[5];
float temp = 0;
if (msb > 127) {
temp = (65536 - msb * 256 - lsb) * -0.1f;
} else {
temp = (msb * 256 + lsb) * 0.1f;
}
if (temp < -20 || temp > 200) return NAN;
return temp;
The OLED + Status LED#
The SSD1306 runs in portrait mode and shows two sections: HYDRAULICS (boiler temp, solar temp, pump speed, pressure) and CLOSET (ambient temp, humidity, heater state). Values blink when abnormal. A NeoPixel LED mirrors the status — green when everything is normal, red when anything needs attention. Glanceable from across the room.
The Automation: Smart Heater Control#
With all sensor data in Home Assistant, the electric backup heater gets a proper controller:
flowchart TD
A["Every 5 min\n18:00-22:00"] --> B{Sensors OK?}
C["Temp > 58°C"] --> D["SAFETY OFF"]
E["On > 1.5 hours"] --> D
B -->|No| F["Log + notify, stop"]
B -->|Yes| G{Temp < 40°C\nAND heater off?}
G -->|No| H["Skip"]
G -->|Yes| I["Turn ON"]
I --> J["Wait for 50°C\nor 60 min timeout"]
J --> K["Turn OFF + notify"]
style D fill:#991b1b,stroke:#ef4444,color:#fff
style I fill:#166534,stroke:#22c55e,color:#fff
style F fill:#854d0e,stroke:#eab308,color:#fff
Evening schedule: Every 5 minutes between 18:00–22:00, if the boiler is below 40°C and the heater is off, turn it on and wait until 50°C (or 60 min timeout). If solar thermal already heated the water, it does nothing.
Safety (24/7): Over-temperature (>58°C) or stuck-on (>1.5 hours) — immediate shutoff + Telegram notification.
Sensor validation: If the temperature sensor or heater switch is unavailable, the automation refuses to act and notifies you. A smart boiler that can’t read its own temperature is more dangerous than a dumb one.
The key pattern is wait_template with a timeout — heat until done, never longer than the safety limit:
- wait_template: "{{ states(boiler_sensor) | float(0) >= target_c }}"
timeout: "01:00:00"
continue_on_timeout: trueAfter the wait, wait.completed tells you whether you hit the target or timed out.
The Result#
With everything wired up on a single ESP32, Home Assistant sees: solar collector temp, boiler temp, pump speed, thermal power (W), cumulative heat energy (kWh) for the Energy Dashboard, operating hours, error codes, hydraulic pressure, closet climate, and full heater audit trail via Telegram.
We never waste electricity on heating the boiler — we only heat it when there wasn’t enough sun during the day to do the job, and only to the exact temperature we need. We never touch the boiler manually anymore. It’s one less thing to think about — the system guarantees hot water is ready every evening without anyone having to remember to flip a switch.
The cost savings are real too. Before this, the boiler’s 5kW heating element would run for hours because nobody remembered to turn it off, or the mechanical thermostat would overshoot well past what we needed. Now we have a closed-loop system that heats to exactly 50°C and stops — no more paying for electricity to keep water at 70°C when 50°C is perfectly fine.
And beyond the heater control, we’re closely monitoring both the boiler and solar collector temperatures around the clock. If the solar panels overheat, if the boiler gets stuck at a weird temperature, or if hydraulic pressure drops — we know immediately, not when someone complains about cold water.
