Heat Pump Cycling and Minimum Modulation

Note: this article (created September 2025) forms part of a wider series of Vaillant Arotherm information and guides.

Vaillant Arotherm Heat Pump Guide: Weather Curve, Hot Water Modes, SensoCOMFORT, COP & SCOP

Table of Contents

This article about heat pump cycling is a comprehensive read.

Use the Table of Contents below to jump straight to the section that matters most — or scroll through at your own pace.

What is Weather Compensation?

Weather compensation is a control strategy built into most modern heat pumps. It automatically adjusts the target flow temperature of the water going into your radiators or underfloor heating depending on the outdoor temperature.

• On a cold day, weather compensation raises the flow temperature so the house gets enough heat.

• On a mild day, it lowers the flow temperature because the house doesn’t need as much heat.

This sliding curve helps the system run efficiently and avoid wasting heat. With the aim to create enough heat to match the amount being lost to the outside world. But it has limits.

As the weather gets warmer, weather comp keeps reducing the flow temperature. Eventually, the house’s heat demand can fall below the minimum heat output of the heat pump. When that happens, the unit can no longer run steadily so it has to cycle on and off.

You can find a full explanation on flow temperature curves in this dedicated article:
Vaillant aroTHERM weather curve information

Note: heat pumps should always be controlled using the original manufacturers controls and not by any third party systems (like Hive, Nest).  These add-on third party controls can act like on/off switches which goes against the whole principal of low and slow that heat pumps love.

Note: Havenwise, a remote optimiser we mention in other Vaillant articles if fine to use.  Despite it being ‘third party’, it uses the Vaillant controls and absolutely sticks to the principals of weather compensation and low and slow heat output.

Minimum Outputs

Every heat pump (and gas boiler) has a minimum heat output. That’s the lowest amount of heat it can put into your system. If your house needs less than that, the pump has to cycle off for a bit, then back on again.

⚠️ This is heat output, not electrical input. Manufacturers quote minimums as heat delivered, not how many watts the unit is drawing.

A quick refresher on Coefficient of Performance (COP), this is a point-in-time snapshot of system efficiency:

COP = Heat Output ÷ Electrical Input​

Example: if the unit is delivering 2280 W of heat while drawing 455 W of electricity, then:

2280÷ 455 = 5.0 COP

So 1 kWh in gives ~5 kWh out.

You can see that behaviour on my live monitoring here: https://emoncms.org/energystatsuk?mode=power&start=1742710290&end=1742715380

👉 Note: SCOP is the Seasonal Coefficient of Performance. It’s the same ratio but measured over a much longer period, such as an entire heating season or a full year.

For the numbers in this article I’ve used Vaillant’s published spec sheets. I’ve written about the “Gold Dust” sheets in detail in my Top of the SCOPs article, which links directly to the official PDFs.

Typical minimum heat output of each Vaillant model:

• aroTHERM plus 3.5 / 5 kW → ~2.2 kW

• aroTHERM plus 7 kW → ~3.2 kW

• aroTHERM plus 10 / 12 kW → ~5.8 kW

These are rough guides. The actual number depends on outside temperature, flow temperature, and compressor speed. Check your own spec sheet to be sure.

Note: The 3.5kW and 5kW are bascially the same hardware, so have the same minimum modulation.  The only difference is that they are capped at their maximum output.  The situation is the same with the 10kW and 12kW units.  Same minimum output, but capped maximum output.

How other brands compare

The new Grant R290 models are some of the best right now at turning down low, which means more steady running and less cycling in mild weather:

• Grant Aerona R290 4 kW max output → 1.25 kW minimum

• Grant Aerona R290 7.6 kW max output → 2.4 kW minimum

• Grant Aerona R290 9.4 kW max output → 2.5 kW minimum

• Grant Aerona R290 16 kW max output → 4.6 kW minimum

Here is a snippet from their tech spec sheet showing maximum heat output in red and minimum heat output in blue.

I’m not going to supply much more information for the Grant as this article is primarily focused on the Vaillant data that I know well.

If you want to keep up with a couple of Grant R290 installations, then firstly look at Glyn Hudson 9kW unit that he featured on his LinkedIn post from September 2025;

Designing and Installing the Ultimate Heat Pump for My House

And Paul Spence from the Heat Pumps U.K. Facebook group has been waxing lyrically about his too.

