Heat only boiler

Cardew

HappyMJ wrote: »

The average gas user now has an efficient boiler....it was only 5 years ago when the average gas usage in the UK was 20,500kWh per year. It's not that the weather has got warmer it's the average household is replacing old boilers with new boilers and improving insulation measures. Someone with an old boiler might be using 20,500kWh a year and could reduce that to 16,500kWh a year by changing the very old boiler and improving insulation.

I take your point, but it shows how useless it is to quote average figures without giving detail of the calculation.

Even your example of a 25% saving from 20,500kWh to 16,500kWh with a new boiler means a saving of 4,000kWh which represents approx £160 not the average figure of £300 that the EST state.

I tend to think that the Royal Society of Chartered Surveyors figure of £95 is closer to the mark.

Ben84

Cardew wrote: »

Any idea what the 37.7% figure represents?

I think it's the efficiency of the hot water supply and it takes in to account losses from the hot water cylinder as it's comparing against on demand heaters. Hence the low figure. I expect a fair bit of guess work goes in to this and hot water cylinders vary a lot, from bare copper to well insulated with foam, and how people use their hot water varies a lot too.

Anyway, heating a hot water cylinder is never going to be that efficient. Trying to accumulate lots of heat in a small volume of water will result in pretty hot water returning to the boiler as the cylinder heats up, which will increasingly reduce the efficiency of heat transfer. I checked a few modern system boilers on the site and even they have low figures in the 50% range. So, given the type of installation, your old boiler isn't performing that badly. It does however raise the question, would you be better off using the electric immersion heaters if you have them? They won't suffer any reduction in efficiency from trying to accumulate heat in a small volume of water.

Yolina

C_Mababejive wrote: »

I too have my doubts about the claimed 25% saving. As others have said,you cannot conveniently also discount such things as cost of new boiler,installation,annual contract as they are expensive to repair, the limited life of new boilers etc.
My annual gas usage is probably about 10,000 Kw pa and i am in the business of managing it down so i wont be rushing to ditch my boiler. In fact ill keep it going until it falls of the wall !

I'm with you there... 22 year old boiler, use around 6000Kw per year. It's staying until I have no choice - even if I calculate with a generous 25% saving, at today's gas prices it would probably take something like 30 years for me to break even :rotfl:

grahamc2003

Cardew wrote: »

Any idea what the 37.7% figure represents?

It's not explicit, but I would say that's the efficiency you get in summer when just heating the hot water tank and no rads.

Assuming a non-modulating boiler, I bet if you felt the exhaust temperature in summer when just heating the hw tank, it would be very hot, due to the high return temperature almost all the time (even with the hot water tank cold). I'd expect the figure for a modulating boiler to be much higher in this regime.

Basically, if the boiler is producing heat at 18kW for example, even if most of it goes into the circulating water (i.e. with the return temp cold), then there's no way it can be dumped through the coils in the hw tank at anything like a rate of 18kW - simply not enough surface area to do that. So much of the heat would remain in the circulating water meaning the return temp would be high, lowering the efficiency. As the water in the tank heated, the return temp would also rise further, leading to the very low efficiency. Rads could dump most of the 18kW input, whereas a hw tank alone can't. Which is why for some with old boilers, it may not be a good idea to use them in the summer just for hot water.

rogerblack

grahamc2003 wrote: »

It's not explicit, but I would say that's the efficiency you get in summer when just heating the hot water tank and no rads.

The most efficient way to heat hot water with gas is to have the hot water heating turned on only just long enough to heat the whole cylinder.
Otherwise, every time the boiler goes on to top up the heat loss from the cylinder, it has to add not only the heat lost from the insulation of the cylinder, but reheat the water in the pipes that will have cooled down to cold

This is especially true if the boiler and cylinder are not close to each other.
One hour a day, shortly before your peak demand for HW may be appropriate.

C_Mababejive

I havent looked at the SEDBUK website but again,they use the word efficiency but are they specific about what they mean?