Appendix: Full ESPHome Configuration#
Click to expand — ESPHome YAML
substitutions:
name: esphome-boiler
friendly_name: ESP32_Boiler
esphome:
name: ${name}
friendly_name: ${friendly_name}
esp32:
board: esp32-c3-devkitm-1
framework:
type: arduino
logger:
api:
ota:
- platform: esphome
wifi:
ap: {}
captive_portal:
web_server:
version: 3
# --- VBus connection to Resol DeltaSol ---
uart:
id: resol
rx_pin: 4
baud_rate: 9600
vbus:
uart_id: resol
sensor:
# --- Closet climate ---
- platform: dht
pin: 10
model: DHT22
temperature:
id: closet_temp
name: "Closet Temperature"
humidity:
id: closet_hum
name: "Closet Humidity"
update_interval: 30s
# --- Hydraulic pressure (0-100 PSI transducer) ---
- platform: adc
id: pressure_hydralics
pin: 0
name: "Hydraulic Pressure"
unit_of_measurement: "Bar"
samples: 5
attenuation: 12db
filters:
- calibrate_linear:
datapoints:
- 0.49175675 -> 0
- 4.54175676 -> 100
- lambda: |-
float bar = x * 0.0689476;
return (bar < 0) ? 0 : bar;
# --- Resol DeltaSol main data ---
- platform: vbus
model: custom
dest: 0x10
source: 0x2271
command: 0x100
sensors:
- id: temperature_solar
name: "Solar Collector Temperature"
device_class: temperature
unit_of_measurement: "°C"
lambda: |-
unsigned lsb = x[4]; unsigned msb = x[5];
float temp = (msb > 127)
? (65536 - msb * 256 - lsb) * -0.1f
: (msb * 256 + lsb) * 0.1f;
return (temp < -20 || temp > 200) ? NAN : temp;
filters:
- sliding_window_moving_average:
window_size: 30
send_every: 30
- id: temperature_boiler
name: "Boiler Temperature"
device_class: temperature
unit_of_measurement: "°C"
lambda: |-
float temp = ((x[7] << 8) + x[6]) * 0.1f;
return (temp < 0 || temp > 120) ? NAN : temp;
filters:
- sliding_window_moving_average:
window_size: 30
send_every: 30
- id: pump_speed_1
name: "Pump Speed"
unit_of_measurement: "%"
lambda: return x[20];
- id: operating_hours
name: "Operating Hours"
device_class: duration
unit_of_measurement: "h"
lambda: return ((x[35]<<24)+(x[34]<<16)+(x[33]<<8)+x[32]);
- id: error_mask
name: "Error Mask"
entity_category: diagnostic
lambda: return ((x[75]<<24)+(x[74]<<16)+(x[73]<<8)+x[72]);
# --- DeltaSol SLL heat meter ---
- platform: vbus
model: custom
dest: 0x10
source: 0x2272
command: 0x100
sensors:
- id: sll_power_w
name: "Solar Thermal Power"
unit_of_measurement: "W"
device_class: power
lambda: |-
if (x.size() < 8) return NAN;
return (float)((uint32_t)x[4]|((uint32_t)x[5]<<8)
|((uint32_t)x[6]<<16)|((uint32_t)x[7]<<24));
- id: sll_heat_kwh
name: "Heat Quantity Total"
unit_of_measurement: "kWh"
device_class: energy
state_class: total_increasing
lambda: |-
if (x.size() < 4) return NAN;
uint32_t wh = (uint32_t)x[0]|((uint32_t)x[1]<<8)
|((uint32_t)x[2]<<16)|((uint32_t)x[3]<<24);
return wh / 1000.0f;
- id: sll_heat_today_kwh
name: "Heat Quantity Today"
unit_of_measurement: "kWh"
device_class: energy
state_class: total
lambda: |-
if (x.size() < 12) return NAN;
uint32_t wh = (uint32_t)x[8]|((uint32_t)x[9]<<8)
|((uint32_t)x[10]<<16)|((uint32_t)x[11]<<24);
return wh / 1000.0f;
# --- OLED Display (portrait) ---
i2c:
sda: 6
scl: 7
frequency: 350kHz
font:
- file: "gfonts://Poppins"
id: my_font
size: 11
display:
- platform: ssd1306_i2c
model: "SSD1306 128x64"
address: 0x3C
update_interval: 1s
rotation: 90
lambda: |-
it.printf(0, 8, id(my_font), TextAlign::BASELINE_LEFT, "HYDRAULICS");
it.printf(0, 20, id(my_font), TextAlign::BASELINE_LEFT,
"BOIL: %.0f C", id(temperature_boiler).state);
it.printf(0, 34, id(my_font), TextAlign::BASELINE_LEFT,
"SOL: %.0f C", id(temperature_solar).state);
it.printf(0, 48, id(my_font), TextAlign::BASELINE_LEFT,
"PUMP: %.0f%%", id(pump_speed_1).state);
it.printf(0, 62, id(my_font), TextAlign::BASELINE_LEFT,
"PRS:%.1fBar", id(pressure_hydralics).state);
for (int x = 0; x < 128; x++) it.draw_pixel_at(x, 66);
it.printf(0, 80, id(my_font), TextAlign::BASELINE_LEFT, "CLOSET");
it.printf(0, 94, id(my_font), TextAlign::BASELINE_LEFT,
"TEMP: %.0f C", id(closet_temp).state);
it.printf(0, 108, id(my_font), TextAlign::BASELINE_LEFT,
"HUM: %.0f%%", id(closet_hum).state);
it.printf(0, 122, id(my_font), TextAlign::BASELINE_LEFT,
"HEATER:%s", id(switch_shelly_rack_switch_2).state ? "ON" : "OFF");
# --- Status LED ---
light:
- platform: neopixelbus
variant: WS2811
pin: 8
num_leds: 1
name: "Status LED"
id: esp32_rgb_led
# --- Heater switch (from HA) ---
switch:
- platform: homeassistant
name: "Heater Switch"
entity_id: switch.your_boiler_relay
id: switch_shelly_rack_switch_2Click to expand — HA Automation: Smart Boiler Controller
alias: "Smart Boiler Controller — Evening + Safety"
description: >-
18:00-22:00: every 5 min, if boiler < 40°C and heater off, turn ON
until 50°C or max 60 min. Safety: over-temp >58°C and stuck-on >1.5h.
