Author Topic: Gas vs wood fuel technical question  (Read 7197 times)

Mrs Pingu

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Gas vs wood fuel technical question
« on: December 10, 2015, 09:46:16 pm »
Due to our damp chimney probs I've been looking into reinstating some sort of fire in the hole.

Completely ignoring the aesthetic and other similar considerations, I want to know what's going to be best for our chimney in terms of a) drying it out and b) making least amount of new condensation. This assumes a closed wood burning stove (burning briquettes rather than damp wood) vs either a gas stove or a glass fronted high efficiency gas fire.

Also, do the gas stoves & fires have some sort of vent to allow draw up the chimney?
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Re: Gas vs wood fuel technical question
« Reply #1 on: December 10, 2015, 10:10:11 pm »
For proper function and efficiency, a liner is recommended for wood-burners. When we had one properly fitted at the old house, they dropped the liner down the chimney from above, and then back-filled with vermiculite or some such magic fluff.

Anything you put in that results in fumes in the chimney will lead to moisture, especially gas I would have thought. At least you get the option to burn dry wood with a wood burner.

Mrs Pingu

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Re: Gas vs wood fuel technical question
« Reply #2 on: December 10, 2015, 10:26:11 pm »
Everything makes H2O, I have it in my head that gas would be worse but not sure why, possibly difference in chimney temp. Or I could be making it all up.
Gas would fit our lifestyle better (lazy sods in a 1 bed flat) but I'm not convinced it's best for the building.
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Kim

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Re: Gas vs wood fuel technical question
« Reply #3 on: December 10, 2015, 10:28:58 pm »
If there's going to be a liner, then the flue gasses aren't actually in contact with the chimney.  It's just a big pipe-shaped heater...
I do find anything involving ball bearings oddly satisfying

hellymedic

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Re: Gas vs wood fuel technical question
« Reply #4 on: December 10, 2015, 10:29:51 pm »
Maybe cos gas is mostly methane CH4 but there's no H in coal...

Kim

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Re: Gas vs wood fuel technical question
« Reply #5 on: December 10, 2015, 10:31:42 pm »
there's no H in coal...

Fairly sure there is.  Bake it off and you get coke...
I do find anything involving ball bearings oddly satisfying

Mrs Pingu

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Re: Gas vs wood fuel technical question
« Reply #6 on: December 10, 2015, 10:43:16 pm »
If there's going to be a liner, then the flue gasses aren't actually in contact with the chimney.  It's just a big pipe-shaped heater...

Well ok, some page on the Internets reckoned that gas exhaust is cooler hence you can use PVC pipe through a wall, but that was the Internets. ...
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Kim

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Re: Gas vs wood fuel technical question
« Reply #7 on: December 10, 2015, 10:48:11 pm »
If there's going to be a liner, then the flue gasses aren't actually in contact with the chimney.  It's just a big pipe-shaped heater...

Well ok, some page on the Internets reckoned that gas exhaust is cooler hence you can use PVC pipe through a wall, but that was the Internets. ...

I'm sure the internets know more about the subject than I do (which is limited to applied common sense).  PVC certainly seems to be common on boilers, so that's a fair point.
I do find anything involving ball bearings oddly satisfying

Jaded

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Re: Gas vs wood fuel technical question
« Reply #8 on: December 11, 2015, 06:51:18 am »
The liners can be insulated with vermiculite, but cam also be wrap insulated themselves. This makes them a bit wider to fit down the flue, but lessens the risk of loads of vermiculite escaping into another flue or your fireplace.
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Re: Gas vs wood fuel technical question
« Reply #9 on: December 11, 2015, 07:15:46 am »
Although the kids would love it if Santa came down the chimney accompanied by a snow fall.

Vince

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Re: Gas vs wood fuel technical question
« Reply #10 on: December 11, 2015, 07:15:56 am »
Don't modern installations use a balanced flu with the warm exhaust running down the middle and the incoming air down the outside, which is why the outer tube can be PVC. I would have thought if the intent is to warm the chimney to dry it out, neither gas or wood burning stove would be that effective. An open wood/coal fire would be effective, but messy.
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Re: Gas vs wood fuel technical question
« Reply #11 on: December 11, 2015, 07:40:24 am »
Probably the best (for the building) solution is to open the flue and leave it open, allowing warm air from the room to pass out of it - in other words ventilation. Not very energy efficient of course, but it's what we do in our cottage. And we're fortunate enough to be able to burn wood in an open fire occasionally, which certainly warms the chimney breast through.
We are making a New World (Paul Nash, 1918)

Re: Gas vs wood fuel technical question
« Reply #12 on: December 11, 2015, 07:56:42 am »
Don't modern installations use a balanced flu with the warm exhaust running down the middle and the incoming air down the outside, which is why the outer tube can be PVC. I would have thought if the intent is to warm the chimney to dry it out, neither gas or wood burning stove would be that effective. An open wood/coal fire would be effective, but messy.
Spot on. The gas exhaust flue is metal, the outer pipe can be plastic because it is for intake air.

