How many bends can you have in a combi boiler flue?

Scranner · 3 Nov 2007

Hi
I'm planning on having a gas combi installed to replace an ancient open system and I have a mate who's just Corgi qualified to fit it etc. The boiler will be something like a Potterton Performa 28HE or the like. He says that we're only allowed a single 90deg bend in the balanced flue, but this is really limiting where I can put the thing. What's the regs on this, please - I reckon I'll only need two 90 deg bends to sort it, and a metre and a half of flue in total to fulfil the terminator position requirements? If it's limited to one I'm gonna have to do a lot of digging up of concrete floors etc!

Cheers

doitall · 4 Nov 2007

The maximum flue length for the performa is 3m.

from that you have to deduct 0.5 for each 93deg bend or 0.25 for each 45deg bend. they will allow 2 bends.

I'm not into domestic boilers but there are boilers on the market that will do what you want.

sambotc · 4 Nov 2007

depends on boiler manufacturers. Check the technical data on flueing options before you buy.

Scranner · 4 Nov 2007

Thanks folks - quick replies given the time of day!
I understood that the max length would be reduced according to number and type of bend, so it's good to have that confirmed. I hope I can convince my mate to think again!

Blasphemous · 4 Nov 2007

Not sure about your mate's advice scranner. doitall is spot on.

I have just consulted the MIs for the Performa 28HE. Here's the Fluing details:

Standard concentric flue diameter 100mm
Standard flue installed length 685mm
Maximum equivalent length horizontal 3m
Maximum equivalent length vertical 3m
Optional twin pipe vertical flue diameter 80/80mm
Maximum equivalent length 24m

The two things that I have highlighted in Bold are the lengths. Any additional “in line” bends in the flue system must be taken into consideration. Their equivalent lengths are:

Concentric Pipes 135° bend - 0.5m
93° bend - 1.0m
Twin Flue Pipe 135° bend - 0.25m
90° bend - 0.50m

The elbow supplied with the standard horizontal flue is not included in any equivalent length calculations.

So the tables above tell you that the maximum length is 3 meters. However, if you insert a 90° elbow you will need to reduce the total length of the flue by 0.5 of a meter. This is due to the restriction the bend places on the flue.

The position of the flue terminal is something you/he also need to consider. Your mate will have covered Flue Terminal Positions in his course and/or at the very least at his ACS assessment. If he's struggling tell him to look it up in the Corgi Book (Essential Gas Safety). But even the MIs for the Performa give Minimum flue terminal clearances. So he should be able to work it out. He'll get there.

Scranner · 4 Nov 2007

Thanks, Blasphemous.
If I use the concentric system does that mean I could have a 2m flue (assuming the bend out of the top of the boiler comes 'free') if I have a 93deg bend?
I've notice that the 'HE' version of the Potterton 28 has an equivalent flue length of 10metres! Is that because it's fan driven or something? (excuse the ignorance!)
Sorry for the cascade of questions - only just discovered this forum this week and it seems pretty well used - a great resource, so thanks!

Scranner · 4 Nov 2007

I was looking here when I quoted the flue length of 10m:

http://www.potterton.co.uk/products/literature/Potterton_Gold_Combi_HE_Brochure.pdf

- as I noticed that Blasph had kindly looked up this model too and confirmed the 3m equivalent flue length. I must be reading something wrong!

chrishutt · 4 Nov 2007

the Potterton Gold (10 metre max flue length) is a band A boiler, whereas the Performa (3 metre max flue length) is a band B boiler.

Band A boilers use an advanced burner/heat exchanger design and are more efficient and expensive. They have fans that vary their power to control the heat produced, so can cope with a much greater range of flue lengths. If the flue offers more resistance the fan automatically works harder to overcome the resistance.

