New boiler £££ - is it actually worth it?

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We have a 1989 boiler (Glow Worm Space Saver MKII) - gravity fed system.

I am lead to believe this is about 65% efficient.

We used 20,000 kWh of gas last year - which I calculate will cost us around £2,200 this year.

Our usage seems high compared to national averages - we are a family of 4 in a 4 bed detached but I am assuming this is all down to boiler efficiency, or lack of!

If we changed our boiler our energy consumption should be more like 16,00kWh with a new boiler? this is 20% less.

Calculation - 95% efficiency, less 10% lost during installation = 85% efficiency. A 20% improvement)

This would save us 20% only - which is less than £500 a year. 

Are these calculations about right?

Also I understand radiators are more efficient now, but I assume this is simply in terms of time to heat-up. Not energy efficiency to warm the actual house?

There are some other benefits in a new boiler, possible increase in water pressure (lord do we need it for showers!) and some space saving in the loft and airing cupboard, but don't really want to chuck money at something unessecerily.





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  • BUFF
    BUFF Posts: 2,185 Forumite
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    edited 10 October 2022 at 1:27PM
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    If your boiler was 65% efficient when new it's almost certainly less now.
    I think that, if properly set  up & used correctly, you will get 90% efficiency out of a new boiler (maybe a bit less on DHW if you go for a combi). No permanent pilot light.
    I imagine that your controls are fairly dated too? Again, modern controls properly used will save more in energy costs over their life than they cost to buy. With much better control you may also be able/decide to make some behaviour changes.
    I would be surprised if you didn't save 30% in gas consumption & potentially more (I saved ~50%).
    At current gas rates I would expect break even in under 10 years i.e. within boiler's lifetime - quite possibly much less.
    Your current boiler is simpler than a modern boiler, so inherently more reliable but I imagine that parts may be hard to get for it in the event that it does go wrong?

    Modern radiators use less water content so heat up more rapidly & tend to give more output for the same physical size (especially with fins). Given that you will want to run your central heating at lower temps to max efficiency on a condensing boiler this could be important.
    As an idea, in England new installations (yours will be classed as a replacement install) are now required to be designed to run at 55C, your existing was probably designed for 75 or 80C

    As for a combi v a replacement regular tank-fed boiler:
    combi will ideally require replumbing/repiping to your outlets & possibly a larger gas supply pipe so installation costs probably higher. Boiler probably dearer to buy as well.
    Combi should give you better pressure (subject to mains supply) but combis are less efficient for DHW production. You could similarly get higher pressure by going system boiler with an unvented tank (you would be able to get rid of header tank). 
    The next replacement cycle will almost certainly require stored DHW i.e. putting back what was taken out for a combi. How long do you plan to stay in this building?

    If/when you do sell your house a modern, efficient heating system will make it more attractive to buyers.
  • pensionpawn
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    We have a 1989 boiler (Glow Worm Space Saver MKII) - gravity fed system.

    I am lead to believe this is about 65% efficient.

    We used 20,000 kWh of gas last year - which I calculate will cost us around £2,200 this year.

    Our usage seems high compared to national averages - we are a family of 4 in a 4 bed detached but I am assuming this is all down to boiler efficiency, or lack of!

    If we changed our boiler our energy consumption should be more like 16,00kWh with a new boiler? this is 20% less.

    Calculation - 95% efficiency, less 10% lost during installation = 85% efficiency. A 20% improvement)

    This would save us 20% only - which is less than £500 a year. 

    Are these calculations about right?

    Also I understand radiators are more efficient now, but I assume this is simply in terms of time to heat-up. Not energy efficiency to warm the actual house?

    There are some other benefits in a new boiler, possible increase in water pressure (lord do we need it for showers!) and some space saving in the loft and airing cupboard, but don't really want to chuck money at something unessecerily.





