Gas Fire Efficiency - How?

[twentysix by twentyfive]

Mrs 26x25 has decreed that we should get a new gas fire (the old one works fine ). So in an attempt to be green and cost efficient I said that we should require the most efficient gas fire possible. Efficiency of a gas fire should be simple should it not? Wrong. Manufacturers quote numbers like 89% but when asked how that is measured one gets a different story depending on who you ask. One really bad answer was

"We burn gas at rate X and put a thermometer about 3 feet from the radiant elements. It gets to Y degrees celcius so our efficiency is 89%."

What? The thermometer will attempt to get into thermal equilibrium with the temperature of the radiant element temperature (any meteorologist knows this when measuring temperature - do it in the shade not in direct sunlight) so any efficiency calculated that way is just WRONG. So can we actually get at the correct efficiency? A physicist chap I had a report of did so with an "89%" efficient fire and actually measured 11% (that'll be 89% up the chimney then ). He had it taken back.

So how to get at the correct numbers from the manufacturers before dishing the dosh? I've set Mrs 26x25 the task of contacting all likely manufacturers to try to solve this one. But why are we treated like this by the industry? Do they think we are all stupid - or is it just them, they haven't a clue how to measure the correct numbers? We are supposed to have choice but choice can't exist in gas fires when this situation prevails.

Anyone out there know anything about real gas fire efficiencies? I'd be interested in any knowledge that might be available.

I define fire efficiency as (Room Temperature rise x thermal capacity) divided by (Gas Flow x calorific value) all in appropriate units. Ie useful heat power out /gas power in. For short times and small temperature rises thermal losses from an insulated room can be ignored. Simples?

 

[philipbh]

I am looking into replacing our current Flued Gas Fire - which has an anedotal 50% or less efficiency

I understand these new fangled Flueless types claim 90% +

Not sure how they measure it though - but with no heat disappearing up your flue, then they "must" be better than the Flued type

£500 quid seems about the entry level though - payback time within in the year??

 

[twentysix by twentyfive]

90%+ I don't think so. Ask how it's measured.

It should be more efficient using what might otherwise go up the flue but appearances in the catalogues can be deceptive.......

 

[jimboalee]

http://www.hse.gov.uk/RESEARCH/rrhtm/rr023.htm

 

[Beardie]

What is a 'flueless' gas fire? It needs a flue for the combustion gases to escape, otherwise they would kill you.

 

[Davidc]

Flueless just means it uses a fan and an inlet/ outlet terminal. Some fires are now condensing and can reach over 90%.

The measure is seasonal efficiency, a figure allowing for the range of operating conditions it is likely to meet. The efficiency varies with operating conditions hence the need for a figure which allows for that.

The efficiency is heat delivered to the room as a proportion of the total energy released by burning the fuel.

 

[twentysix by twentyfive]

The efficiency is heat delivered to the room as a proportion of the total energy released by burning the fuel.

Well I'd buy that (it ignores unburnt fuel) but apparently that isn't what happens in the brochures as I understand the situation

I'm still hunting for some sense of sense (as it were).

 

[Davidc]

Well I'd buy that (it ignores unburnt fuel) but apparently that isn't what happens in the brochures as I understand the situation

I'm still hunting for some sense of sense (as it were).

I should have put "total energy which would be released by completely burning the fuel". Unburnt fuel is still input to the fire but doesn't contribute to output and is therefore reduces efficiency.

 

[threefingerjoe]

I don't know all of the technical issues of how it is measured, but a lot of different things go into determining efficiency rating. My old flued gas furnace not only required a metal flue (which got very hot, hence wasted heat going up the flue), but also required (already heated) room air for combustion. That means that even more heat was wasted. That room air had to be replaced. In other words, cold air was sucked in through every nook and cranny. Then, that fresh, cold air had to be heated. These issues are eliminated with the newer "flueless" furnaces.

Combustion air is drawn in from outside the house, through a PVC pipe. The exhaust is then pumped out through another PVC pipe. Not only is the exhaust cool enough to be exhausted through a plastic pipe, but the furnace is sealed, so that no already-heated room air is lost. No cold air is drawn into the house, which then has to be heated. The MOST efficient units use multi-speed forced air blowers, to draw even more warmth off the heat exchanger.

There is one negative factor: Old gas forced air furnaces only had the forced air blower (to blow air over the heat exchanger and move the heat through the house). The new ones also have a (small) combustion blower to draw air in for the fire. Soooo....they may use a little more electricity, though less gas.

I'm told that all of these things are included in the calculations to determine "efficiency". I would disregard any actual numbers given, but use the numbers simply to compare different units. If there are no government standards on those numbers, then you have to figure in the "marketing factor". ;-)

Still, when you can observe that the exhaust is considerably cooler than the old furnace, it stands to reason that less fuel is being wasted.

 

[twentysix by twentyfive]

What is a 'flueless' gas fire? It needs a flue for the combustion gases to escape, otherwise they would kill you.

Apparently there is a catalyst that removes the CO by turning it into CO2. But then the CO2 builds up in the room as well as NO2 (and presumably H2O). So a Vent is required.

But if the catalyst stops working

 

[twentysix by twentyfive]

I'm told that all of these things are included in the calculations to determine "efficiency". I would disregard any actual numbers given, but use the numbers simply to compare different units. If there are no government standards on those numbers, then you have to figure in the "marketing factor". ;-)

Unfortunately different manufacturers use different methods so comparisons are pretty useless.

Time for some correct numbers using good government standards thinks I.

 

[vernon]

Unfortunately different manufacturers use different methods so comparisons are pretty useless.

Time for some correct numbers using good government standards thinks I.

Out there are some wall mounted gas fire manufacturers claiming 100% efficiency......