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Author Topic: Quick Cost Comparison: Wood vs Heating Oil  (Read 13791 times)
dan_c
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« on: April 18, 2012, 04:36:17 PM »

Afternoon all,

I am in the process of deciding exactly what I am going to do to replace/supplement my current oil fired combi setup and thought I'd do a quick cost comparison of wood vs oil in terms of fuel costs...if the worst came to the worst and I had to buy all of my logs "retail". My plan is to get off oil as much as possible and make the move to a solar and wood burning setup (I'm desperate to get a Wallnoefer Walltherm in my kitchen) Smiley

After a quick think I did a very simple spreadsheet to compare fuel costs to see if my desire to go greener has the potential to bankrupt me even further than I will already be after investing in all the initial purchases. This ignores things like capital cost, losses, maintenance etc.

Bottom line is that the wood looks about 18% cheaper than oil based on the prices I could buy both at today. (approx £0.65 per litre of oil or £140 per cubic metre of hardwood logs when spending approx £300-400 per "refuel").

I assume other people have done this sort of thing so I am not thinking I've done anything clever here - I just wanted to see if anyone can point out any errors where I'm way off in terms of calorific values etc.

Thanks
Dan


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dhaslam
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« Reply #1 on: April 18, 2012, 05:14:38 PM »

The wood stove is probably not as efficient as you estimate.    I was getting about 7 kW  to water from mine  when I tested first but that ws with fairly fresh ash.     I have since got some beech and  larger  pieces of ash which seem to burn better but is still too wet, over 30% moisture,  to test properly.  It may reach an output to water of  10 kW with good dry wood.  I am presently setting upthe fuel shed to dry the wood with warm  solar heated air and to also draw warm air  from the stove  through the inside store into the heat recovery system .   I think that the chimney draught may be on the low side, it seems to work better on windy days.     It might be wort lining and insulating the chimney at some stage.

Where you may make savings is on the wood price.    I just bought a full lorry load recently that was nominally 12 cubic metres for €680 and I think that will do the full winter  but I also get some pallets free that  are good for lighting the fire.     The big wood  sellers have machines to cut and  chop wood so  they can do big volumes quite cheaply.    The same supplier can  deliver an artic full for about €1200  that would  do for  five years if I could  store it.



The big difference from an oil boiler is that  you have a fire to look at.       


* Walltherm.jpg (31.78 KB, 491x900 - viewed 3054 times.)
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DHW 250 litre cylinder  60 X 47mm tubes
Heating  180,000 litre straw insulated seasonal store, 90X58mm tubes + 7 sqm flat collectors, 1 kW VAWT, 3 kW heatpump plus Walltherm gasifying stove
billt
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« Reply #2 on: April 18, 2012, 05:36:33 PM »

That looks as good a guess as anyones. http://www.nottenergy.com/energy_cost_comparison/ gives 5.8p per unit net seasoned wood and 7.47p per unit net for kerosene which is in the same ball park. There are too many variables to be precise.

Having moved from oil to wood/solar, I would think very hard about moving from oil if you have to buy in wood. The savings are small, but moving wood, firing the boiler and cleaning up the mess does become a chore. It's not a huge commitment, but it's a lot more than picking up the phone to order oil and let the heating sort itself out.
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dan_c
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« Reply #3 on: April 18, 2012, 05:50:36 PM »

Thanks for the replies - this is just the sort of feedback I was after.

I suppose my wood costs are quite expensive and some sort of economy of scale could easily be achieved as I do have more space for storage - not quite an artic but certainly something like 12cubic metres would be possible.

At the moment I have no problems sourcing wood for free (or in some cases for donations to parish councils etc) and I enjoy the chopping and storing and sorting of it. (Sounds a bit sad really!). I wanted to get a feel for what would happen if I wasn't able to get wood or process it any more so basically some sort of worst case comparison.

