efficiency of gas boilers

[brackwell]

I recently came across this report from Energy saving Trust  http://www.energysavingtrust.org.uk/Publications2/Energy-efficiency/In-situ-monitoring-of-efficiencies-of-condensing-boilers-and-use-of-secondary-heating-trial-final-report  which tested 40+ boilers in real use.

It is a very informative read and shows the lie of "the 90% + efficient boiler"

As we insulate more and use less and less gas for CH the impact of DHW increases and of course this is mimicked by summer use when the efficiency drops dramatically.

My conclusion is that heating DHW with a inline electric heater has a lot of merit in the non CH phase

Ken

[odbob]

wholeheartedly agree, I have a tiny storage type electric heater though, rather than instant, more than adequate for summer use and definately less cost

[KLD]

Ken,

Good find and thanks for posting the link. It makes indeed an interesting read. How significant the findings are relative to the entire housing stock in the UK is not quite so clear, since the sample size was rather small.

The gas boilers monitored in this study were all fairly new, and all of the condensing type. Sadly, no attempt was made to investigate at what percentage of time the boilers were operating in condensing mode? Anyway, the mean efficiency was indeed lower than the laboratory results, but is this surprising? And the shortfall was just 5 percentage points.

With respect to energy requirements for space heating versus DHW, most houses in the study burnt between 10000 and 20000 kWh of gas per year, with only about 1/10 of that amount going into DHW. A "regular" boiler in DHW only mode still operates at around 80% efficiency (Figure 22). Further standing losses form the cylinder can be minimised (for instance by only heating the cylinder when DHW is needed).

Let's have a quick look at the carbon emission factors: on page 24 they use CEF = 0.192 and 0.568 kg CO2 per kWh gas or electricity. Your advocated method of using an electric inline heater, only becomes "better" when the gas boiler method drops below 34% system efficiency.

Klaus

[Philip R]

A good way of heating water on demand is with a multipoint gas water heater. Very efficient and cheap to run. However, somewhat bulky and no good for central heating!

Electric in line water heaters, very compact, are grossly innefficient in the use of infrastructure (cabling and distribution, requiring a large incoming cable/ cutout to feed them. Also a peak capacity margin is required to cater for these peaky loads. These tend to use lower merit order fossil fuelled plant as they are used when the system is at high load, not off peak.)( Personnaly I regard these devices as the work of the devil.)

The lie of 90%+ efficient boilers, as Ken puts it is partly due to retro fitting these efficient devices into old fashioned, badly specified inadequately sized systems with poor heat exchange performance. Take the standard Hot water cylinder and also some of the solar cylinders sold with a standard 0.44m² primary heat exchanger. They meet requirements of L1A, but needs to be bigger, to reduce boiler short cycling and maintain boiler in condensing mode for the bulk of the cylinder warm up phase. These require high performance exchangers like the supercyl ( multiple microbore type tube nests)
The same can be said of undersized radiators and high primary circulation temperatures.

I do not believe the report drilled down to the underlying causes.

PhilipR

[odbob]

A good way of heating water on demand is with a multipoint gas water heater. Very efficient and cheap to run. However, somewhat bulky and no good for central heating!

Electric in line water heaters, very compact, are grossly innefficient in the use of infrastructure (cabling and distribution, requiring a large incoming cable/ cutout to feed them. Also a peak capacity margin is required to cater for these peaky loads. These tend to use lower merit order fossil fuelled plant as they are used when the system is at high load, not off peak.)( Personnaly I regard these devices as the work of the devil.)

The lie of 90%+ efficient boilers, as Ken puts it is partly due to retro fitting these efficient devices into old fashioned, badly specified inadequately sized systems with poor heat exchange performance. Take the standard Hot water cylinder and also some of the solar cylinders sold with a standard 0.44m² primary heat exchanger. They meet requirements of L1A, but needs to be bigger, to reduce boiler short cycling and maintain boiler in condensing mode for the bulk of the cylinder warm up phase. These require high performance exchangers like the supercyl ( multiple microbore type tube nests)
The same can be said of undersized radiators and high primary circulation temperatures.

I do not believe the report drilled down to the underlying causes.

PhilipR

A good way of providing the right equipment for heating and hot water is to fully understand the wishes of the occupier, in my case small electric storage heater is more than adequate for summer use, whilst in the winter we obtain our hot water from the waste gasses of an 85% efficient standard gas boiler. This little lot though, could be horrendously inadequate for another occupier

[brackwell]

Klaus,
Hello again.

I am not taking any sides as i believe there are many different usage patterns and there is not one correct answer and indeed we are only considering summer DHW

However the average efficiency (gas in v hot water output) of the 13 regular condensing boilers was 26.8% ( worst 11%, best 49%) and the 30 combis  48.8% (worst 28%, best 72% ) , but very dependant on the amount drawn off on each occasion.

I personally do not think that these efficiencies  are acceptable but am struggling with the answer.    I favour a regular boiler and have witnessed big benefits from reducing the tank storage temp to low 40sC but i doubt many people are doing that.

Philip
You touch on another point that bugs me and that is the coil in my tank is not really transferring enough heat -in my case it may be a bit furred up but are new ones really up to the task either?  I note that s.s mains pressure tanks are fitted with a s.s coil. The thermal conductivity of s.s is a fraction of copper.  Is there a standard defining heat exchange rates in tanks? I agree that you need zillions of micro bore tubes.

The big lessons to be learnt (perhaps not for the learned people here) is dont wash hands in hot water and with a standard boiler try to match hot water heated to hot water required.

OR use a inline electric heater that heats precisely the amount of water required to just the right temp.

Ken

[KLD]

Ken,

Could you help me along, I struggle to find the system efficiency figures you quote. Since in this EST report there are only 10 regular boilers (at least only 10 with a complete 12 month data track), you might refer to a different report?
Ta
Klaus

[Philip R]

Point taken about the right hot water heating methodology, one solution, does not fit all.

With respect to standards of heat exchange, depends on the differential temperature profile within a stratefied cylinder. I think we could go off into the realms of some differential calculus, my mathematics skills have long since degraded so I will not dwell.

Suffice to say, that the bigger the heat exchange area, the better heat transfer should occur. Hence the Supercyl type tube nest. ( In a very small cylinder.) Saying that, maybe larger surface area is not everything, a small surface area, combined with a tiny heat exchanger volume ( i.e. small cross section area pipes, lots off, so low pressure differential and adequate residence time ensure good heat transfer.

A few design lessons here from other types of heat exchangers, i.e. condensing boilers, car radiators etc. The reason not used is due to hardness scale, renderring these ineffective very quickly.

With respect to copper v stainless steel. Basically of little consequence as wall thickness very small. Many Unvented st/ st cylinders have very large primary heat exchange area, more than typical copper cylinder.

PhilipR

[martin W]

going to have a read in a minute of the linked report....

Last year I posed a question on real efficencies of gas boiler. My halstead boiler (officeially 84% efficent) worked out at around 44% efficent when heating a known quantity of water (172 litres) up by 10-20 degrees C.

general conclusion was that yup the boiler really was onkly about 50% efficent when the outside air temp was -10°C.

[desperate]

Hallo MartinW,

Surely then you are looking at the efficiency of the whole system , not just the boiler, you need to take into account all the various losses to calculate the boiler efficiency. Similarly with an in line electric heater, most contain a couple of litres of water, so if you only require a couple of litres of hot probably half the water you heat will be wasted. All systems will be very wastefull when just a small amount is used, combis probably waste a bit more than than a small electric heater in that situation, but it is not correct to assume the efficiency is that low in all running situations.

Desp