Is My Heat Pump Undersized? 5 Signs to Look For

If you’re wondering whether your heat pump is undersized, you’re not alone.

It’s one of the most common concerns homeowners have — especially during colder weather when the system seems to be working hard or running constantly without improving comfort

The truth is, most heat pumps aren’t actually undersized. But some are. And the signs are usually consistent.

Here are five things to look for.

1. The House Never Quite Reaches Set Temperature

If your thermostat is set to 21°C but the house regularly stalls at 18–19°C during cold weather, that can indicate insufficient output.

Important context:

• During very cold snaps (-3°C to -5°C), a slight drop can be normal.

• But if it consistently can’t reach the temperature in average winter conditions (0°C to 5°C), that deserves investigation.

• Or issues with how heat is being distributed around the system
   

The key question is:
Is the system running at full capacity and still not keeping up?

2. Flow Temperatures Are Extremely High

Heat pumps are most efficient at lower flow temperatures (typically 35–45°C for well-designed systems).

If your installer has set:

• 55–60°C flow 

• Maximum weather compensation curve 

• Or “high temperature mode” just to maintain comfort
   

…it may suggest the heat emitters (radiators or underfloor) are struggling to deliver enough heat, which can sometimes mask undersizing.

High flow temperature doesn’t automatically mean the unit is too small, but it can be a red flag when combined with other symptoms.

3. It Runs Constantly — But Still Feels Cold

Modern heat pumps are designed to run for long periods. Continuous operation is not a fault.

However, if:

• It runs 24/7

• The compressor is near maximum frequency most of the time 

• And indoor temperature still drifts downward
   

That suggests the system output may not match the heat loss.

Continuous running and maintaining the temperature is normal.
Continuous running and losing temperature is not.

4. Excessive Use of Backup Heater

If your system has an electric immersion or backup heater and it activates frequently in normal winter conditions, that can indicate the main heat pump is struggling and can contribute to higher electricity usage

Occasional backup use during very cold spells is expected.

Frequent reliance on it at 0–5°C outside is worth reviewing.

5. The Original Heat Loss Calculation Was Rushed or Incomplete

Under sizing often comes back to design stage errors:

• No proper room-by-room heat loss 

• Assumed insulation values

• Radiators not upgraded

• System sized purely on “like-for-like” boiler replacement
   

If the design was based on estimates rather than measured fabric data, the installed capacity may not reflect the actual heat demand.

Before You Assume It’s Undersized

A system can feel undersized when the real issue is:

• Incorrect weather compensation settings 

• Poor hydraulic balance 

• Oversized buffer tank affecting performance 

• Incorrect pump flow rate 

• Air in the system 

• Radiators not sized for lower flow temperatures
   

In other words: configuration matters.

Many systems that appear “too small” are actually incorrectly set up.

What Is Normal?

A properly sized and configured heat pump should:

• Maintain target temperature during typical winter conditions 

• Run steadily at moderate compressor output 

• Avoid constant immersion heater usage 

• Operate at sensible flow temperatures
   

It does not need to cycle aggressively.
It does not need to “blast heat” like a gas boiler.

Heat pumps are steady, not explosive.

It is also worth considering that what looks like an undersizing problem on the surface can sometimes reflect a gap in how the system was handed over. If no proper commissioning record exists — no flow temperature log, no weather compensation settings, no record of how the system was configured at sign-off — it becomes very difficult to tell whether the unit is genuinely too small or simply running on incorrect settings. Our Installer Handover Pack outlines exactly what should have been documented when the system was installed, which can be a useful starting point for working out where the problem actually lies.

If you are still at the consideration stage and have not yet committed to an installation, our honest guide on whether you should get a heat pump in the UK covers the conditions under which the technology works well and the situations where it is worth pausing before proceeding.

Need an Independent Technical Review?

If you’re unsure whether your heat pump is correctly sized or configured, a structured technical review can provide clarity.

In many cases, what appears to be an undersized system is actually an issue with setup, flow, or heat distribution — but without reviewing the system properly, it’s difficult to know.

👉 You can request a full heat pump performance review, where we assess system sizing, flow temperatures, and overall performance to identify exactly what’s happening — and what (if anything) needs changing.

No sales.
No installation upsell.
Just clear, independent technical advice.