Balancing of Central Heating Systems
The job of balancing radiators properly appears to have become a dark and forgotten art, despite the fact it remains the key to an efficient system. Certainly if using condensing boilers, heat pumps, or district heating it is most likely to be one of the main causes of inefficiency if not addressed properly.
This article will cover the best practices for the installation and balancing of radiator systems in modern properties, with the aim of achieving low return temperatures - the key to efficiency.
Best Practice Summary
We would recommend the use of Danfoss RA-DV Radiator Valves.
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The is sometimes a misconception that balancing radiators (i.e setting a lockshield) assists TRVs. That's not the case. It simply provides an upper limit on flow for a given pump setting, balancing one radiator against another. In operation, the TRV has little effect until its orifice is smaller than the lockshield, but for any reasonable pump head (>5 kPa) that's lower than the smallest setting on a TRV head, so the other job of balancing is ensuring the DP across the system allows the selected TRV to perform as intended, using test points and suitable equipment.
The problem with balancing on a lockshield is as soon as either the head changes, or the lockshield is isolated/opened it all goes out of calibration anyhow.
But, with pressure independent TRVs with a presetting, the pump can be whatever it wants to be, and one can isolate/open without loss of settings. Absolutely no need to balance a system, just preset against radiator size. The most important thing is however, the TRV remains in control - it is not operating on tiny movements any more.
Settings on a RA-DV correspond to 700, 875, 1050, 1400, 1750, 2625, 3325, 4375 Watts (30C drop).
That's all you need to know to put radiator balancing to bed once and for all.
Selecting the right radiator is the remaining task. For example a radiator working on 65-30 will output 38% the nominal output.
A 600mm double panel radiator puts out 3.1kW for 1800mm of radiator at 50C difference, or 0.666W/mm for a 65-30C system. So radiators match TRV settings as follows:
| Setting | Power | 600mm Radiator Length |
|---|---|---|
| 1 | 700W | 1050mm |
| 2 | 875W | 1300mm |
| 3 | 1050W | 1580mm |
| 4 | 1400W | 2100mm |
| 5 | 1750W | 2630mm |
| 6 | 2625W | 3940mm |
| 7 | 3325W | 5000mm |
| N | 4375W | 6570mm |
Regarding HIUs, direct or indirect makes no difference really. You still face the same fundamental issue that if the radiators do not regulate flow properly, power output will be limited for a given return temperature limit, and an engineer may be needed to re-balance if the system is to cope with cold weather.
Flow rates in modern properties
When the book was written on balancing systems, heating loads were significantly higher, and the issue of designing for low flow conditions was not as important as it is now. Where before one would be designing at 10kW plus, you now need to ensure systems work well at loads under 2kW.
There have also been changes in pump and valve technology that change the playing field. These low flow rates make balancing a system using traditional radiator valves almost impossible, and it is important to look at the latest technology that is been used to control at these lower flow rates.
By-passes and towel rails
As far as we are concerned, it is a cardinal sin of plumbing to have open or uncontrolled by-passes on heating systems. Towel rails are the most common culprit, not been fitted with a TRV or properly balanced, they just let hot primary flow water into the return pipework, destroying decent return temperatures.
By-passes are often excused on the basis of protecting a pump from a dead heat situation, especially when using zoned control. Modern self regulating pumps can wind themselves down and will not incur damage from short periods against valve closure. Furthermore, they can be protected with an auto-bypass valve that will only open when needed.
Radiator Sizing
http://checalc.com/solved/lmtd.html
kW, Delta T, Flow Rate, Kv and Pressure Loss
These absolutely must be understood by anyone taking any responsibility for the selection of equipment used in heating systems. Almost all heating calculations use these figures.
We start with the power output in kW. Such as a radiator load of 1.5kW
The Delta T refers to the change in temperature, and often referred to as the temperature drop, or rise. Measured in degrees C. For a radiator one would expect to be designing for a drop of at least 20C, an selecting radiators accordingly. It is quite possible in modern properties to have return temperatures from radiators below 35C if done properly, and with a flow of 70C this would mean a 35C drop. This article will not go into the science of radiator selection. For another day.
From the kW an Delta T we can work out the Flow Rate using the following equation:
Flow Rate [litres/second] = kW / (4.2 x Delta_T)
Example: 1kW radiator, 20C drop (65-45)
Flow Rate = 1 / (4.2 x 20) = 0.012 litres/second = 0.71 litres/minute
Kv is a measure of the resistance to flow - how big the holes are in a valve - and allows us to calculate pressure losses (or Differential Pressure).
In our example, with a flow of 0.012 litres/second, you can see that a valve with a Kv of 1 m3/h has a pressure drop of around 25 kPa or 2.5 metres head.
Pump head settings
Modern pumps allow for low and constant head settings that in turn allow the use of larger bore TRVs.
If you think you can leave a pump on full setting against standard TRVs in a modern dwelling, then think again.
A modern pump generates up to 7 metres head.