I don’t think Grant have discovered a miracle here though, as there are many brands with low modulation ranges.  Most modern heat pumps are all in the same sort of ballpark.  There’s just too many brands to cover them all within the scope of this article.  Be sure to go search out the technical documents are brands you are interested in.

Using heatpumpmonitor.org To Look For Minimum Outputs

If you’re interested in the performance of a heat pump model other than a Vaillant or a Grant, then it’s well worth heading over to heatpumpmonitor.org

As of September 2025 there are over 175 heat pumps that have 365 days worth of real world data and over 360 heat pumps in total.

I find being able to dive in and interrogate this data invaluable for noseying around other heat pumps.

Is all heat pump cycling bad?

Everything we talk about in this article assumes a well-designed, well-functioning installation.  Properly sized, running with weather compensation on manufacturers controls (not on/off controls), adequate system volume etc.

I split cycling into two types:

• Normal cycling – what I call system-defined cycling. The gentle on/off in mild weather when the house hardly needs any heat. Totally normal.

• Short cycling – the bad kind. Constant on/off even when it’s cold outside. Usually caused by oversizing, high flow temps, not enough water volume, or TRVs shutting rooms down.

This article is about the first type: normal cycling in mild weather.

What is short cycling vs normal cycling?

Short cycling is when the unit is switching on and off more often than it should,  sometimes every few minutes. That usually means something’s not right.

Normal cycling in mild weather is expected. When the house barely needs any heat, the pump can’t turn down far enough, so it ticks on and off gently. That’s not a fault.

Heat loss vs. heat pump output

• Every home has a heat loss rate, which is how much heat it needs to stay warm as the outside temperature drops.

• Every heat pump has a minimum output, the lowest it can go before it has to stop.

Think about when you had a gas hob and you played the game to see how low you could get the flame before it gave up.  I like to think of minimum output like that.  Eventually you went so low it just went off.

In cold weather, the house needs lots of heat, so the heat pump normally runs steadily (as long as the unit is correctly sized and the demand is above its minimum output).

In mild weather, the house needs less heat, sometimes less than the heat pump’s minimum.

When this happens, the heat pump turns itself off to stop the house overheating.

Quick check: does this apply to your home?

You only need two numbers:

1. Your heat loss at design temperature. From your MCS or installer report. It will read something like 4.8 kW at –3 °C outside.

2. Your heat pump’s minimum output. From the spec sheet.

Then:

• Work out the temperature difference between indoor and outdoor design temperature. Example: 20 °C indoors and –3 °C outdoors = 23 °C difference.

• Divide your design heat loss by that number.  This gives you the heat demand per degree difference (your house’s “kW per degree”).

• Take your heat pump’s minimum output and see how many degrees of demand that would cover.

• Subtract that from your indoor setpoint to find the outdoor temperature where cycling will stop.

Above that temperature, the pump will cycle. Below it, it should run steadily.

We can look at some examples below for different Vaillant models if all that seems a bit confusing.  But these calculations work for any heat pump as long as you have the initial data.

Minimum Modulation Example

Worked example (house with a 4.8 kW heat loss at 20°C Indoors, -3°C Outdoors)

• Step 1: Design temperature difference (Indoor and Outside)
  20°C minus (–3°C) = 23°C degrees difference.

• Step 2: Heat demand per degree (Heat Loss / Temperature Difference)
  4.8 kW ÷ 23°C = 0.209 kW per degree.

• Step 3: Compare with heat pump minimum
  Minimum output = 2.2 kW (assuming 3.5 or 5kW Vaillant).

• Step 4: Find the cycling point (minimum output / kW per degree)
  2.2 ÷ 0.209 = 10.52 degrees difference.

• Step 5: Deduct Degrees Difference from Indoor Design Temp
  20°C – 10.52°C = 9.48 °C outside.

So, in a house with a 5 kW heat loss, the 5kW heat pump should stop cycling whenever it’s colder than ~9.5 °C outdoors.

