How efficient is an open fire?

[Woodworks]

2 hours ago, Haironyourchest said:

Ihing I wonder is: is it better to pipe the air into the stove from outside ("mushroom" inside firebox) or let the fire draw air from the room and pull in new cold air through gaps or a cracked window? If the air in the room is being slowly replaced, then the temp will be lower, but the air fresher. If drawn from an isolated source via a feeder pipe, then won't the air in the room be stuffy?

Is this for a stove or open fire?

 

We have a stove with a closed combustion system ie it brings it's combustion air in from outside through a pipe out the back. Maybe it's the stove but cant say I am impressed but was the only legal option as it's in an open plan room including a kitchen with extractor fan. Much preferred the old Woodwarm that used the air from the room. 

[Stubby]

3 hours ago, onetruth said:

I don't have much experience with stoves, but if you've got a flue with an additional internal wall (most have), the outside will be much cooler than the inside, so it's probably not as efficient as it seems.

 

As for where you draw the air from, I've never seen stoves that draw in their own air from outside.  It sounds like an expensive installation and I can't imagine it would bring much benefit (not in our climate anyway).  I'd expect a very slight efficiency boost by using room-temperature air instead of cold air, but you should get your secondary burn whichever way.  You could always open a window for a bit if it gets too stuffy!

Its quite common on modern stoves to have a kit to draw cold air in from the outside . Most stoves over 5KW have to if there is no room vent . Its actually preferable to have cold air as its denser than warm air so contains more 02 for combustion ,

[Al Cormack]

Our wood burning boiler stove draws it's air from the outside via a vent and tube. No droughts in the sitting room. It also has a door that sides up inside, so can be like an open fire. Best of both worlds!

[Woodworks]

1 hour ago, Stubby said:

 Its actually preferable to have cold air as its denser than warm air so contains more 02 for combustion ,

I have read that but not convinced it's so good in reality. Cant see that freezing cold wet air is perfect for the fire and as I say our stove with it is fairly poor by modern standards IMO.

[Al Cormack]

Same as an engine. Colder air combusts better.

[Woodworks]

2 minutes ago, Al Cormack said:

Same as an engine. Colder air combusts better.

But if the secondly air does not get hot enough  before it enters the firebox it does not combust. It may be that our stove is not great anyway but our old Woodwarm which drew the air from the room was far better at getting that rolling flam above the logs.

[difflock]

Meh, the open fire in the bungalow I grew up in had an underfloor air supply, with a sunken ashbox as well.

The bungalow was built in the middle/late 1950's.

All our Morso stoves draw air from the room/house, though a 4" duct lurks beneath the recent garden room extension,

I tiled over the top of it and marked the tile with a drill bit, just in case.

It was not needed.

There is sufficient air leakage about our 1995 built dwelling to provide sufficient combustion air.

Burning this internally sourced air probably, hopefully, may also removes some of the doggy odours.

The 3 recently installed state-of-the-art CO alarms have never even whimpered.

P.S.

I recall a figure of 30% efficiency, for an open fire,

compared to say 75% for any decent stove.

I always intended to get a temperature reading from the  gases exiting the top of our flue, after they have travelled 10 to 12 m up through a clay lined, but vermiculite insulated traditional masonry built flue.

Be interesting. 

P.S.

Since I had, on several occassions, the 5' of single wall flue pipe directly above the Morso stove in the living room glowing dull cherry red, I therefore suspect our stoves, bytimes, or even mostly, run hotter than 250 deg Centi.

mth

Edited January 30, 2018 by difflock

[Ashbrooke]

6 hours ago, Haironyourchest said:

I just bought an infrared remote thermometer in Lidl. Been checking my Jotul stove, the firebox is like 250 degrees and the metal stovepipe neat the ceiling is about 35. So it appears most of the heat is being trapped in the house, which is good. Thing I wonder is: is it better to pipe the air into the stove from outside ("mushroom" inside firebox) or let the fire draw air from the room and pull in new cold air through gaps or a cracked window? If the air in the room is being slowly replaced, then the temp will be lower, but the air fresher. If drawn from an isolated source via a feeder pipe, then won't the air in the room be stuffy?

Are you sure the readers working right? 35 deg is really cold for a flue pipe. I've got a flue thermometer on mine that usually reads between 200-500deg

[Haironyourchest]

29 minutes ago, Ashbrooke said:

Are you sure the readers working right? 35 deg is really cold for a flue pipe. I've got a flue thermometer on mine that usually reads between 200-500deg

Yeah its cold alright. Just took some readings, all at the same distance, about two meters.

 

Stovetop: 196 degrees celcius

Top of pipe: 40 degrees

Own hand: 34 degrees.

[Ashbrooke]

9 minutes ago, Haironyourchest said:

Yeah its cold alright. Just took some readings, all at the same distance, about two meters.

 

Stovetop: 196 degrees celcius

Top of pipe: 40 degrees

Own hand: 34 degrees.

If it's a single wall pipe all the flue gasses will be condensing on the inside of the pipe. Have you got the fire going well or just smouldering?