What size of woodburner?

[Zebra]

Any expertise and experience of those in the know would be appreciated.
I want to get a woodburner. The kilowatt calculators that you get on the websites of retailers are helpful, but are only based on the size of the room that the stove is in. I have a rather open plan house, with areas opening off the living room, and an open stairway to the bedroom, all of which I am hoping will benefit from the heat from the fire.
However, the room that the fire is in has a low ceiling and I don't want it to get so hot in there that I can't sit by the fire!
Working it out on the room alone, using a kilowatt calculator online, gives an output of 4kw.
But including the upstairs gives 8kw.
So I was thinking I'd get a Villager B at 8kw. But my sister advises me to get a really big one saying that hers is supposedly oversized for her room but doesn't really give out as much heat as she had expected.
The big Villager A is 14kw. Should I go for a really big one or would I be roast alive?

 

[Flyfisher]

In my limited experience, wood stove outputs are often less than people expect in practice because they don't like running them hard because they then burn the wood too fast for their liking!

A stove may be rated at a certain figure but that will usually be the maximum, based on the amount of wood it can hold at any one time. There's no need to run a stove at the maximum rate so if it's too hot then just close it down . . . except that may be less efficient and lead to smoky/tarry flue gases.

I find it useful to consider things from the perspective of the fuel itself. From trawling the web, wood seems to have a calorific value of about 3Kw/h per Kg, i.e completely burn 1kg of wood and you'll release about 3kW of heat. Do this in one hour and that's 3 kW/h.

This figure can be quite variable depending on the moisture content of the wood (the above figure is for about 40% moisture). The drier the wood the higher the effective calorific value because energy is not wasted boiling off the moisture. The type of wood makes very little difference to the calorific value of the wood when compared to the moisture content.

So to get,say, 4kWh from a stove you'll need to burn about 1.3 kg every hour.

To get 8kW will require about 2.7 kg every hour and 14kW will require about 4.7 kg every hour.

I'm afraid it's all a bit black art-ish so it's probably worth talking to your stove supplier as well as trawling through the mass of information on the web. This example might help:
http://www.biomassenergycentre.org.uk/pls/portal/docs/PAGE/RESOURCES/REF_LIB_RES/PUBLICATIONS/FC-BEC-INFOSHEET-WOOD-AS-FUEL.PDF

 

[MdB]

How well insulated is your house and is it supplemental heating or the only heating?

Our new (ish) stove is rated at 4.5kW which is low but we have a fairly well insulated house (which will be improved next year) though open plan and a heat pump (6kW) as our main heating. Everybody I spoke to advised on not getting too large a stove as you can end up over heating. Stoves also work best when operating at full blast and don't like being throttled back so go small rather than large within reason - this especially applies to the glass which I'm told blacks up quicker when you partially close the air vents. I'm not so sure of this myself but we have only had the stove going for a couple of weeks so I'm not an expert yet.

Our neighbours house on the other hand has a larger stove in a smaller room and the room was still cold this evening. This was partially due to the stove not being cleaned out for a while so air wasn't getting to it and partially due to the lack of insulation.

 

[biffvernon]

If you are sawing your own wood the most important metric is the length of log that will fit in.

 

[sidd]

As Flyfisher says, it is a very black art. Consider borrowing a few 2kw electric heaters and running them full tilt for a few hours to see what happens. Even the most controllable woodburners are very difficult to get right. Inevitably, there is a filling cycle, and the heat in the room is slow to respond.

In general, I totally agree about undersizing. Go for the smallest you can get away with and burn it hard. We have a Morso 04, which is 4kw, but because it has a lot of of exposed flue, it radiates a hell of a lot, and it is over 80% efficient, so it doesn't need much filling up. It's in a cold room about 4m x 5m x 2.5m It heats up the room fast, and keeps it very warm through the whole day.

We also have a 6kw Firebelly in a sitting room half the size. The room is the coldest in the house, so the job of the Firebelly is to warm it up fast in the evening for a few hours. The firebelly was previously in the drawing room, (6x6.5x3.5), and even running absolutely full tilt chewing an enormous quantity of wood, it was unable to get the room to a decent temperature in the depth of winter with no heating. Now the drawing room has background heating, and a Jetmaster tops it up and makes it toasty.

If the stove needs to warm up a room fast, then you need a bigger one, but if it is going to be running all day, then maintenance heat is all you need, and you had better get a smaller stove and sacrifice warm-up time.

