Ok so we have just installed a Mansfield , love the look bit of a learning curve compared to our PE Summit . We have heated with wood for 40 some odd years hear in Canada , the cat option my jury is out on … doesn’t create any more home heat in my observations. Now someone commented that a BK cat stove does indeed create more in house heat , my question is how do they differ ?
In my observation, the cats on the truhybrid are most useful/active towards the end of the off-gassing phase as the flames are puttering out and after they have gone out. In most burn cycles, after the flames go out, there are still lumps of wood in the box at this point that have a fair bit of pulpy tree carcass goodness still in them, but it's buried behind ash and coal and often in the back corners of the box away from the direct air blowy hole. The rate of wood gas release at this point is too low to sustain a continuous flame in the firebox, but it is plenty to put the cats to work, and to work they go.
Lots of clink clink tink tink noises coming from the cats around this part of the burn. This is where the emissions reduction and thermal advantages of the cats come into play. The amount of combustion that will take place in the cats during this part of a unmanaged (fixed rate) burn cycle will vary depending on your fuel type, fuel moisture, fuel loading density, fuel loading arrangement, and selected burn rate. Selecting a higher fixed burn rate will get more of the wood gas out during the active flaming session. Selecting a lower fixed burn rate will cause the combustion to transition to the cat earlier in the burn cycle. An actively managed burn cycle, with the operator adjusting air and re-arranging the fuel in the firebox, can maintain active flames in the firebox until nearly all wood gases are expelled from the tree chunks. When operated in this manner, the cat in these stoves doesn't do much because it isn't fed much fuel to do anything with.
An "active operator" can push a non-cat stove into the emissions and efficiency territory of a cat-stove with fuel and air manipulations during the burn cycle. Leaving chunks of tree in a non-cat stove (or bypassed hybrid) to smolder without adjustment after the flames have dissipated is probably the primary contributor to the gap in thermal efficiency and emissions between cat and non-cat stoves. If you are reading this, and want to run your non-cat stove closer to the thermal efficiency and emissions of a cat-stove, you can. When the flames are about to drop out of your fire, or just after they have, get in there with a poker and pull the chunks into a pile up in front of the air inlet on the stove, open the air slightly from your usual burn rate setting and close the door. This should get some flames rolling for another 30 minutes or so, rather than send that wood gas up the chimney.
The EPA test process for taking various emission and efficiency measurements does require selecting a fixed air-control-setting after a specified startup procedure and stabilization time. This is sort of the "set it forget it" burn strategy that folks are likely to settle into if they are loading the stove before going to work or going to bed. In these conditions, the new Mansfield and other hybrid stoves like it are likely to do better than previous models in terms of thermal and emissions performance with most burn cycles if the operator dials in a good loading methodology and burn rate selection for these unmanaged burns.
The position and mounting strategy and thermal coupling of the cats in the TruHybrid design is a novel set of compromises that is not the normal strategy found on other wood stove designs. The cats are further down-stream and have less thermal coupling to the stove. This has advantages and disadvantages. The cats in this position are a bit harder to activate and keep active, and are more likely to get sooted/plugged up, requiring more maintenance, however, the position also protects them from flame licking, which may extend their usable life, and contributes to maintaining higher EGT's while wood gas is still being burned, which should help reduce chimney deposits and promote a steady draft through the completion of the burn. The hearthstone dealer here who I purchased our Mansfield from, who also services chimneys for a wide variety of wood stoves and fireplaces, including older non-cat hearthstones, mentioned that creosote is basically a thing of the past in chimneys above these new truhybrids. He only ever gets the usual fluffy soot from the pine everyone here burns that falls right off the chimney with a quick agitation from a gentle brush.
Testing for efficiency:
The "chimney" system defined in the CAN/CSA-B415.1-92 standard used by the EPA for stove testing includes single wall pipe from the top of the appliance to a "simulated" 8 foot ceiling, then insulated chimney above that. Efficiency in the EPA test configuration is measured by means of taking EGT's at the ceiling height in the chimney system, and then combining that data with the flow rate to determine energy loss, and then calculating energy input by measuring the rate of weight loss in the stove from fuel burn-off. The single wall stove pipe in the test configuration is part of the measured heat output of the system. If you have double wall, or less pipe above the stove than this, then you will get lower system efficiency than the EPA measurements when burning at the same rate as their test.
