Gas Boiler Efficiency

[Munroast]

How accurate are the efficiency ratings given to boilers ? Mine is Mexico Super CF 3/70 and is rated here as 72.7%

Many new condensing boilers are rated considerably higher but how accurate are these figures in reality. If I was to get a modern boiler of 92.7% that would give me a mathematical saving of 21.5% - and if I assumed another 50% increase in gas prices the pay back on a £2,500 boiler would be just over 5 years. This sounds worthwhile but how would it work out in reality? going from 72.7% to 92.7% would I really get a 21½% reduction in gas use...

 

[bernardgreen]

most efficiency values are either calculated using theory or actually measured in a perfect system set up in the test laboratory.

There is one manufacturer who has in the past claimed their boilers were more than 100% efficient.

I wrote this some years ago...

Heating 100 litres of water in one continuous draw will achieve a much higher real efficiency (*) than heating 100 litres of water in 20 draws each of five litres separated by a few minutes. ( between draws the mass of the heat exchanger and the water in it will be cooling down, that heat energy being lost to the atmosphere )

(*) real efficiency is calculated from the amount of gas burnt, the calorific value of the gas, the volume of water heated and the temperature rise of that water.

 

[muggles]

It depends on how well it is installed. A high-efficiency condensing boiler does not condense out of the box, and when not condensing the efficiency is significantly reduced. Gas boilers only start to condense when the water returning to them from the heating system is below 52 degrees, and only fully condense when it is below about 30 degrees. In order to consistently achieve this target, you need a boiler which has a good range of modulation (power outputs), is correctly sized for the property, and has good controls which can vary the flow temperature according to the requirements of the property on any given day. Radiators sized to be able to give the required heat output at these lower flow temperatures will also help. If you want an installer who understands all of this, head to either Heat Geek or The Heating Hub.

 

[DP]

What no one tells you is that a singing dancing condensing boiler has estimated 10 year life not forgetting the cost of parts that Mexico CF does not require (though the mexico has ability to kill you) and Mexico will keep on chugging

A useful point to note- look at the heat input and output for the Mexico
A condensing boiler has near enough the same heat output as the input

If I had a Mexico and it was to keep chugging away, I would install additional controls to keep it in check and also heat the rooms as they are occupied. Did that for a customer with that boiler and control system is Evohome to heat water and radiators

 

[gas112]

condensing boilers are working the vast majority of the time at just over 80% and as DP has said the additional parts cost will also eat into your savings .
Dont believe all the hype about saving 20/30 % of gas bill

 

[Munroast]

What no one tells you is that a singing dancing condensing boiler has estimated 10 year life not forgetting the cost of parts that Mexico CF does not require (though the mexico has ability to kill you) and Mexico will keep on chugging

A useful point to note- look at the heat input and output for the Mexico
A condensing boiler has near enough the same heat output as the input

If I had a Mexico and it was to keep chugging away, I would install additional controls to keep it in check and also heat the rooms as they are occupied. Did that for a customer with that boiler and control system is Evohome to heat water and radiators

My gas fitter (who I have known for years) reckons the mexico will easily last another decade or two, and if it was his he wouldn't change it until forced to, but does say a new condensing one would use a fair bit less gas.

Gas prices are just going to go higher and higher, so I need to look at my options, but even if I got all of the theoretical 21½% saving - its just on the limit of being worthwhile.

 

[kidgreen61]

In my opinion, the consumption figures for new gas boilers are somewhat theoretical.
It's a bit like fuel consumption figures for cars, it's v difficult to achieve the MPG manu's claim.
As said, the Mexico will outlive your grandchildren, and 2 modern replacement boilers, up to you to decide.

 

[cdbe]

Surely the government could do some real world analysis of the actual gas savings? Using data from the utility providers/"smart" meters, not that nonsense from the energy saving trust. With millions having gone from band G to A there should be a noticeable/quantifiable reduction in average consumption per property.

If the new boilers aren't delivering the promised efficiency (and the old ones are - presumably fairly likely if well maintained) then the savings in energy are probably marginal (I think most sensible people recognise that whole life cost financial savings are fairly spurious), especially when you consider the human factors - new efficient boiler, let's throw away the jumper and have the heating on a bit more!

