how do we measure and work out the flow rate we need around a system?
This article is supplementary to the ultimate guide to balancing, please also check that out!
Many modern condensing boilers will do this within a certain degree but perhaps not optimally. There is a very practical and simple tool to accurately measure flow rates in heating systems that doesn’t involve installing flow gauges or any unnecessary fittings. Delta T.
Delta T (or DT) simply refers to the temperature difference. If we measure the temperature difference either side of an emitter or heat source, we can work out the flow rate in litres per second through it, providing we know the heat input or output of that component. For the purpose of this article I’m not going to go into the maths, but there will be an article on mass flow rate available soon. Or watch our mass flow rate Youtube Video and article coming soon.
To keep it simple, a faster flow rate will give a smaller DT across your heat source or emitter and a slower flow rate gives a wider DT. If you have the correct speed at your pump you will have the correct DT at the heat source and importantly, vise versa.
Because the flow rate and DT are completely linked in this way we can simply target a DT at our emitters and heat source rather than working out any specific flow rates. Generally speaking with heat pumps we would aim for a DT of 5-7°c and for gas boilers a DT of 20°c, turn up the pump if the DT is too wide and down if too narow.
Many modulating boilers automatically adjust the pump speed to target dT20, these can be quickly checked, and many have settings for refinement. All heat only boilers will of course need the pump to be set manually.
You may have some individual radiators on the system that have too much temperature difference and some with too little, but if your overall flow rate is correct you will have still have the correct DT at the heat source as the return temperatures will equalize when they tee in together. This is why balancing radiators only has a minimal effect on increasing condensing.
Finally then, how do we put this into practice?
- First fire your heat source to its ‘design operating temperature’. *Important, do not put it into maximum power output i.e. chimney sweep mode. The design operating temperature is the temperature the system is designed to run at maximum demand. So the flow temperature required when it’s a worst-case scenario of -2 outside. Most of the time this will be around 70oc, bigger overall radiators will require a lower flow temperature, and smaller radiators a higher flow temperature. Your design operating temperature can also be worked out using method 2 of our range rating method, but a guess is fine if your experienced. You can do this by turning your thermostat up and adjusting your maximum flow temperature. Make sure your rooms don’t get above 20/21°c by opening the windows if and when necessary. This ensures we are balancing for worse case demand.
- Place temperature probes on the flow and return pipes at the heat source.
- Turn on your system and adjust your target flow temperature if necessary. Open some windows if required as you don’t really want the room to exceed your target room temperature of say around 20°c.
- Adjust the pump parameters or settings if needed until you have an approximate DT of 20°c (or the figure calculated below) Increase speed to reduce Delta T, Decrease speed to increase delta T.
Also bare in mind this does not need to be 100% accurate!!. Read more at Do we really need DT20?