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[merrellroofing]

       I hooked up my unit heater in my shop. My buddy got it for me off a demo he did. It is 78k btu.  I just have it hooked up on a switch because I may go a week not being in there. I have only run it 3 times and have not been real impressed. Granted it's been very cold and the wind seems to blow right thru. Yesterday I ran it for about four hours and my temp dropped from 175 to 150 and was burning the whole time obviously. Cb is natural draft, so is that why I can't keep up?  Or are there other things to look at?  Speed of pump?  Speed of fan? 
     My stove sits in a 40x60 3 sided metal barn, just outside my shop which is 40x30x14. 50 of logstor comes down then loops up to the open ceiling and over into my shop.  Then comes thru the shop ceiling to my heater. Couldn't go underground for preexisting concrete. The pump is a taco 006.(I bought a used unit and just hooked up the pumps that came with it.) 
     My main line is 165' of themopex with a sidearm and a 12x25 w/a exchanger and taco 009.

    Any help is good.   

[capitalpyro]

Granted it's been very cold and the wind seems to blow right thru. Yesterday I ran it for about four hours and my temp dropped from 175 to 150 and was burning the whole time

My first thought after reading the above is that you are trying to deliver more BTU's than the stove can produce. You say "the wind blows right through". Is the space insulated with a ceiling etc...?

[yoderheating]

 Yes your furnace can't keep up with your heat load. One of the down falls of a natural draft furnace is that it limits you on those times you need lots of heat quickly. On a unit with a forced air draft, especially a draft that comes in under the fire, you would be able to produce the heat and it would just reduce the burn time.

[slimjim]

Hold on guys, He didn't tell us where the temp drop is, if it's at the wood boiler then I would agree, he may be checking it at the unit heater(supply and return), if so then increase circulator size and check for air in the loop.

[fireboss]

You can put on a draft inducer (fan) for around 130$ I just put one on my friends it took 5 min to install,!

[cantoo]

I have the same issue with my shop. Only need heat once in awhile so it's on a switch. I think it's just because it's suck a high temperature increase it will take a lot of wood to get it up there. For instance - 20 C outside, inside shop is likely close to that maybe -15 or so and you want to work in it so +20C, that's a huge increase. My shop is 24x56, that's a lot of air to warm up that many degrees. Means big exchanger, big fan, big pile of dry wood, and time.

[merrellroofing]

       I ran my heater some more today.  If I leave the door cracked I can maintain temp but can not gain. Ran for 2hrs then I shut it off and closed the door for 30 minutes and gained 8'. Then turned it back on and cracked the door again for 1hr and maintained again. 
      So I need more air but not all the time. I know I can put a fan in the door but the classics are not really designed for that IMO.  I just see it blowing over the fire and blowing heat right out the stack.
     Coul I add a fan and unhook it unless needed?  Or would that block the natural draft too much wen not in use?