I replaced my G with a B and didn't notice huge reductions in consumption - what there was is probably largely to do with getting rid of the pilot light, crap controls and gravity hot water - countered by the human factors (wife) and lovely gas guzzling showers from the unvented.

It now looks like gas (in the same way as ICE) is now considered a dirty legacy product to be forgotten about and quickly replaced by solar and heat pumps.

 

[vulcancontinental]

Surely the government could do some real world analysis of the actual gas savings? Using data from the utility providers/"smart" meters, not that nonsense from the energy saving trust. With millions having gone from band G to A there should be a noticeable/quantifiable reduction in average consumption per property.

If the new boilers aren't delivering the promised efficiency (and the old ones are - presumably fairly likely if well maintained) then the savings in energy are probably marginal (I think most sensible people recognise that whole life cost financial savings are fairly spurious), especially when you consider the human factors - new efficient boiler, let's throw away the jumper and have the heating on a bit more!

I replaced my G with a B and didn't notice huge reductions in consumption - what there was is probably largely to do with getting rid of the pilot light, crap controls and gravity hot water - countered by the human factors (wife) and lovely gas guzzling showers from the unvented.

It now looks like gas (in the same way as ICE) is now considered a dirty legacy product to be forgotten about and quickly replaced by solar and heat pumps.

There's lots of information out there from government, manufacturers and independent organisations.

Your analogy about savings being lost in increased consumption is my experience. Put a new boiler/controls in and the customers watch the bills first year. They get happy then leave windows open and wear T shirts and shorts in January. People heat to a budget not the kW.

 

[ScottishGasMan]

a boiler claiming 92% eff for example, will generally state the flow/return temperatures that this figure reffers to.

So for condensing boilers, typically their stated at 50 degree flow and 30 degree return, at those temps you will have at least in that ball park off efficiency, vs maybe 80-85% if you stick it straight onto a system set for 75/65.

The issue here is that its down to the system and installation, not the individual boiler.

For example, my system, although not complete yet, is designed to hopefully run about 45 degrees at maximum flow temperature. And it will only do that when the outdoor temperature is minus 5. when its plus 5 outside, it may run about 39 degrees, at 10 it may run about 32 degrees and so on, so its always running as low a temperature as whats needed to replace the heat in the house, as opposed to running 75 all year round and just cutting out when rooms over heat as we traditionally have done.

Those controls are called "Weather Compensating" controls. And will work even on high temp systems. so even if your system is designed to 75 degrees at -5 outside, it wont really condense in the coldest days of winter, but in spring and autum when its on the controls will lower the flow temperature to achieve a higher chance of condensing.

 

[Munroast]

Yes, I totally get that something claiming 92.7% will probably achieve less in real world situations, but would that also no apply to the mexicos claimed 72.7% - hence the condensing boiler will still be 20 points more efficient. ?

I am also getting the impression from some of the answers here is, probably the best savings may be to make my current system perform better - where do I start ?

 

[Johntheo5]

How accurate are the efficiency ratings given to boilers ? Mine is Mexico Super CF 3/70 and is rated here as 72.7%

Many new condensing boilers are rated considerably higher but how accurate are these figures in reality. If I was to get a modern boiler of 92.7% that would give me a mathematical saving of 21.5% - and if I assumed another 50% increase in gas prices the pay back on a £2,500 boiler would be just over 5 years. This sounds worthwhile but how would it work out in reality? going from 72.7% to 92.7% would I really get a 21½% reduction in gas use...

I have never come across any boiler manufacturer that gives the efficiency rating based on the boiler return temperature (or more accurately, the flue gas temperature). A lot of condensing boilers are never in condensing mode and the remainder will very rarely, if ever, be in fully condensing mode. The really effortless big gain in a condensing vs non condensing boiler, more accurate IMO to call them High Efficiency (HE) vs Standard Efficiency (SE), a HE efficiency boiler will have a flue gas temperature as low as 80C even with a return temperature of ~ 60C, a SE boiler like mine (oil fired) will be around 230C no matter how low the return temperature, this gives the HE boiler ~ 7% gain in efficiency, one can then recover some of the latent heat in the flue gases to get still higher efficiencies.

The Mexico's efficiency of 72.7% could be indicative of a flue gas temp of ~ 400C.