Which? Boiler test results

[dawoodseed]

Thought some of you might like to know the best buys in Which? Best combis:

Baxi Duo-tec Potterton Gold Vaillant ecoTEC Vaillant ecoTEC(Larger model)

The Baxi and Vaillant were the only two boilers that met the claim of

91% efficiency.

Bottom of list were: Ideal Isar Worcester Junior (Always thought these boilers had an over- inflated opinion) Potterton performa Forgot the rest, but these boilers did not meet efficiency claim.

No open vent boilers were best buys but the top of list was the Vaillant and at the bottom was the Potterton promax

The next boiler after the 4 best buys was a Boulter Buderus

[Doctor Drivel]

All very interesting. Buderus is good. How about long term reliability? Baxi/Potterton don't look good then. Did they test an Atmos, I doubt it. Buy an Atmos.

[John]

Mmm - were the tests biased towards those that a small child could not poke a finger inside.

[Doctor Drivel]

Every Which report on boilers I have come across have been crap. They are very naive.

[geoff]

In message , [e-mail] writes

I bet the Suprima was up there when it first came out ...

[John Rumm]

What was their selection criteria?

[Lobster]

Why didn't they like Worcesters? Every fitter I come across seems to swear by them (Ok, maybe easy to fit...?)

David

[John Rumm]

Having just had a glance at the non subscription bits, I get the impression that they are not too on the ball anyway:

"Nearly all new boilers are now 'condensing' types. Condensing boilers are far more energy-efficient than traditional boilers, as they cunningly reuse heat that would otherwise be wasted.

Some heat generated by traditional boilers disappears up the flue in the form of hot waste gases. But condensing boilers use some of the heat from these flue gases to heat water, making the boiler far more efficient."

That would be a "woosh" then ;-)

[Andy Hall]

"Statistical analysis suggests that if two boilers have SEDBUK values 3 percentage points apart then there is 95% confidence that the boiler with the higher value is more efficient."

What they don't say is what the confidence level is when the figures are only 1 - 1.5% apart. Also, one wonders what the accuracy of measurement is. Better than 1%? That would be surprising. Which? being able to make the measurements? Even more surprising.

[neverwas]

From the online version they appear to have made no comment on or allowance for (i) the effect of the return temperature on efficiency or (ii) the cost of upgrading radiators in order to make it practicable to reduce the return temperature. Is it naughty of them to show potential savings on heating bills based on 90% efficiency and costs for boilers alone?

For me (with 20+ year old albeit unvented system and separate multipoint water heater) in a small Victorian terrace with suspended floors the need to upgrade radiators in order to get anywhere near 90% has been a big factor in doing nowt. Or (as is increasingly likely) have I misunderstood what wd be involved?

[Andy Hall]

The range of efficiencies of modern condensing boilers is in the range from 90 - 91.5%.

SEDBUK say that where there are two boilers with 3% difference there is a 95% confidence that one will be better than the other. They don't say what the confidence is when the difference is only 0.5% or 1%, but obviously less. can measurements be made consistently to a fraction of a percent? Seems unlikely considering the many factors involved.

Consider also the energy cost saving between 90 and 91% (assuming the figures are reached).

A much more important set of criteria at this point becomes build quality and servicability.

[Andy Hall]

The issue is a comparative one.

Increasing radiator sizes does allow the return temperature to be reduced for a given heat output. However in terms of what is needed and achieved, there are a number of factors:

- Assuming that the radiators were only just adequately sized for 82/70 conventional operation in coldest weather, then the boiler will still run at relatively low temperatures during the 6-8 months of the year when only small amounts of heat are needed - i.e. max operation is only usually for 1-2 months of the year.

- Some or all of the radiators may well be oversized anyway.

I found that I was able to redesign for 70/50 operation by leaving most radiators alone, moving three to new positions and replacing those.

In any case, even if the 90% figures are not met if the existing boiler is one of the older wall mounted natural ventilation types like I had, the starting point was 65%, and there would easily be an improvement in that.

[Lobster]

Well AIUI in science if you're using a statistical test to determine whether A is different from B using whatever measurement or assessment, then if there is less than a 95% (tytpically) chance of the measurements being different then you cannot claim that there is any difference.

If so, then the above would indicate that there is actually no statistically significant difference between the efficiencies of any modern condensing boilers.

Does that sound about right?

David

[Andy Hall]

I think that at least it puts it into the context of being a second order issue.

However, if the manufacturer was claiming 90.3% and the boiler only does 85% it would be a different matter, but I think that that would be surprising.

[Andrew Gabriel]

At University, in the department of Physics and Astronomy, the business of being within 3 standard deviations was often quoted. The Physicists were required 3 SD's in the mantissa, but the Astronomers were allowed their 3 SD's in the exponent ;-)

That's probably right for the basic combustion -- I suspect they all eject the flue gasses at around only 5C hotter than the return water temperature..

However, there's much scope for the control system to optimse the efficiency.

e.g. inside the boiler by accurately setting the flow and return temperatures to their optimim values (low as possible whilst getting exactly the right amount of heat into the house). I don't think many of the condensing boilers (particularly the cheaper ones) even attempt this. I see lots of condensing boilers with the water temperature still set high when it's not necessary -- I suspect few owners actually understand what the controls do in any detail.

Then there are the external control systems -- timeswitches and crude thermostats can be bettered with control systems which implement setbacks, and know about the occupancy of the house and even occupancy of separate zones. Mine is integrated with my burglar alarm (as it also knows things like occupancy of areas of the house), but that kind of system design is still very rare.

[Andy Wade]

That's alright, isn't it? Water vapour is a gas and latent heat is heat.

[Doctor Drivel]

You are better off sealing the loft from the rooms below and have 1 foot of insulation in the loft. That will bring better returns. Also have better controls on the CH system.

[Doctor Drivel]

This is where a weather compensator shines. They mate well with condensing boilers.

Even a BIASI model has an integral weather compensator.

[Ed Sirett]

I would have to say to you that doing nothing because of trying to upgrade to a perfect installation is unwise. There are real and substantial saving to be made with a modern boiler and controls.

Most of the boiler improvement comes not from the condensing itself but form a better heat exchanger in which condensing can and does happen. The condensing is a bonus and increasing the emitters to reduce the return temp is a further bonus.

[neverwas]

Thanks. Understood and fully accepted. (A fuller account would have gone on to say that I would expect to be able to get around 80% which will be worth doing now we do not get the benefit of trogging off to heated offices - but only after we have sorted out the roof, which comes after sorting out the subsidence recently discovered..... )

In passing, I have now also read the printed report in Which? and still think they are naughty not to explain what is required to get to 90% efficiency.