If you’re wondering how to choose the best speed setting for your central heating pump, then you’re not alone. It’s one of the most frequently asked questions about central heating pumps.
Although it appears confusing at first, finding the most efficient speed setting for your pump is actually quite simple – and this article will show you how to do it.
What does the central heating pump do?
A central heating pump’s job is to pump the hot water that’s just been heated by the boiler around the radiators in your home, and back to the boiler to be re-heated. Read more about Central Heating Pumps.
Why does it have more than one speed?
Many central heating pumps have more than one speed setting, which can be confusing. So why do pumps have multiple speed settings?
Every central heating system is different. Different numbers and sizes of radiators. Different room layouts and lengths of pipe run. Different diameters of pipe and different sizes of boiler. Etc, etc.
All these things affect the amount of friction between the water and the system, which the central heating pump has to overcome.
It stands to reason that there’s no one perfect pump speed that works for every central heating system – therefore it makes sense for manufacturers to allow you to adjust the speed of your central heating pump so it’s just right for your system.
What if the pump is running at the wrong speed?
If the original installer set the pump speed correctly for your system, then it’s best not to change it – but since many installers seem to always set the pump to “medium” and hope for the best, many people are tempted to try to optimise the speed themselves.
If your central heating pump is running too fast, then it’s a waste of electricity, since it uses more electricity to pump the water at high speed rather than a lower speed. So your electricity bill will be higher than it needs to be.
If on the other hand your pump is running too slow your radiators may not heat up properly.
- Wastes Electricity
- Noisier – Pipes as well as pump itself
- Boiler may cycle rapidly if return temperature is too high (water has moved through the radiators too fast, so the water hasn’t had long enough to lose much heat to the radiators)
- Air in system – one radiator often needs bleeding
- Cavitation in pump (wears more rapidly)
- Radiators don’t warm up properly – water is spending too long in the radiators, and cooling down too much
- Boiler overheats and cuts out
- Return temperature too low
How much electricity does a central heating pump use?
As you might expect, this varies with the size and model of central heating pump.
It also varies significantly with the pump speed – which is why, other things being equal, a lower speed setting is desirable.
As an example, this Grundfos pump (pictured) has three settings:
1. 40 Watts
2. 65 Watts
3. 105 Watts
So the highest speed uses more than double the electricity of the slowest speed.
Some modern pumps claim to use far less power, eg the Wilo Stratos PICO Commercial Circulating Pump claim to use as little as 3 Watts, though we recommend treating such claims with caution.
What different types of speed setting are available?
It’s probably still possible to buy a central heating pump that only does one speed, but they’re not very common. Most pumps now on the market offer at least 3 distinct speeds.
Multiple fixed speed
Most central heating pumps, like the Grundfos UPS2 15-50/60 have three individual speed settings to choose from. A few have more, like the Grundfos Alpha 2L 15-60 which offers 7 pre-set speeds.
Some modern pumps like the Wilo Stratos PICO Commercial Circulating Pump offer just a continuously variable speed setting.
Some brands, eg Wilo offer a combination of three fixed speeds or a continuously variable speed.
How to set central heating pump speed
The actual mechanism for setting the speed varies depending on the brand and model of the pump in question. Usually there’s a control fairly clearly visible on the pump.
For example the Grundfos UPS2 15-50/60 has a button and three lights to indicate the selected speed.
Other pumps, like the Flomasta range have a rotary dial on the front of the pump which allows you to select the desired pump speed.
How to find the best speed setting
The simple answer is to use the lowest speed at which the system works properly without any problems – as that will save electricity.
So you can try reducing the speed of your pump from high to medium or medium to low, and check that all the radiators still get hot, and that the boiler doesn’t overheat and cut out. You can always increase the pump speed again if you encounter any problems.
In theory, the most efficient way to set up a central heating system is so that there’s an 11 or 12 degree difference in the temperature of the water leaving the boiler, and the water returning to the boiler from the radiators. It may not always be possible to achieve this in practice.
Changing the speed of your central heating pump will affect the temperature of the returning water.
If you have a condensing boiler, you’ll want to make sure that the return temperature doesn’t exceed 55 degrees C, or the boiler will be unable to work efficiently.
If you can measure the flow and return temperatures accurately with a thermometer, you can try the available pump settings, to see which one gets closest to the ideal efficiency.
Obviously you need to also look out for other undesirable effects, like radiators not warming up properly, or excessive noise, either from the pump, or the pipework, or any funny noises from the boiler.
Also, don’t forget that higher speed settings use more electricity to pump the water, so choose the slowest setting that works well.
Signs that pump speed may need adjusting
If your radiators have been bled, but still aren’t getting hot, it may be a sign that the pump speed is too low. Try increasing it and see if it helps, but watch out for the boiler overheating and cutting out, additional noise, and radiators needing bled more frequently.
If the pump or pipework is noisy, or one or more radiators often need to be bled, it may be a sign that the pump speed is too high. Try reducing it and see if the problems go away – but check that the radiators are heating up properly and the boiler is not cycling too fast.
Central heating pump speed selection may seem confusing at first, but once you understand why different speeds are available – and some of the problems you’re likely to see if the speed is too high or too low – it becomes quite straightforward to try the different speed settings your pump allows, and find the best speed for your system.