My shop is aprox 5000 sf cement block building 16or 18 foot cielings and 5 12 foot overhead doors that are constantly opening and closing . We are on our second waste oil heater the curent one we have is an energy logic 340000 btu It is awsome clean it at the start of the season nd keep the tank full and it just runs and makes heat. we have installed a large truck luberfiner filter between the tank and the spin on filter that came with the unit and we have a vaccum and pressure gauge so te fillter can be monitored the large filter is dirt cheap and lasts a year if nobody puts antifreeze in the tank. we paid north of 10k for the unit 5 years ago , and it replaced an older smaller unit that was a pos that required constant babysitting . The pluses for us are we were using 500 gallons of oil every 3 weeks which gets old when oil prices are high, and at that we were keeping the shop uncomfortably cold . With the waste oil we keep it toasty warm . the down side is you need a supply of good used oil taking oil from other shops/ people you need to be carefull of what they give you a little coolant will cause you fits one guy tried to give us paint thinner oil mix, you need a tank an the extra oil has to be stored inside if using barrels I dont care how tight the cover is outside they get water in. We have a 300 gallon bench type tank and keep barrels in a storage trailer . I always keep 3 barrels ahead in te building and the tank topped off , we used to wait till we needed oil and bring it in and found that if you put 250 gallons ofstone cold oil in the tanks it causes trouble ith the way the heater runs . . another thing to consider is some units require shop air to run and some have built in compressors , I would only consider the ones with there own compressor. If the payback is tere for you buy a new unit and buy quality over price , if its not there a cheap or used unit is likely more trouble thn its worth