Asking what is 55oC flow temperature has become much more common in the last year. Most gas boilers now can benefit from low flow temperatures and the extra efficiency it provides.
Very short summary
Set your boiler thermostat to 60oC and your boiler will be more efficient.
A 60oC flow temperature will (probably) give a return temperature below 55oC. This means the boiler will be in condensing mode most of the time.
Why 55 degrees?
55°C is the dew point temperature of water. Running your boiler below 55°C allows the water in the flue gases to condense inside the boiler.
The “smoke” you see coming out of the boiler flue is a mixture of:
- carbon dioxide
- a small amount of nitrogen oxides
- loads of water vapour.
If you can get the “water vapour” to turn back to “liquid water” inside the boiler less gas is wasted.
When a gas turns back to a liquid it releases “latent” heat. This is the encyclopedia Brittanica explanation of latent heat:
In the past this change of state would have happened out in the atmosphere outside your house. With a condensing boiler and a low flow temperature the transformation happens in your boiler. This means you can keep this heat and put it to use.
If all the water vapour is condensed inside the boiler efficiency is increased by 11%. In the real world it’s very difficult to achieve the full 11% while still heating your property. But you can get close by lowering the return temperature as much as possible..
Do I have a condensing boiler?
It should be easy to find out if your boiler is a condensing model. Follow our easy steps below.
Read your boiler instruction manual
If you have your instruction manual read the first few pages it will tell you in there. Look for the following key words:
- Condensing
- High efficiency
- Greenstar
- HE
Look for a white pipe
Under the boiler there will be a white plastic pipe that is roughly 22mm or 3/4 inch. This is the condensate pipe and should lead either inside or outside to a drain.
Look at the boiler flue
If the flue is plastic it is very likely to be a condensing boiler.
Is there a plume of steam?
When the boiler is running is there a plume coming from the flue. None condensing boilers run so hot there is no visible sign of it running.
How do I get to 55 degrees?
55oC is the temperature you are aiming for on the return pipe to your boiler from your heating system.
There will be two bigger (22mm) pipes (normally) on the the left and right hand sides of your boiler.
When you turn your heating on, hot water is pushed through the flow pipe to your radiators. This water loses its heat in your radiators and comes back in the return pipe.
You want the return pipe to be 55oC or below.
To measure this I recommend getting a pair of clip on pipe thermostats.
This type is from amazon and is good enough for getting below 55 degrees.
Fit one on the flow pipe and one on the return. When the heating is turned on the flow pipe will heat up first. The return will slowly heat up as the water “returns” from the radiators.
Most boilers don’t have a return temperature control. You can normally only change the temperature of the water leaving the boiler in the flow pipe.
A good starting point is to set the boiler thermostat at 60oC. Most heating systems will remove at least 5oC so the return will end up being below 55oC.
The return temperature is determined by:
- the flow temperature set at the boiler.
- how much heat is lost from the water as it travels through your heating system.
Why should I choose 55oC?
- Higher efficiency means lower heating costs.
- More time condensing helps to clean the inside of the boiler. The condensate is mildly acidic and helps to keep the boiler heat exchanger clean.
- Lower temperatures mean less stress on components inside the boiler and heating system.
- Cooler radiators mean there is less overheating before the room stat turns off. This again helps to reduce fuel usage.
- The boiler starts and stops less at lower flow and return temperatures. This reduction in cycling saves gas.
- Using less gas is good for the environment. If every property in used 11% less gas there would be a massive reduction in CO2.
Why should I not choose 55oC?
Most heating systems would benefit with having a 55oC return temperature. However there are circumstances where 55oC return isn’t the best idea.
Undersized radiators / pipework
Reducing the temperature that the heating system works at reduces the amount of heat the radiators give off.
The radiators are so small in some heating systems that a 55oC return temp won’t heat the house.
From experience I would say this is for a minority of systems mostly on the coldest days of the year.
Slower heating times
As the radaitors are slightly less hot the time to heat a property may be longer.
Hot water tank supplied from same boiler
A system or heat only boiler is used to heat up both the heating system and the hot water tank.
The flow temperature of the boiler is the maximum temperature the hot water tank will achieve. Setting a low flow temperature like 55oC means the water in the hot water tank can’t go above that temperature.
The problems this can cause are:
- Less hot water available to use. Having very hot water that can be cooled with cold gives a bigger volume of usable hot water.
- Lower temperature hot water. Not hot enough for baths or washing dishes.
- Lower temperatures don’t kill bacteria as fast. Possible problems with legionnaires disease.
- Hot water thermostat never be satisfied. This may cause heating problems. On hot water priority systems the heating can’t come on until the tank is fully hot.
Summary
Almost every property could benefit from reducing their return temperature to below 55oC.
The fuel usage would be reduced and most customers don’t even know the difference.
If you found this confusing:
If you found this confusing. Please let us know in the comments section so we can clear things up for you. We love feedback and we are trying to make the best possible website. If you don’t understand then others don’t as well.
If you found this useful:
If you found this useful maybe check some of our other pages.
- My radiators are too hot
- Vaillant F28 fault
- Common causes of a Vaillant F75 fault
- Boiler pressure dropping
- E160 Fault on Baxi or Potterton Boiler
- Expansion vessel problems
- F1 low pressure fault on an Ideal Logic boiler
- Filling loop left open
- Do I need a powerflush?
- E50 fault
- Hello deary