triggers:
- trigger: time_pattern
minutes: /5
id: evening_check
- trigger: numeric_state
entity_id: sensor.your_boiler_temperature
above: 58
id: temp_too_high
- trigger: state
entity_id: switch.your_boiler_relay
to: "on"
for: "01:30:00"
id: on_too_long
conditions: []
actions:
# --- Validate inputs ---
- choose:
- conditions:
- condition: template
value_template: "{{ states(heater_switch) in ['unknown','unavailable'] }}"
sequence:
- action: notify.send_message
target:
entity_id: "{{ notify_entity }}"
data:
title: "Boiler"
message: "Cannot control boiler: switch is {{ states(heater_switch) }}."
- stop: "Heater switch unavailable"
- conditions:
- condition: template
value_template: "{{ temp_now is none }}"
sequence:
- action: notify.send_message
target:
entity_id: "{{ notify_entity }}"
data:
title: "Boiler"
message: "Cannot read boiler temperature: {{ states(boiler_sensor) }}."
- stop: "Sensor unavailable"
# --- Safety (24/7) ---
- choose:
- conditions:
- condition: template
value_template: "{{ trigger.id in ['temp_too_high','on_too_long'] }}"
- condition: template
value_template: "{{ is_state(heater_switch, 'on') }}"
sequence:
- action: switch.turn_off
target:
entity_id: "{{ heater_switch }}"
- action: notify.send_message
target:
entity_id: "{{ notify_entity }}"
data:
title: "Boiler"
message: >-
Safety OFF ({{ trigger.id }}).
Temp: {{ (states(boiler_sensor)|float(0)) | round(1) }}°C.
- stop: "Safety handled"
# --- Evening heating ---
- choose:
- conditions:
- condition: template
value_template: "{{ trigger.id == 'evening_check' }}"
- condition: time
after: "18:00:00"
before: "22:00:00"
- condition: template
value_template: "{{ temp_now < min_temp_for_on }}"
- condition: template
value_template: "{{ is_state(heater_switch, 'off') }}"
sequence:
- action: switch.turn_on
target:
entity_id: "{{ heater_switch }}"
- action: notify.send_message
target:
entity_id: "{{ notify_entity }}"
data:
title: "Boiler"
message: "ON — target {{ target_c }}°C (now: {{ temp_now|round(1) }}°C)."
- wait_template: "{{ states(boiler_sensor) | float(0) >= target_c }}"
timeout: "{{ max_on }}"
continue_on_timeout: true
- variables:
temp_after: "{{ states(boiler_sensor) | float(0) }}"
reached: "{{ wait.completed }}"
- action: switch.turn_off
target:
entity_id: "{{ heater_switch }}"
- action: notify.send_message
target:
entity_id: "{{ notify_entity }}"
data:
title: "Boiler"
message: >-
OFF — {{ temp_after|round(1) }}°C
(target {{ target_c }}°C, reached={{ reached }}).
variables:
boiler_sensor: sensor.your_boiler_temperature
heater_switch: switch.your_boiler_relay
notify_entity: notify.your_telegram_bot
min_temp_for_on: 40
target_c: 50
max_on: "01:00:00"
temp_now: "{{ states(boiler_sensor) | float(none) }}"
mode: restart
max_exceeded: silent
