Weather Compensation Heat Pumps

29 Dec

Weather compensation is one of the most important heat pump settings. It helps your system run efficiently by automatically adjusting the heating water temperature (flow temperature) based on outdoor weather.

Done well, weather compensation makes your home more comfortable, reduces running costs, and keeps the heat pump running steadily instead of constantly switching on and off. Done badly, it can make rooms too cold, too hot, or cause confusing behaviour that looks like a fault.

What is weather compensation?

Weather compensation means the heat pump uses an outdoor temperature sensor (or weather data) to decide what flow temperature it should produce.

The idea is simple:

When it’s mild outside, you only need a low flow temperature.
When it’s colder outside, you need a higher flow temperature.

This is usually controlled by a weather compensation curve (sometimes called a heating curve).

Why weather compensation matters on heat pumps

Heat pumps are most efficient when they can run with a low flow temperature and a steady output. If the flow temperature is set higher than necessary, efficiency drops and running costs rise.

Weather compensation helps because it:

Keeps flow temperatures as low as possible (better efficiency).
Reduces room temperature swings (more comfort).
Helps prevent short cycling (better reliability).
Automatically responds to changing weather without you constantly adjusting settings.

Weather compensation vs room thermostats

Weather compensation and room thermostats can work together, but they do different jobs:

Weather compensation controls how hot the water is (flow temperature).
Room thermostats decide when heating is allowed on (on/off demand).

Many heat pump systems run best when weather compensation is doing most of the work, with room thermostats used lightly (or set high) so the heat pump can run steadily.

If your system has lots of zones and thermostats (common with underfloor heating), you can still use weather compensation — but it becomes more important that zoning and valve behaviour don’t restrict flow too much.

What is a weather compensation curve?

A weather compensation curve is a line (or set of points) that links:

Outdoor temperature (what the sensor reads)
Target flow temperature (what the heat pump aims to produce)

Most controllers let you adjust the curve in one of these ways:

Slope (how aggressively flow temperature rises as it gets colder)
Shift / offset (moving the whole curve up or down)
Design points (setting a flow temperature at a specific outdoor temperature)

How to tell if weather compensation is set wrong

Weather compensation is often blamed for problems that are actually caused by thermostats, zoning, or flow issues. But these are common signs the curve needs adjusting:

Home too cold in mild weather (curve too low / too flat).
Home too hot in mild weather (curve too high).
Home warm enough in cold weather but not in mild weather (curve shape needs changing).
Radiators always very hot (flow temperature likely set higher than needed).
Heat pump constantly stopping and starting (often too high a flow temp, or too much thermostat control).

How to adjust weather compensation (simple homeowner method)

The goal is to find the lowest curve that still keeps the house comfortable. Adjust slowly and give the system time to settle.

Pick a stable period (avoid adjusting during extreme weather swings).
Make small changes (one step at a time, not big jumps).
Wait 24–48 hours after each change before judging results.
If the house is too cold most of the time, increase the curve slightly. If it’s too warm, decrease it slightly.
If it’s fine in cold weather but not in mild weather (or vice versa), you may need to change slope rather than just shifting the whole curve.

If your controller uses “slope” and “offset”, a good rule of thumb is: use offset for small comfort tweaks and use slope for seasonal behaviour changes.

Common mistakes with weather compensation

Turning the curve up too high because rooms feel cold temporarily. Heat pumps heat slowly — sudden changes often make things worse.
Fighting the system with thermostats (thermostats constantly shutting zones and causing cycling).
Assuming hot radiators = correct. Heat pumps often run best with radiators that feel only warm.
Ignoring flow problems. Blocked filters, closed valves, or air can mimic “wrong curve” symptoms.
Changing settings too frequently. You need time to see the effect of each change.

Weather compensation and running costs

If your flow temperature is higher than necessary, the heat pump works harder to lift the temperature and efficiency drops. Weather compensation aims to keep the system running at the lowest useful temperature for the current weather.

If you want to reduce running costs, weather compensation is often a better place to focus than constant thermostat tweaking.

When to call an engineer

You should contact a qualified heat pump engineer if:

You can’t find weather compensation settings on the controller.
The system never reaches temperature even with a higher curve.
You’re getting repeated flow or temperature-related fault codes.
The system has never been balanced or properly commissioned.
Your system has complex zoning and behaves unpredictably.

If weather compensation never seems to work well, the issue may be system design, sensor placement, flow rate, balancing, or commissioning — not just a setting.

Key takeaway

Weather compensation is how a heat pump “knows” how much heat to deliver as the weather changes. The best results come from finding the lowest curve that keeps your home comfortable, and then leaving it alone.

If your system is cycling, running expensive, or never feels stable, weather compensation is one of the first settings worth checking.

Jack Mundinano