What Happens During a Heat Pump Cooling Conversion? A Step-by-Step Guide

What Happens During a Heat Pump Cooling Conversion? A Step-by-Step Guide

Most people start with one question: can my heat pump even do cooling? The short version is usually yes.

But "yes, in principle" isn't the same as "here's what happens in your actual house over the next fortnight," and that second answer is the one most people never get. This guide walks through the real sequence, from the first assessment through to sign-off, and points out where these conversions tend to go wrong along the way.

The short version

The straightforward jobs (underfloor downstairs, a cooling-capable unit already fitted, controls-only work) often wrap up inside two days. Once fan coils come into it, you're realistically looking at a week or more spread across several visits.

Step 1: Check suitability properly, not by model number

A lot of heat pumps get described as "reversible." Far fewer of them are actually ready to run cooling in a specific British house.

Three things need to line up here. First, the unit needs a reversing valve and a compressor rated for cooling duty, and this varies within a manufacturer's own range, not just between brands. Second, the condensate needs somewhere to go, either a gravity fall to a gully or a pump and trap if that's not available. Third, and this is the one people underestimate, the property needs enough humidity headroom for the system to run cold without wetting anything.

That third point is where UK conversions really part ways with continental ones, because our summers are humid rather than dry. On a muggy August afternoon at 26°C and 65% relative humidity, the dew point sits around 19°C. Drop below that on any surface and water starts forming on it. On a drier day, say 24°C at 55% RH, you've got until roughly 14°C before the same thing happens. Same house, same equipment, a ten-degree swing in what's actually safe depending on the weather that week, which is why a single fixed flow temperature setting can't be trusted to protect against it on its own.

When this step gets skipped, the problem doesn't show up at commissioning. It shows up in late July, as a damp patch on a ceiling under a bathroom pipe run, or a dark stain creeping across engineered oak flooring where a manifold happens to sit.

Step 2: Assess what your emitters can actually deliver

This is usually the part homeowners least expect to be the hard one, but it often is.

Underfloor heating is the obvious fit for cooling. A large surface running at a gentle temperature, comfortably above dew point, spread across the whole floor. It genuinely works, though the numbers involved are much smaller than most people assume going in. A screed floor that puts out 50 to 70 W/m² when heating will typically only manage 15 to 25 W/m² in cooling mode, because you can't push the floor surface much below 19 to 20°C without it feeling unpleasant to walk on and risking condensation. That's enough to take the edge off a room over the course of an evening. It won't rescue a south-facing loft conversion at four in the afternoon in August.

Radiators are where things get genuinely difficult. Run chilled water through a steel panel radiator and its surface drops below dew point almost straight away, and you get exactly what that sounds like: beading, then dripping, then a stained floor underneath. There's no fault code that flags this, because as far as the system's concerned, it's doing its job. Standard radiators simply aren't a cooling emitter, and no amount of clever controller tuning changes that fact.

Fan coil units are what you reach for when you need real cooling capacity somewhere. They pull chilled water in at around 7 to 12°C, blow room air across the coil, and drain off the condensate that process deliberately creates. One well-placed unit can do more for a hot bedroom than an entire floor's worth of underfloor cooling. They cost more, need a drain, and sit visibly in the room, but they actually deliver.

In practice, most conversions end up as a mix: underfloor cooling downstairs for background comfort, one or two fan coils upstairs where the heat genuinely piles up.

Step 3: Reconfigure the controls

Cooling isn't a switch you flip. It's effectively a second operating mode with its own rules, and the controller has to be told every one of them.

On a typical job, that involves:

• Enabling cooling in the installer menu, which is often hidden behind a service code, and some units need a firmware update before the option even shows up.

• Setting cooling flow temperature limits, roughly 16 to 18°C for underfloor circuits and lower still for fan coils. These sit much closer to room temperature than any heating setpoint you'll have seen, which is what catches people out.

