Heat Pump Controls Setup: Weather Compensation, Boost Modes, Smart Thermostats and TRV Compatibility
Quick Answer: Weather compensation is the most important control setting for heat pump efficiency — it automatically reduces the flow temperature as outdoor temperature rises, improving COP by 0.3–0.8 SCOP points compared to a fixed flow temperature. The heat pump controller should be set with a weather compensation curve (heating curve) matched to the building's heat loss characteristics and the emitter system. Most modern smart thermostats are compatible with heat pumps, but conventional TRVs need careful management to avoid over-restricting flow.
Summary
Heat pump controls differ fundamentally from boiler controls. A boiler responds to on/off demand from a thermostat; a heat pump is most efficient when it runs continuously at a modulated output, varying the flow temperature in response to outdoor conditions. Controls that are optimised for a boiler often work poorly with a heat pump.
For heating engineers, configuring weather compensation correctly at commissioning and handing over a customer who understands how their heat pump works is a professional obligation. Heat pumps running at an unnecessarily high flow temperature due to incorrect controls setup are a common cause of poor COP and customer dissatisfaction.
Key Facts
- Weather compensation (WC) — automatically reduces the heating circuit flow temperature as the outdoor temperature rises; the relationship between outdoor temperature and flow temperature is defined by a "heating curve" or "weather compensation curve"; requires an outdoor temperature sensor
- Heating curve — defined by two points: (1) the flow temperature at the design outdoor temperature (e.g., 45°C at -3°C outdoor); (2) the flow temperature at a mild outdoor temperature (e.g., 25°C at 20°C outdoor); the heat pump interpolates linearly between these points
- Load compensation — an alternative (or complement) to weather compensation; adjusts flow temperature based on the room temperature relative to the room setpoint; requires a room temperature sensor; can cause instability with heat pumps (hunting); weather compensation is preferred
- Frost protection — the heat pump controller should maintain a minimum flow temperature (e.g., 15°C) to prevent the heating system from freezing in unoccupied properties; separate from the main heating schedule
- Fixed flow temperature — setting the heat pump to always deliver a fixed flow temperature (e.g., 50°C) regardless of outdoor conditions; less efficient than weather compensation; still common in poorly configured installations; should be avoided
- Setback vs off — heat pumps are more efficient when they run continuously at reduced output (setback) rather than switching off overnight and reheating in the morning (on/off); a setback of 2–3°C overnight is preferable to a complete off period; avoids morning "recovery" demand spikes that require higher flow temperature
- DHW priority — when DHW demand occurs, the heat pump controller diverts output to DHW cylinder heating; space heating pauses; configure DHW priority window to avoid morning peak (programme DHW heating overnight)
- Smart thermostat compatibility — most modern smart thermostats (Nest, Hive, tado°, Honeywell T6R) are compatible with heat pumps but must be configured for heat pump operation; avoid "temperature swing" settings that cause frequent on/off cycling
- TRV (Thermostatic Radiator Valve) — conventional TRVs close individual radiators when the room reaches setpoint; closing multiple TRVs simultaneously reduces circuit flow and can cause the heat pump to short-cycle or exceed its maximum differential pressure; requires management (leave some radiators on permanent, fit bypass valve)
- Outdoor temperature sensor — required for weather compensation; typically a simple NTC thermistor sensor; must be sited on a north-facing wall, out of direct sunlight and away from heat sources (flue, exhaust, solar gain); an incorrectly sited sensor produces erroneous compensation
Quick Reference Table: Weather Compensation Heating Curves
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Try squote free →| Emitter Type | Flow Temp at -3°C Outdoor | Flow Temp at +15°C Outdoor | Curve Notes |
|---|---|---|---|
| UFH (35°C max) | 35°C | 25°C | Flat curve; UFH already low temp |
| Large radiators (45°C max) | 45°C | 28°C | Moderate slope |
| Standard retrofit radiators (50°C max) | 50°C | 30°C | Steeper curve |
| Legacy radiators (55°C max) | 55°C | 35°C | Steep curve; SCOP penalty |
Detailed Guidance
Configuring the Weather Compensation Curve
Setting the design point (cold end): The design flow temperature at the coldest design outdoor temperature is determined by the emitter sizing. If the heat loss calculation (BS EN 12831) shows the building requires 45°C flow to deliver the design heat load at -3°C outdoor, then the curve's cold end is set to -3°C → 45°C.
Setting the mild end (warm end): The flow temperature at mild outdoor conditions should be sufficient to provide a small heating output but no more. Typically 25–28°C at 15–20°C outdoor is appropriate for most UK systems. Setting the warm end too high wastes energy by over-heating on mild days; too low and the building loses heat on cool autumn days.
Adjusting the curve in practice: After commissioning, the customer should monitor indoor comfort. If rooms are:
- Too cold in cold weather → increase the cold-end flow temperature
- Too warm in mild weather → reduce the warm-end flow temperature
- Consistently comfortable → the curve is correctly set
Most heat pump controllers allow the curve to be shifted up/down (parallel shift) or the slope changed. A parallel shift upward increases the flow temperature at all outdoor temperatures uniformly — useful if the emitters are marginally undersized.
