[Stoves] Oversized stoves / thermal insulation

[ajheggie at gmail.com]

On Wednesday 03 November 2010 10:41:18 Boll, Martin Dr. wrote:
> Dear Crispin and Andrew,
>
>
>
> The problem of "oversized" stoves is rising in Germany. The new houses
> are built regarding low-energy standards, made by the government to
> bring down CO2 emission.( German key-words for different grades of
> energy-saving houses: Niedrigenergiehaus, Passivhaus, Nullenergiehaus).

I think we have much the same in UK

Low-energy building which tends to be fairly conventional but using 
materials with low embedded energy and good insulation. Often nowadays 
this means one is required to have fairly small windows or a novel means 
of heating as once you've got the walls and roof down to .3W/m2.degC 
there's not much more you can do and even triple glazed windows 
are .6W/m2/deg C. I'm all in favour of external insulated shutters.

Passivehaus must be one that is designed to collect solar heat or stored 
summer heat.

And zero energy development  is more of a wish. I have worked with people 
aspiring (and claiming) to build zero energy developments but they missed 
their target in my opinion. I still maintain a pellet boiler in a well 
insulated actively solar heated house and not only did they get their 
sums wrong they don't maintain the solar thermal array such that the 
boiler wore itself out in 5 years.


> Their main problems/difficulties/not-low-tech-solutions are
> condensing-water and the need of forced change of air with its
> heat-recuperation.

Yest once you have  high insulation then the heat carried away from the 
warm house in the air becomes the biggest loss. It needs careful control 
to prevent the atmosphere becoming stuffy. This is often done with whole 
house ventilation systems whereby the house is well sealed and the fans 
maintain the correct air changes to prevent stuffiness. A heat exchanger 
recuperates heat from outgoing air and warms the incoming cold air.

Once you seal a room and control air changes any cold bridges can lead to 
condensation, which is why it is necessary to have a vapour barrier 
between the room and the insulation, otherwise water vapour from cooking 
and breathing migrates through the insulation and condenses on to a cold 
surface, possibly leading to mould and rot. Exterior walls need to 
breathe also to allow atmospheric moisture to dissipate as the air 
temperature changes overnight.
>
>
>
> In that houses, the old "Kachelofen" with its high mass and slow
> radiation of heat does not fit, even it is worse, because of its good
> heat-radiation, which helps better in bad insulated old houses to keep
> down
> heating-energy-needs.

I've worked on tiled pellet stoves where the tiles act as storage heaters, 
those in North America will be familiar with the similar idea of masonry 
stoves, a quick hot efficient batch burn at full blast with the thermal 
mass of the chimney breast being designed to absorb heat just fast enough 
to ensure heat transfer but not so fast as to quench the flames and 
allowing the flue temperature to be just enough above boiling to prevent 
condensation and not so hot as to cause unnecessary flue heat loss. Our 
old fashioned bread ovens fired by faggots were similar.
>
>
>
> Many people like to have a stove apart from their central-heating. But
> the current commercial stoves, which are used in normal
> thermal-insulated houses have a normal working output ( Nennleistung )
> of at least 4kw.

Yes I can understand this, I have had the smallest Jotul stove for over 30 
years and without a means of moving heat around this old, poorly 
insulated house it gets uncomfortably hot near it and cold elsewhere.

> Naturally they can be choked, mostly with less 
> efficiency and less cleanliness.

Turn down ratio is an issue with stoves, also as you get smaller the 
surface area to volume ratio changes and the heat exchange surface is 
often the cast iron walls, so the smaller stove is actually difficult to 
keep the fire hot enough for a clean burn.

I'm not the fan of flue dampers that Crispin is, I prefer to maintain the 
maximum depression in the secondary combustion area but this does require 
good sealing to prevent excess air ingress. Maximum depression means the 
combustion air can have a good velocity for turbulence and mixing with 
the offgas from the primary combustion.


> The only commercial solutions for 
> those new houses seem to be pellet-stoves.

Not necessarily so, I believe Nat at worldstove has demonstrated one of 
his small burners run as a batch burn inside the body of a pellet stove 
with good results. 

I think adding an extra cast iron liner to a simple stove will also 
achieve results the other thing is to reduce the log cross section and of 
course use dry logs.
>
> I did not check how low they can be run, because pellets are not my
> favour, but I saw a pellet stove in working, burning only about some 5
> or 10 pellets at once.

Yes this is how pellet stoves work, in fact the smaller diameter the 
pellets the easier it is to optimise heat output because their is less 
mass per pellet and similar surface area for burning. The modern ones 
couple the fan speed to the feed auger and have secondary air fine tuned 
by a wideband lambda sensor in the flue. This means the biggest losses 
from earlier designs, which was often mandated excess air, are avoided.

My pellet stove probably has a 4 to 1 turn down but I have no means of 
testing how this affect the CO2:CO ratio, which should be a measure of 
good combustion.

> -By the way, Crispin, there are projects in Austria building
> well-insulated gers as well ( ger in German mostly called: Jurte ).
>
Yurt in UK and they have become fashionable at fairs, exhibitions and 
wedding receptions.

AJH