• Wiring and configuring dew point protection, using a humidity and temperature sensor placed in the room, feeding a calculated dew point back to the controller so it can raise the flow temperature or stop cooling before condensation forms. On underfloor systems, this isn't a nice-to-have.

• Handling the changeover between modes, so the system isn't heating at 7am and cooling at 2pm on the same mild shoulder-season day.

• Rescheduling hot water, since a domestic heat pump only has one compressor circuit and can't make 50°C cylinder water and 12°C chilled water at the same time. It has to time-share, running cooling for most of the day and stepping out briefly to reheat the cylinder. Leave the old cylinder schedule untouched and you'll get hot water priority barging into your cooling all afternoon.

The wiring and dew point step is the one that most often gets left on its factory default. When that happens, it's rarely a decision anyone made on purpose. The safety margin that was meant to stop condensation was simply never switched on.

Step 4: Commission it in cooling, not in theory

Whatever was checked during heating commissioning tells you nothing useful about how the system behaves in cooling. It has to actually be run cold and watched doing it.

What should get checked and written down:

• Flow and return temperatures under real load, held long enough to mean something

• That dew point protection genuinely cuts in, ideally by provoking it rather than just assuming it will

• Condensate actually draining away, not pooling somewhere nobody's thought to look

• Floor surface temperatures, where underfloor cooling is involved

• Room temperature across a full day, since the number that matters is the overnight result, not what happens in the first half hour

This is where a design assumption that looked fine on paper either survives contact with the real building or doesn't. Catching a problem here costs an afternoon. Catching it next August costs a floor.

Two things worth knowing before you book anything

Cooling sits outside MCS and outside the grant. MCS certification and the Boiler Upgrade Scheme only cover space heating and hot water. A cooling conversion is separate, self-funded work, and it isn't covered by the same scheme rules, so the quality assurance that applied to your original install doesn't automatically carry over to this. Any electrical work should still go through a qualified electrician, who can tell you whether it counts as notifiable under Part P.

Sort the heating side first. If your system's already underperforming, short cycling, running up higher bills than expected, or struggling to reach temperature, then bolting on a second operating mode only makes diagnosing the real problem harder. Deal with that first through our Fix My Heat Pump service.

Why "your installer says it can do cooling" isn't the end of the story

They're probably right about the hardware. Plenty of reversible units genuinely can do it.

What they're not answering is whether your specific emitters, your pipework, and your particular house can run that safely through a humid English summer. That's a question about your property, not the product, and it's not something you'll find printed on any datasheet.

That's the gap our Heat Pump Cooling Assessment is built to close. We confirm suitability, hand over the wiring diagrams and controller settings your installer will need, and stay reachable by phone or video through the conversion and commissioning itself, not just a one-off opinion at the start.

Common questions

How long does a cooling conversion take?
Controls-only jobs usually take one to two days. Once fan coils, new pipework, or rewiring are involved, expect multiple visits spread across a week or more.

Will it feel like air conditioning?
Underfloor cooling won't, not really. It takes 2 to 4°C off a room over several hours and holds it there, which is genuinely noticeable overnight but subtle during the day. Fan coils get much closer to a proper air conditioning feel.

Does running cooling damage the heat pump?
No, as long as it was designed for reversible operation and commissioned properly. The failures we actually see are condensation and controls problems in the system around the unit, not damage to the unit itself.

Do I need planning permission?
Generally not, if you're not adding any new external equipment. If you are adding visible outdoor hardware, or the property is listed or in a conservation area, it's worth a quick call to your local planning authority first.

Can I add cooling to a split system rather than a monobloc?
Often, yes. Keep in mind that any work touching the refrigerant circuit on a split system needs an F-gas qualified engineer, though enabling cooling on a unit that already supports it usually doesn't involve refrigerant work at all.

Considering cooling? Book a Heat Pump Cooling Assessment and find out exactly what your conversion would involve: what has to change, what it will cost, and what to expect from start to finish.

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