The only safe way to use a fuel inside a closed space is to provide 
combustion air and exhaust for the combustion process. To use the same 
air as the space air is inefficient and marginal safety. Direct vent is 
the best way to go. They are not cheap, probably in the $400 - $500 
range, haven't checked recently. Many are made for use in RV's on lower 
voltage, but there are models that operate on 120VAC.

I have 2 Dynavent DNV40 direct vent 85% efficiency gas furnaces, one for 
the garage, and one for our small greenhouse. They go through the wall 
like a window air conditioner. They have both heated and combustion air 
fans, separate thermostat. Have had them for years and really like them.
http://www.dynavent.com/

Also check out
http://www.grainger.com/Grainger/wall-furnaces/heating-equipment/hvacr/ecatalog/N-90f?op=search&sst=subset

On 6/2/2010 8:19 AM, Olin Lathrop wrote:
> Gerhard Fiedler wrote:
>    
>> I didn't think of CO. It's the room that has good ventilation. Venting
>> the flame would probably kill my idea of using a simple burner.
>>      
> Ventillating the room enough to get rid of combustion gasses would also be
> very inefficient as it would vent the warm air out too.  If you're making
> room heat with combustion, you really need to vent the combustion gasses
> (unless perhaps you are using hydrogen as a fuel).
>
>
> ********************************************************************
> Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
> (978) 742-9014.  Gold level PIC consultants since 2000.
>    
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