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Why is the heating engineer telling me I need to replace all my radiators?

Updated: Jul 16

Radiators of different sizes

You’re in discussion with a green technology company about installing a heat pump, the heat loss calculations have been carried out and now the engineer is saying you need to replace all the radiators with larger units. Worst case, they also tell you that you need to replace the existing piping system for pipes with a larger bore. “Why?” you ask the engineer. “Because the heat pump works at a low temperature”, they reply. This is true, but does it really explain why, so that you can make an informed decision? I’ll try and explain the ‘why’…


Conventional heating systems are designed to run at a boiler output temperature of around 70°C to 80°C and, until recently, heat pumps could only produce an output flow temperature of 50°C to 55°C. Heat transfers from a hot body to a colder body. The rate of transfer is affected by many factors but, for this discussion, the temperature of the water flowing in the radiator and surface area of the radiator is key. The higher the water temperature in the radiator, the greater the flow of heat into a room in a set time. Conversely the lower the water temperature, the lower the flow of heat in a set time. Likewise, the larger the surface area of a radiator, the more heat it can transfer in a set time, and the smaller the surface area, the less it can transfer.


Conventional radiators are designed to give a defined output for a specific flow temperature; the convention is Delta T (∆T). Delta T is the average water temperature flowing through the radiator minus the room temperature. So, if the average water temperature is 70°C and the room temperature is 20°C, the Delta T is 50. The average water temperature is the average of the temperature of the water flowing into the radiator and the water flowing out. If you look at the specification for radiators, you will most likely see output heat values for either 50 or 60 ∆T. When the flow temperature is reduced, the amount of energy the radiator will emit, in a set time, is also reduced. To compensate for this reduction, the radiator output is generally oversized by a correction factor. Here are two examples:

The heat pump is set to give a flow temperature of 55°C and the temperature drop across the radiator is anticipated to be 10°C, so the average radiator temperature is 50°C. The room temperature is 20°C so the Delta T is 30°.  There are lots of correction tables online. The one provided by Radiators Online gives the correction factor as 0.51. This means that a radiator with an output of 2.5kW ∆T 50° would have an output of 2.5 x 0.51 = 1.275kW at a Delta T of 30°. So, to have the same output in a given time would need the output of the radiator to be oversized by 2.5/0.51 = 4.9kW – 1.96 times the original output.
The heat pump is set to give a flow temperature of 45°C and the temperature drop across the radiator is anticipated to be 10°C, so the average radiator temperature is 40°C. The room temperature is 20°C, so the Delta T is 20°. The correction factor this time is 0.3. This means that a radiator with an output of 2.5kW ∆T 50° would have an output of 2.5 x 0.3 = 0.75kW at a Delta T of 20°. So, to have the same output in a given time would need the output of the radiator to be oversized by 2.5/0.3 = 8.3kW – 3.33 times the original output.

It doesn’t necessarily mean you need to add a radiator 1.96 or 3.33 times the size of the original unit, but it could. There are many types of radiator on the market with differing constructions and so differing outputs. Looking at the Stelrad website, they make four versions of the panel radiator: the K1, K2, K3 and P+.

Radiator types

The difference between the K1 and K3 is that the K3 outputs 2.4 times the heat of the K1, so the choice of radiator type may reduce the degree of oversizing required.


Other options could be to use underfloor heating or fan-assisted radiators:

Underfloor heating gives the largest surface area for the radiator, effectively the floor. This enables lower flow temperatures of around 35°C to be used.
Fan-assisted radiators have a more efficient heat exchanger built into the body of the unit with a fan to force the air through the heat exchange. For example, with a flow temperature of 50°C, a Stelrad K3 measuring 600mm by 1200mm has an output of 1,462W, whereas a similar-sized Panasonic Aquarea has a heat output of 3,190W.

The energy a heating pipe can carry depends on the size of the pipe, the temperature of the water and the flow rate of the water. The flow rate of the water has a practical maximum as it starts to become turbulent at a certain point, and the more turbulent it becomes the more energy is required to move it around the system. So, if you drop the temperature of the water, it may not be able to carry enough energy to heat the property.

Additionally, heat pumps for the most part require a significant flow through their heat exchanger to the buffer (or equivalent), so 28mm piping is needed for most systems but possibly larger if the run is long or has restrictions such as multiple bends.


There is a building regs requirement – and rightly so – to limit the flow temperature to 55°C, and new installations should adhere to this. Ideally, the flow temperature of the heating system should be designed to the minimum as practical as this will increase the efficiency of the heat pump and so reduce the running costs. Where the installation is a retrofit, it may not be practical to replace the wet system (or the client may not be able or willing to accept the increased costs). The new R290 heat pumps generally have a maximum stated flow temperature of 70°C to 75°C and, while not ideal, these now offer a viable solution for a retrofit. I will talk about this in future posts and provide an overview of a retrofit into a 20-year-old microbore system and the importance of installing a system with temperature/weather compensation.



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