    I've just replaced my 1996 boiler which I believe was around 50% efficient. Yours will be less. My new Viessmann bolier is 94% efficient and should almost half my gas usage, which in your case would be 10,000 kWhrs at around 10.5p / kWhrs = £1050 pa which repays in 3.5 years. It's a no brainer.
  • Eldi_Dos
    Eldi_Dos Posts: 1,686 Forumite
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    BUFF said:

    The next replacement cycle will almost certainly require stored DHW i.e. putting back what was taken out for a combi. How long do you plan to stay in this building?


    I wonder if you could explain your thoughts on this.I am in similar position as Op  and would appreciate hearing more about this aspect.
  • [Deleted User]
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    As above.  Cast iron heat exchange boiler heating systems were historically set up and sized for a flow temperature of 82C and a return temperature of 71C. 

    A modern condensing boiler needs a 20C temperature differential. Even, if the radiators are adjusted to provide this flow temperature differential, owners may find that their boiler still isn’t condensing or that their radiators are under sized if the flow temperature is lowered. Despite what many people think, plumes of white vapour being emitted from the boiler flue is not confirmation that the boiler is condensing.

    A boiler will not condense until the boiler flow temperature is below 56C: the lower the flow temperate, the higher the boiler efficiency.



    PS. It is also a very bad idea to add a new boiler with an aluminium heat exchanger to an old set of pipes and radiators without the system being given a very though clean. Boiler warranties do not cover failures due to disturbed system sludge etc.
  • TheBanker
    TheBanker Posts: 1,912 Forumite
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    I had my old boiler replaced with a new combi boiler in 2020 because it kept breaking down and was beyond repair.

    Last winter my gas bill was about about half what it was the year before, without any other major changes to my usage. I think the big saving for me is hot water - I am a single person household so don't use a great deal, but I was heating a whole tank of water every day. Now I just heat enough to meet my immediate needs. 

    My only regret is that I didn't do it sooner. I must have wasted quite a bit of money on gas, and on repairs to the old boiler which only lasted for a few months. 

    My advice would be to get a few quotes - the first quote I got was more than double what I ended up paying.
  • Coffeekup
    Coffeekup Posts: 661 Forumite
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    I replaced my 20+ year old biasi boiler 4 years ago which was around 80% efficient iirc. It got replaced with an ideal mini same size 24kwh 97% efficient.
    After having 7 years of gas usage date on spreadsheets I was keen to see the the difference only to find out a year later it was minimal. 
    Not saying you shouldn't get one, but if your's has a pilot light you will be saving some usage all year round. But the biggest saving to my gas usage over the 12 years of spreadsheets I have was loft insulation... It save me around 30% of my annual usage of gas and that was whilst I had the old biasi boiler.
  • kittennose
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    TheBanker said:
    I had my old boiler replaced with a new combi boiler in 2020 because it kept breaking down and was beyond repair.

    Last winter my gas bill was about about half what it was the year before, without any other major changes to my usage. I think the big saving for me is hot water - I am a single person household so don't use a great deal, but I was heating a whole tank of water every day. Now I just heat enough to meet my immediate needs. 

    My only regret is that I didn't do it sooner. I must have wasted quite a bit of money on gas, and on repairs to the old boiler which only lasted for a few months. 

    My advice would be to get a few quotes - the first quote I got was more than double what I ended up paying.
    Yes we have our water set to 1hour each day but don't always use it all. Have just set to 45 mins and lowered the themp but now we sometimes run out if 2 of us want a bath.


  • kittennose
    kittennose Posts: 142 Forumite
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    edited 25 October 2023 at 9:41PM
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    As above.  Cast iron heat exchange boiler heating systems were historically set up and sized for a flow temperature of 82C and a return temperature of 71C. 

    A modern condensing boiler needs a 20C temperature differential. Even, if the radiators are adjusted to provide this flow temperature differential, owners may find that their boiler still isn’t condensing or that their radiators are under sized if the flow temperature is lowered. Despite what many people think, plumes of white vapour being emitted from the boiler flue is not confirmation that the boiler is condensing.

