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Hello folks

Got a bit of a dillema which i can't seem to make a decision on:

I'm currently building a house, very slowly - and am wanting to put in a WBS; ideally this winter so i can keep it warm when working in there. It's not passivhause, but is to (or exceeding) current LABC so 4" celotex in walls*, 4" celotex +2" EPS in floor (had 2" spare so filled it with insulation) and will have 7"+ of celotex in roof when 2nd layer is installed - currently 4" between beams. Triple glazing.
*actually it's not 4" celotex, but 6" or dense rockwool due to celotex being unavailable when the walls were going up, but it's about the same Insulation level.

I have two WBS 2nd hand sitting around: Esse Ironheat which i reckon is 7.3kw & 82% efficient (burning wood) and an Dovre 500 which is apparently 8kw.

When i've talked to installers they say these are 'far too big' for a newbuild house and recommend 5kw instead (incidentally pointing out that a >5kw stove needs a vent but 5kw or less doesn't).

My thinking is that a stove doesn't have to be run 'flat out' , although i accept there must be a limit to how low you can run it and still maintain a clean burn, and that if a 5kw stove isn't too big then a 7.3 used 'lightly' should be OK. I was also considering the possibility of reducing the firebox size of the esse by adding more firebrick/vermiculite (although that's going down the route of 'modification' which might be frowned upon). If i installed the esse, the flue would be 6", straight up, with no bends and 5m + the cowl to get it to 600mm above to ridge.

my experience of WBS makes me think that a good insulated flue with a good draw and dry wood are more important to a clean burn than the size of the stove, but i stand to be corrected...

I’ve read that reducing the firebox size with additional firebricks is ‘acceptable’.

For me, the important thing is to have a good clean burn - just enough oxygen to burn efficiently without too much heat disappearing up the flue - but enough to keep the flue working efficiently.

Good quality, dry wood and an insulated flue would support this.

Think your proposal sounds very reasonable

OK, good, one vote in my favour , although I still may have to find an installer who's happy to fit it and sign off, eventually.

It was only a decade or so ago people were really buying ironhearts & the likes, commanding high prices on ebay, lately not much so.
Maybe seek out an older guy / shop owner & talk to them about it, you might get somewhere based on it being somewhat of a longstanding classic, explain what you wish to do with the firebox reduction, & maybe speak to ESSE themselves, they know their network of dealers & fitters i'm sure.

good ideas, thanks.

Hi Marcus, glad you found us.

I have a duel fuel Stovax and they do produce fire bricks to reduce its size when burning coal and could obviously be used for reducing the size for wood.
I think this would be a very sensible idea for yourself.

Heat output is largely controlled by the surface area available to burn and available air (and of course good dry wood). It is unlikely that you can reduce the area of wood and air enough whilst maintaining effecient high temp burning without reducing its firebox size.
Fire brick material in sheet form is not expensive (see Mole Valley for eg) so go for a bit of that see what you have and go from there.

Cant comment on the stoves but if they lined, good sealing, large doors and made from welded sheet then cannot see how it can make any difference.

I am concerned you might make too good a job of building the house and too air tight for a wood burner ? regardless of H+S. Could they be fed from outside air? Too air tight and it could be difficult to burn and even reverse flow in the wrong wind conditions.

PS Immersun working perfectly.

Ken

Hi Marcus,

As you are building new and will be subject to building inspection, you've got to do it right whichever way you go? We installed a WBS 1 year ago and it took a while to find a fitter to work with us.

A lot of companies we contacted were very hard and fast to the rules with no margin to shade into grey. We evntually found an acreditted company who were happy to work with us for the look and position we wanted. Best advise I can give is try many companies. Are your stoves roomsealed? If not and on a tight house I agree with Ken, you really need to be either roomsealed or air brick. Airbrick isn't going to be the way you want to go on your lovely new tight home so i'd advise a roomsealed stove of 5kWh or slightly less and draw the air from subfloor allowing the perimiter airbricks to be your multiple supplies of fresh air. I hope one of your stoves has the capacity to be roomsealed and you can drop the box size with insulating material. All the best on the build.

Hi Marcus,

We have a Dovre 500 (8kW) in the lounge and the smaller 5kW in the dinning room (kids play room these days). The 500 warms up the living room (approx 7m long 3.4m wide) to 20 degrees in about an hour to an hour and a half (starting temperature 17.5 degrees today IIRC) The house is solid stone walls 2.5 ft thick 100mm EWI the two doors to the room are never shut and warm air rises through the house to a degree while colder air from upstairs can be felt rolling over the stairs down across the floor of the lounge to the stove, aided and abetted by the front door cap flap

Living here has taught us to have upstairs windows on vent to avoid the dreaded black mould and even with the windows shut the cottage can best be described as leaky despite years of closing up draughts and such in walls floors etc etc. It was literally more hole than wall when we first moved in !
So with this in mind I would say the Dovre 500 would be far too much in a tight well sealed home unless of course you enjoy Saharan temperatures?

The 5kW fire box measures approx 100mm less in width but depth and height are roughly the same - to the best of my knowledge you cant plumb the 5kW into be room sealed. This smaller fire is often lit early spring and late autumn instead of the main fire to warm the house without overheating and its a great little fire that kicks out the heat but it runs best when run hard so again it may not lend itself well to modification.

Rather than reducing the firebox have you considered adding thermal mass as Joeboy has done ? a short hard burn will not then overheat the room but would soak the thermal mass and release the heat over a longer period while allowing the stove to operate un- modified and at its optimum settings ?

Moxi

I'm also building a new house and will be interested to see how you get on.

I would have liked to take my Brosley Hercules 30 boiler stove to it but just don't think its possible for a number of reasons.

Firstly, it's going to be too big. I'm told I should only need a heat pump of about 6kw to heat the house. (I'm not sure if that included DHW).
Secondly, it's not sealed and for a new house, which I'm aiming to be passive house, but not bothering with certification, that's a requirement.

I would like a boiler stove as i'd like it to do the DHW in the winter and to do towel rails in the bathrooms. I have a Dunsley Yorkshire stove with the sealed air kit but to be honest I don't like the look of it and the air kit is a pretty poor attempt at sealing it. My wife also doesn't like it!

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