One of the most common concerns we hear about retrofit underfloor heating is heat loss.
If the heating system sits on the floor, particularly above a cold subfloor, it could seem logical to assume that a large amount of the heat will simply disappear downwards instead of warming the room.
But that is not really how heat loss works, and definitely not how Wunda’s Rapid Response system works.
What do we actually mean by heat loss?
When we talk about heating a room, the most important heat loss is the heat leaving the room or property through its walls, windows, floor, roof and ventilation.
The job of any heating system is to replace that heat.
If a room is losing 1,000 watts of heat, then whether we use a radiator or underfloor heating, the heating system needs to deliver enough useful heat to compensate for that loss.
While the heat from the heating system might be experienced differently by the user (radiant vs convection heating), the room’s needs in kWh are the same to maintain the desired temperature.
Heat does not simply get “drawn” into a cold floor
Heat transfer is mainly governed by two things:
- the temperature difference between two materials;
- the thermal resistance between them.
A cold subfloor can therefore create a route for some heat to travel downwards, from it’s surroundings.
But the fact that the subfloor is cold does not mean that much of the heat will travel down rather than into the cold room.
The thermal resistance between the heating pipe and the structure beneath it matters enormously.
This is one of the reasons Rapid Response panels incorporate insulation beneath the heating pipe. The purpose of the panel is not simply to hold the pipe in place. It also provides resistance to downward heat transfer.
Above the pipe, the conductive aluminium surface helps spread heat across the floor, following the path of least resistance.
The result is a very large heat emitter designed to transfer heat into the room above.
A cold room actually provides a strong upward heat path
If the room is cold, the floor surface and the air above it are cold too.
That means there is also a significant temperature difference between the heating system and the room above.
Heat is therefore travelling in all directions relative to the temperature differences and resistance in each path, resulting in the majority of the heat being directed out and up.
As the floor warms, heat begins transferring into the room. That is precisely how underfloor heating works.
Being on the floor does not automatically make a heating system more lossy
A floor can have a cold boundary beneath it, particularly over an unheated void or poorly insulated solid floor.
But orientation (across the floor or on a wall) itself does not make a heat emitter more prone to heat loss.
What matters is the temperature difference and the thermal resistance between the heat source and the colder building fabric around it.
Radiators are not inherently protected from this simply because they are mounted on a wall.
In many homes, a radiator sits against an external wall, which may itself be one of the coldest surfaces in the room. The radiator heats the room, but also some heat transfers towards the wall behind it.
Heat entering the building fabric shouldn’t be thought of as ‘wasted’
There is another important distinction. While heat entering a floor or wall is not in the room where it is intended to be, it is not necessarily wasted or the same as heat leaving the building.
Some heat may enter the surrounding structure and remain within the thermal envelope of the property, later contributing to the overall temperature of the building, depending on the conditions of the other materials.
The more useful question is :
“Can the heating system deliver enough useful heat into the room to meet that room’s heat loss?” –
If the answer is yes, then the system can heat the room.
And not:
“Does any heat travel downwards?”
Because some heat does – just as some heat from a radiator travels towards the wall behind it.
The advantage of using the whole floor as the emitter
A radiator concentrates its heat output into a relatively small surface area. While underfloor heating takes the opposite approach.
By turning a large proportion of the floor into the heating surface, the same room heat requirement can often be delivered using a much lower surface temperature and lower water temperature.
This is particularly useful with modern condensing boilers and heat pumps, which generally operate more efficiently at lower flow temperatures.
Low-temperature radiators can of course achieve the same objective, but they normally do so by becoming larger.
Underfloor heating simply uses the largest surface already available in the room, the floor. The heating method is also different to radiators, which use convection, which circulates heated air initially up to the ceiling and then around the room, whereas UFH systems gently and evenly warm the room throughout.
So is heat loss through the floor something to worry about?
It is something that should be considered when designing any heating system.
The fabric of the property, condition of the floor, what lies beneath it, the insulation available, the floor finish and the required room output all matter.
But it should not be assumed that retrofit underfloor heating is inherently more wasteful than radiators simply because it sits on the floor.
Both systems transfer heat into a room while sitting next to colder building fabric.
The real questions are how well that heat transfer is controlled, how much useful heat reaches the room and what temperature the heating system needs to operate at.
If Rapid Response can deliver the output required to meet the heat loss of the room, then it can heat that room just as effectively as a correctly sized radiator.
The difference is that it does so using a much larger emitter area, typically at a lower water temperature, while providing even heat distribution and leaving the walls free.
The floor is not a problem that underfloor heating has to overcome. It is the reason underfloor heating can work so effectively.