Heat pump readiness
A heat pump is not a boiler that runs on electricity. It operates at lower flow temperatures, which means tighter margins on every radiator in the house. Getting the heat loss right — room by room — is what separates a comfortable heat pump installation from a disappointing one.
This page explains why accurate heat loss estimates matter more for heat pumps than for boilers, and where the common sizing mistakes happen.
Why heat pumps need accurate heat loss estimates
A gas boiler typically delivers water at 70-80°C. At those temperatures, most radiators have plenty of spare capacity. If the heat loss estimate is slightly off, the boiler just runs a bit longer and the room still gets warm.
A heat pump typically delivers water at 35-55°C. At those temperatures, radiator output drops substantially — a radiator rated at 1,200 W at Delta T 50 might only deliver 650 W at Delta T 30 or 400 W at Delta T 20. There is much less margin for error.
If the heat loss estimate is wrong — too low because it missed a poorly insulated wall, or too high because it assumed worst-case construction everywhere — the heat pump will be the wrong size and the emitter check will be unreliable. The consequences are real: cold rooms, wasted money, or both.
Why oversizing and undersizing are both problems
Undersizing is the obvious problem: the heat pump cannot deliver enough heat on cold days. Rooms do not reach their target temperatures. The backup immersion heater or auxiliary electric heating runs constantly, destroying the running cost advantage that justified the heat pump in the first place.
Oversizing is less obvious but still costly. An oversized heat pump costs more to buy and install. It short-cycles — switching on and off frequently because it reaches the target temperature too quickly and cannot modulate down far enough. Short-cycling reduces efficiency, increases compressor wear, and can cause uncomfortable temperature swings.
The goal is to match the heat pump capacity to the actual heat loss as closely as possible. That requires a realistic heat loss estimate — not a worst-case guess and not an optimistic underestimate.
Low flow temperature and emitter sizing
The critical link in any heat pump assessment is between heat loss, Delta T, and whether each existing radiator can still do the job.
At a flow temperature of 45°C with a return of 40°C in a room at 21°C, the Delta T is only about 21.5 K. A radiator's output at Delta T 21.5 is roughly a third of its catalogue output at Delta T 50. That means a radiator that was comfortably oversized for a boiler may be significantly undersized for a heat pump.
Some rooms will be fine — a small, well-insulated bedroom with a large radiator may have enough margin. Others will not — a draughty living room with a modest radiator and a large external wall area may need a bigger emitter, underfloor heating, or a higher flow temperature (at the cost of efficiency).
The only way to find out is to check every room: heat loss on one side, corrected emitter output on the other. This is the emitter comparison that sits at the heart of heat pump assessment.
Why room-by-room matters for heat pumps
A whole-house heat loss figure can hide individual room problems. A house with a total heat loss of 7 kW might have six rooms that are fine and one room that needs a larger radiator. The whole-house total does not reveal this.
With a boiler, a slightly undersized radiator in one room is often masked by the high flow temperature — the radiator still has enough output because the water is very hot. With a heat pump at lower flow temperatures, every room's margin is thinner and the problem room becomes the cold room.
Room-by-room calculation is not optional for heat pump assessment. It is the only way to identify which rooms need attention before the heat pump is installed, rather than after.
Why rules of thumb are risky
Rules of thumb — "100 watts per square metre", "just get a 10 kW heat pump for a 3-bed semi" — ignore the specific building. They ignore insulation levels, window areas, exposure, ventilation rates, room temperatures and emitter sizes. Two houses with the same floor area can have heat losses that differ by a factor of two or more.
For a boiler replacement, a rule of thumb might get you close enough because boilers are cheap relative to heat pumps and the flow temperature gives generous emitter margins. For a heat pump, the stakes are higher: the equipment costs more, efficiency depends on getting the flow temperature right, and every radiator needs to be checked at that flow temperature.
A proper room-by-room heat loss calculation is the minimum responsible starting point. Not because it gives a perfect answer — it does not — but because it gives a structured, reviewable answer based on actual survey data rather than a guess.
How Heatworx helps prepare the survey data
Heatworx produces a room-by-room heat loss estimate based on surveyed or manually entered geometry, editable construction assumptions, and ventilation evidence. For each room, it compares the corrected emitter output at the planned flow temperature against the room's design heat loss.
You can set the flow and return temperatures to match the planned heat pump system and immediately see which rooms have enough emitter capacity and which do not. Change the flow temperature and every room's emitter comparison updates — so you can explore the trade-off between efficiency (lower flow) and emitter adequacy (higher flow).
This gives a heating engineer or installer the survey foundation they need: room-by-room heat loss, emitter status at the planned operating temperatures, and transparent assumptions that can be reviewed and adjusted.
Heatworx does not design the heat pump system itself. It does not select the heat pump, specify the buffer or cylinder, design the pipework, or commission the controls. Those decisions require professional judgement and site-specific knowledge. What Heatworx provides is the structured heat loss and emitter data that those decisions depend on.
Heatworx helps produce and review a structured heat loss estimate. It does not replace professional judgement, manufacturer requirements, regulatory obligations, or a full heating system design where one is required.
Frequently asked questions
Why do heat pumps need a heat loss calculation?
Because heat pumps run at lower flow temperatures, every radiator produces less output than its catalogue rating suggests. You need to know each room's heat loss to check whether the existing emitters can deliver enough heat at the planned flow temperature. Without that check, you are guessing — and the cost of getting it wrong with a heat pump is higher than with a boiler.
What happens if a heat pump is oversized?
An oversized heat pump costs more upfront and tends to short-cycle — repeatedly switching on and off because it reaches target temperature too quickly and cannot modulate low enough. Short-cycling wastes energy, increases wear on the compressor, and can cause uncomfortable temperature swings. It also means the system spends more time at higher flow temperatures than necessary, reducing overall efficiency.
Why does flow temperature matter for heat pumps?
A heat pump's efficiency (COP) improves at lower flow temperatures. But lower flow temperatures also reduce radiator output. The design challenge is finding the lowest flow temperature at which every room's emitters can still deliver enough output to meet the heat loss. Getting this balance right requires accurate, room-by-room data — not a whole-house average and not a rule of thumb.
Can Heatworx design my heat pump system?
No. Heatworx provides room-by-room heat loss estimates and emitter comparisons at your planned flow temperature. This is essential input for heat pump system design, but system design also requires selecting the heat pump unit, specifying the hot water cylinder or buffer, designing pipework, planning controls and commissioning — all of which need professional judgement and are beyond what a survey app can or should do.