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Effect of log guard on woodburner performance?

Posted: Fri Jan 24, 2014 4:55 pm
by mikepepler
Over the past 3.5 years the cast iron log guard in our Woodwarm woodburner slowly bowed backwards, presumably because the front was always cooler than the back, with the fire at the back and airwash at the front.

Anyway, I'd ordered a spare, and yesterday the original one finally broke, and I dropped the spare in. I was surprised at how much hotter the fire is burning now, it's quite noticeable! It has angled slots in it, which must direct the airflow, but I'd never had thought they'd make that much difference...

Shame the component doesn't last longer, at over £70 for a replacement...

Posted: Fri Jan 24, 2014 9:28 pm
by RenewableCandy
Not quite clear which part you mean, Mike, and indeed whether our little machine has an equivalent.

Posted: Sat Jan 25, 2014 1:23 am
by vtsnowedin
8) Burn out is a common problem with wood and coal stove grates and can be made much worse when operators fail to understand the need to remove the ashes daily from the face of the grates. Of course something is apt to work much better as originally designed vs. old and warped out of shape.

Posted: Sat Jan 25, 2014 11:21 am
by adam2
RenewableCandy wrote:Not quite clear which part you mean, Mike, and indeed whether our little machine has an equivalent.
Most, but not all solid fuel stoves have a log gaurd.
Generally a removable component of fairly lightweight cast iron, located at the front of the stove to allow the fire to built up higher without risk of logs or other burning material falling out when the door is opened.

Most types lift out, but are not inded to be regularly removed.

Posted: Sun Jan 26, 2014 2:40 pm
by RenewableCandy
Ah right, thanks. Yes ours has one then. Now to go and see if it is (a) more, or (b) less, warped than my sense of humour...

Posted: Mon Jan 27, 2014 8:47 am
by mikepepler
vtsnowedin wrote:8) Burn out is a common problem with wood and coal stove grates and can be made much worse when operators fail to understand the need to remove the ashes daily from the face of the grates. Of course something is apt to work much better as originally designed vs. old and warped out of shape.
That's interesting, I'd not heard that before... but this is the guard at the front of the fire, we don't have a grate (it's a wood-only stove, not multifuel). Would the same apply there? It didn't so much burn out as just slowly bend until it cracked.

Posted: Mon Jan 27, 2014 10:39 am
by vtsnowedin
mikepepler wrote:
vtsnowedin wrote:8) Burn out is a common problem with wood and coal stove grates and can be made much worse when operators fail to understand the need to remove the ashes daily from the face of the grates. Of course something is apt to work much better as originally designed vs. old and warped out of shape.
That's interesting, I'd not heard that before... but this is the guard at the front of the fire, we don't have a grate (it's a wood-only stove, not multifuel). Would the same apply there? It didn't so much burn out as just slowly bend until it cracked.
Burn out happens when you let ashes block the air passages and the grate gets covered with hot coals. You get temps above red hot 1400 deg.F and spots to White hot where melting occurs. The warping comes from repeated heat cycles cold to hot then cold again. Not as much of a problem on stoves that are run constant and even. To illustrate how it works imagine a block of iron set into a strong vice. Now add heat from a torch until the block is red hot. The block can't expand against the vise but dose in all the unrestricted directions. Now let it cool, it is free to contract in all directions and dose and soon falls out of the vise on its own.
When part of your log guard or grate gets hot the resistance of the cooler ends resist the expansion as does the cooler air side of the grate so to allow the hot side to get just that little bit longer it bows in towards the fire. Every cycle of the fire repeats the process and eventually it adds up to a warp you can see.
Of course if it's hot enough to warp it you are close to hot enough to burn it out so you can do both at the same time.

Posted: Thu Jan 30, 2014 8:59 pm
by mikepepler
Ah OK, that makes sense. The ash problem doesn't apply to our log guard - it's above the fire. But the warping happens eventually anyway... Oh well, at least I know it'll get through 3 more winters at least before it needs changing again.

Posted: Sun Apr 06, 2014 4:22 pm
by adam2
The sensible prepper would keep a spare log gaurd to hand, the doomer would keep several, and also other parts of a stove liable to wear or breakage.
Although spares are readily available now, it would be unwise to count on this in the long term.

Posted: Sun Apr 13, 2014 7:35 pm
by mikepepler
adam2 wrote:The sensible prepper would keep a spare log gaurd to hand, the doomer would keep several, and also other parts of a stove liable to wear or breakage.
Although spares are readily available now, it would be unwise to count on this in the long term.
My thinking exactly! Not that I've ordered a spare yet... but I think I will, if only to have time to shop around and get a good price rather than have to buy one in a hurry.

Posted: Sun Jun 08, 2014 11:29 pm
by adam2
I have recently inspected the log gaurd on Mothers stove, it is very slightly bowed as described above.
The distortion is very slight, less than 1mm, and would not have been noticed if not specificly looked for.

Will order a spare, and possibly also replacement glass for the door.