Home Heat Pump Guide

Heat Pump Constant vs Setback Heating

If you have switched from a gas boiler to a heat pump, your old heating habits need to change. With a boiler, you probably had the heating on a timer — off overnight, blast it in the morning, off when you leave, on again when you come home. That pattern worked because boilers are cheap to run at high output for short periods.

Heat pumps are different. They prefer to run gently and continuously, maintaining a steady temperature rather than recovering from deep setbacks. This guide explains why, and how to set your heat pump controls for the best balance of comfort and efficiency.

Why Gas Boiler Habits Do Not Work with Heat Pumps

A gas boiler can heat water to 70°C in minutes. It fires up, blasts heat through your radiators, and the room reaches 21°C quickly. It then switches off and waits. The gas burned during the recovery period is relatively cheap, and the boiler does not care whether it starts from cold or from warm.

A heat pump works on fundamentally different physics. It does not burn fuel — it moves heat from outside to inside using a refrigerant cycle. Its efficiency (COP) is directly linked to the temperature difference between the heat source (outside air) and the heat output (flow temperature to your radiators).

The COP Problem with Setbacks

When you let your house cool by 5°C overnight and then ask the heat pump to recover in the morning, several things happen:

  • The heat pump needs to raise the flow temperature to recover the lost heat quickly — perhaps from 40°C to 50°C or even higher
  • Higher flow temperatures reduce the COP — from perhaps 3.5 at 40°C flow to 2.5 at 50°C flow
  • The heat pump runs at maximum output for an extended period to reheat the entire thermal mass of your home — walls, floors, furniture, and air
  • On very cold mornings, the backup electric heater may kick in to assist, running at a COP of 1.0 (pure resistance heating)

The energy saved by switching off overnight is largely (or entirely) consumed by the inefficient recovery period. In many cases, the total energy consumed is higher than if the heat pump had simply maintained temperature throughout the night.

What Constant Heating Actually Means

Constant heating does not mean your heat pump runs at full power 24 hours a day. It means the heat pump maintains a target temperature continuously, modulating its output to match the heat loss from your home.

On a mild autumn evening, the heat pump might run at 30% capacity, using very little electricity. On a cold January night, it might run at 70% to 80% capacity. The key point is that it never needs to sprint — it always jogs, and jogging is what heat pumps do most efficiently.

How Weather Compensation Helps

Modern heat pumps use weather compensation — an outdoor sensor that automatically adjusts the flow temperature based on outside conditions. On a 10°C day, the flow temperature might be 30°C. On a -2°C day, it might rise to 45°C. This automatic adjustment keeps the heat pump running at the most efficient flow temperature for the current conditions, without any manual intervention.

Weather compensation and constant heating work hand in hand. The system continuously adjusts to deliver exactly the heat your home needs, no more, no less.

The Case for a Small Setback

Constant heating does not necessarily mean maintaining exactly 21°C at all times. A small overnight setback of 1°C to 2°C can save a modest amount of energy without significantly impacting efficiency or comfort.

Why 1-2°C Works

A small setback means the heat pump needs only a slight increase in output to recover in the morning. The flow temperature stays low, the COP stays high, and the recovery takes 30 to 60 minutes rather than 2 to 3 hours. The energy saving from reducing heat loss overnight slightly exceeds the extra energy needed for recovery.

Why 5°C+ Does Not Work

A deep setback of 5°C or more creates a large temperature deficit that takes hours to recover. The heat pump must run at maximum output with elevated flow temperatures, reducing efficiency significantly. The energy consumed during recovery often exceeds what was saved overnight. You also wake up cold and uncomfortable.

The Sweet Spot

Most heat pump manufacturers and energy advisers recommend:

  • Daytime: 20°C to 21°C in living areas
  • Overnight: 18°C to 19°C (a 1-2°C setback)
  • Away from home (short absence): 18°C to 19°C
  • Extended absence (holiday): 15°C to 16°C (frost protection with gentle recovery before you return)

What the Data Shows

The Energy Systems Catapult Electrification of Heat trial monitored hundreds of UK heat pump installations and found that homes using constant or near-constant heating strategies achieved better seasonal efficiencies than those using deep setback schedules.

The key findings:

  • Homes with no setback or setbacks of 1-2°C achieved the best seasonal performance factors (SPF)
  • Homes with setbacks of 5°C or more had measurably lower SPFs
  • The efficiency penalty of deep setbacks was most pronounced in poorly insulated homes with high thermal mass (solid walls, concrete floors)
  • Well-insulated lightweight homes (timber frame, modern construction) could tolerate slightly deeper setbacks without as much efficiency loss

Running Costs: Constant vs Setback

Here is a simplified comparison for a typical three-bedroom semi-detached house with an 8kW heat pump.