Auto-adaptive weather compensation: Some advanced heat pump controllers (Daikin Altherma, Vaillant arotherm, Mitsubishi Ecodan) include auto-adaptive weather compensation, which adjusts the heating curve automatically based on room temperature feedback. This can simplify commissioning but should be monitored to ensure it does not ratchet upward excessively.
Smart Thermostat Configuration
Compatible smart thermostats:
- tado° — specifically marketed as heat pump compatible; uses OpenTherm protocol for modulating control; recommended
- Nest (Google) — compatible with heat pump systems via relay output; does not natively support OpenTherm modulation; functions as an on/off thermostat
- Hive — relay-based control; works with heat pumps as on/off controller; some models support OpenTherm
- Honeywell T6R — relay-based; straightforward compatibility; no modulation
OpenTherm protocol: OpenTherm is a standard communication protocol between a room thermostat and a heating appliance. It allows the thermostat to request a flow temperature (rather than simply switching the heat pump on/off). OpenTherm-compatible heat pump controllers and thermostats communicate to modulate the heat pump output and flow temperature — this is more efficient and avoids stop/start cycling.
Where the heat pump controller supports OpenTherm and the smart thermostat also supports OpenTherm, connect via the OpenTherm bus (two-wire) rather than the relay output.
Temperature swing: Avoid smart thermostats set to allow large temperature swings (e.g., heat to 21°C, allow to drop to 18°C before heating again). This causes frequent on/off cycling, which reduces SCOP and causes wear. Set a tight deadband (0.5°C or 1°C swing) or use a thermostat that modulates the heat pump continuously.
TRV Management in Heat Pump Systems
The problem: Conventional TRVs close the radiator valve as the room warms up. If multiple rooms reach setpoint simultaneously and all their TRVs close, the circuit flow drops. The heat pump may exceed its minimum flow rate, triggering a fault or thermal cutout. The heat pump short-cycles until zone demand resumes.
Solutions:
- Thermostatic bypass valve — fit a pressure-differential bypass valve on the primary circuit or manifold; opens automatically when circuit pressure rises above a set point (all TRVs closing); maintains minimum flow through the heat pump; essential for systems with multiple TRV-controlled radiators
- Leave at least one radiator on permanent (no TRV) — designate one radiator (typically the hallway or a large room) as a permanent open radiator with no TRV; this provides a minimum flow path at all times
- Smart TRVs with open window detection — smart TRVs (tado°, Netatmo) can be programmed with a higher setpoint so they never fully close; reduces the risk of all zones closing simultaneously
- Single zone thermostat approach — remove TRVs from all but the largest rooms; use a single room thermostat to control the heat pump; most efficient approach for a small property
Zone Control for Larger Properties
In properties with multiple heating zones (e.g., ground floor UFH + first floor radiators, or separate lounge/bedroom zones), zone control via motorised valves and zone thermostats is common. Each zone thermostat controls a motorised valve; when the zone calls for heat, the valve opens and the heat pump runs.
Minimum flow protection: Always include a bypass valve (or a permanently open zone) to prevent all zones closing simultaneously and starving the heat pump of flow. The heat pump should have a minimum flow rate requirement specified in its installation manual (e.g., 0.3 l/s for an 8kW unit); the circuit must provide at least this flow at all times when the heat pump is running.
Frequently Asked Questions
The customer has a Nest thermostat from their old boiler. Can they keep it?
Yes, but configure it correctly. Nest operates as an on/off relay with the heat pump. Set the temperature swing to minimum (1°C or less). Ensure weather compensation is configured at the heat pump controller (Nest will not override it — the heat pump still controls its own flow temperature via weather compensation). The heat pump will modulate its own output based on outdoor temperature; Nest provides the room-level setpoint.
Should weather compensation be disabled if the customer uses a room thermostat?
No — weather compensation and a room thermostat are complementary, not mutually exclusive. Weather compensation sets the flow temperature based on outdoor conditions; the room thermostat provides a setpoint for room comfort. Use both. Weather compensation handles the bulk of the modulation; the room thermostat provides the final call for heat.
The heat pump keeps cycling on and off every 5 minutes. What is the controls issue?
Short-cycling is usually caused by: (1) the heating circuit volume being too small for the heat pump's minimum run time; (2) all TRVs closing, starving the pump of flow; (3) an oversized heat pump reaching setpoint too quickly. Controls fixes: check that a bypass valve is fitted; ensure at least one permanently open radiator; check that setpoint hysteresis is not too small. If the system volume is the cause, add a buffer tank.
Regulations & Standards
MCS 007 — requires weather compensation to be configured and functioning at commissioning; controls setup recorded in commissioning documentation
Building Regulations Approved Document L — heating controls requirements; time and temperature controls for all new or replacement heating systems
Vaillant arotherm controls guide — weather compensation setup guidance
Heat Geek — Heat Pump Controls Guide — practical controls configuration for heat pump efficiency
tado° heat pump compatibility — OpenTherm and relay control configuration
heat pump cop scop explained — how weather compensation improves SCOP
radiator sizing for heat pumps — emitter sizing to allow lower weather compensation setpoints
underfloor heating heat pump — UFH zone control and bypass valve requirements
air source heat pump installation — commissioning sequence including controls setup