Note: These figures assume perfection. ie, at 9.5 °C outdoors the amount of heat generated the heat pump (2.2kW) matches exactly what is being lost from the house.  But we know that isn’t always the case.  If it’s a sunny 9 °C day the house may need less then 2.2kW because of solar gains.  Other examples of internal gains could be cooking or having a large amount of people in the house.  On the flip side, your house may be a little draughty, so on a dull but windy 9 °C day your heat demand may be slightly higher.

Minimum Modulation Calculator

Case study: Vaillant 5 kW unit (minimum 2.2 kW output)

Watts Per Degree For 5kW, 4kW and 3kW heat loss (20°C Indoors, -3°C Outdoors)

• 5,000W ÷ 23 = 217 W/°C or 5kW ÷ 23 = 0.217 kW/°C

• 4,000W ÷ 23 = 174 W/°C or 4kW ÷ 23 = 0.174 kW/°C

• 3,000W ÷ 23 = 130 W/°C or 3kW ÷ 23 = 0.130 kW/°C,

Heat demand table (kW), Target 20C Indoors (excerpt, 1 °C steps)

With a 3.5 / 5 kW unit that can’t go below 2.2 kW output:

Case study: Vaillant 7 kW unit (minimum 3.2 kW output)

Now let’s scale things up. Same 20 °C indoors, –3 °C outdoors design, but three houses with 7 kW, 6 kW, and 5 kW heat losses.

Watts Per Degree For 7kW, 6kW and 5kW heat loss (20°C Indoors, -3°C Outdoors)

• 7,000W ÷ 23 = 304 W/°C or 7kW ÷ 23 = 0.304 kW/°C

• 6,000W ÷ 23 = 261 W/°C or 6kW ÷ 23 = 0.261 kW/°C

• 5,000W ÷ 23 = 217 W/°C or 5kW ÷ 23 = 0.217 kW/°C

Heat demand table (kW), Target 20C Indoors (excerpt, 1 °C steps)

With a 7 kW unit that can’t go below 3.2 kW output:

What this shows:

• In the 7 kW house, cycling starts once it’s warmer than ~9 °C outdoors.

• In the 6 kW house, the cycling point is ~7 °C.

• In the 5 kW house, you’ll wait until ~5 °C before the pump stops cycling.

So the better your fabric (lower heat loss), the more of the year you’ll spend in cycling mode. That’s not a fault, it’s just because your house doesn’t need much heat until it’s really cold outside.

Case study: Vaillant 10 / 12 kW unit (minimum 5.8 kW output)

Finally, the big units. Let’s look at houses with 12 kW, 11 kW, 10 kW, 9 kW, and 8 kW heat losses.

Watts Per Degree For 12kW, 11kW, 10kW, 9kW and 8kW heat loss (20°C Indoors, -3°C Outdoors)

• 12,000 W ÷ 23 = 522 W/°C, 12kW ÷ 23 = 0.522 kW/°C

• 11,000 W ÷ 23 = 478 W/°C, 11kW ÷ 23 = 0.478 kW/°C

• 10,000 W ÷ 23 = 435 W/°C, 10kW ÷ 23 = 0.435 kW/°C

• 9,000 W ÷ 23 = 391 W/°C, 9kW ÷ 23 = 0.391 kW/°C

• 8,000 W ÷ 23 = 348 W/°C, 8kW ÷ 23 = 0.348 kW/°C

Heat demand table (kW), Target 20C Indoors (excerpt, 1 °C steps)

With a 10 / 12 kW unit that can’t go below 5.8 kW output:

What this shows:

• In a 12 kW house, cycling starts once it’s warmer than ~8 °C outdoors.

• In an 11 kW house, the cycling point is ~7 °C.

• In a 10 kW house, the cycling point is ~6 °C.

• In a 9 kW house,  the cycling point is ~5 °C.

• In an 8 kW house, you’ll wait until ~3 °C before the pump stops cycling.

Just as with the smaller units, you’ll see more cycling unless your house is heat loss is towards the maximum output.

Is Minimum Modulation Important?

Is minimum modulation on a heat pump important?  Yes and No.

Of course, being able to go really low like the new Grant R290 heat pumps mean they will spend a lot more of the winter months in steady state running mode, which is exactly how you want to run a heat pump.

