My Heat Pump Runs Backwards (And Yours Could Too)

04/08/2026

Rupert Cook is an architect and partner at MCMa. He and his family self-built their EnerPHit home in Winchester, monitored publicly at [Open Energy Monitor - emoncms link]. This is his take on adding the 'cooling chip', in July this year to his ASHP, 11 months after the original installation.

Let me start with a confession that undermines my own headline: the single most effective cooling measure in our house has no moving parts, uses no electricity, and isn't the heat pump at all. It's the external shading. But we'll get to that — because the heat pump trick is the one few people seem to know about, and it deserves to be told.

British summers are changing faster than British houses. The Met Office's latest State of the UK Climate report, published this month, records 2025 as the UK's warmest year in a series stretching back to 1884 — the sixth time the record has been broken this century — and finds that days over 30°C, and nights over 18°C, have more than quadrupled in Greater London compared with the 1961–1990 period [source: Met Office, 14 July 2026]. Our response so far has been panic buying: during June's heatwave, Currys reported fan sales up nearly 3,000% in a single weekend and air conditioning unit sales up 330% [source: Currys/press, July 2026]. Only around 4% of English homes currently have air conditioning, yet the National Housing Federation predicts 90% of UK homes will overheat by 2050 [source: Univ. of Reading / NHF]. Something is going to fill that gap — and the portable units flying off the shelves are the worst of all worlds: 1,000 watts or more of electricity, a hose out the window undoing half the work, and a noise like a taxiing aircraft in your bedroom - that may be an exaggeration, but they are noisy! 

Here's the thing almost nobody tells you: if you have an air source heat pump, you may already own most of an air conditioning system. It's just been told not to.

The machine you already have

A heat pump is a refrigerator turned inside out. Your fridge pumps heat out of a cold box and dumps it into your kitchen; a heat pump pulls heat out of the outside air and pumps it into your home. Same physics, same components, opposite direction.

Which raises the obvious question: can it reverse? For many models, yes — mechanically it always could. The manufacturer simply ships it with cooling disabled.

We never designed our house for cooling. When we selected the heat pump — a Vaillant Arotherm plus, a common unit in UK installations — cooling wasn't part of the brief. But the capability was sitting there anyway. Unlocking it means fitting a small component installers casually call "the cooling chip" (properly, a coding resistor) and switching cooling on in the installer settings. Mine was done on 4 July by Erik Johansen of AE Renewables, a Heat Geek trained installer. The invoice reads: the part cost £56, the labour £100 — £187.20 all in, including VAT. Six days later it was 32.4°C outside. Total cost of turning a heating system into a cooling system: less than a single decent portable air conditioner. For completeness: the heat pump itself was installed with a Boiler Upgrade Scheme grant, as most domestic installations now are. The £187.20 is what the cooling upgrade cost on top of a system that was already there.

The catch — and it matters

The chip is the cheap bit. What decides whether cooling works in your house is what's in your floors and on your walls.

Cooling works by circulating cool water — around 17–18°C — through your heating system. Underfloor heating is ideal: a large, gentle heating surface becomes a large, gentle heat absorber. Fan coil units (radiators with a quiet fan inside) also work, but most likely is that would be a new piece of kit. Ordinary radiators are the difficult case — cold water through a radiator risks condensation: drips, damp, eventually mould. Difficult, but not impossible: Adam Chapman, who founded Heat Geek, has run cooling through a radiator-only system in his own home for several years having written and posted on YouTube publicly about it. The lesson from that experience is that careful dew-point settings and gentle, slow cooling are what make it work. Our system guards against condensation properly: humidity sensors inside and outside the house track the "dew point" — the temperature at which water vapour condenses — continuously, and the water is never allowed closer than 2°C above it.

That safeguard is also why surface area matters so much. With heating, an installer can compensate for a poor design by turning the water temperature up. With cooling there is nowhere to go — the water can't get any colder — so output depends almost entirely on how much cool surface you have. Our ground floor happens to have roughly 520 linear metres of pipe at 100mm centres under 52 square metres of open-plan floor. That density was specified for efficient low-temperature heating; it turned out, unplanned, to be exactly what radiant cooling needs. Good fabric-first design pays twice, even when you didn't know you were paying for the second thing.

Radiant cooling only works directly where the water goes. Downstairs, that's the whole slab. Upstairs, our only wet emitters are three bathroom towel rails, which in cooling mode absorb — being generous — about as much heat as one seated adult gives off. So how do the bedrooms stay habitable? Not through the pipes.

The house as a system

Three things keep the upper floors civilised, and the heat pump is only indirectly one of them.

First, the shading — the heavy lifter. Downstairs, external fabric sails shade the glazing. On the upper floors, the windows carry an external screen of woven bronze micro-louvres — MicroLouvre, made by Smartlouvre just down the road in Havant, from a design first invented in the late 1930s. Each tiny louvre is fixed at 17°, so the high summer sun is blocked completely before it ever reaches the glass, while views, daylight and breeze pass straight through the 80% open weave. Internal blinds, by contrast, are largely theatre: once sunlight is through the glass, most of its heat is already in the room. External shading is unglamorous, needs no electricity, and does more than any machine.

