# Would a heat pump actually save you money? > Enter what your boiler burns today and what you would replace it with. Get the seasonal COP, the new running cost, the CO2 cut and the payback - for a full swap or for a hybrid that keeps the boiler. [View the web page](https://heatuneed.com/heat-pump-savings-calculator/) ## The answer depends on three numbers, not on the brochure A heat pump replaces one bought kWh with roughly a third of a kWh of electricity, but electricity costs two to four times what gas costs. Whether that trade works out depends on the seasonal COP you can actually reach, the price ratio you pay, and how much the installation costs after grants. This tool works all three out from your own figures. ## How the calculation works The tool rebuilds your heat demand, walks a whole year of outdoor temperatures one degree at a time, and prices both systems against the same demand. 1. **Your heat demand** — Fuel bought times its energy content times the seasonal efficiency gives the heat the boiler actually delivered. Hot water is separated out at 750 kWh per person per year; what remains is space heating. Describing the home instead uses floor area times a demand per square metre. 2. **A COP for every outdoor temperature** — Space heating is spread across a normal distribution of outdoor temperature in one-degree bins. In each bin the weather compensation curve sets the flow temperature, and the COP is a fixed fraction of the Carnot limit for that lift. Air source units also carry a defrost penalty between -7 and 5 C, where frost forms fastest. 3. **Splitting the load with the boiler** — A hybrid runs the heat pump at full output down to the changeover temperature and lets the boiler cover the shortfall below it. Because demand below that point is rare, a unit sized for 0 C typically still covers most of the year's heat. 4. **Money, carbon and payback** — Each system is priced on its own fuel and its own standing charge, then compared. Payback is the cost after grants divided by the yearly saving, and the break-even electricity price tells you how much headroom the answer has. ## What the calculator asks for ### Current heat sources and their seasonal efficiency - Condensing gas boiler — Seasonal efficiency 0.85, Natural gas - Older non-condensing gas boiler — Seasonal efficiency 0.68, Natural gas - Condensing oil boiler — Seasonal efficiency 0.85, Heating oil - Older oil boiler — Seasonal efficiency 0.72, Heating oil - LPG / propane boiler — Seasonal efficiency 0.82, LPG / propane - Wood pellet boiler — Seasonal efficiency 0.78, Wood pellets - Electric resistance heating — Seasonal efficiency 1.0, Electricity ### Fuels, default price and CO2 factor - Natural gas — 0.06 USD/kWh, CO2 0.202 kg/kWh, Yearly standing charge 120 USD - Heating oil — 0.11 USD/kWh, CO2 0.267 kg/kWh, Yearly standing charge 0 USD - LPG / propane — 0.13 USD/kWh, CO2 0.234 kg/kWh, Yearly standing charge 60 USD - Wood pellets — 0.075 USD/kWh, CO2 0.04 kg/kWh, Yearly standing charge 0 USD - Electricity — 0.17 USD/kWh, CO2 0.37 kg/kWh, Yearly standing charge 0 USD - Electricity price — 0.17 USD/kWh, CO2 0.37 kg/kWh Currency: USD, EUR. Only the currency the figures are shown in changes. The prices stay exactly as you typed them, so set them to what you actually pay. ### Heat pump types - Air to water (air source) - Ground source (borehole or collector) ### Emitters and design flow temperature The flow temperature sets the seasonal COP. Dropping from 55 to 35 C is worth roughly 40 percent more heat per kWh of electricity, but retrofitting underfloor heating is also the most expensive change on the list. - Existing radiators, 55 C flow - Larger low-temperature radiators, 45 C flow - Oversized low-temperature radiators, 40 C flow - Underfloor heating, 35 C flow ### Climates Each climate is a normal distribution of outdoor temperature (annual mean and spread) plus a design temperature. The calculator walks it in one-degree bins. - Mild - coast or south, frost is rare - Temperate - a few weeks below freezing - Cold - long winters, hard frosts - Severe - mountain or continental ### Insulation levels used when estimating demand - Uninsulated, built before 1975 — 180 kWh/m2 - Partly insulated, 1975 to 2000 — 120 kWh/m2 - Modern build or insulated since 2000 — 80 kWh/m2 - Deep retrofit or low-energy build — 45 kWh/m2 - Passive house standard — 20 kWh/m2 ### Full replacement or hybrid A hybrid keeps the boiler for the coldest hours. The heat pump is then sized for the changeover temperature instead of the design temperature, so it is smaller and cheaper, but the fuel connection and its standing charge stay. - No - heat pump only, boiler removed - Yes - hybrid, boiler covers the cold snaps — Boiler takes over below In hybrid mode the gas or oil connection stays, so its standing charge is still paid. The heat pump is sized for the changeover temperature only, which is why it is much smaller and cheaper than a full replacement. ### Who makes the hot