But all is not lost with heat pumps like Vaillant models because of the way they handle cycling.  In the next section we will take about how Vaillant models control cycling via Energy Integral.

As the old Pirelli tyre advert said “Power is Nothing Without Control”.

What is Energy Integral?

Energy Integral is Vaillant’s internal counter that decides when to start and stop heating cycles.

It works like this:

• The controller constantly compares the actual flow temperature (water in your rads/UFH) against the target flow temperature (from the weather curve and Room Temp Mod).

• The bigger the gap, the faster the counter adjusts up or down.

• Because that difference can be either positive or negative, the counter can move in both directions.

Default behaviour:

• Heating cycles start at –60 (this is the default, but it can be adjusted by an installer down to –100).

• Heating cycles stop at 0.

For most homeowners, you don’t need to worry about Energy Integral, it works quietly in the background.

But Energy Integral does a very good job of controlling cycling and so makes the Vaillant Arotherm one of the better heat pumps.

This what I refer to as “system-defined cycling”.  As long as it’s the heat pump that is deciding to cycle, in the case of the Vaillant, via the Energy Integral, that’s normal behaviour.

Note: there are some circumstances in Room Temp Mod: Expanded mode where Energy Integral is ignored and this interferes with normal cycle operation.  So I’d suggest reading all about Room Temp Mod in our dedicated article to get a better idea which mode to choose.

SensoCOMFORT Room Temp Mod: Inactive, Active or Expanded?

Note: On other brands of heat pump you may hear terms like Degree Minutes to describe a similar control strategy to Energy Integral.

Firmware quirks: Vaillant aroTHERM

On firmware version 351.06.07, there was a bug where the Energy Integral effectively double-counted, leading to more frequent cycling than necessary.

This was fixed in version 351.09.01 and later. So if you’ve got a newer install, April 2023 onwards, you shouldn’t be affected.  But just double check your firmware version if you are borderline around that date.

As described above, the Energy Integral counter normally does a good job of controlling cycling.

You can see in this example of my 5kW Vaillant.

It’s around 7-9 °C outside (house heat loss at design ~4 kW) cycles around 4–5 times in 5–7 hours.  Which is exactly what you’d expect.

You can view a direct link to that particular day via my own Open Energy Monitor page.

For a monster deep dive into Energy Integral (explainers, firmware details, how to check your own system etc) see my dedicated article:

Vaillant Arotherm Firmware 351.06.07 Problems (Energy Integral)

How to check your cycles per hour

If you don’t have live monitoring like Open Energy Monitor, you can still get a ballpark view of cycling from the Vaillant VWZ AI controller.

Menu → Installer Level (17) → Test Menu → Statistics

Note down:

• Compressor Starts

• Compressor Hours

These figures are across the lifetime of your system. Ideally you’d take the reading after at least a whole year of heating. If you take it too early (e.g. just after a summer install) the results will look skewed, because pumps naturally cycle more in the shoulder months.

The formula is simple:

Compressor Starts ÷ Compressor Hours = Average cycles per hour

Example (my system):

• Compressor Hours: 4040

• Compressor Starts: 2575

• 2575 ÷ 4040 = 0.64 cycles per hour

That would generally be considered pretty good.

Take these numbers with a pinch of salt:

• If your heat loss is close to your pump’s maximum output, you’ll probably see fewer cycles (in your favour).

• If your heat loss is much lower, you’ll see more cycling — even by design.

Remember: this calculation only makes sense across a whole year. Don’t judge the system just on spring or autumn behaviour.

How often should my heat pump cycle?

There’s no single right answer, but as a rule of thumb (averaged across a whole year):

• Less than 1 cycle per hour → good

• 1–2 cycles per hour → fine

• More than 2 cycles per hour → worth looking into

On mild days you’ll always see more cycling. What matters is how the system behaves in winter.

Why system design matters

Your system design plays a big role in cycle frequency. Emitter sizing, radiator output, flow temperature, and total water volume all affect how smoothly the pump can run.

A well-designed system spreads heat evenly and allows longer run times between cycles. Poor design can cause unnecessary short cycling.

I covered system design in depth and why it is important in Heat Pump Myths Busted: Real Insights from a UK Family Home.

