The message from "Doctor Drivel" contains these words:
Hah, I read that as plumbing and had visions of gangs of unruly plumbers fitting a bathroom on the street corner and giving the wife a loutish plungering.
The message from "Doctor Drivel" contains these words:
I'm fairly sure that the coolth of the incoming charge is only of consequence on an expansion engine, where the idea is to get the smallest most dense charge into the engine, then expand it to the maximum amount practicable to extract the most energy possible.
Boilers are different - provided the fuel is fully burnt at the stochiometric point or better (leaner), then all the thermal energy will have been liberated and now the task it to extract as much as practicable. This means that the closer the exhaust temperature is to ambient (above, of course) the more energy has been retained for useful purposes.
If you look at the design of boiler systems for power stations you'll see that they take great trouble to preheat the imcoming air for combustion to the maximum practicable. This is a much closer analogy to a domestic boiler than an internal combustion engine.
What with twiddling screws to get the gas-air mix right, poking things in the exhaust pipe and spark plugs that don't they are going that way!
Yep.
Not really, although external combustion is far more efficient than internal explosions.
A pre-mix fan can only deliver a ratio of air/gas because of the fan design. Put more oxygen insthe air and more efficient combustion. Cold air is denser containing more oxygen.
All the sensible heat has been liberated, not to recover the latent heat.
It means more latetent heat has been extracted
It depends on the engines being used. Turbines are different again.
Yes *but* the Zenex design looks to be very much optimised for gas to water heat exchange, not gas to gas. Now, what proportion of a typical domestic combi boilers gas consumtion goes on heating hot water for the taps and how much on heating for the radiators? 5%, 10%, 20%?
The water to gas heat exchange is going to do absolutely diddly if you're not drawing hot water and would be useless on a non-combi boiler. (Okay, you could run the SH return through it but the gains are going to be small). A gas/gas heat exchanger would improve the efficiency *all* the time the boiler is running.
Tim
Read their web site and HV article again.
If you take the density in its gaseous state at 1.82 kg/m^3 - your 3:1 water to gas weight ratio above would yield 5.46 L of water vapour. Which seems a bit much...
Are you sure that 36g is the mass of just the water? If it were the total mass of the combustion product (i.e. inc CO2) that would make more sense with a water vapour yield of 3.64 L
(if you take the latenet heat of vapourisation of water as 2.26 MJ/kg the 5.46L figure would give you a total of 12.3MJ as oposed to 8.23MJ for the lower figure)
Since the figures are normally quoted at about 36 / 43MJ for the nett / gross energy yield of natural gas the latter one seems closer.
It is under very little pressure when metered (0.021 Bar) so you can probably discount that.
Which bit didn't you undersand? It preheats your cold water. Ergo, if you're not drawing any cold water at times when your boiler is running for the CH, its effectiveness will be greatly reduced. If you're using a combi
*just* for hot water, then yes, it will improve efficiency.Tim
and that the logistics of running the flue worked.
It depends on how much and the context of the rest of the house. The amount of surface area required to achieve this amount of temperature drop could become quite substantial.
Although the contact area is then not that substantial in comparison to something with baffles etc.
It comes back to whether people would want the extra cost and hassle of running a long flue for the small amount of energy involved.
We were looking originally at conventional boilers.
Generally those that are already condensing models.
I would look elsewhere first
Wrong (partly) again. I guess you've never experienced "carb" icing then?
As you seem to have such a close relationship with this company, would you care to locate the relevant patents that they claim to have?
The mumbo jumbo quoted on their website, together with the performance "test" that has more holes in it than a colander hardly inspire confidence in the claimed performance.
Sounds like you have been reading glossy brosures again, get ready for the incoming barrage of dodgy statistics....
What did I say! What does that mean exactly? 30% of what?
From the description it sounds like the best that would be able to achieve would be recovery of 30% or the un-recovered heat remaining in the flue gasses - or put it another way, for a top efficiency boiler you are only talking about perhaps 3% over and above what the boiler can extract anyway. Note also the gain is only applied to HW use and not CH
- so again there is a proportionate reduction in return there.
100% - 92% = 12% hmmm...The idea looks ok in priniple, but I can't see how you will ever achieve any payback given its £800 cost (probably more like £1100 once fitted for most buyers). Unless you have a gerriatric old boiler with very poor efficiency - in which case the cost of the top box would buy you a whole new boiler.
