Heaterman, you have nailed a key point inherent in my comment on the combined system. Perceptive on your part.
Since I was not the person responsible for choosing the systems, installing them, determining the control system, and now operating them, although I had plenty of input, I want to do the best I can to be objective in stating any of my observations, which to be fair, also are based on incomplete information. In any case, these are just my observations, and I do not judge myself to have been competent to do all of the above anyway.
The institution has two primary heated structures, each previously served by LP boilers in each structure and not interconnected. One structure has 5 staged LP boilers, and the other has a single LP boiler firing system (may be more than on LP boiler). This is a pressurized system. The two wood boilers were installed in a new, separate building, located about 60' from the tie-in point to one of the heated structures and about 150' from the tie-in point to the other structure. Each structure had its own control system, and install of the wood boilers required a more complex control system. Both heated structures are served primarily by hot water unit heaters, with some of these being on the cool side of fairly long zones. I don't believe either structure is more than average in energy efficiency.
Garn - disadvantage. Setting aside what I regard to be errors in the installation, which if corrected would at least to some extent ameliorate the one issue now mentioned, a disadvantage with the Garn results from 1) the need to use a hx to interface the non-pressurized Garn with the pressurized system, and 2) need for high temperature hot water in the unit heaters to meet demand, particularly at times when outdoor winter temps are in the 0F to -40F range experienced in this area. To my knowledge expected hx performance is a 10F approach temp. Operating the Garn to maintain a continuous 180F supply to the system requires continuous 190F-200F Garn operating supply temp to the hx, and this is not practical. This also defeats the purpose of the storage capacity of the Garn, as there is little effective storage in these operating conditions.
Garn - advantage. The mentioned disadvantage of the Garn diminishes as heat demand diminishes with warmer outdoor temps. The Garn will much better serve its heat storage function at times other than high heat demand. The Garn also provides some advantage in providing pre-heated water to the Wood Gun and to the system in high heat demand periods, including when supplemental heat is needed from the LP boilers.
System design flaw. One goal of the Garn/Wood Gun wood boiler system was to have the Garn also serve as peak storage for the Wood Gun at times other than high heat demand. Due to design flaws, IMO, this function will be met partially, at best, and little if any, at worst.
Wood Gun - disadvantage/advantage. These are pretty much the opposite of the Garn. The Wood Gun performs well to provide continuous high temp supply, and therefore does well at times of high heat demand. As heat demand drops, the Wood Gun begins to idle and idling increases as heat demand continues to drop. The goal of the Garn serving as storage was intended to allow the Wood Gun more continuous high temp operation, but this goal was not realized. Separate dedicated storage for the Wood Gun would be very helpful and increase the operating efficiencies and flexibility in use of the Wood Gun.
There are many other issues with each system, including how it was installed and the control system. At this time I would tend to put some of these other issues in the "learning curve" arena of wood boiler operation, although some may be more substantive. As I do not operate the system, without more information from the institution and personal verification, I do not think it is reasonable to comment on these now.
Overall - Operating the system as it now is along the lines outlined by Heaterman is the likely direction for the near future. The institution may be able to obtain additional grants, and much further discussion will result as to whether to expand the system in the Garn or in the Wood Gun with storage direction.
A large wood boiler system using round wood, which must be dry, is a large issue. This institution has available a relatively inexpensive "endless" supply of wood. This is the north woods of MN, after all. LOL. Labor in obtaining wood also is not a large issue. But the time to dry wood, the space required to maintain wood in the drying stage until it becomes usable plus current usable wood, protecting the drying wood from the weather, locating drying area space near the wood boilers, moving wood to the boilers, etc. have become a much larger issue than was foreseen. In the case of this institution, the issue is available land area.
Smoke/odor etc has not been an issue with either boiler. The boiler building is located close to occupied buildings.
Staff to fuel the boilers has not been an issue, except at night and some weekends. This is where greater storage capacity would be very useful, particularly in periods of less than high heat demand, as storage could be charged and then provide supply during non-firing periods. Currently the LP boilers supply heat at these times, and although large savings in LP usage are being experienced, greater savings are possible.
A caution for any person/institution contemplating a wood boiler install is the experience, more likely the lack of experience, of the HVAC contractor. Understanding of how these systems need to operate and interface with conventional boiler systems, including such "simple" things as sizing the hx, proper install of the hx, calculating head loss, flow rates, return water protection (Wood Gun), system shock to the conventional system, lack of modulation, high operating temps, etc. -- may not be within the experience or understanding of the HVAC contractor, and may result in design, install and control systems which defeat the purposes of wood boiler operation.
