Efficient burning

[Stereo]

Not sure where to put this really so I put it here. It's something I have wondered about for years.

 

Say I have a wood stove with a back boiler. We know that wood has stored energy which is released when burnt. Hopefully the stove sends as much as possible to the room / boiler and as little as possible up the chimney.

 

So, I have 20kg of dry ash. Is it better to get a nice fire going in the base of the stove and chuck a log on every now and again? Or is it better to fully load the thing and burn down to embers and then start again? Which method gets more heat to the water / air?

 

The time seems to be the same. If I half load my Esse it takes more or less the same time to combust all the fuel as if I fully load it. If I do 2 half loads, will I send more or less heat to the water than 1 full load?

 

I understand that there are many variables but generally, anyone clued up on this stuff?

[RichardS]

Get a stove pipe thermometer and maintain constant temperature. You'll waste a lot of energy reheating the cooled stove if you burn to embers.

[Stereo]

OK, lets not say embers, lets say a few 80% burnt logs to the point where new fuel will immediately catch without the need to open vents etc.

[RichardS]

This is mine as we speak. Glowing as you suggested but at the bottom end of what the stove pipe thermometer suggests is efficient. Time for more wood!

 

 

Image isn't showing. Here's link

 

https://www.flickr.com/photos/richardsanderson/16147885501/sizes/m/

Edited December 30, 2014 by RichardS
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[NoRush]

Just a suggestion. I wouldn't store wood that close to your fire it could cause trouble. Just had a customer who's wooden mantle caught above he fire catch fire when she was asleep. Luckily he husband came home to discover it

[RichardS]

Don't worry, It's not left unattended like that - just trying to dry it out whilst I'm in all day.

[teepeeat]

Not sure where to put this really so I put it here. It's something I have wondered about for years.

 

Say I have a wood stove with a back boiler. We know that wood has stored energy which is released when burnt. Hopefully the stove sends as much as possible to the room / boiler and as little as possible up the chimney.

 

So, I have 20kg of dry ash. Is it better to get a nice fire going in the base of the stove and chuck a log on every now and again? Or is it better to fully load the thing and burn down to embers and then start again? Which method gets more heat to the water / air?

 

The time seems to be the same. If I half load my Esse it takes more or less the same time to combust all the fuel as if I fully load it. If I do 2 half loads, will I send more or less heat to the water than 1 full load?

 

I understand that there are many variables but generally, anyone clued up on this stuff?

 

Interesting one Stereo

I have a supposedly high efficiency 8kw German unit that is designed to give 70% of the heat to the boiler and 30% to the room.

Their manual - from hazy memory - recommends using relatively small, well seasoned logs with a total of I think 2,5kg per load. The object of the exercise supposedly to burn hard and fast for maximum efficiency. Clearly this works best with a thermal store to store the energy.

I do tend to split fairly small logs anyway as this gives more surface area for seasoning as well as for improved burning.

I dont tend to weigh the wood, but did experiment in the early days and 2,5kg only filled the box about 40% which ties in with your half load.

From observing the thermal store temperatures it seems that the half load burn maintains the best temperature vs wood consumption although I find that I do need to load slightly more often when compared to a full load.

 

No doubt the Germans have tested it to the n'th degree to come up with their recommendations and works for me.

[Stereo]

Thanks Teepeeat. That's kind of what I do as well. I suspect that will a full charge, the machine can't get all the heat into the boiler and the burn is certainly not twice as long as a half load. I think the main thing is to get a good hot burn going rather than a heap of smoldering damp logs just to make you feel better. I know when my Esse is working right as I open the door and the whole charge is glowing almost white hot, like the fires of hades. Then you know you are getting good combustion.

[cornish wood burner]

I would think similar principles applies as in a larger chip boiler. A constant fuel supply gives constant O2, constant exhaust temp and a constant heat output. Fuel input every 10 seconds for maximum output. Obviously your stove will have bigger gaps between stoking but little and often seems the right way to me.

[teepeeat]

I think that stoking the fire is always a compromise as opening up and reloading will always affect the temps/balance of what is going on, but is of course a necessity. The more wood you put in at any one time, the greater the effect - ie there is more wood to get up to temperature, but when it does there will be a lot of energy so I am guessing there will be greater fluctuations and extremes in Big J's case.

 

Cornish Wood Burner's chip boiler I guess relies on automatic fuel feeding, but would indicate a little and often approach where the manufacturers of my stove have moderated that approach for practical reasons related to manual feeding of logs.

 

As Stereo has said, and my experience backs up, a half load burns for almost as long as a full load, meaning the extra energy is just going up the flue.

Having a boiler attached, I guess there is also the rate of transfer of energy to consider, which will have limits and excess energy again just flies by up the flue.

My stove is insulated to the extent that I can put my hand on the side when it is roaring away, so burning more does not translate to more heat to the room to any large degree as the only transfer to the room is via the door, but then it is designed to put the majority of the heat to the boiler.

 

The OP was looking at achieving fuel efficiency, This is going to vary with different set ups, however I think the general trend for efficiency is to feed little and often depending on your setup as regards heat transfer to a boiler or the room.

 

For the record Big J, my 8kw stove does the hot water and heating for a 200m2 house. It is located in the upstairs open plan living area and the heating in that space has never to my knowledge kicked in, although this is more to do with the levels of insulation we have. I can burn for an hour in the evening and then come upstairs the next morning and it will still be 18-20 deg C in the space, although this is helped by the underfloor heating from the bedrooms down stairs migrating up. Bedroom thermostats set around 15-16 deg C normally.

Edited January 1, 2015 by teepeeat