Tips to help reduce unnecessary cycling

• Don’t use TRVs or zoning to shut down rooms — the more water “in play” in the heating system the better. The heat has to go somewhere.

• Add system volume if required (a volumiser can help as can bigger radiators).

• Always use manufacturers controls and enable weather compensation. Avoid using third party controls like Nest and Hive.
• Tweak the weather curve so the heat pump only delivers what the house really needs.  See the Vaillant Weather Curve Guide for more help with that.

If you’re seeing lots of short cycling, the best option is to get a competent heating engineer to take a look.

Effects of short cycling on efficiency

Frequent short cycling can reduce efficiency because every start-up wastes energy and adds wear to the compressor.

Normal mild-weather cycling only has a small effect. The real issue is constant stop-starts in all conditions.

Heat pump sizing and oversizing

Sizing makes a big difference. Oversizing, where the minimum output is very close to, or higher than, your home’s maximum demand means the unit will cycle even on the coldest days.

Get the sizing right and the pump will run steadily for more of the heating season, with fewer cycles and better comfort.

What if I add insulation to my house later?

That’s a good thing. Better insulation lowers your heat loss, which means your heat pump won’t have to work as hard. Less energy in equals lower bills, so you’ll save money.

But there is a flip side: by reducing the heat demand, you also lower the point where your pump runs steadily without cycling. In other words, it will now need to be colder outside before you reach steady-state always-on running.

You can see this effect in the case study tables earlier — the 3 kW house tips into steady running much later (colder) than the 5 kW house. The same thing will happen if you improve your fabric: your cycle point moves further down the temperature scale.

For example:

• A 5 kW house with a 5 kW Vaillant unit (minimum 2.2 kW) hits steady running at about 9 °C outdoors.

• If you improved the house and cut heat loss to 4 kW, the same unit wouldn’t hit steady running until around 7 °C outdoors.

• Drop the loss further to 3 kW and you’d be waiting until about 3 °C outdoors before the cycling stops.

Cycle points with improved insulation (Vaillant 5 kW, min 2.2 kW)

So you’re saving energy overall, but you’ll see a bit more cycling in spring and autumn.

Is cycling a problem?

• System-defined cycling in mild weather = normal.

• Frequent short cycling in all conditions = not normal. That’s when you need to investigate.

The compressors in heat pumps (and air conditioning units) are as tough as old boots.  They are built to withstand so much hammer over a many many years, so I don’t think we need to worry about compressor wear and tear.

But it’s surely common sense (at least to me) that it’s better to not have something mechanical turning on and off all the time?

Another point to remember is that as it gets colder and you get closer to the point where steady state running occurs, the number of cycles should decrease and the length of the cycles should increase.

• Close to Cycle Point (colder outside): Longer cycles
• Further away from the Cycle Point (warmer outside): Shorter cycles

As a quick example from my system (4kW heat loss, 7C cycle point)

• 14C outside, 20 min on (60 min off)
• 13C outside, 35 min on (40 min off)
• 10C outside, 45 min on (40 min off)
• 9C outside: 1hr on (30 min off)
• 8C outside, 1hr 20 min on (30 min off)

Then when my system gets below 7C the cycles start to be “always on”.

Obviously, the cycle point on your system may be different as we’ve covered earlier.

So cycling isn’t bad, it needs to be there, “system-defined cycling” is fine and to be expected.

Conclusion

Cycling in mild weather isn’t a fault — it’s just how heat pumps work when the house only needs a small amount of heat.  Hopefully this homeowner focused guide has helped understand the basics.

Frequent short cycling in colder weather is the red flag. That’s when something may be wrong.  It’s unlikely it will break the heat pump or the compressor, but it points more to a system design issue.

Understanding the link between your home’s heat loss and your unit’s minimum output helps you tell the difference between normal behaviour and a problem worth checking.

Reminder: never use third-party controls like Nest or Hive with a heat pump as they act like big on/off switches, always look to use the manufacturers controls and set them up to use weather compensation.

Ultimately, heat pump cycling isn’t too much of a problem if you’re getting good performance (COP/SCOP) and your bills are low.  It’s only when you are not, that you might want to investigate further.