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Re: Gas vs wood fuel technical question
« Reply #13 on: December 11, 2015, 08:31:35 am »
Don't modern installations use a balanced flu with the warm exhaust running down the middle and the incoming air down the outside, which is why the outer tube can be PVC. I would have thought if the intent is to warm the chimney to dry it out, neither gas or wood burning stove would be that effective. An open wood/coal fire would be effective, but messy.
Spot on. The gas exhaust flue is metal, the outer pipe can be plastic because it is for intake air.

And limited in length to (as far as my google-fu can tell) 6m, presumably for a fanned flue.
We are making a New World (Paul Nash, 1918)

Mrs Pingu

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Re: Gas vs wood fuel technical question
« Reply #14 on: December 11, 2015, 09:01:42 am »
Probably the best (for the building) solution is to open the flue and leave it open, allowing warm air from the room to pass out of it - in other words ventilation. Not very energy efficient of course, but it's what we do in our cottage. And we're fortunate enough to be able to burn wood in an open fire occasionally, which certainly warms the chimney breast through.

We're doing that at the moment. I need some extra heat in the living room though - it's got the smallest radiator in the flat (possibly because the previous owner had a ££££ burning open gas fire) and it's therefore the coldest room in the place. Brr.

Maybe things will be better when I've had the pointing redone and the water can escape back out of the wall.  :-\
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Jaded

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Re: Gas vs wood fuel technical question
« Reply #15 on: December 11, 2015, 09:17:16 am »
Open chimneys leach heat out of the room. In the cottage we use the living room would be warm when the fire was lit and cold (not as cold as Aberdeen, but close) an hour later. With a woodburner installed the heat stays way longer. Magnitudes longer.
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LEE

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Re: Gas vs wood fuel technical question
« Reply #16 on: December 11, 2015, 09:28:13 am »
We had a log-burner installed last year. Our external chimney stack had previously been taken down, back into the loft floor. They dropped a flexible steel liner tube down our old chimney and bridged the gap between loft floor and the roof tiles with stainless steel "double wall" tube.

We have a Burley wood stove which are claimed to be around 89% efficient when up to temperature.  Since their operating temperature is many hundreds of degrees I imagine the steel tube in the old chimney must be doing a good job of drying out any damp. 89% efficiency is up there with modern Gas boilers (though I expect there are all sorts of caveats if I expect to achieve that figure). They use secondary combustion to ignite unburnt gases.  The way logs burn in an efficient sealed stove is very different to how they burn in an open fire.

Burley call it "the fireball effect".  The logs aren't burning as you'd normally expect, they just get so hot that it "squeezes" the gas out of them and you see the gas burning.

I was so impressed I took a video of the effect... Fireball 2ndary combustion
The log just slowly turns to dust over a period of about an hour and kicks out 5kW (I have a very small stove which can turn out north-facing front room into a sauna if we leave the door closed).

You also have a 30Kg lump of iron acting as a radiator in the morning, many hours after the actual fire has gone out.

I clean the ash out my my log burner about once a month, because there isn't any ash left to speak of, possibly a few tablespoons of it. It burns logs to dust. They are very efficient.

Open fires are extremely inefficient unless you view them as an efficient way of getting rid of surplus wood rather than an effective way of heating a room.
Open fires are about 10% efficient and a really bad one, with a poorly matched chimney, can actually draw more heat out of the room than it puts back.

Cleaning a steel tube will also be less hassle than cleaning an old chimney I expect.
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Re: Gas vs wood fuel technical question
« Reply #17 on: December 11, 2015, 09:35:18 am »
IF going down the open fire route should check flue lining. An unused sealed non ventilated flue will produce interstitial condensation.

CICO chimney linings retrofit new refractory lining via chimney breast.

http://www.chimney-problems.co.uk/

I am sure that there are other companies out there this is just one I know of.
'Absence of evidence is not evidence of absence'.