Are we talking purely about combustion efficiency i.e how effectively the appliance converts raw fuel into useable heat or are we talking about efficiency when considering a closed system and if so,what is the system that they use as a standard for comparison with others?

HappyMJ

A boiler first has to heat the circulating water between the boiler and the cylinder before the heat of the water is above the temperature of the water in the cylinder and the heat is transferring from the circulating water to the cylinder. As the temperature difference is quite low at about 10 degrees then the return water temperature will still be quite hot. The losses take into account the waste heat in summer that is emitted into the house from the cooling water in the pipes.

rogerblack

C_Mababejive wrote: »

I havent looked at the SEDBUK website but again,they use the word efficiency but are they specific about what they mean?

Are we talking purely about combustion efficiency i.e how effectively the appliance converts raw fuel into useable heat or are we talking about efficiency when considering a closed system and if so,what is the system that they use as a standard for comparison with others?

A useful study in this regard
http://www.energysavingtrust.org.uk/Publications2/Housing-professionals/Heating-systems/In-situ-monitoring-of-efficiencies-of-condensing-boilers-and-use-of-secondary-heating-trial-final-report
I think someone posted this on here a yearish ago.

7.1.2 Combination boilers efficiency Annual heat efficiencies for the thirty one individual sites with combination boilers are distributed between 89.7% and 68.6% (see Table 9 below). The mean efficiency of the sample set of combination boilers was 82.5% with a standard deviation of 4.0%.
The mean of the SEDBUK rated seasonal efficiencies of the combination boilers was 90.3% with a standard deviation of 0.9%. The mean of the trial data suggests that thein situ performance of the boilers is significantly less than the rated SEDBUK
seasonal efficiency. This data set suggests that the SEDBUK rated seasonal efficiency is unlikely to be observed amongst in-situ boilers.
SEDBUK tests combination boilers in space heating mode. When operating in hot water mode with intermittent demand and short draw-off periods a higher proportion
of heat is wasted. Trial data represent a combination of space heating and hot water and as such does not provide a direct like for like comparison with SEDBUK, as is the case with regular boilers.

(Or the set of boilers performs 8% worse than the spec.)

Also, at 7.2.4 - it mentions that efficiency may actually be higher with the conventional boilers.

Sweey_as_a_nut

I had a new boiler put in a week before xmas, I wrote about it on here.
Yesterday before I put my heating on I took a reading and this morning I took a reading again and I found I had burnt 5 units. It was on until I switched it off at 11.00pm I had the temperature set at 18.5 at the highest temperature and 15.5 at the lowest, I had the temperature on the boiler set at 65deg. the amount that cost was £2.55. I am in my seventies and fit...so far. I do dress for it and move about.

grahamc2003

C_Mababejive wrote: »

I havent looked at the SEDBUK website but again,they use the word efficiency but are they specific about what they mean?

Are we talking purely about combustion efficiency i.e how effectively the appliance converts raw fuel into useable heat or are we talking about efficiency when considering a closed system and if so,what is the system that they use as a standard for comparison with others?

Yeah, efficiency can be defined any way we wish, so we'll probably just have to guess at what they mean.

My guess is that it is a theoretical efficiency for that particular boiler under that particular regime. As I expect you know, calculating eta (some sort of standardised efficiency) is a pretty standard (though very complex) calculation for engineering undergrads and a typical exam question (or used to be!).

The quoted efficiency can't be just the conversion of the gas into heat, as these days that part would be pretty much 100% (for a well serviced boiler). Losses would occur absorbing the heat from the hot gasses into the circulating water - obviously the colder the circulating water enters, the more heat will be absorbed and therefore the higher the efficiency. Further losses would occur for non-stochiometric combustion, with excess air, which simply lowers the temperature of the gases after combustion.

The efficiency numbers are only guides of course, the actual efficincy would vary in realtime with many parameters such as ambient temperature, pressure and humidity, but the most effect on efficiency coming from how much excess air beyond stoichiometric is ingested, and i expect that is determined during servicing.