Which route fits your situation?
"We're doing a big refurbishment"
Everything is on the table. If your house is semi-detached, terraced or a flat, permitted development allows only one outdoor unit — so the single-unit routes are the shortlist. Detached, or happy to go through planning? The two-unit route joins it.
"We're replacing the boiler with a heat pump — can we add cooling?"
Yes — the easiest win in this guide. As part of the heat pump installation we re-use your existing pipework: UFH cools as it is, and radiators simply swap for fan coils. Full £7,500 grant, least cost and disruption of any route.
"We already have a heat pump — can we add cooling?"
Usually, yes — cooling rides on the system you already own. If the house is heated by underfloor heating, the floor can already add some cooling with no pipe changes at all. In rooms with radiators — or where the UFH sits underneath timber or carpet, floor finishes that don't conduct the cooling well — fan coils replace the radiators, or are added to the room, on the existing pipework. That's option 1 retrofitted; want stronger cooling and room for a second outdoor unit? Add separate AC alongside.
"The boiler's still fine — we just want cooling"
If you'd rather keep the boiler for now, add dedicated air conditioning alongside it — the strongest pure cooling, though without grant support. And when the boiler does eventually go, the AC carries on working alongside its replacement.
The six approaches
The heat pump's pipework diverts at the tank location into the hot water cylinder, or into one shared circuit — underfloor heating, radiators and fan coils together, no buffer needed.
- One outdoor unit covers heating, hot water and gentle whole-house cooling
- Fan coils heat too — they can top up undersized radiators or replace them entirely
They are a godsend — the lounge with two units in has remained comfortable throughout. The best thing is the bedrooms are cooler at night — nothing worse than trying to get to sleep when you can't cool down.
A homeowner who swapped every radiator for fan coils on this circuit, by email during the June heatwaveComfort cooling keeps the pipe water above the room's dew point (around 19 °C), so it removes heat but not moisture. Rooms drop a very comfortable 5–6 °C — but on a genuinely muggy day the air stays humid: a 25 °C room at 65% humidity cooled to 22 °C sits at about 78%. Cooler, yet slightly close.
A household of four also adds 5–7 litres of moisture a day indoors. Homes with MVHR ventilation track the outside air (~5 affected days a year); without it, moisture builds up (~20 days) — and a small dehumidifier (roughly £25–40 a year to run) clears the lot. Every other route in this guide removes moisture as it cools.
The system here is great — unless the dew point is above, I'd say, 15-ish… so it can't really fight those humid hot days.
The homeowner from the case study below, being honest with usCase study — real Econic monitoring data
The hottest day of 2026, in a 300 m² pre-1900 semi
24 June 2026: London hit 35.8 °C. This Econic-installed 12 kW Vaillant Arotherm+ cools the whole house through the underfloor heating alone — one zone, no fan coils.
36 outside, 26 in our hottest room. The feels-like temp is much less though — this inverse radiation, or however you describe it, is great.
The homeowner, messaging us mid-heatwave · 26 June 2026This system publishes its performance openly — view the live data on heatpumpmonitor.org.
One outdoor unit, two circuits: water to the tank and radiator/UFH circuit, refrigerant straight to indoor air-to-air units — capacity time-shared across all three duties.
- Often the best overall technical fit where it's available — for example the Samsung EHS Quint
- BUS certification isn't expected before January 2027 — we'll confirm at quoting stage
The same start as option 1, but a buffer feeds the fan coils with genuinely cold water — the strongest cooling a heat-pump-led system can deliver, dehumidifying as it goes.
- Needs proper condensate management and a buffer on the cooling circuit
- The chilled water is far too cold for underfloor heating or radiators — they would stream with condensation — so the UFH/radiator circuit stays heating-only and all cooling comes through the fan coils
- The fan coils heat as well as cool — they can replace radiators entirely
Case study — real Econic monitoring data
Fan coils pulling a room back on a 34 °C day
23 June 2026, a London townhouse with an Econic-installed 7 kW Vaillant and dedicated fan coils: the room had drifted to 25.6 °C by early afternoon. Chilled water at 8–13 °C pulled it down 4.3 °C in about four hours — still 30+ °C outside — then held it at a steady 21.3 °C through a 27 °C night, draining condensed moisture away as it went.
It's been a revelation… an absolute lifeline this year.
An Econic customer on his living-room fan coil — 21 °C indoors while 35 °C outside — featured in The Guardian, August 2026A split AC unit has refrigerant at around 5 °C inside its coil, so a small box moves a lot of heat. A hydronic fan coil gets 7–12 °C water and delivers roughly half the cooling for the size — Daikin's own catalogue makes the point:
So air conditioning can be reactive — switch it on when you're hot and it drags the room down in minutes — while hydronic fan coils are best run continuously, so heat never builds up in the first place. One sprints, the other keeps a steady pace.
