TLDR. If you have a condensing boiler turn down your flow temperatures especially in mild weather, it could save 10-15% on your heating costs.

Longer explanation. Reading energy saving advice from the big energy suppliers is frustrating, it’s all expensive stuff like install more insulation/replace your boiler, or negligible stuff like put a lid on your pans.

You can make a fairly substantial saving with what you likely already have. Most heating engineers install new boilers and rarely mess with flow temperatures because the last thing they want is to come back out to a boiler in warranty with people saying they’re cold. The result is most boilers are up at 70°C+ flow temperature.

Condensing boilers as the name suggests condense the steam produced while burning gas because that steam includes a lot of heat energy. That steam is condensed on a secondary heat exchanger preheating returning heating water before it reaches the primary heat exchanger over the gas burners.
https://diyheatingtips.files.wordpress.com/2012/03/condensing.jpg

The more water you can condense back the more energy recovered. The way to do that it to lower outbound flow temperatures so that the returning water from the radiators is as cool as possible so it can condense as much water as possible.
https://ars.els-cdn.com/content/image/1-s2.0-S019689041730016X-gr2.jpg

Your heating will run for longer, your radiators won’t be as hot, but you’ll actually use less energy because you’re condensing. If the house starts to feel cold turn up the flow temperatures a bit.

This is essentially the operation of a genuinely good smart thermostat wired using OpenTherm. It allows the thermostat to vary your boilers flow temperature based on various factors. So if you have an OpenTherm thermostat it is hopefully already doing this. You can read more about that here.
https://theevohomeshop.co.uk/content/21-What-is-OpenTherm-and-why-use-Honeywell-evohome-with-it
https://support.google.com/googlenest/answer/9259109?hl=en-GB

And if you’re wondering, Hive, owned by British Gas and their extremely popular Thermostat does not support OpenTherm. Because of course it doesn’t. And what actually prompted this post is visiting my parents this weekend, they have an OpenTherm compatible boiler and thermostat with a fixed flow temperature of 75°C because the installer wired it for simple on/off, not OpenTherm.

If you want to know more about all of this I learnt most of it from Heatgeek on Twitter and YouTube. Octopus also have it as their number 1 tip.
https://octopus.energy/blog/winter-workout-gas-saving-tips/

And if anyone actually finds this useful and saves some money and thinks about switching to Octopus here’s my referral.
https://share.octopus.energy/loyal-bell-924

21 comments

  2. The ground floor of my home is open plan (the result of several renovations from previous owners) and there is one small radiator in the living room, and one in the dining. Neither rad is adequate for the job, the heat from the dining mostly goes right up the stairs. This means the downstairs never meets temperature. Put that thermostat to 20 degrees in the winter and it will stay on forever. I’m looking at replacing two downstairs rads with a double and triple model, then considering the half wall and banister on the stairs with a full floor to ceiling wall to keep the heat downstairs, maybe adding an electric heater in the kitchen to work alongside the radiator in the dining area. It’s a ball ache, but I sure as shit ain’t moving house.

  3. It’s a good idea, although I just have different small GPU mining farms in most rooms to heat the house now lol as efficient as electric heaters but generate more money than they cost in electricity

  4. Great advice. Well done for putting this together. Re: flow temperatures, I think most of us can barely perceive the difference between 55C and 80C. Both are too hot to touch!

  5. If you have the means a wood burner is a very good way to heat a house. Fuel is cheap even if you have to buy it yourself and it’s cheaper to install then other fancy ways of heating like heat pumps. A smaller house is going to be easier to heat then a larger house obviously and will save you a good amount of money. Also oil can be more beneficial then gas as you can buy oil when it’s cheap instead of having to get gas as you use it.

    Insulation is really important but so is ventilation, I wouldn’t sacrifice ventilatiuon for increased insulation as it can cause other problems. Still insulation is very important.

  6. What about combi boilers? I’m always running mine at Eco mode as I understand running it lower costs more as it has to work harder to do less.