Big shout out to the guys at Open Energy Monitor for their heat pump monitoring system. This article would never have been possible without their kit.

You can monitor my system live here: emoncms.org/energystatsuk
My system is also part of the growing dataset at heatpumpmonitor.org.

And for more Vaillant Arotherm specific information, check out the Vaillant Arotherm Heat Pump Guide page.

FAQ: Heat pump cycling explained (COP, SCOP, efficiency, design, oversizing)

Why does my heat pump turn on and off so often?

In mild weather, your home loses so little heat that the pump’s minimum output is still too high. It cycles on and off to avoid overheating.

At what outdoor temperature will my heat pump stop cycling?

It depends on your house’s heat loss and your pump’s minimum modulation. For example, with a 5 kW heat loss house and a Vaillant 5 kW unit (minimum 2.2 kW), it will run steadily once it’s colder than about 10 °C outdoors.

Is frequent cycling bad for a heat pump?

Occasional mild-weather cycling is fine. Frequent short cycling in all conditions can reduce efficiency and lifespan.

How many cycles per hour is too many?

As a rough guide (averaged across a whole year):

• < 1 cycle per hour → good

• 1–2 cycles per hour → fine

• 2 cycles per hour → potential concern

Do outside temperature fluctuations affect cycling?

Yes. Rapid swings around mild conditions can trigger more cycling because the house’s heat demand is constantly crossing the pump’s minimum output point. In cold weather the pump usually runs steadily — as long as the system is sized correctly and the demand is above its minimum output.

What’s the difference between design day and mild day operation?

A design day is the coldest outdoor condition your system is sized for (often –3 °C in the UK). On those days the pump runs steadily at or near maximum output. On mild days, the heat demand is far lower, so cycling is more likely.

Is efficiency higher in mild weather?

Yes. COP is often highest in mild conditions because the pump can deliver heat with very little effort. Even with some cycling, overall efficiency can be excellent compared to colder conditions.

What’s the difference between COP and SCOP?

COP (Coefficient of Performance) is a point-in-time snapshot of efficiency. For example, if a heat pump is delivering 2280 W of heat while drawing 455 W of electricity, the COP is 5.0.

SCOP (Seasonal Coefficient of Performance) is the same idea, but measured over a much longer period — usually an entire heating season or a full year. It gives a more realistic view of long-term efficiency.

Do all heat pumps cycle the same way?

No. Every brand publishes a minimum output figure, and they vary a lot. For example, Vaillant’s minimums are around 2–6 kW depending on the model, while the new Grant R290 range can turn down much lower. A unit that can modulate further will cycle less in mild weather.

👉 I’ve written more about this in Top of the SCOPs, which links directly to the official Vaillant spec sheets.

Does system design affect heat pump cycling?

Yes. Things like radiator sizing, emitter output, flow temperature, and water volume all influence how smoothly a heat pump can run. Poor design can cause unnecessary short cycling. See Heat Pump Myths Busted.

What should I do if I think my heat pump is cycling too much?

If it only happens in mild weather, that’s normal. If it’s happening in winter too, speak to a competent heating engineer.

Is minimum modulation on a heat pump important?

Yes and no. A low minimum output allows more steady running and less cycling in mild weather, which is desirable. The new Grant R290 models, for example, can modulate down very low. However, Vaillant’s Energy Integral control does an excellent job of managing cycling, so the way the unit handles modulation is just as important as the raw numbers. As the old Pirelli advert said: “Power is nothing without control.”

What is Energy Integral?

Energy Integral is Vaillant’s internal counter that governs when heating cycles start and stop. It measures the difference between actual flow temperature and the target flow temperature, adjusting a counter accordingly. Heating starts at –60 (default, can be set lower by the installer) and stops at 0. For most homeowners this happens in the background, but it’s key to preventing unnecessary short cycling. This is what we call system-defined cycling.

Does adding insulation affect heat pump cycling?

Yes. Adding insulation reduces your home’s heat loss, which is always a good thing — it means lower bills and better efficiency. But it also lowers your heat demand, so your heat pump may cycle more in spring and autumn. That’s not a fault, just the maths in action: the unit now needs colder outdoor temperatures before it can run steadily without cycling.