I have a 7kW Westfire Two stove in my living room and we love it. It replaced a horrible old open fire and I am so glad we did it. I am now at the point where I need to make some sensible plans related to my 16 year old Worcester combi. The easiest option would be to replace it with another combi and call it a day at that. However, we will be fittting another stove in the kitchen anyway and I am thinking that this is my opportunity to make a change and go for a thermal store type installation. This opens up my options for solar and wood contributing to the overall heating and DHW needs and will drastically reduce the amount of work the aged combi has to do.

Whatever happens we will need to keep some sort of oil fired boiler for times when I am am away and we need some automation or the wife can't be bothered to faff about with the wood boiler stove.
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dan_c
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« Reply #4 on: April 18, 2012, 09:30:16 PM »

The wood stove is probably not as efficient as you estimate.    I was getting about 7 kW  to water from mine  when I tested first but that ws with fairly fresh ash.     I have since got some beech and  larger  pieces of ash which seem to burn better but is still too wet, over 30% moisture,  to test properly.  It may reach an output to water of  10 kW with good dry wood.  I am presently setting upthe fuel shed to dry the wood with warm  solar heated air and to also draw warm air  from the stove  through the inside store into the heat recovery system .   I think that the chimney draught may be on the low side, it seems to work better on windy days.     It might be wort lining and insulating the chimney at some stage.
       

I kind of passed by your feedback on efficiency - sorry. Very good points, I was actually basing my 85% on the Walltherm but point taken that I may be a bit too optimistic there.

Which Walltherm do you have - is it the "insulated" version or the normal? I think the normal is supposed to put 10-11kW to the water isn't it?

When do you notice the chimney lacking in draw, is it when you are running in downdraught mode or just most of the time?

Could you post up any more photographs please?  Tongue
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Bodidly
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« Reply #5 on: April 18, 2012, 10:08:38 PM »

Hi Dan

We sell logs and I think your figures look pretty accurate. It might also be worth looking at pellet boilers as pellets are about £210 per ton.
When the weather is cold we get through about 20kg of logs per day to heat our house (around 300 m3 fairly well insulated)
A few of our our customers are using wood burners instead of their oil and they have said this does save them a bit.

Hope this helps
Beau
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acresswell
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« Reply #6 on: April 19, 2012, 06:07:49 AM »

We're working from a blank canvas rather than any experience of oil, but I'd echo Bo's suggestion to look at pellets.

We have our own woodland to provide us with logs, only requiring labour.... so the obvious first thought was to go log gasification. However, we're now actively looking at pellets precisely because it then offers automation without the need for a second boiler to do so.  I shall continue to harvest logs (though maybe not quite so energetically) and will use them in our stove (both for the attractive flame feature, and to add heat to the house which will reduce the boiler's consumption of pellets).   Any excess logs should be pretty easy to sell, off-setting the cost of the pellets. A side benefit is that a pellet boiler needs a much smaller accumulator tank (some installers reckon you can manage without one, but the manufacturers' small print usually says that one is needed and other installers certainly say that things are more efficient with one) and that will free up some space inside the house. We do have the advantage that our house will be very well insulated and therefore the heat demand will be low (could heat the whole house with only around 2 tonnes of pellets per year) so even though bagged pellets are more expensive than bulk I probably won't spend money on a sil.o

Whatever happens we will need to keep some sort of oil fired boiler for times when I am am away and we need some automation or the wife can't be bothered to faff about with the wood boiler stove.
I've got a similar situation in that if stick with log gasification I'll need to install a 2nd boiler (mains gas in our case) to keep my wife happy.  I'm very tempted by combined log/pellet boilers such as http://www.thewoodheatingcompany.co.uk/store/biomass_boilers/smoke_control_(defra)_approved/solarfocus_therminator_ii_log_and_pellet_22-60kw
which would provide the ability to use as many logs as are available and can be loaded, but also top up with pellets when needed. If I'm going to have to buy fuel in, I'd rather have pellets which are more energy-dense.
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renewablejohn
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« Reply #7 on: April 19, 2012, 09:08:57 AM »

