Refurbishing a used vacuum oven

Wow, vacuum ovens are sure neat, albeit expensive for a new one and the old used ones frequently can’t hold the temperature in the low ranges that we use for purging and refining cannabis essential oils.  What to do?

A simple vacuum pot sitting on a hot pad does the trick, so our inclination to spend much money was low, and we decided to refurbish a used Napco 5851 vacuum oven, which we scored for $550.

As received, it wouldn’t hold within -7F/+60F of the 115F set point, so I first replaced the capillary tube type thermostat with a Panasonic AC100 self teaching Temperature Controller, in conjunction with a Type K thermocouple.  To my chagrin, while that helped, the temperature still swung wildly, because of the overall oven design.

The Napco oven is a simple vacuum can, wrapped in insulation, with an outer metal surround.  The inner can has two 500W strip heaters bolted to the bottom, and they heat the entire can through convection.  That clearly leaves a wide disparity in temperature within the oven, plus the inertia  of the two 500W heaters significantly overshoot the upper control end.

Soooo, I took the oven completely apart, and after replacing the stiffeners that were in the way, I relocated one 500W heat strip to the top of the oven and centered the remaining one under the bottom.  I added Type K ring thermocouples to the upper and lower can surfaces, and added another Panasonic AC100 to control the upper heating element.

I inserted my first Type K thermocouple into the center of the oven, through a compression fitting in the rear oven port, but left it to just hang at this point.

In both cases, I added Crydom solid state relays, to control the oven current with the low Panasonic controller voltage.

Alas, still too much heat strip inertia to control at 115F, so next I disconnected the upper heat strip completely, and hooked the thermocouple in the center of the oven to the Panasonic controller, in read only mode.

Finally I was able to control the oven in the 85F/115F range that we were shooting for, so next I will pick up a nice box for the two Panasonic controllers and finish up the oven.  In addition to using one controller to run the lower heating element, and the other to read out the actual inside oven temperature, I will add individual switches to the upper and lower heat strips, so that I can switch one or both on or off at will.

That will allow me to use the upper heating element to speed up startup, and switch it off during actual processing.

Here are some pictures.  I will follow up when we complete final packaging.

vacuum oven upgrades-1-1vacuum oven upgrades-1-2vacuum oven upgrades-1-3vacuum oven upgrades-1-4vacuum oven upgrades-1-5Oven shakedown-1-6

14 responses to this post.

  1. Posted by Eric Twelker on March 18, 2016 at 9:49 AM

    I’m looking for a power switch for my 5851. Does anyone have a clue about where to find one?


  2. Posted by Daniel on November 26, 2015 at 12:24 AM

    Can you use Mild steel to build a chamber? I have a heraeus 5042 Oven. Now its not designed to hold vacuum so i want to build a new chamber to put inside and remod the door to seal properly. Or should I go with SS304? And what thickness is good enough?



    • Mild steel is strong enough, but if you use stainless you don’t have to worry about finishing the inside so it doesn’t rust.

      I would use 10 gauge 304L for the shell,with a mild steel tubular frame on the outside for stiffening.


  3. Posted by Twotonepunk on March 1, 2014 at 9:30 PM

    Hey there GW!

    partner and I just scored a mint conditionoven for $100! It’s pristine aesthetically, and seems to be in full functioning order. (Holds vacuum, heats up, yadda yadda) Sweet deal!

    It’s a NAPCO 58501 875 watt, that from the outside, to our very untrained eyes looks identical to yours except for the controls on the front.

    My search had turned up no manual or literature on this model. My question for you is: do you have any insight in this particular model and how it could be used/modified to work for our purpose?



  4. Posted by kev on December 22, 2013 at 9:52 AM

    sorry to revive a old post, i just got a napco 5850 and i want to rebuild it with digital controls as well. im making up a parts list so far ive got 2 controllers, 2 soild state relays, new ring thermocouple, am i missing anything? thanks for the help i think this will work great for my 5850 rebulid


  5. Posted by vaco on November 28, 2013 at 3:53 PM

    any experience with gravity convection vac ovens?


  6. Posted by JohnnyBased on July 19, 2013 at 2:59 PM

    Recently got a used VWR 1410 oven for fairly cheap, only problem is it has a noticeable amount of ‘chemical dirt’ inside and on the rack. Any tips on cleaning it and the smell up?


    • I infer that it has a stainless steel liner and a glass front.

      The fastest way to clean the stainless liner, regardless of what is on it, is plain old emery cloth. Wear a respirator if you don’t know what the baked on chemicals are. Just polish it up, and the contaminants will come off.

      Use glass cleaner and soft scrubber on the glass.

      I picked up new stainless racks that fit ours from Goodwill.

      If you want to salvage yours, I would start by soaking the rack in hot soapy water and use a stainless sidewall brush to clean off what will come off. I would then soak it in a strong solvent like acetone or MEK and brush it, to see what will come off.

      Next you can soak it in an acid like Muriatic acid to remove what is left, followed by a strong base rinse, such as NAOH or KOH (Draino).

      You can also run it through a conventional oven during a 750F cleaning cycle, to reduce any organics to CO2 and water vapor.

      You will probably have to replace the silicone rubber door gasket, as you don’t know what it might be soaked with.


      • Posted by DENNIS on November 30, 2014 at 1:03 PM

        I bought a napco but the gasket on the door seems to leak do you have any suggestions on buying or building one?


        • Well same problem with me so i took out the gasket and added a layer of 100% silicone froma caulking tube from Home Depot and allowed it to cure . Cleaned it up and reinstalled and I then attempted vacuum and it was much better but it still leaks slowly so i must keep pump on .\new one is on my list, lot of work in rebuilding one..


  7. Posted by tylerdurden71 on July 5, 2013 at 12:15 PM

    is there much advantage to using a vac oven versus a diy vac setup like your other thread? it seems like being able to use multiple racks within the oven would be fantastic, but most ovens only use bottom heat.

    on my diy setup, i use a presto grill with a large pan of water on it. i use glass marbley things that are flat on one side to elevate my vac pot and use a Ranco ETC-111000 to control the temp of the water bath. i can keep it within 1 degree of my set point.


    • Multiple racks sounds like a better idea than it actually works out to be, because of the need to watch what is going on.

      An oven has more even heat top and bottom than a chamber and ours will hold larger Pyrex trays.

      Both work slick!


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