Another reason to undersize is that the calculations usually assume you have no background central heating.

Size it for normal conditions in the winter, the way you will usually be running it. Then if it gets much colder, you can put on a coat or an electric heater, without wasting efficiency the 90% of the time the weather is not extreme.

But above all, go for a very efficient stove, it saves so much hassle and wood.

As an aside, I built the Morso up on a timber plinth in the corner with a slab of slate on the top, partly to get over the power and network ports there, partly to store wood underneath, partly to radiate from a higher level in the room, and partly to avoid bending down so much. Overall, we are incredibly happy and warm with it

 

[biffvernon]

Another reason to undersize is that the calculations usually assume you have no background central heating.

Our one IS the central heating - it has a backboiler that does our hot water and radiators. And it takes logs 50cm long.

 

[Flyfisher]

Nice stove Biff. I'm also planning for a stove with a back boiler and to link it all to a 'heat store', with radiators in other rooms fed from the heat store - the idea being to be able to operate the stove at maximum efficiency and store the excess heat for later use, e.g. the mornings. The heat store will also have additional coils to allow for possible future solar thermal heating and maybe (sorry!) even a gas boiler.

Trouble is, I'm finding it very hard to find someone local who can design and install such a system.

 

[Penners]

If you are sawing your own wood the most important metric is the length of log that will fit in.

Not sure I understand that observation. If you're sawing your own wood, don't you simply saw the correct length of log for the stove that you have?

 

[Flyfisher]

:lol: :lol: :lol:
And you, Sir, owe me a new keyboard because I had just taken a mouthful of tea before I read that.

 

[biffvernon]

If you are sawing your own wood the most important metric is the length of log that will fit in.

Not sure I understand that observation. If you're sawing your own wood, don't you simply saw the correct length of log for the stove that you have?

The OP was looking for advice about choosing a stove. Others discussed the theoretical output of stoves and sizes of rooms. My point is that, in the circumstance where you are sawing your own logs rather than buying ready cut up wood, the length of log the stove can take is a very important factor in choosing a new stove. My stove can take logs 50cm long, for which I feel grateful when sawing. If your stove only took short logs you would have to do more work. Of course in your position, Penners, it is all irrelevant because your man would just do it for you.

 

[Penners]

My stove can take logs 50cm long, for which I feel grateful when sawing. If your stove only took short logs you would have to do more work. Of course in your position, Penners, it is all irrelevant because your man would just do it for you.

But of course. I leave all that kind of thing to Lady Penners - she instructs the gamekeeper who cuts the logs (this briefing must be very complicated, because it seems to take a considerable amount of time).

 

[Feltwell]

Lady P is so lucky to have the services of a young man who knows how to wield his chopper.

(Sorry, couldn't resist).

 

[philpjuk100]

If you are sawing your own wood the most important metric is the length of log that will fit in.

Not sure I understand that observation. If you're sawing your own wood, don't you simply saw the correct length of log for the stove that you have?

The OP was looking for advice about choosing a stove. Others discussed the theoretical output of stoves and sizes of rooms. My point is that, in the circumstance where you are sawing your own logs rather than buying ready cut up wood, the length of log the stove can take is a very important factor in choosing a new stove. My stove can take logs 50cm long, for which I feel grateful when sawing. If your stove only took short logs you would have to do more work. Of course in your position, Penners, it is all irrelevant because your man would just do it for you.

Confucious say man who chops logs for fire warms himself twice!

 

[Zebra]

Sorry chaps, not checked for replies, been building reclaimed slate hearth ready to install fire! Damned hard work! So, a smaller one and burn it harder perhaps. At the moment the house is completely uninsulated although as time goes by I will improve this. There is also no central heating. I was going to go for a Villager BH - 8kW but with a high canopy so it might not look pathetic in the huge inglenook.
I hope this will be OK. The room is 4m x 7m with a low ceiling, but as I mentioned, I'm hoping that the heat will also make its way to other areas of the house. Thanks for your advice. As I'm hoping to buy a reconditioned one, I'm not getting advice from retailers. There are so many stoves for sale on ebay where people are selling them because they're too big/too small, I hope to get this right first time!

 

[Penners]

Zebra, there's one point about woodburners that I don't think we've covered yet. It's the matter of flue temperature. If you run a woodburner well below its capability, the flue gases don't get the flue liner hot enough. This can result in combustion products condensing on the inside of the liner and forming acidic tars which can eat through the liner - sometimes surprisingly quickly.

