I also thought this was interesting from Jim in another thread as well. I hope you don't mind Jim.
The fun never ceases. My experience is limited to a Tarm Solo 40 (my own), now in its 5th heating season; a Wood Gun E500 and a Garn WHW3200, both now in their 3rd heating season. The WG and Garn are installed at Deep Portage. Given what I’ve done, I can support simplicity and I also can support some key add-ons, or upgrades if available from the mfr.
As to the Tarm, the controller shuts the draft off when the fire is out, adjusted by its own thermostat, and it shuts the Tarm down completely on overheat. Simple and effective. I also have added 1000 gal pressurized storage. My only changes are wiring the controller to also shut off the circulator when the fire is out, and a stack overheat controller that shuts the draft fan off it the stack temp gets too high. Plus I monitor key system points and data logging as needed, both of which have greatly helped me to understand how the Tarm functions. Since I started weighed wood burns last year and learned that I now can always load the Tarm so that it burns full out and never idles, I am continuing weighed wood burns. Storage makes this possible, along with a couple of sensors and digital panel meters to monitor the storage tank. I have not had any repairs, wear-outs, replacements of any kind with the Tarm.
As to the WG, I know that the mfr says storage is not needed and letting the WG cycle is fine, but I also know that since DP added 4000 gallons of pressurized storage, WG operation has become a breeze and nothing could be simpler. The WG now always takes a full load of wood (100 lbs+) and then walk away. It no longer ever cycles. In this respect it operates much like the Garn, the primary differences being that the WG is kicking out 160F+ water nearly from the start—no need to first heat the tank before hot water is delivered to the system—and that the WG is designed to continuously put out high temperature water, 180F+ or so. The only add-on required for the WG was a motorized boiler return water protection valve, as I think all wood boilers require. Due to installer errors or ignorance, this was not installed or controlled correctly, but that now has been solved through re-plumbing, adding a 2nd sensor and reprogramming the Tekmar controller. What would be very beneficial for the WG is a draft fan and circulator shut down when the wood load has burned out. I’m waiting for the go-ahead from DP to add this, which must be done by or approved by an electrician. There have been no operational or mechanical problems with the WG, other than the mechanical timer failing, which DP did not replace.
As to the Garn, it functions with simplicity: throw in a full load of wood (100+ lbs) and walk away. What also would be very beneficial for the Garn is a draft fan and circulator shut off when the wood load has burned out. DP is following up with the mfr on this. Keeping the draft fan operating (now a mechanical timer) when the wood load has burned out continuously moves lots of air through the Garn tank, and I can’t help but believe that this throws lots of stored btu’s up the stack. Of course, the Garn does not deliver hot water immediately because it needs to heat 3200 gallons of water first. Garn also has advised that efficiency declines in applications where water temp greater than 140F is needed. There have been no operational problems with the Garn, but the draft fan motor has failed 3 times, I believe, and Garn now is providing a different motor than the one that was originally supplied. This new motor was just installed last week. Hopefully this motor will be more dependable.
I would think that lambda or other more sophisticated controllers, including variable speed draft fan, would result in all 3 of these boilers operating more efficiently. But with sufficient storage and the ability to always burn a full load of wood without idling or cycling, I tend to think that all 3 of these boilers are operating at their design best. It would be interesting to see some data on the efficiency gain provided by more sophisticated controllers vs how these 3 operate now.
