Surely if I reduce the temperature on my combi boiler…

[HairyKitty]

to the slightly more economic slightly lower temperature, it will just take longer for the room to reach the thermostat temperature and therefore the boiler will be running for longer? Resulting in the same or maybe even increased gas use? Anyone know more?

It doesn’t really answer from my point of view, it doesn’t explain how running the system for longer due to cooler rads will cost less. Unless we can explicitly say that despite running for longer, less gas will be used overall?
Perhaps the “save money by reducing the boiler temp” thing means reducing boiler temp only til it doesn’t reduce the temperature of the radiators? I would understand this.

To clarify, yes the explanation does explain how using for the same length of time would be cheaper as it’s running more efficiently. But it doesn’t explain how/whether it is actually cheaper overall since it will have to run for longer iyswim.

You would be reducing the boiler flow rate NOT the thermostat for your heating. Yes it will take longer for the house to heat up to your desired temperature but because the boiler is working more efficiently it will actually use less gas.

Condensing boilers achieve high efficiency by condensing water vapour in the exhaust gases and recovering the latent heat which would otherwise be wasted and this uses less gas. If the flow rate is too high the boiler doesn’t condense and so burns more gas.

Condensing boilers significantly reduce carbon emissions and can save up to 1,200 kg of carbon a year. This is why they were made mandatory in 2005. Unfortunately this benefit is completely negated if it’s not operating as it should be.

I have my combi water set to 55 in the summer
And as it gets colder i put it up to 65/70

Heating is set to 65/70 depending on the coldness outside

I also have a weird boiler that has a back boiler thing and was on constantly unbeknown to me! Hot water instantly, cost me a fortune in gas

You need to keep water at 55oC or risk legionella. Heating should be no less than 50oC or else you'll cause probs inside your system - sludges will accumulate and it'll cost £000's to fix.

Hope this helps

Think so.

I've just lowered mine (about 5 days ago) after reading something about it and have managed to tweak it so I don't need to add cold.

Lets say your bath takes 80l of water and you want it at 40 degrees. If your hot water is set to 70 degrees and the ambient temperature is 20, you will need to mix 32l of 70 degree water with 48l of 20 degree water to get your 80l of 40 degree water.

So you’ve heated 32l (plus a bit extra that cools in the pipes) by 50 degrees…. and if I could remember how to convert that to joules and kilo watts I would, but it’s basically 1600 whotsits (joules?)

If your hot water is set to 60 degrees, then you will need to mix 40l of hot with 40l of cold to get your 80l of water at 40 degrees. So now you’ve heated 40l by 40 degrees which is…. also 1600 whotsits.

Hmmmm…..

But the water left in the pipe after you turn the tap off was also heated, and it will have taken more energy to heat that to 70 than to 60.

Please check my workings, someone, cos O level physics was a long time ago. At the moment the only difference I’m seeing is that bit left in the pipes, and any used by someone who has the egregious habit of rinsing something under a running hot tap.

@BailOutChapsGingersGornSquiffy you seem to know your onions. You keep on talking about flow rate. I thought it was flow temperature you need to lower to enable the boiler to reclaim heat. Or is it just another term for the same thing?

@BailOutChapsGingersGornSquiffy is bang on the money, and yes, it's flow temperature that can be turned down (I don't think you can change the flow rate manually for combi boilers).

This is not the same as turning down the thermostat in the house, as the OP (and others) have spotted it does reduce the temperature of the hot water flowing to your rads, so all else being equal they will remain 'on' for longer to get the same heating effect.

However, because the boiler is more efficient when there is a lower flow temperature it means that for every unit of gas that you burn you will get more heat out of it. If your boiler was installed using the typical default settings and you can turn down the flow temperature then you can probably get between 5-8% more heating output for every unit of gas consumed.

And I'm delighted to find someone else who finds Heatgeek as fascinating as I do, @BailOutChapsGingersGornSquiffy!

There's a really nice play list here of some of their energy saving stuff: worth a browse

I finally get it! Thanks @BailOutChapsGingersGornSquiffy and @gizmo!

@PuzzledObserver thank you for taking the time to work that out.. I'm impressed!

To me it was just a case of getting a bath of water at a certain temperature is going to take a certain amount of energy no matter how you do it.. you've reassured me I may not be completely mad.

When our gas engineer came last week to do the annual service I asked him to change any central settings to the most economical.

If you are due an annual service (which you should get done before winter anyway) get him to do the same and explain to you what he's done.

Lots of confusion here. One thing that crops up on many sites is people believing the longer the boiler runs the more gas it will burn. The best I can offer is my own experience below.
My boiler was serviced in November 21 and the engineer, I’ve known for years, asked if I always run the boiler at this temperature. It was around the 78c mark. He pointed out it would work fine at 68c and was set at that. With the 22 gas price hike I started investigating. The government and experts were saying 60*c and I aimed for. It was working fine in all but the coldest days.
My energy suppliers app keeps my consumption records. The three years before my change, 2019, 20 and 21 my average annual consumption was 14673kWh. The consumption for 2022 was 11941kWh. A huge saving although 22 was a relatively mild winter. 2023 consumption was in line at 12237kWh.
Spurred on by the results over 2023 I changed many of the twenty plus years old radiators for triple panels where they doubles and doubles where they were singles. Many were second hand but less than a year old. I now run my heating at 56c all winter and in spring and autumn around 48c. It works fine and I don’t have great plumes of water vapour exiting the boiler flue other than at start up.
I have made other changes changes including a modern thermostat, and improved the insulation to the rear of my Edwardian terraced house.
Lowering the flow temperature works for me. The boiler runs longer but at a much reduced flame rate. My background is 40 years Marine Engineer operating, amongst other things, large heating and cooling plants.
Before I sign off two names have been mentioned previously. HeatGeek on YouTube and the Heating Hub online. Recommend both but try to forget the idea that running a boiler for longer burns more gas. Set the timer to start the heating 30 minutes earlier.

I wonder if one cancels the other out, ie running the boiler on a lower temperature might mean less gas used, but surely it means more wear and tear and in the long-term more repairs.

I shouldn't think it would lead to more repairs - lower heat will lead to less stress and temperature variation which will
Reduce failures

Calling it the flow rate is very confusing it's the flow rate temperature, ie, the temperature of the water flowing through the radiators, that you may need to lower to optimise the efficiency of the boiler.

Your husband is correct but I’m sure he’s referring to the temperature of the water returning to the boiler. He will know there is a flow temperature, which can be adjusted by the homeowner, and the return temperature that comes out of the radiator after giving up some heat to the room.