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Re: Gas vs wood fuel technical question
« Reply #18 on: December 11, 2015, 09:40:34 am »
Open chimneys leach heat out of the room. In the cottage we use the living room would be warm when the fire was lit and cold (not as cold as Aberdeen, but close) an hour later. With a woodburner installed the heat stays way longer. Magnitudes longer.

I guess it depends on how much air is getting in to the room, to slow the circulation to leach the heat out.  Our grate is a Baxi one - no back-boiler, but rather a pit sunk into the floor (which houses an ash-can), and a vent to the outside wall, with a flap valve on it operated at the grate. Open the valve to start the fire (increase the draught) then close once established. The ash can will take a weeks worth of evening fires. The fireback can still be so hot in the morning that a new fire can't be laid as the paper would char, and it does do a fair job of putting heat into the chimney breasts. But ours isn't the main form of heat, the GFCH is. We just like the atmosphere it gives. If it were the main then we'd open out the fireplace again and install a wood burning stove.
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Mrs Pingu

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Re: Gas vs wood fuel technical question
« Reply #19 on: December 11, 2015, 10:35:56 am »
An open fire is not under consideration here. Only a closed gas fire/stove or a woodburner, which would have to use briquettes as we don't have the storage to season wood.
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Aunt Maud

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Re: Gas vs wood fuel technical question
« Reply #20 on: December 11, 2015, 10:38:01 am »
Spiral wound, flexible steel flue liners need to be replaced after a few years, as they can burn through. Considering the glass on a modern wood burning stove can reach over 1000ºc the flue liner has to take a considerable amount of heat.

Also if there is rain coming down the chimney it creates a very aggressive environment for steel when the water comes in contact with the products from combustion that line the chimney.

I built both our chimneys from scratch after taking down the existing ones when I took the roof off. They're made from two layers of precast elements that stack on top of each other and create a double lined flue. You could probably line your existing flue with the inner precast units I used and backfill with vermiculite. As both the precast clay liner and vermiculite absorb moisture, they would dry the flue out when you lit a fire in the burner.

It's essential that the wood fuel is seasoned correctly, as unseasoned wood coats the flue in tar, which can lead to a chimney fire....exciting, but not so good.

The units are called Isokern and are made from Leca type clay pellets with a cement binder.

Re: Gas vs wood fuel technical question
« Reply #21 on: December 11, 2015, 05:54:19 pm »
An open fire is not under consideration here. Only a closed gas fire/stove or a woodburner, which would have to use briquettes as we don't have the storage to season wood.

We used briquettes in ours - you might find some posts from me somewhere on the matter.

Anyway - they burn with thermonuclear heat, but quite quickly; it takes some getting used to because they're nothing like logs. The knack is to get a really small, but really hot fire, and keep it that way. Which means loading one or two more briquettes on, frequently. Wood burners are labour intensive.

Feanor

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Re: Gas vs wood fuel technical question
« Reply #22 on: December 11, 2015, 06:19:15 pm »
Wood burners are labour intensive.

And even morsø if you chop your own wood.

I put my back out last weekend splitting some large rounds which have been seasoning for a year or so.

Re: Gas vs wood fuel technical question
« Reply #23 on: December 11, 2015, 07:07:56 pm »
there's no H in coal...

Fairly sure there is.  Bake it off and you get coke...
Depends on the coal.  Anthracite is pretty much the same chemical composition as coke, just harder.  Bituminous coal, which is the usual stuff, contains quite a lot of hydrocarbons.  Still a lot less than gas per kWh of heat.

The composition of the coal affects how it needs to be burnt; anthracite needs lots of primary air (from under the firebed) whereas coal with a lot of volatiles needs this too.  but also a lot of secondary air (air introduced above the firebed) to burn the hydrocarbon gases driven off by the heat.  Wood only needs secondary air. because the stove basically cooks the wood to gasify it and the gases are what is burnt.
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Aunt Maud

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Re: Gas vs wood fuel technical question
« Reply #24 on: December 11, 2015, 08:54:18 pm »
Wood burners are labour intensive.

And even morsø if you chop your own wood.

I put my back out last weekend splitting some large rounds which have been seasoning for a year or so.

Hope you're back feels better soon Feanor, large rounds are bloody heavy work. I've been splitting 4 tonnes of fresh Beech for the last two weeks or so, with another 15 cubic metres to fell and split before christmas. Despite being a very dense wood, it splits like a dream when green and makes for one of the best firewoods.