Only if you put the wrong calorific value (i.e. the nett value) for gas into the equations... do the sums correctly and you can't.
Hype is certainly the word....
(I suppose we are talking about 35 degree rise over incoming temp here)
Are you sure that is not 12 lpm *peak* rate - i.e. by dumping its reserve of stored heat into the first few litres demanded?
Otherwise where is the other 14.4kW coming from if you are talking about susstained performance?
"Straws", clutching at perhaps ;-)
Why not leave it up to the control systems in the boiler? They seem to do a pretty good job of modulating the gas rate to achieve best efficiency anyway, and the efficiency ought to take precedence over the visibility of the plume.
Andy, I don't *know* if my idea would be viable but you seem to be taking the position "It hasn't been done so it can't work" which may be right most of the time but certainly not all of the time.
There's nothing logically wrong with my idea that you've pointed out. I see no reason why a cheap co-axial flue extension shouldn't reclaim a significant amount of waste heat. It doesn't need to be complicated and if the bore diameters were reasonably dimensioned, it seems unlikely that it would greatly increase the amount of backpressure over what a modern fan flues boiler can cope with.
Tim
Lord Hall, the point is combustion. I doubt the penny will drop.
Lord Hall, I have no direct relationship with them.
GasTec, a testing house in Holand, confirnen their claims.
Johnson & Starley sell a unit that converts a non-condensing conventional flued forced air unit into a condenser doing what you proposed. It is a box in the flue pre-heating air.
The message from Tony Bryer contains these words:
ISTM you don't need be a scientist to solve the problem. Being an engineer is quite sufficient. The key is the difference between the Higher and Lower Calorific Values of North Sea Gas. 38500 - 34500 = 4000 or about 10.4% of the higher value.
I could be wrong but I thought the figures usually bandied about for boiler efficiency were based of the HCV. Dribble says different but he can't be trusted at that level of competence.
As Tim pondered it should be worthwhile reducing exhaust gases to input air temperature and while that would appear impossible in an gas to air heat exchanger surely if achieved that would mean a fuel efficiency of
100%. But if that is the case then in the manner of heat engines if some way was used to reduce the temperature of the exhaust gasses to less than that of the incoming air then efficiency in excess of 100% does become a possibility. Not technically true of course but on a par with using the LVC to calculate boiler efficiencies.
Chav, this is your way of envy of my knowledge, as you tend to know sweet FA about thsis game.
£600 and they hope to half this price once production is up.Unless you have a gerriatric old boiler with very poor
Chav, tell that to GasTec in Holland who did the testing
Chav, you clearly haven't got it.
You could buy a two pipe plastic flue combi and make you own pipe in pipe flue, mains water pre-heater. Totally legal to do so. These plastic flue boilers are expensive. Or buy one of these top-boxes for an Ariston and the total cost is less than the Keston, Eco-Hometec, etc. And you claw back a hell of a lot of wasted energy, converting 12 l/min combi to an 18 litres/min combi and fill your bath pronto
Then, add a GFX drain water heat reclaimer, solar panels too and DHW will not cost too much at all. Have all this in a highly insulated and pretty air-tight house, where DHW is the bulk of heating energy used, and your overall gas bills will be very, very, low.
I don't disagree Tim.
I was taking a top down view on the basis of how much energy currently would go out of the flue with a condensing boiler.
In round numbers, with a SEDBUK efficiency (which takes climate into account) of 90%, this means 10% of input.
Averaged over the year, and considering the modulation of burner in a condensing boiler, let's say that the rate is 7kW while the heating is running. That's probably about right for the statistically average house if it's moderately well, but not excessively insulated.
For heating running for a third of the day, the implication would be of 5.5kWh per day. However, it's unlikely that all of the heat could be recovered - perhaps 30% - so we are talking about say 2kWh/day.
This works out to about £15/year.
Of course, this is estimation, but I think it's reasonable approximation within half an order of magnitude
There are other places to obtain a better ROI.
You probably are self deluded enough to believe that...
+VAT + the adaption kit for connecting to the boiler gets you to £788. Still 'kin expensive at half the price.If there are worthwhile gains to be had then you can expect a boiler manufacturer to include the same concept in the box without needing anything like the price jump proposed here.
Chump, if it is what you have, then I clearly don't want it!
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