    A boiler will not condense until the boiler flow temperature is below 56C: the lower the flow temperate, the higher the boiler efficiency.



    PS. It is also a very bad idea to add a new boiler with an aluminium heat exchanger to an old set of pipes and radiators without the system being given a very though clean. Boiler warranties do not cover failures due to disturbed system sludge etc.
    Not sure I fully understand this, but would the advice be to replace all rads if we opted for a new combi? I think if we used BOXT they would include a power flush and all the sludge in the rads would clearly be removed with the rads themselves - that was one of the reason I wanted to update them.

  • brewerdave
    brewerdave Posts: 8,526 Forumite
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    edited 25 October 2023 at 9:41PM
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    As above.  Cast iron heat exchange boiler heating systems were historically set up and sized for a flow temperature of 82C and a return temperature of 71C. 

    A modern condensing boiler needs a 20C temperature differential. Even, if the radiators are adjusted to provide this flow temperature differential, owners may find that their boiler still isn’t condensing or that their radiators are under sized if the flow temperature is lowered. Despite what many people think, plumes of white vapour being emitted from the boiler flue is not confirmation that the boiler is condensing.

    A boiler will not condense until the boiler flow temperature is below 56C: the lower the flow temperate, the higher the boiler efficiency.



    PS. It is also a very bad idea to add a new boiler with an aluminium heat exchanger to an old set of pipes and radiators without the system being given a very though clean. Boiler warranties do not cover failures due to disturbed system sludge etc.
    Not sure I fully understand this, but would the advice be to replace all rads if we opted for a new combi? I think if we used BOXT they would include a power flush and all the sludge in the rads would clearly be removed with the rads themselves - that was one of the reason I wanted to update them.

    The payback will be longer if you replace all rads , TCVs and controls as well as the boiler.
  • [Deleted User]
    [Deleted User] Posts: 0 Newbie
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    edited 10 October 2022 at 3:17PM
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    It is not a question of replacing all the radiators but it is worth checking, for example, that radiators in say your lounge will emit the heat that you want at a lower temperature. In my previous home, I just replaced two radiators with a double panel. The thing that took me the most time was balancing the radiators to ensure that the system heated up as it should. Modern boilers have a lower pump pressure which on some boilers also modulate.

    I have no experience of BOXT. My advice would be to speak to three installers. I looked at Atag; Intergas and Viessmann boilers. I choice Atag but today I would probably go for Viessmann because of the excellent modulation ratio:

    https://www.viessmann.co.uk/en/products/gas/vitodens.html#product-range

    It is also worth bearing in mind that most boilers are massively too big for the property that they are installed in. Eventually, we went for a 18kW boiler for a 2000 build 5 bed property.

    The graph. A condensing boiler uses condensing technology to make better use of the heat it generates. When a fuel such as gas or oil is burned, gasses are released via a flue. In a non-condensing boiler system these gasses would be released into the atmosphere and their heating potential would be lost. 

    However, in a condensing boiler the hot gasses are captured and recycled back into the system through a heat exchanger into whats known as the primary circuit. Depending on the boiler type, the primary circuit then moves the heat to either the heating circuit (radiators) or the domestic hot water.

    For condensing to happen, the temperature of the water returning to the boiler must be <56C. The lower temperature, the greater the efficiency. During this process, litres of water are extracted from the exhaust plume which are then siphoned into a drain. 

    Modulation ratio. All boilers modulate to a degree. In effect this means they become smaller boilers as the heating demand reduces. A boiler that modulates down to 4kW is better than one that can only modulate down to 7kW. The latter will cycle once the heating demand drops below 7kW. Cycling reduces boiler life.

    Finally, for maximum efficiency choose a boiler that has Opentherm control. Suffice to say that it is akin to driving through London with all the traffic lights on green. Opentherm is now mandated in many European countries which is why I chose an Atag boiler for my previous home.


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