Like many people in the UK over the last few years, I’ve become a lot more interested in how much energy my house actually uses. Gas prices have been very volatile due to the conflict in Ukraine, and now with further conflicts in Iran it looks like prices are expected to rise again.
Living in a Victorian home, which we are currently renovating, means I keep an extra watch on the heating bill. And, while our current set up works well, I started wondering whether the way in which the boiler was set up was affecting bills.
The boiler flow temperature is the one boiler setting that a homeowner can easily control, and one that can have an effect on how the boiler operates, and the energy it consumes. Here's how I changed mine, and what happened when I did.
Article continues belowWhat is the flow temperature and why does it matter?
Put simply, the flow temperature is the temperature of the hot water leaving the boiler and circulating around the heat emitters, such as radiators. Traditional thinking is that a higher temperature heats the radiators quickly, and brings rooms up to temperature fast.
This means when your boiler is installed, the plumber often sets the flow temperature high at 70°c, or occasionally even 75-80°c.
But, newer thinking suggests that running your gas boiler at a lower flow temperature can improve efficiency, particularly with modern condensing boilers which are designed to recover extra heat from exhaust gases.
The theory states that boilers actually run more effectively when the water returning to the boiler from the radiators is relatively cool, around 50°c. So, if the flow temperature is very high, the return temperature tends to be high as well, which then means the boiler spends less time operating in its most efficient condensing mode.
How I adjusted my flow temperature
Our heating system consists of a Baxi 24Kw EcoBlue condensing gas boiler, with cast iron column radiators and a Nest smart thermostat. The system works well, but I started wondering if it could operate more efficiently.
I decided to try an experiment to see if the theory about flow temperature worked in a real life situation. I adjusted the boiler settings to reduce the flow temperature to 63°C, and ran the heating system as normal for a full month throughout January 2026, to see if it would make any measurable difference.
The thermostat schedule stayed the same, and we heated the house exactly as we usually did. This meant the heating was on in the morning from 6.00am-9.00am, again at 12.00pm-2.00pm, another burst at 4.00pm-6.00pm and then finally again in the evening from 9.00pm-11.00pm. This maintained a steady temperature of between 19°C-21°C during the day, and around 18°C-19°C overnight.
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What happened when I changed my flow rate
Lowering the flow temperature definitely changed how the system behaved. The radiators no longer blasted out intense heat quickly after the boiler fired up. Instead, they warmed up more gradually, and most importantly, the heating cycles ran for longer, but at a gentler level of output. I could also tell that the boiler was running in condensing mode, mostly because it was a lot quieter, which was an added benefit.
In theory this is exactly what you want. A condensing boiler operating steadily at lower temperatures should be more efficient. But did this translate into actual financial savings in our energy bills?
I tracked gas consumption through the app provided by my energy provider. It certainly looked lower. And there's no doubt that the house was warmer in a more constant way.
I looked back on the app and was able to compare January usage across multiple years. It's a good month for a test because it’s typically the coldest, and heating demand is high. The results were as follows:
- January 2026: 2,246 kWh
- January 2025: 2,412 kWh
- January 2024: 2,303 kWh
At first glance 2026 appears slightly better than 2025, but not dramatically different from 2024.
What do the results mean?
In summary, the change in flow temperature didn’t produce a huge drop in overall gas consumption. The difference between years was only a couple of hundred kilowatt-hours across the entire month.
Initially this seemed disappointing, but we have to factor in weather differences. 2025 wasn't an especially cold winter, so that almost certainly accounts for part of that variation, as even small changes in average outdoor temperature can shift heating demand.
In terms of actual money, the difference across the years I measured was around £40 for the month. This is still a good saving, but as gas prices in early 2026 were lower than in the previous couple of winters, there is a chance the cost comparisons were affected by the change in energy tariff changes, rather than the operation of the boiler. So even if the gas bill looked better, the actual gas usage was similar.
So did lowering the flow temperature help? Possibly a little, but not dramatically in terms of energy usage and costs.
Will I keep my flow temperature lower?
The financial gains however are not the end of the story. What did clearly change was how the heating system behaved. Instead of rapid bursts of heat followed by the boiler shutting down, (short cycling) where the boiler and the thermostat try to chase small 0.5°c increments, the system ran in longer, steadier cycles which is much more efficient. These longer runs are also easier on the boiler and are generally considered good practice, especially for condensing systems.
Another potential advantage is comfort. Lower flow temperatures often produce a more even indoor temperature rather than sharp swings between hot and cool. This is also seen in heat pump and underfloor heating set ups.
As a result, I believe that adjusting the flow temperature is still worth trying, especially because it’s simple and easily reversible. I've left my boiler at 63°c and I'm happy with that.
I had hoped for greater savings, but ultimately became more about fine-tuning an already functioning system, than achieving dramatic reductions in energy consumption.
The system continues to run reliably and comfortably at the lower setting, and the boiler is definitely running more smoothly. This is also evidenced by the pressure, which at 75°c often got up to 2.5 bar plus, while at 63°c it runs at no more than 2.0 bar. This again puts less stress on the system and reduces the risk of boiler problems or extra maintenance.
Overall, I'm pleased with the experiment as the house is consistently warmer, and more comfortable. The bills have reduced – even if only slightly, and the boiler is working more efficiently in its optimal range. For very little effort the rewards are quite impressive and substantial.
For more ways to reduce your energy bills, take a look at these energy saving tips, find out when to turn off your heating in spring, and make sure your draught-proofing measures are all up to scratch.