Band B boilers have fans that operate at a fixed power input and so can cope with only a limited range of flue lengths. They are basically the same technology as older, non-condensing boilers with the addition of a second heat exchanger or recuperator. Consequently they are less efficient and a bit cheaper.

bengasman · 4 Nov 2007

chrishutt said:
Band A boilers use an advanced burner/heat exchanger design and are more efficient and expensive. They have fans that vary their power to control the heat produced, so can cope with a much greater range of flue lengths. If the flue offers more resistance the fan automatically works harder to overcome the resistance.

Band B boilers have fans that operate at a fixed power input and so can cope with only a limited range of flue lengths. They are basically the same technology as older, non-condensing boilers with the addition of a second heat exchanger or recuperator. Consequently they are less efficient and a bit cheaper.

Is that ALWAYS true? Admittedly I have never bothered to look into it as I only use band A boilers. Intuitively I would say there are other ways to squeeze out the few percent that bring a band B to band A. very interesting, please tell. Is there any documentation about the technical aspects of this? Other than manufacturers propaganda I mean.

chrishutt · 4 Nov 2007

bengasman said:
Is that ALWAYS true?

I don't know, I just make it up as I go along. Most of the time I get away with it, but now and again someone will catch me out.

bengasman · 4 Nov 2007

chrishutt said:
bengasman said:

Is that ALWAYS true?

Click to expand...

I don't know, I just make it up as I go along. Most of the time I get away with it, but now and again someone will catch me out.

Sorry if it came across that way, I do not question your integrity. I thought that the band system ONLY depended on the percentage of output versus input. As far as I know, if you make the heatexchange path long enough, at least theoretically the flue gas will come out stone cold which would mean your output is virtually the same as your input. And yes, I have noticed that theory is something that is only theoretically correct. The whole thing about saving energy is a bit theoretical anyway as it is perfectly okay to leave your house draughty, uninsulated, single glazed with the thermostat on 26 and the timer on constant. Not to mention the compulsory trvs that are set to max in most houses where I work. Nevertheless, interesting subject. If nothing else, an rgi should at least know how it SHOULD be done and why it works that way.

Gasguru · 5 Nov 2007

Band A boilers use high pressure fans that allow ridiculously long flue lenghts. The impellers are similar to water pumps (backward curved blades). They often run at twice the speed as traditional fans and can be twice as efficient (energy wise).

Band B boilers using traditional impellers run at lower speeds, are not very effcient (shaded pole motor) and generate lower pressures hence the limited lenght of flues.

And, due to the variable speed of the Band A boiler fan, excess air is limited to the minimim over the whole modulation range increasing boiler efficiency.

Band B boilers with fixed fan speeds become more inefficient as the burner modulates down since excess air increases.

Dew point varies slightly depending on the excess air, Band B boilers have a slightly lower dew point (reducing condensing opportunity).

chrishutt · 5 Nov 2007

bengasman said:
Sorry if it came across that way,...

I wasn't trying to be sarcastic. I really do make it up, but based on a fair degree of background knowledge.

As far as I know most (maybe all) band A boilers use the zero governor pre-mix fan driven burner system with one heat exchanger (usually cylindrical - aluminium alloy or stainless steel - often made by Giannoni) and most (if not all) band B boilers use traditional technology (copper heat exchangers) with the addition of a second heat exchanger (recuperator, where condensation takes place) to extract the extra heat.

I take your point that the efficiency of the traditional design could be brought up to band A with a bigger recuperator. However it appears to be uneconomic to take that approach further when the more advanced technology is available to deliver the higher efficiency. Also good point about excess air made by gasguru above.

This illustrates the technology of the advanced burners quite well.

I'm very cynical about the whole energy saving thing. There is a simple mechanism for reducing the use of fossil fuels and hence CO2 production and that is seriously increased taxation. But our politicians are afraid to go down that road and prefer to make token gestures, such as making condensing boilers compulsory. It's part of the strategy of failure that governments invariably follow.

namsag · 5 Nov 2007

RE band A boilers and fan speed when they are going full pelt do others find they are excessively noisy far more than band B and older standard efficiency boilers.

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How many bends can you have in a combi boiler flue?

namsag

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