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Stelrad Radiator

Heat Emitters

A heat emitter is anything that dissipates the heat generated by the heating appliance, this includes any pipework which inadequately insulated but primarily we are referring to radiators, wet under floor heating or fan coils.

White Panel Radiator

Various types of radiators are available, in widths and heights to suit most applications. The styles availale will differ greatly and assuming the heat output is as required, will make no little impact on the performance of the system.

It should be noted that the quality of radiators will vary depending upon the manufacturer and the warranty periods will vary too. Most manufacturers insist on filling/flushing to BS7593:2006 and water treatment to ensure that excessive corrision can not take place, this should be also be periodically retreated.

The radiator to the left is a Stelrad Compact 600x700 P+ Radiator and it is rated by the manufacturer at 942watts, based upn a 50 deg delta t.

What does 50 Deg Delta t mean?

It represents the difference between the room temperature and the average temperature across the radiator.

75 Deg C (Boiler Flow Temp) - 65 Deg C (Boiler Return Temp) = 70 Deg C (Average)
70 Deg C (Average) - 20 Deg C (Room) = 50 Deg Delta t

Manufacturers generally publish the heat outputs of their radiators at 50, 40 and 30 Deg Delta t, although it is possible to calculate any Stelrad calculator with the information in their radiator book and this can be downloaded free of charge from the Stelrad website.

Radiator Correction Factor

How does this help us with our Condensing Boiler?

The Heat Loss Calulation for a room has resulted in a 842 watt head loss, if you are unsure as to how this is calculated, click on the link below;

Using the manufacturers technical information for 50 deg delta t, we simply find a radiator which dissipates above 842 watts and on this occasion, a 600 x 900 K1 (882 watts) radiator could be selected.

To heat the given room to 20 Deg C at our external design temperature, the flow temperature would require 75 Deg C and the return temperature would be 65 Deg C, based upon 1.26 litres per minute flow rate.

We will now apply an "Oversize Factor" from the chart to the left, based upon a 30 deg delta t and this gives a factor of 0.515. The actual design heat loss of 842 watts should now be divided by the correction factor, 0.515.

The corrected rating of the radiator is now 1634 watts, based upon a 50 deg delta t.

Vertical Designer Radiator

Again, using the manufacturers technical information for 50 deg delta t, we now need to find a radiator which dissipates greater than 1634 watts and a 600 x 1000 K2 (1732 watts) radiator could be selected, assuming it would be physically capable of fitting the space available.

To heat the given room to 20 Deg C at our external design temperature, the flow temperature would require 55 Deg C and the return temperature would be 45 Deg C, based upon 1.26 litres per minute flow rate.

This would enable the Condensing boiler to run over 20 Deg C cooler through the complete heat exchanger, a reduction of over 26% and the room temperature would remain unaltered!

The resulting decrease of the temperature flowing through the boiler will ensure the efficiency stays higher and the running costs are reduced accordingly.

This procedure is completed for each room of the property and we now have a heating system which can run at lower flow and return temperatures, ensuring greater efficiency.

Is there more that can be done?

Yes, the 55 Deg C flow temperature and 45 Deg C return temperature are only required to heat the room to 20 Deg C when it is cold outside, typically an external temperature of -2 Deg C is used in the South East but it is changed as required.

Therefore, when the external temperature increases, the heat loss of the property decreases and eventually, the temperatures are equal. This reduces the required flow and return temperatures to the optimum and this mode of operation is known as;

Please click on the above link to learn more about weather compensation, how it can benefit your installation and how it can be applied to your system, as the theory behind the use is different for Condensing Appliances to Biomass Appliances.

Confused by your P1's, K1's?

Stelrad Radiator Types
Towel Radiator

The towel rail doesn't get "hot"

It will not. By design we have a low temperature heating system and therefore, the towel rail will only be tepid. Due to the very small surface area of the towel rail, it can not be specified to heat adequately at low temperatures and we would therefore recommend an "Electrical Towel Rail", as pictured to the left.

The electrical towel rail can also be used in the summer if required, without turning all of the heating system on too. We do not recommend dual fuel towel rails as they require user intervention and often remain on permanently, using excessive electricity.

Under Floor Heating

Lots of talk about radiators but nothing about Under Floor Heating, why is that we hear you ask?

Simple really, it can not be changed after it has been installed and is therefore outside the scope of this article, although similar gains are to be made by running underflooor heating at a cooler flow temperature.

If you are purchasing a new installation of "Wet" under floor heating, just ask for a system that operates at a lower temperature and the pipework spacing will be moved closer to work better at lower flow temperatures, this concept is well accepted with Heat Pump Systems but the blending valve must be retained.

We would however recommend using Weather Compensation for existing under floor heating and also, further information on under floor heating problems can be found below;

Power Flushing

If work is being completed to upgrade the Heating System, it is good practice and with regard to some manufacturer warranties, essential to complete a power flush.

A seperate section of the website covers power flushing and should answer all of your questions, plus give you all the information you need to make an informed decision, the link is below;

Any system which is assembled primarily with plastic pipework is also susceptable corriosion due to the steel radiators and any lack of inhibitor water treatment, particularly if the system is microbore.

Further calculations

It is commonly misunderstood that it is only the boiler return temperature that must be below 50 Deg C, this is incorrect.

What is important is the COMPLETE heat exchanger temperature, the heating water returning must be as cool as possible but the water flowing out of the boiler must also be as cool as possible. This will need a little maths but it isnt to difficult to understand

Heat Input (w) = 4190 (Specific Heat of Water) x Temperature Difference (Deg K) x Flow Rate (Litres per Second)

In our first example, we shall install a condensing gas appliance in an older property and use the existing heat emitters (radiators). To enable the property to acheive the desired room temperature, the flow temperature leaving the boiler must be set to 75 Deg C and due to the poor design of the heating system, the water is returning to the boiler at 65 Deg C.

If we apply these temperatures from our example, we arrive at this calculation;

Heat Input (w) = 4190 (Specific Heat of Water) x 10 (Deg K) x 0.334 (Litres per Second)

Heat Input (w) = 13994w or 13.99kw

In our second example, we shall install the same condensing gas appliance but this time, we shall install an "Oversized" radiator system to ensure the heating water temperature leaving the boiler is below 50 Deg C and the water returning to the boiler is now 40 Deg C.

If we apply these temperatures from our example, we arrive at this calculation;

Heat Input (w) = 4190 (Specific Heat of Water) x 10 (Deg K) x 0.334 (Litres per Second)

Heat Input (w) = 13994w or 13.99kw

That is not an error, cooler radiators can provide the same quantity of heat to a property but a specific heat loss calculation must be applied to every room or area being heated.

An additional benefit of specifying heat emitters which are capable of operating at lower temperatures, is the ability to operate at lower flow rates as shown in the next example;

In our third example, we shall now install a PWM heating circulation pump, Heat Lab Ltd PWM Controller and Buffer Tank to example 2. We can now program a lower flow rate to the heating circuit.

If we apply these temperatures and reduced flow rate from our example, we arrive at this calculation;

Heat Input (w) = 4190 (Specific Heat of Water) x 15 (Deg K) x 0.222 (Litres per Second)

Heat Input (w) = 13952w or 13.95kw

We now have the benefit of a Condensing Boiler running much more efficiently and a Heating Circulation Pump using less electrical energy to circulate water around our heating circuit/s.

It is clear that there is no definitive return temperature that a Condensing Boiler suddenly starts to condense and becomes more efficient. It is the average temperature across the heat exchanger that matters and the golden rule is, the cooler the better.

Tel. 01206 432 999