The trick is to ensure that the flue gets hot enough for the gases to escape from the top before they cool enough to cause this condensation.

So, you need to ensure three things. Firstly that the stove itself is correctly specified for the area it needs to heat. Secondly that it is run at a good optimum temperature for maintaining hot flue gases - you can get little gauges to clip to your flue pipe which show the correct temperature range. Thirdly that the flue pipe is well insulated, so the gases remain hot all the way to the top. This is normally done by (a) using a pre-insulated double-skin stainless steel flue liner (probably the best but most expensive solution), (b) using a good quality flexible liner, wrapped with an insulating sleeve, or (c) using a rigid or flexible liner inside your chimney, and then filling the void between the liner and the chimney itself with vermiculite. Needless to say, this requires a register plate to be fitted inside the chimney, just above the fireplace (which is actually a good plan whichever method you use - it may be a requirement of the regulations, but I'm not sure).

 

[Zebra]

Oh I see, so this would be one of the reasons that it's better to run a small one at full blast than a big one half full - to avoid a build up of tar deposits. But would the flue temperature be governed more by the intensity of the fire you burn, or the size of the stove you buy? I mean, would the flue temperature of a 7kW stove at full blast be more than the flue temperature of a 14kw stove running half full?
I plan to do option c, to fill the chimney with vermiculite, because the chimney forms about a quarter of one wall of my bedroom, so I reckon it would be a good thing for it to be full of insulation, to help prevent heat loss from the building.

 

[lou]

Thank you so much, Penners. That's the most helpful piece of information I have read during my tedious hours of research on woodburners. As you are obviously the fount of knowledge on this subject, would you be kind enough to offer me some advice? Our house is stone and will have, any day now..., underfloor heating under flagstones. We want a woodburner both to top up when it gets very cold, but also to be a point of focus in our very large stone fireplace. The chimney place is very large so I would like a large sized burner to take up the space, but am concerned that it will be too hot with the UFH. I suspect I want the impossible. I've seen the Dovre and Jotul stoves that you can use open as well as closed though they are a bit expensive - given that the renovation has guzzled much more money than we expected.

I'd be very grateful (ha ha) for any advice.

 

[MdB]

underfloor heating under flagstones. We want a woodburner both to top up when it gets very cold, but also to be a point of focus in our very large stone fireplace.

That was our situation too. We put some granite in the bottom and then fitted a small stove. It seems to work for us and I wouldn't want anything larger. Opening was originally 1.6m x 0.9m

 

[Penners]

would the flue temperature be governed more by the intensity of the fire you burn, or the size of the stove you buy? I mean, would the flue temperature of a 7kW stove at full blast be more than the flue temperature of a 14kw stove running half full?

That's a fair point, Zebra, and one that I can't really answer with any degree of knowledge. However, my gut feeling suggests that the combustion products from a big stove running "slowly" will tend to mix with more ambient air as they are drawn up the flue, hence they will tend to cool more quickly.

But please seek professional advice on this... in fact I do recommend that you discuss your needs with a proper HETAS registered stove supplier before you make any final decision on your choice of stove. Here's one (of whom I have no personal experience at all):

Signature Fires
11 Page Place
Frogmore
St Albans
Hertfordshire
AL2 2FB
Tel: 01727 874695

 

[Penners]

We want a woodburner both to top up when it gets very cold, but also to be a point of focus in our very large stone fireplace. The chimney place is very large so I would like a large sized burner to take up the space, but am concerned that it will be too hot with the UFH.

For goodness' sake don't regard me as any kind of expert, lou! I'm just someone who's had several woodburners and has tried to understand vaguely the principles involved. I should perhaps add, however, that I've never had underfloor heating!

In the case of your proposed setup, the critical question seems to me to be the degree to which your UFH is thermostatically controlled. If the room in which the woodburning stove is located is a discrete "zone" in the UFH system, controlled by its own thermostat, then presumably once the woodburner gives sufficient heat into the room the thermostat will become satisfied and will shut off that UFH zone. All you have to do is to adjust the thermostat to the point where you find the room temperature remains comfortable, regardless of whether the heat is coming from the UFH or the stove.

Maybe others will have this UFH/woodburner combination and will be able to give you a more expert view.

Are you there, MdB?