We have a big old draughty farmhouse which used to have oil for central heating and an oil Aga for cooking. We are now all wood with a Dunsley Yorkshire replacing the central heating and an Esse replacing the Aga although we have been disappointed with the Esse and will be replacing it shortly with a Lohberger combined pellet and log burner. We also have a Dunsley Highlander in the snug as a focal point.
As for your figures we sell solar kiln dried split logs at £100 per m3 so I think your £140 is a bit over the top.
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brackwell
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« Reply #8 on: April 19, 2012, 09:23:35 AM »

Is the m3 used in fuel density the same as the m3 used in cost of ?   The m3 in cost of is perhaps 50% air !

When i read about all this stove activity and cost and then 20m3/day and the a wheel barrow a day and lorry loads of logs  -  have you guys not heard of insulation? or if things are that bad moving house!
Ken
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renewablejohn
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« Reply #9 on: April 19, 2012, 09:50:57 AM »

Is the m3 used in fuel density the same as the m3 used in cost of ?   The m3 in cost of is perhaps 50% air !

When i read about all this stove activity and cost and then 20m3/day and the a wheel barrow a day and lorry loads of logs  -  have you guys not heard of insulation? or if things are that bad moving house!
Ken

Ken

We live in a grade 2 listed house and have put 600mm insulation in the roof which has made a big difference. Walls are over 2 foot thick and not a lot we can do about them. The big battle is windows which are only single glaze but we have given detailed plans to the conservation officer for triple glaze units directly glazed into the stone mullions of which we have one original as an example but they refuse to give permission. Two other listed buildings in the area have already had there windows done in this manner and unless permission is given soon we will take english heritage advice and install anyway as a like for like replacement which does not need there permission.
As for wood usage we get through 2 boxes per day 600mmx400mmx300mm unless its below freezing in which case we might end up using 4 boxes a day. Very little wood really but all our wood is guaranteed less than 20% moisture content so burns very efficiently.
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billi
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« Reply #10 on: April 19, 2012, 10:12:27 AM »

Quote
The big difference from an oil boiler is that  you have a fire to look at.   


I am jealous   Dhaslam


But happy that the Walltherm stove is in good hands
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dan_c
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« Reply #11 on: April 19, 2012, 10:39:23 AM »

We have a big old draughty farmhouse which used to have oil for central heating and an oil Aga for cooking. We are now all wood with a Dunsley Yorkshire replacing the central heating and an Esse replacing the Aga although we have been disappointed with the Esse and will be replacing it shortly with a Lohberger combined pellet and log burner. We also have a Dunsley Highlander in the snug as a focal point.
As for your figures we sell solar kiln dried split logs at £100 per m3 so I think your £140 is a bit over the top.

Is that in a bag or a stack John?

I should have explained my pricing and volume calcs a little better.......My price was worked out to be the equivalent of a 100% solid fill block of wood. I used prices I could find for neatly stacked logs and then assumed those stacks were only 80% wood. The "as stacked" kiln dried logs I was using as a guide were as per the picture attached - and would cost the equivalent of £112 as a single cubic stack. I hope that makes sense.

Is the m3 used in fuel density the same as the m3 used in cost of ?   The m3 in cost of is perhaps 50% air !

When i read about all this stove activity and cost and then 20m3/day and the a wheel barrow a day and lorry loads of logs  -  have you guys not heard of insulation? or if things are that bad moving house!
Ken

Ken - I didn't explain my volume and costs too well, sorry. I had already allowed for air volume in the pricing. This was an estimate though and I'm not quite sure if 80% is a good number to assume for a neat stack.

The 15kW I used was an arbitrary figure – I’d have been better using 1kW to get a comparable unit cost.

My house is insulated in the attic, and the wall construction is brick, 2” open cavity, 5” turbo block wall, dry lined and then skimmed. I cannot fill the cavities (or more like I haven’t decided if I should go with the minority advice) as the outer brick skin has a recessed joint – on the upside, in terms of water ingress,  the brick is solid with no holes or “frog”.