Constant Heating (20°C, 24/7)

  • Average COP: 3.2
  • Annual heat demand: 12,000 kWh
  • Electricity used: 3,750 kWh
  • Annual cost at 24.50p/kWh: £919

Gentle Setback (20°C day, 18°C night)

  • Average COP: 3.1
  • Annual heat demand: 11,200 kWh (slightly lower due to reduced nighttime loss)
  • Electricity used: 3,613 kWh
  • Annual cost at 24.50p/kWh: £885

Deep Setback (21°C day, 15°C night)

  • Average COP: 2.7 (reduced due to recovery periods)
  • Annual heat demand: 10,500 kWh (lower total but higher peak demand)
  • Electricity used: 3,889 kWh
  • Annual cost at 24.50p/kWh: £953

The gentle setback saves around £34 per year compared to constant heating. The deep setback actually costs £34 more than constant heating, despite delivering less comfort. The numbers vary by property, but the pattern is consistent.

Special Situations

Underfloor Heating

Wet underfloor heating with screed has significant thermal mass — the screed stores heat and releases it slowly. This system responds even more slowly to setbacks, making constant heating the obvious strategy. Some UFH installers recommend no setback at all, letting the thermal mass of the floor maintain temperature naturally.

Radiator Systems

Radiators respond faster than UFH, so a gentle setback is more practical. The heat pump can recover 1-2°C in 30 to 60 minutes with radiators, compared to 2 to 3 hours with screed UFH. A gentle overnight setback makes sense for most radiator-based systems.

Time-of-Use Tariffs

If you are on a time-of-use electricity tariff (such as Octopus Agile or a heat pump-specific tariff), there may be a financial case for boosting heat during cheap-rate periods and coasting through expensive periods. This is not the same as a setback — it is pre-heating. The heat pump runs at slightly higher output during cheap electricity hours (typically overnight) and lets the thermal mass of the house carry through the expensive daytime rates.

This strategy works best with UFH and well-insulated homes that can "coast" for several hours without a significant temperature drop.

Smart Controls and Learning

Some smart thermostats and heat pump controllers can learn your home's thermal characteristics and optimise the heating strategy automatically. They measure how quickly your home loses heat, how long recovery takes, and what flow temperatures are needed. Over time, they find the most efficient balance for your specific property. If your heat pump supports this feature, use it.

How to Set Up Your Heat Pump for Constant Heating

  1. Enable weather compensation — let the heat pump adjust flow temperature automatically based on outdoor conditions
  2. Set your thermostat to a constant target — 20°C or 21°C for living areas, with an optional 1-2°C overnight setback
  3. Set bedroom zones to 17°C to 18°C — bedrooms rarely need to be as warm as living rooms
  4. Disable boiler-style timers — do not use on/off schedules; let the heat pump and thermostat manage output
  5. Check the heating curve — your installer should set the weather compensation curve during commissioning; if your home is consistently too warm or too cool, the curve may need adjusting

Frequently Asked Questions

Should I leave my heat pump on all the time?

In most cases, yes. Heat pumps are designed to run continuously during the heating season, modulating their output to match demand. Switching them on and off like a gas boiler wastes energy during recovery periods and can cause short-cycling, which increases wear on the compressor.

Will constant heating cost more in electricity?

Counterintuitively, no. Constant heating keeps the heat pump running at low, efficient output. Deep setbacks force the heat pump to run at high, less efficient output to recover. The gentle approach typically uses the same or less electricity overall.

Can I turn the heating off when I go on holiday?

Do not turn it off completely — risk of frozen pipes in winter. Set the thermostat to 12°C to 15°C for frost protection. Many smart thermostats have a "holiday mode" or "away mode" that does this automatically. Schedule the heating to start recovering a day before you return.

My heat pump seems to run all the time — is that normal?

Yes, during cold weather. An inverter heat pump modulates its output, running at low power for extended periods. This is efficient and correct behaviour. If your heat pump is running at maximum output constantly and still not reaching the target temperature, contact your installer — the system may be undersized or have a fault.

What setback should I use overnight?

1°C to 2°C is the recommended maximum. For example, if your living room is set to 21°C during the day, drop it to 19°C or 20°C overnight. For UFH with screed, no setback at all is often the most efficient approach.

Does this advice apply to ground source heat pumps too?

Yes. The same principles apply to both air source and ground source heat pumps. Ground source heat pumps have a more stable heat source (the ground stays at 8-12°C year-round), so their COP is less affected by weather, but the recovery penalty from deep setbacks still applies.