There is a complaint heating engineers hear again and again from homeowners in their first winter with a heat pump. The house is at temperature. The thermostat says 21°C. And it still feels cold.
Usually, two separate things are going on — one in the physics, one in the head. Both are worth understanding before you decide the technology has let you down.
The radiator on the wall is barely warm — and that’s normal
A gas boiler typically sends water out at around 70–75°C. A heat pump works most efficiently at 40–50°C, and much of its running cost advantage depends on staying there.
The consequence is that you can put your hand flat on the radiator and feel almost nothing. Skin sits at roughly 33°C, so a radiator running at 42°C reads as mildly warm rather than hot. Nothing is broken. But we have spent our whole lives using radiators as touchable heat sources — draping wet gloves over them, standing with our backs to them after coming in from the rain — and that habit is hard to unlearn.
There is a real perceptual effect underneath the habit, too. Comfort isn’t governed by air temperature alone; it also depends on mean radiant temperature, the average warmth of the surfaces around you. A very hot radiator throws noticeable radiant heat at anyone near it. A lukewarm one doesn’t, so the same 21°C room can genuinely feel less cosy even when the thermometer agrees.
It’s the difference between central heating and an open fire. A fire is a poor way to heat a room and a wonderful way to feel warm — which tells you something about how much of comfort is local, radiant and immediate rather than a number on a wall.
Most people adjust within a season. But adjustment isn’t the whole answer, because there’s often a genuine fault underneath.
The real problem: radiators sized for a boiler
Radiator output is quoted at a standard test condition — usually a mean water temperature 50°C above room temperature. Drop the flow temperature and output falls away sharply, on roughly a power-of-1.3 curve.
In practice:
- At 55°C flow, a radiator gives about 57% of its quoted output
- At 50°C flow, about 46%
- At 45°C flow, about 35%
So a radiator rated at 1,500W on a boiler delivers barely 500W on a heat pump running at 45°C. Fit a heat pump to a house full of radiators sized for a 75°C boiler and you have, in effect, cut the heating system’s capacity by two-thirds.
The house then can’t reach temperature on the coldest days, or only does so by pushing the flow temperature up — which wrecks the efficiency the heat pump was bought for. Many homeowners conclude heat pumps don’t work in British houses. What actually happened is that nobody resized the emitters.
What “oversizing” really means
To hold the same output at 45°C flow you need roughly two and a half to three times the radiator surface you had before. That sounds alarming until you see how it’s achieved — usually not with enormous panels, but with radiators designed to move more heat from less wall.
Two approaches do most of the work.
Aluminium radiators hold much less water than steel and conduct heat quickly, which suits a system running continuously at a low temperature. They also offer high output for their physical size, so you can gain capacity without losing another wall.
Column radiators take the other route: multiple columns front to back multiply the heated surface within the same width, which is why the traditional column shape has become one of the most practical choices for low-temperature systems. Depth is doing the work that height and width would otherwise have to — and in 4 and 6 column radiators in particular, that means high output from a modest footprint.
Either way, the sizing has to be done against the flow temperature the system will actually run at — not the figure on the box. A proper room-by-room heat loss calculation, with the correction factor applied, is the difference between a heat pump that delights and one that disappoints.
Before you blame the technology
If your heat pump feels underwhelming, work through the order: check the flow temperature it’s running at, check whether the radiators were ever resized, and get the heat requirement worked out room by room rather than guessed. Bradleys Radiators supply and fit radiators for low-temperature systems, and there is a BTU calculator on their site to size against your own rooms.
The lukewarm radiator you can rest your hand on isn’t the problem. It’s the point. But only if there’s enough of it.