Second, the ventilation system running "the wrong way." Our whole-house ventilation (MVHR) recovers heat in winter: outgoing stale, moist air warms incoming fresh air through a heat exchanger. Most people assume you switch that off in summer. On a 32°C day, we do the opposite — because heat recovery works in both directions. The outgoing air is at 26°C, and the exchanger uses it to pre-cool the incoming 32°C air, which arrives at roughly 27°C instead (a figure inferred from the unit's rated efficiency, not a measured figure). The cool interior the floor slab creates downstairs is, in effect, exported by the ventilation to every room in the house. The exchanger also transfers moisture, drying muggy incoming air (excess moisture condenses and I drained away) — this lowers the indoor dew point, lowers the humidity level, which in turn lets the cooling water run that little bit colder. Each part quietly improves the others.

Third, the reset. Around 9 or 10 in the evening, once the outside temperature drops below the inside, the strategy flips: bypass the exchanger, open the windows, and purge the day's heat with free night air.

Shade blocks the heat coming in, the cooled slab absorbs what gets through, the ventilation spreads the benefit, and the night flushes the remainder. No silver bullet; a sequence; a process. 

And one component that isn't automated at all: us. After back-to-back showers (family of five, plus occasional guests or a lodger) we press the ventilation's boost button; when a room feels muggy we open a window. Expecting an automated system to cover every scenario in a lived-in house is mad.

Humans are remarkably good sensors of humidity, warmth and coolth — we feel these things long before the sensors and the control panel notices — and a house works best when its residents are part of the system, not spectators of it.

What the numbers say

The heat pump deals with what the shading can't stop — and because I monitor the system continuously through Open Energy Monitor, with the whole dataset public [Open Energy Monitor - emoncms link], I can show you the before and after.

Before: 25 June, the fiercest day of the early-summer heatwave, 35.9°C outside at half past three. The heat pump sat idle — no cooling existed yet — and despite the shading and the ventilation doing everything they could, the living space reached 32°C. We lived at 32°C.

After: the cooling chip was fitted on 4 July. On 10 July, with 31.6°C outside, the living space held 26.2°C — with the system drawing 527 watts of electricity, a hairdryer on its lowest setting, while removing 3,167 watts of heat from the house. Six units of cooling for every unit of electricity (see screenshot of Vaillant Control App below).

The caveat: those aren't identical days — 25 June was 3.5°C hotter and deeper into a heatwave. But the shading, the ventilation and the house were the same on both. The only deliberate change was a £56 part. And the difference between a room at 32°C and a room at 26°C is not subtle: at 32ºC you avoid exertion and struggle on, the other you carry on with your life. Radiant cooling is gentle — typically worth a few degrees, not the arctic blast of a hotel air conditioner — but on hot days like these, a few degrees is everything.

The monitoring software adds some accidental comedy: designed on the assumption that heat only flows into a house, it reports the heat output as minus 3,167 watts and reports that my efficiency is negative. 

And unlike winter heating, summer cooling lines up beautifully with the grid's cleanest hours: the sun overheating your house is simultaneously supplying the grid with solar power. Cooling at 500 watts on a bright July afternoon is about as low-guilt as electricity use gets, (similar to a hair dryer on low setting).

Should you do it?
In order:

  1. Shade first. External shading — awnings, sails, shutters, louvred mesh — stops heat before it enters and outperforms anything you switch on. Insulation helps in summer too: it keeps heat out.
  2. Use the ventilation you have. Close windows by mid-morning; open everything once the evening air is cooler than the house. If you have heat-recovery ventilation, remember it recovers "coolth" too.
  3. If you're planning a heat pump anyway, ask about cooling before the design is fixed. Underfloor heating at tight pipe centres, or fan coils, cost little to specify at design stage and a lot to retrofit later.
  4. If you already have a cooling-capable heat pump and suitable emitters (radiators, underfloor pipework), activation is one of the cheapest comfort upgrades available. Use a properly trained installer — the humidity monitoring and dew-point settings are precise and an important part that an amateur can get wrong.
  5. Check your grant position before you spend. I can't tell you whether activating cooling affects a Boiler Upgrade Scheme grant. The rules are clear that cooling-only systems aren't eligible, and that a funded system must meet all the property's space heating and hot water needs — mine does, and the cooling came eleven months after commissioning. But published guidance doesn't address switching cooling on after the event. If your pump came with a grant, ask your MCS installer to confirm the position with Ofgem before spending anything. Don't take my word for it, and don't take an opinion on an online forum.
  6. Planning rules allow it — with local caveats. In England, a domestic air source heat pump is permitted development provided, among other conditions, it is not used solely for cooling — heating-plus-cooling is stated as fine (see source below). But permitted development rights don't universally apply, they are restricted within the curtilage of a listed building, and in conservation areas — which covers a great deal of Winchester — the unit must not be on a wall or roof fronting the highway. Check the Planning Portal, and when in doubt, ask the local planning authority. Permitted development rights for air source heat pumps (ASHPs) in England changed most recently on 29 May 2025 via The Town and Country Planning (General Permitted Development) (England) (Amendment) Order 2025.

 
The houses we live in were designed for a climate that no longer exists. We can meet that with a million humming plastic boxes wedged into windows — or with shade, night air, and a quiet unit already sitting outside thousands of homes, finally allowed to do the one thing it was built for but never asked to do: run backwards. 

Images:
Top: View of St Catherine’s Hill through external shading Smartlouvre MicroLouvre
Below: Vaillant Arotherm plus control APP, screenshot.
Bottom: Vaillant Arotherm plus ASHP Installed