Combis and showers

[Andy Hall]

There are the proper ones and the toy ones for sale in Argos...

[Doctor Drivel]

Matt, Tower Shower is made by Triton. I think Argos also sell Triton too.

[Andy Hall]

Your point being?

[Ed Sirett]

That's about what I expect IME with a conventional vented stored HW system.

This is compensated to a large extent by the fact that they (along with all modern boiler designs) have modulating burners.

Agreed. This is what us required to transfer less heat to the house.

The better models fully control the pump power according to conditions.

...and no HW warm up time. The disadvantages are lower flow rate & no backup immersion heater & longer delay before the HW comes through.

[Ed Sirett]

All the ones which use forced premixing burners will do so, that most of them except for the crappy, hastily reworked models such the Suprima HE for example.

[Andy Hall]

How do these maintain the correct gas air ratios though? or don't they?

[Owain]

How would you know what Argos sell? ;-)

Owain

[Ed Sirett]

The gas air ratio on a conventional burner is done by control of the supply pressure, the injector size and the size and design of the inlet to the mixing tubes and the tubes themselves. In fact on some very old appliances (e.g. a cooker from the 1950s) the inlets to the mixing tubes are adjustable.

The design is such that over the typical range of gas pressures (say

2-10mb) the mixing good enough that the flames are blue and stable over the whole range. A 5:1 range of pressures give a root(5):1 range of gas flow rates about 45%-100% modulation range.

Sometimes when the gas rate suddenly increases you can see a transient effect where the gas:air ratio rises (and the flames become yellow tinged).

A great deal of extra air (known as the excess air) passes around the burner and through the heat exchanger. This is helpful to keep the dew point of the flue gases lower and so prevent condensation in a non-condensing boiler.

The extra air does have a modest effect on the efficiency. Assuming round figures and assuming non-condensing (so scoring out of 90 instead of 100). Say : Inlet temp is 0 Flame is 2000 C Flue is 100C Then you get (2000-100)/2000 = 95% (of the maximum non condensing 90%) if there is no excess air. About 86%.

If the excess air is 100% (quite plausible at full load) then Flame + Excess averages at 1000C Flue is still 100C So you get (1000-100)/1000 = 90% of the 90% which is 81%.

So the difference is just 5%. But at reduced power the excess air becomes greater still and effciencies drop a bit more still.

[Andy Hall]

I suppose that the situation is fairly similar in a room sealed boiler using natural draught, although I imagined that when running at higher rates, the additional convection would also draw more air through.

Why would that happen if there is plenty of air anyway, or do you mean models where the fan speed is also increased? I thought that this was only done on premix designs.

OK. Wouldn't the flue temperature rise at higher burn rates though?

[John Rumm]

Hmm, do you have an attention span problem?

Why would you be adding cold water to it since the 15.5 lpm is going to be running flat out, and already at bath temperature anyway?

(15.5 lpm @ 35deg rise, which with the supply water at 5 degrees gives a final temperature of approx 40 degrees leaving the boiler)

Or do you have cool baths to help quell your excitement from reading all those biler catalogues?

18 x 5 = 90

So 7.5 mins would be more like it then... for a cool bath. Or about 10 mins for a warm one.

(No need to argue, I have a combi that does 15 lpm so I m speaking from experiance).

Serious question:

Why do you always have to make your position look foolish simply because you can't resist the temptation to exagerate all the time. Tell it like it is, and we might take you seriously once in a while rather than dismissing you as the class clown.

[Doctor Drivel]

Not again. And this one says he is a professional. Well he has only ever fitted Vaillants who don't have high flowrate combis. But at least he can learn, which he appear incapable of doing.

[Doctor Drivel]

Matt, the point being is that you don't have a point.

[John Rumm]

I had a fairly poorly designed storage system that did about 20 - 22 lpm of hot at 65 degrees (had to keep it hot since the cylinder was too small otherwise!) - so I guess that was in the high 30's fill rate with cold added.

That certainly used to be a problem, although does not apply so much with most modern ones that can modulate. Obviously choosing one that can modulate very low gives more flexibility (some will go to under 4kW).

[Doctor Drivel]

The air ports not the injectors. I have never ever seen adjustable injectors, ever, on a cooker. Can you tell me the make and models?

Domestic burners are nearly there efficient. 100% complete combustion is done on large commercial and industrial equipment using CO and NOx sensors feeding back to a controller which adjust the air gas ration to attain the highest efficiency. Similar to a cars fuel management system. Sensors are cheap enough that on larger domestic burners these can be used cost effectively. They can also be used to display the burners levels, which can dictate if the appliance needs servicing; a warning light appears, then if left and out of parameters lockout. Similar to Skoda cars which don't have regular service intervals, only when the management system dictates. Ceramic fibre burners are more efficient on domestic burners, yet only one boiler has these, the Sogno, who didn't design the heat exchanger well enough and match the burner too well to it either. A ceramic fibre burner should be of condensing efficiencies and not condense. Still a good boiler though, that will last and last as the burner burns at a much lower temperature.

[Fred]

It's so nice to see a helpful reply without trying to score points and sounding like an ass.

Many thanks.

[Doctor Drivel]

He said "The disadvantages are lower flow rate". That is sounding just like an ass.

[John Rumm]

It sounds like experiance...

In the vast majority of real world cases, he is right of course. The vast majority of combis sold are not storage combis, and people do buy loads of units with power outputs of 30kW and less.

The fact that you can get more powerful solutions is not relevant to most peoples experience, since they will be guided by cost saving and what their plumber wants to fit.

It is like arguing that small family hatchbacks will do 150mph. Some will, but it is not representative of the real world.

[Doctor Drivel]

Chav, you don't know about these things.

He is not right at all. You wouldn't know this of course. He said all combis has low flowrates. he has been continually told that is not the case, but this so-called professional still prattles the same crap...and yes he is CORGI registered. Yes, go to them for some advise and you get misadvise. Appalling.

** snip Chav drivel **

[Doctor Drivel]

Nice one. He is the only person who shops there in a top hat.

[Ed Sirett]

I did not say the injectors were adjustable did I? I said the inlets to the mixing tubes were. Franklin Chef, various New World models.