Within the test parameters: Stoves that drive higher EGT's will get a bit more of their total thermal transfer and efficiency out of the stove pipe than out of the stove body. Stoves that are physically shorter will get a bit more stove pipe length to transfer energy through. Tiny stoves will undoubtedly get a significant portion of their thermal transfer from the single wall pipe section, as the surface area of the pipe is almost certainly greater than the tiny stove! The efficiency of a lot of tiny stoves is probably being somewhat disproportionately boosted by this test configuration compared to the larger stoves.
This isn't to say that you shouldn't use double wall stove pipe where appropriate to achieve the desired total system behavior, burn rates, draft, deposit control, and installation clearances.
BK efficiency vs Mansfield / everything else?
The CAN/CSA-B415.1-92 standard testing procedures measure thermal efficiency at the lowest manufacture prescribed burn rate that the stove can sustain combustion at. EGT's as low as 115C (239F) measured at the transition from stove pipe to the insulated chimney system are permitted in this test.
BK stoves are engineered to operate as wood gasifiers. They can maintain clean combustion at lower burn rates than hybrid or non-cat stoves. BK stoves are very well suited to produce very high efficiency in a low-burn-rate test, because the stove can maintain combustion from a huge fuel load at a fuel consumption rate that is comparable to the low burn rate setting of a traditional, non-cat ~1-cubic-foot wood stove.
Imagine taking a normal, non-cat 1 cubic foot wood stove, and then attaching it to as much steel as is normally used to make a 3 cubic foot firebox, then running that stove on its lowest burn rate that maintains clean/hot combustion. That's essentially what is happening in the test conditions that produce the BK stove efficiency numbers. Big heatsink, tiny fire. The combustor in these stove designs is further "upstream" than in the Hearthstone truhybrid designs, and in close proximity to the more conductive steel stove body for better thermal transfer.
The minimum burn rate of a Mansfield and most other hybrid stoves of similar size was established ~20-40% higher than the minimum burn rate on similar size BK stoves in EPA testing. The Hearthstone Mansfield, BK PE32, and BK 30.2 based products, achieve 78%, 80%, and 76% HHV efficiency respectively on the EPA test bench and all achieve impressively low emissions. The Mansfield is competitive in this test despite being assembled from material of lower thermal conductivity and despite being operated at a higher burn rate. Like the BK stove with its ultra-low-burn-rates strapped to a huge heatsink, the Mansfield also has a way to "cheat" the test. 600lbs of soapstone/iron thermal mass starting off from warm but not hot (coal bed start), provides a place to dump thermal energy into through the duration of the test burn cycle.
Both of these "systems" have their own exploit to push the limits of the test, but those exploits may or may not be representative of the way you use the stove.
The thermal efficiency reported by the EPA does not consider any sort of BTU normalized configuration and doesn't consider continuous feeding scenarios or scaling up to higher output. As burn rates are increased and "cycling" from warm to hot and back down is removed, the efficiency of most stoves is likely to decline. Both the BK stoves and the Hearthstone stoves have compromises in their design that limit their capacity for scaling to high BTU output.
The Soapstone construction of the Mansfield is less conductive than steel, which doesn't help. The design decision to get the cats further downstream likely forced the enginerds to insulate the path to the cats above the baffle in the stove. The underside of the stones on the top of this stove actually have a heat-shield under them, so there's actually a fair bit of missed opportunity here for thermal extraction on the top plate. I rarely see the stones on top of this thing over 350F. Unfortunately, removing that heat shield would very likely cause the cat not to work when it is needed most.
The BK stoves do not have any secondary combustion tubes, so their burn rate is limited to what can be safely managed as primary combustion directly off the logs and through the cat as secondary. Without burn tubes, the cat is forced to deal with a significant combustion workload when the stove is cranked up higher. The cat can not tolerate the sheer volume of combustion that secondary burn tubes can. Like the Mansfield, the inside of a BK stove has various efforts in place to balance insulation with conductivity so that its low burn rate is functional and clean. Lots of firebricks line the bottom and sides of the firebox to help hold a smolder in the fuel, and various heat shielding in the firebox ensure combustion gases reach the cat hot enough to keep it running at the low-burn-rate spec.
Technically the Mansfield achieves slightly higher BTU output in the high-burn-rate EPA test. Probably made possible by the secondary combustion system. The differences aren't significant though, and may again be somewhat skewed by the additional thermal mass available to sink energy into. The test standard doesn't appear to account for continuous output, but cycled output. In a continuous output test the BK may well achieve higher usable BTU output due to being more conductive.