Vacuum kilns

Choosing between a vacuum and dehumidification kiln. July 24, 2001

Can anyone comment on the pros and cons of vacuum kilns--especially the Wood Mizer design? I'm going to set up a small charge sized kiln in the near future, and am nearly certain that it will be of the dehumidification type. However, when I factor in the cost of construction, assembly time, etc., I wonder if a used vacuum kiln might be the way to go. I've been told that Wood Mizer no longer manufactures these kilns, and I wonder why that is.

Forum Responses
I'm looking at kilns, too. I hear Wood Mizer doesn't make that kiln anymore because it is too small for a large sawmill and too costly for the small mill to buy. Another vacuum kiln company told me I would have to pre-dry oak before kiln drying. Sounds like a lot of trouble. Now, like you, I am leaning towards dehumidification. I have looked at Nyle kilns and like what I see.

In case you haven't heard, Wood Mizer has a new kiln with hot water as the heat source. I understand that they will also be handling DH kilns (already or very soon).

I ran a Wood Mizer vacuum kiln for 18 months. Although the initial investment was high for a vacuum, the annual cost (depreciated over 10 years) per BF was very favorable. Energy costs were double the DH kiln ($70 vs. $35). Drying times were very short (5 days vs 30 days), but MC variation was higher than a DH kiln. We found it was difficult to estimate the final MC in the vacuum kiln. Labor to load was higher for the vacuum kiln, but no stickers were needed, saving money. Drying times for some species that are not porous (like white oak) were exceptionally long in the vacuum. Leaks at the gasket were a common problem with the vacuum, but I understand that later models were better in this regard. Overall, the vacuum was a good dryer for some species and some thicknesses; likewise, the DH is a good dryer for some species and thicknesses.

Gene Wengert, forum technical advisor

How much wood can you sell or use in a short amount of time? Why dry 10000 board feet a month if you are only going to use 1000 board feet? The cost of a vacuum kiln just doesn't add up. If you are going to dry "hard to dry" wood, maybe look into it.

I ran a Wood Mizer for 6 years. Talk about one big pain in the butt! If you do buy one, figure on a bigger vacuum pump right from the get go. The tiny pump that the Wood Mizer came with was a joke. The "tits" that held the seal in place would rot off in a matter of months. Make sure you can still get the heating blankets. These had a very short life. I think the last one that I bought was around $400. Wood Mizer quit making the kiln for a reason.

The small vacuum kiln that I have built is wonderful. Now if I can get the cost down.

The Wood Mizer vacuum kilns didn't work because of a woefully inadequate vacuum system. The vacuum pump was way too small. It had no cooling system. The pump was destroyed by organics from the wood. There is no pressure control. A timer cycles the pump when you set a duty cycle. Temperature control of the blankets can be a nightmare. Thermisters are a problem. Unequal heating is a problem. Programming the old computer to control heat can be a problem.

A few years ago, Wood Mizer hired me to redesign their kiln. They were thinking about shutting production down but decided to look at my design. I used as much of their old design as possible but I added what was needed. Their engineers were enthusiastic and the prototype was proceeding into operation ASAP. I've been told the project is dead because of hot spots in the blankets and that if I came up with a fix, it may be put back into production. I came up with an idea, the engineers were enthusiastic and the product remained dead. I heard from them a couple more times but only to see if I wanted to buy the production line. I've sold a couple of the prototype designs. They work.

By the way, any so-called vacuum kiln manufacturer who tells you to pre-dry oak does not know anything about building a vacuum kiln. If you're planning to dry a little lumber, DH is the way to go.

The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor A:
I own a Wood-Mizer vacuum kiln. I was told it was the second Mod 2000 that was produced, the first one going to Wood-Mizer's northeast branch in Hanable, NY. In 16 years of operation, Wood-Mizer has been supporting my kiln, though they stopped building them. The Wood-Mizer vacuum kiln is a wonderful machine. It takes time to understand and learn how to use the machine. I have a spare seal for my kiln (still in its box) and I think I'll install it soon. The original (16 years old) looks a little brittle but still works well. The original brass pumps wore out quickly. The stainless pumps are superior. Mine is over 10 years old and still pumps down to 1.5". I just bought all new blankets. Over the years, we have replaced them one or two at a time. This is the first time I've replaced all of them at one time. I was told (by armchair professors of all sorts ) that the blankets were the "weak point of the design". That's simply not true. When used as designed, the blankets will opperate for years. If you abuse them (all kinds of ways to do that), they will be ruined in no time.

I have no connection with Wood-Mizer other than being a customer. I'm on my third Wood-Mizer mill. We sawed with an old Frick handset mill for a few years but had to go back to the bandsaw as the timber prices have gone through the roof. I live and work in Western NY and we pay dearly for electric, but drying in the vacuum kiln is quick and reliable. We get even dryness with no hot spots. I've dried cherry, white and red oak, basswood, poplar and hard maple. Most all we dry now is hard maple and we make nice white maple year round.