water? - The heat pump - The existing boiler - An electric immersion heater - No hot water ### How the size is worked out Space heating is spread over the temperature bins in proportion to the degree difference, which gives the heat loss coefficient of the house in kW per kelvin. The size shown is that coefficient times the difference between the heating base temperature (15.5 C) and the changeover or design temperature. ## Your result ### How the verdict is decided The verdict follows the simple payback on the cost after grants: under 7 years is a strong case, 7 to 12 years is worth doing, 12 to 20 years is borderline, and beyond that it does not repay on money alone. If the running cost rises instead of falling, the verdict says so directly. - **Strong case - swap it** — The running cost falls far enough that the installation pays for itself well inside the equipment's life. - **Worth doing** — A payback in this range is normal for a heat pump retrofit, and the unit should still be running long after it breaks even. - **Borderline - do it when the boiler dies** — The saving is real but slow to repay. This is the classic case for waiting until the boiler needs replacing anyway, so you only pay the difference. - **Hard to justify on money alone** — At this price ratio and this installed cost the heat pump does not repay itself in a reasonable time. Lowering the flow temperature or finding more grant money is what moves this. - **This would raise your running cost** — With your electricity and fuel prices, at the seasonal COP this design reaches, the heat pump costs more to run than the boiler. It can still be the right call for carbon or for getting off fuel, but not for the bill. Lower the flow temperature, or check whether a cheaper tariff is available. Running cost goes up with these inputs. The CO2 figure is still worth reading: on a low-carbon grid a heat pump can cut emissions sharply even when it does not cut the bill. Estimates from a bin model of your climate, a Carnot-fraction COP that follows the weather-compensated flow temperature, and seasonal efficiencies on the same energy basis as your bill. Real results move with installation quality, control settings, tariff structure and how the house is actually run. ## Common questions ### Will a heat pump work with my existing radiators? Usually yes, but at a flow temperature around 55 C, which costs you roughly a third of the seasonal COP compared with underfloor heating. Radiators sized for a 70 C boiler give out about half their rated output at 55 C, so it works in houses whose radiators were generously sized to begin with. Enlarging the two or three worst rooms is normally far cheaper than replacing the whole system, and the calculator lets you price that as the low-temperature option. ### Is a hybrid better than going all in? It depends on what you are optimising. A hybrid sized for 0 C still covers most of the year's heat, costs much less to install, and needs no radiator work, so its payback is often the shortest of any option here. What it does not do is get rid of the fuel connection, so you keep paying the standing charge and you keep a boiler to service. Switch between the two in the calculator and compare the payback line. ### Why does the calculator say my bills would go up? Because the price ratio beat the COP. If electricity costs four times what gas costs and the seasonal COP is 3, every kWh of heat gets more expensive, not cheaper. The break-even electricity price tile tells you exactly where the crossover sits. Lowering the flow temperature is the lever with the most room in it, and a cheaper off-peak tariff is the other. ### What seasonal COP should I actually expect? Field studies of real installations mostly land between 2.5 and 3.2 for air source on radiators and between 3.4 and 4.2 on underfloor heating, with ground source around a point higher. The figures here sit in that range because the COP is derived from the Carnot limit and then cut to a realistic fraction of it, rather than taken from a datasheet tested at 7 C. ### Should I insulate before I switch? Insulation lowers the demand and lets you run the same radiators at a lower flow temperature, which raises the COP - so it improves both terms at once. It also shrinks the heat pump you need to buy. Run the calculator at your current insulation level and then one level better to see how much of the case it moves. ### What about hot water? Hot water needs around 50 C all year, so its COP is lower than heating and it does not benefit from a low-temperature emitter circuit. The calculator treats it separately and lets you leave it on the boiler in a hybrid, which is often the pragmatic answer for a large family in a cold region. ## Want the room-by-room version of this? Heatuneed scans each room with LiDAR and works out its heat loss through walls, windows, doors and roofs - which is what tells you whether your radiators can run at 45 C. - [Check app availability](https://heatuneed.com/app-store/)