« Last Edit: April 19, 2012, 12:29:59 PM by dan_c » Logged
renewablejohn
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« Reply #12 on: April 19, 2012, 11:30:56 AM »

Dan c

My m3 is actually IBC containers therefore the m3 is accurate but is lose fill so air is likely to be 50%. In your stacked example air will probably be as low as 10%.
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dhaslam
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« Reply #13 on: April 20, 2012, 12:44:15 PM »

Which Walltherm do you have - is it the "insulated" version or the normal? I think the normal is supposed to put 10-11kW to the water isn't it?
When do you notice the chimney lacking in draw, is it when you are running in downdraught mode or just most of the time?

It is the insulated version and  it is supposed to output about  12 kW to water and 3 kW to the room.    It does make the room very warm, some of the  stove surface, particiularly the upper part of the sides,   is quite hot, over 100C,   and  the flue pipe is normally over 100C as well      The draught is fine when the stove runs in normal mode, when warming up,   and even in that mode it gets the circulation pump started at about 60C water temperature.    After switching to sasification mode the fire can slow up for a while.  It is partly the cooling effect of the large  water jacket at the back but the chimney probably has a bearing as well.      The last few days had strong winds from the south and that  does increase the draught a lot with the flue temperature  goint to about 150C constantly instead of  the more usual 130C.    The lack of draught becomes more obvious when the flues at the back of the stove need cleaning.   The glass blackens a little  and it is much harder to get the stove to gasify when starting up.      The flues are quite hard to clean because the brush  is quite stiff .  It  needs to be  pushed to the bottom  in order  to change the bristles direction to pull it out again.     They need cleaning about every month in winter and less often in milder weather.

Using about 12 cubic metres of wood each year is a bit more than I expected.   The first few hours burning goes mainly to heating DHW.   The transfer is  regulated by a difference controller but at present the  DHW coil  returns to the bottom of the buffer tank so  it slowly warms the bottom of the buffer tank as well.  I am going to change the  return from  the DHW coil to the second row of connections on the tank.  That will return the fairly hot water to about one third way down so   it will heat the DHW tank quicker and then  have a smaller amount  of hotter water for underfloor heating as well.  


The way that the  plumbing is set up for the stove is not ideal.   The outlets are all on the right hand side of the stove and the  pipes to the buffer tank  are on the left.  Also because the outlet pipe exits so high up  the pump station  would be very high on the wall if the pipes were set up for gravity feed.   As  the photograph shows the outlet from the stove  goes under the return pipe to the pump station.     The smaller copper pipes on the left are for the quench coil, the water for the quench coil comes down from upstairs and back up to exit at the eaves.      I think that the gravity flow would probably    stay going if the pump is stopped provided it is already circulating.        


* Plumbing.jpg (37.73 KB, 464x900 - viewed 2706 times.)

* PumpStation.jpg (31.24 KB, 559x900 - viewed 2560 times.)
« Last Edit: April 20, 2012, 12:46:58 PM by dhaslam » Logged

DHW 250 litre cylinder  60 X 47mm tubes
Heating  180,000 litre straw insulated seasonal store, 90X58mm tubes + 7 sqm flat collectors, 1 kW VAWT, 3 kW heatpump plus Walltherm gasifying stove
dan_c
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« Reply #14 on: April 27, 2012, 12:02:25 PM »

Thanks for the photos and info dhaslam - much appreciated. I do like the functional look of the Walltherm. Smiley

In a previous thread about it (http://www.navitron.org.uk/forum/index.php/topic,14757.0.html) you said it takes about six hrs to get your 860litre tank from 20-60degrees C with no load on the cylinder. Is that still the case or have things changed at all with drier wood or more food in the firebox?

Are you still putting up with the 60degree C return temperature or have you had some luck finding out whether it should be able to go higher?

You also mentioned something about the flow from the stove being around 60degrees C as well - are you able to increase that much and if so, will the return temp increase accordingly?

Thanks, and sorry for the raft of questions!  Lips Sealed
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