Heating and cooling both come straight from the AC outdoor unit — no radiators or wet pipework anywhere. Hot water is a separate immersion tank (an AC-integrated cylinder exists on some products).
- No gas boiler can remain — the gas system is fully removed, which is also a condition of the expected grant
- Highly disruptive as a retrofit (all wet pipework stripped out); far more straightforward in a new build
Option 1's heat pump for heating and hot water, plus a completely independent AC system for cooling — two outdoor units sharing no pipework.
- The boiler is fully retired, so this route keeps the £7,500 grant — unlike option 6
- A second outdoor unit is only permitted development on a detached house; anything else needs a full planning application
The gas boiler keeps doing heating and hot water; a separate AC system handles cooling.
- Currently the strongest pure cooling of any option here
- Doesn't reduce gas use or carbon footprint, and doesn't qualify for grant funding
Heating peaks on winter evenings, when solar panels produce the least. Cooling is the exact opposite: demand rises with the sun. We ran the simulation engine behind our proposals — 17 years of hourly London weather against a 10-panel (4.6 kWp) array and 60 m² of cooled space — and roughly 70% of a summer's cooling electricity is drawn in hours when the panels are generating. And because we only fit solar together with a battery, the midday peak in the chart below isn't wasted — the surplus is stored and carries the cooling on into the evening.
How each system is run compounds this. Reactive air conditioning is often switched on in the evening — when you get home and the house is at its hottest — precisely when solar generation has faded. Hydronic cooling's slow-and-steady style is the better match: running continuously through the day, it removes heat while the sun is paying for it, so the house never reaches the point of needing a hard evening pull-down.
Side-by-side comparison
| Option | Heat source | Hot water | Cooling style | Radiator support | Grant | Disruption & cost | Permitted development |
|---|---|---|---|---|---|---|---|
| 1. Comfort cooling | Heat pump — whole house heating | Heat pump | Light — mostly via UFH, fan coils supplement | Fan coils heat too — supplement or replace radiators | £7,500 BUS | Lowest of the six (as an add-on to a planned heat pump) | 1 outdoor unit |
| 2. Integrated system | Single unit — heat pump + air-to-air | Heat pump | Integrated, time-shared | Depends on system design | Certification earliest Jan 2027 | Medium; cost depends on grant | 1 outdoor unit |
| 3. Active cooling | Heat pump — whole house heating | Heat pump | Active / deep — dehumidifies too | Fan coils heat too — supplement or replace radiators | £7,500 BUS | High — among the most involved | 1 outdoor unit |
| 4. AC-only system | AC unit — heating & cooling, no radiators | AC system (some products) or a separate tank on immersion | Air-to-air — combined heating + cooling | Not applicable — no radiators in this system | ~£2,500 expected | Highest if retrofit; medium if new build | 1 outdoor unit |
| 5. Heat pump + AC | Heat pump — rads & UFH (boiler retired) | Heat pump | Air-to-air — cooling + top-up heating (separate unit) | AC can top up underheated rooms | £7,500 BUS | Medium–high (two installs); medium–high cost | 2 units — detached only |
| 6. Boiler + AC | Gas boiler — rads & UFH (unchanged) | Boiler | Air-to-air — cooling + top-up heating | AC can top up underheated rooms | None | High (cooling) / none (heating); medium–high cost | 1 outdoor unit |
A couple of terms explained
Dew point
The temperature at which moisture in the air starts to condense. Cooling below it removes humidity but produces condensate to manage; cooling above it is gentler and condensation-free.
Boiler Upgrade Scheme (BUS)
The current £7,500 government grant for heat pump installations — amounts and eligibility can change, and not every product is approved yet (the integrated unit, option 2, is awaiting certification). We confirm eligibility at quoting stage.
Air-to-air grant
A smaller grant (~£2,500) expected for air-to-air systems where the gas heating is fully retired. Not live yet — we'll confirm before quoting.
Permitted development (PD)
Under GPDO Class G, most outdoor units need no planning permission provided they also provide heating (cooling-only units don't qualify), stay under 1.5m³, meet noise limits (≤37dB(A) at a neighbour's window) and aren't mounted on a pitched roof. PD allows one unit on most homes, two on a detached house — beyond that, a full planning application. Confirmed at site assessment.
Every property is different — construction, pipework and room usage all shape what's practical and what it costs. The best next step is a site assessment for a property-specific recommendation.
This brief is general guidance and not a quotation. Grant names, rates and eligibility are subject to change by government; actual eligibility is confirmed at quoting stage following a site assessment. Prepared by Econic Energy.