Gas heater flue height

The manual for our gas heater recommends a flue height of 3m "for best venting performance". We have installed it downstairs in our two-storey house, and the final flue height is about 5m. Clearly it vents well, but I've started to wonder if it's possible to vent too well, i.e. to send too much heat up the flue. This thought occurred to me because the heater (rated at 7.8 kW) doesn't put out as much heat as I had expected.

Reply to
Gib Bogle
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What sort of flue (i.e. balance, concentric, twin pipe etc), what sort of heater, and is this height above ground, or height difference between the heater and the flue terminal?

Reply to
John Rumm

It is a simple 100 mm tube with two right-angle bends, the heater is an Envirogas 828 made by Sherwood Industries, Canada, and I interpret "height" to mean height above the heater.

Reply to
Gib Bogle

As in:

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much heat did you expect I suppose is the first question?

These flame effect fires are notoriously inefficient, and this one will be made even more so but not having a balanced flue. Hence it has to consume (heated) room air for combustion which it then throws outside - only to have it replaced by cold air from elsewhere.

I would not be surprised if the actual heat output to the room was less than 3kW in total.

When first lit from cold, does it give a nice blue flame seated on the burners? (if so it would suggest that gas pressure, and air availability are in the right ball park). You would need to test at the flue terminal with a combustion analyser to be sure.

Reply to
John Rumm

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>> How much heat did you expect I suppose is the first question?

Hard to quantify that. More.

Makes sense. I'm now having second thoughts about the economy of this device vs. an electric heater. Too late now.

The flame looks fine to me. What my original post was getting at was the idea that a flue longer than 3m might create more updraft than desirable, sucking hot air up the chimney too fast. Is that possible? I'm probably not going to go to the trouble of testing with a combustion analyser, because there probably isn't anything to be done at this stage anyway.

Reply to
Gib Bogle

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I would be surprised if it's more than 2kW from the fire, and possibly more from the the flue pipe if exposed. If you can find the instructions for sale in the UK, they will give the heat output too. The fanned one will probably give more output.

Some of these types of fire are designed to burn with a slightly white flame, for effect.

Reply to
Andrew Gabriel

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>>> How much heat did you expect I suppose is the first question?

I can't seem to find a UK spec manual for it alas. However looking at:

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claim efficiencies of over 80% which seems suspiciously good for this class of device.

Indeed, although they normally get most of that from the glowing of the "embers" and "log" once up to temperature.

Reply to
John Rumm

Isn't 80%+ pretty common these days for outset or freestanding fires which have good heat exchangers to promote convection? Eg Valor claim

88%/79% for one I've been looking at
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you, I've no idea if Canada measures efficiency in the same way and with the same SAP net/gross conversion factors (factors the origins of which I don't understand.)
Reply to
Robin

Possibly, I had not looked recently. I may be maligning them unreasonably. The previous generation (i.e. radient heat but not HE as such) were woeful. These could well be much better.

The net/gross bit ought to be the same wherever you are (allowing for some variation in the energy content of the gas).

Assessments made with the gross value ought to give the right answer (ones made with the "nett" (i.e. incorrect energy density of gas) figure will come out with the wrong answer).

Reply to
John Rumm

I've a lot more confidence in the credibility of the gross efficiency figure over the net, it doesn't seem realistic to me.

I wonder which equivalent input kW figure we pay for mains gas at, full fat gross or lean, fiddled, net :-?

Reply to
fred

No well any calculation that can conclude an efficiency of up to 109% is going to lack credibility! ;-)

Need you ask? ;-)

Reply to
John Rumm

This discussion has been very interesting. But nobody has addressed my original question. Perhaps nobody knows.

Reply to
Gib Bogle

Well part of the discussion was working out how the heater in question was aspirated. Had it have been a balanced flue, or fanned flue then the answer would have been an easy "no".

Given its not room sealed, and is natural draft, then the answer is perhaps not as clear cut, and I can't give a definitive answer. However if its burning well and the flames are not lifting off the burners etc, then chances are its not being over aired either. The presence of a heat exchanger would also suggest that the efficiency ought not be affected by the flue length or height particularly.

Reply to
John Rumm

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