Another woodburner question

Ok, we have to replace an existing woodburner and have some ideas around space, but is there a formula for calculating the heat loss (kw of heat) between a dry model and a wetback model? The house is a difficult one to heat as it's got rooms in strange places, but is quite well insulated. Also it's split level, so some rooms are destined to remain cool in the winter.
For instance, the Wagener butler multi, is a able to burn wood and coal. Estimated output is 14kw in the dry model, and 6 kw for water and 8 for heat in the wetback model. Magnum P300 (wood only) on the other hand is rated at an estimate of 19kw, but they don't give figures for the fall in heat if a wetback is fitted.
So, I wonder how to convert easily so as to compare other models.

There is no set formula - it depends on the model. If the heating coil is in the back of the fire chamber then at least 50% of the heat are used for the wet back.

If the coil is above the fire chamber just before the heated air enters the flu as in our Lady Kitchener then the loss to water heating is minimal.

The old coal range we had used 80% for the wet back.

So, does that mean if they are talking about a wetback model being 65% efficient, does that mean that 35% is lost to water heating.....or t'other way around?

If energy is used to heat water it isn't lost. If it goes out the chimney then it is lost.

I don't think this efficiency figure gives an indication about the proportion that goes into water heating.

You should consider a number of issues.

If heat lost to the flue is retained by having an internal flue and if a 2nd story can have the flue pass through, then you don't lose as much as first thought.

If your % loss of heat to a wetback is utilised with efficient radiator heaters then again, you may not lose as much as thought.

If your wetback heats water that is not used, then you may well be wasting heat.

If your fireplace is to cook with, and your wetback consumes too much heat, then you may face baking issues.

Each house design, hotwater and cooking needs and budget will effect the "calculation"

Also, the availability of fuel is an important issue.
If you have ample firewood, then why pay extra for a unit that also burns coal when you will never buy same.

If you use the unit for cooking then it is likely you would never run out of hot water and if hot water is circulating through the wetback, then maybe the heat loss will not be as great as expected for a unit just used in the evening.

Personally I would not take the slightest bit of notice about so-called "efficiency" figures. I suspect they may be worked out with "average" moisture wood. There is a very experienced heating engineer in Nelson that is having a continual disagreement with the city council about efficiency figures and smoke emission. I believe him rather than the fireplace makers when he says that by burning dry wood the efficiency is very high, and the smoke very low. Thus the by-law should be about dryness of firewood rather than whether the fire is rated as "cleanburning". Cleanburning means that the fire runs faster, so cannot be damped down.
Our Lady Kitchener, which might actually be a Contessa, has the water heating above the fire, unlike our previous fire. When the water is cold it does steal lots of heat, but if this is first thing in the morning when the fire is roaring, this does not matter. But last thing at night, just as the fire is damped down, will make the smoke colder and not as well burnt, so more smoke.