  7. Don’t most boilers have an ‘Economy’ setting that basically does this automatically if set to?

  8. My local council (Barking and Dagenham) are desperate to give away free solar panels it’s worth checking what you can get from them in terms of energy saving bits.

  9. I have district heating/cooling/hot water in this studio apartment and it costs me £60 a month just for that, along with £60 a month for my electricity… Just a shame there’s nothing I can do about it 😢

  11. I got my plumber to build a split valve system for upstairs and downstairs with 2 Google learning thermostats, within a week it worked out to drop the temp upstairs to fuck all during the day and downstairs over night, it knows when we get up and gets the right areas back up to 19.5° in time. It knows when we go out and drops it then too. So far seen quite a reduction in heating oil usage with them.

  12. Plus it can actually be much more comfortable when the radiators are less hot. When the radiators are too hot it can dry out the air which causes nasal congestion; you can end up feeling miserable because you’re struggling to exchange CO2 without realising why.

    It didn’t get bad enough for me to figure it out until I got long Covid, but since I turned the heating from 80°C to 55°C I’ve felt so much happier and more comfortable, and the radiators still get the flat up to 21°C just fine.

  13. The UK nest thermostat also supports opentherm, mine is wired in this way. Much better control because flow temperature reduced as setpoint get nearer. No overshoot.

  15. This is just the right thread for me because I spent the last few months insulating and maximising thermal efficiency of my house with great success, including converting from oil to gas heating. Here are my tips:

    1. Insulate your attic with at least 270mm of insulation. Very cheap and super effective. Will cost you about £100-150

    2. Check seals of your door and windows. Easy job.

    3. Bleed your radiators, key to bleed it costs around £1

    4. Balance your radiators, again super easy but a bit time consuming. Check YouTube on how to do it. You need a screwdriver/plyers and £16 IR Thermometer.

    5. As OP said, lower your flow temperature. Or get a plumber to rewire your thermostat to use OpenTherm controls if possible. I had it done last week and it’s great. Heating runs for much longer but temperature is more constant and saves gas.

    6. Replace extractor fan outside vents with one of those with a flap to prevent cold air getting back into your house. Will cost you max £10 per vent.

    > negligible stuff like put a lid on your pans.

    That’s actually a very good one if you have crappy ceramic hob like I do. Can’t wait for it to break so that I can justify getting induction hob.

    > And what actually prompted this post is visiting my parents this weekend, they have an OpenTherm compatible boiler and thermostat with a fixed flow temperature of 75°C because the installer wired it for simple on/off, not OpenTherm.

    I had exactly the same situation when I had my conversion done 3 weeks ago. I got the plumber back in to rewire it to OpenTherm controls last week. He didn’t seem to be too sure what it is but I made him do it anyway. I would have done it myself but I didn’t want to void my warranty.

  16. FWIW, Worcester Bosch boilers don’t use Opentherm so if you happen to have one of these (which we do) you’re out of luck – which sucks.

  17. Our boiler guy did this when he installed our combi boiler, he said what’s the point in heating the water to a higher temperature than you’d ever use for showering or doing dishes, only to then mix it with cold water to bring it down to a bearable temperature? Complete waste of energy.

  18. I fitted the Drayton Wiser system to our house, so every room is individually controlled and it’s supposedly already saving us money on the gas heating. Wiser supports opentherm, but our Worcester boiler, fitted just 2 years ago, does not, which is a bit of a bummer. I will drop the flow temp though as it’s set to 70C at the moment and see if that makes any difference.

  19. Surprised you shared tips to get your free entries into the winter workout thing. I just pressed all the buttons regardless.

  20. I think mine’s set at like 63C. It means I can lean back against the radiators and not feel like I’m burning … When it gets properly cold outside that only just manages to keep the house warm so if you don’t have a smart thermostat and have to manage it yourself, you probably need to adjust it in winter.

  21. Nice post. Opentherm is an option on a lot of boilers (as in an add-in circuit board). We have the Honeywell smart system and looked at getting it for our boiler, but it was about £400 and not (officially) available in this country.

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