Hello Firespeaking team and forum members. I am in the process of building the “Shop Cabin Stove” designed by Max (I believe?) at Firespeaking, but my permit officer is asking what the surface temperature of the stove would be at peak firing. This will affect the clearance to other combustibles in my house. It’s either going to be 4’ or 4". Big difference depending on the surface temperatures!
Still learning to manage mine. After my longest burn so far (4 hours) brick facade peak temp was 135F.
Steel top directly over the fire was 700F. I did not measure the temp of the fire door. Has warmed up here in Texas but may fire it again in a few days as temperature drops. Will do some spot checks.
@Miles, We have been in the office and on site this week so I haven’t fired our stove up this week but I would say that @Danocon’s observations are probably spot on. The metal cooktop really makes the unit ambiguous in its definition. A masonry heater is defined in the code as a stove weighing more than 1760 lbs and pre-dominantly of masonry construction. It goes on to specify a surface temperature of less than 230 deg. F.
The cooktop and the temperatures it generates potentially kick it out of this definition but unfortunately there is nothing other than the generic extreme clearances call for, probably as a reference to the NFPA 211 code. This is probably what your code officer is referring to.
From using our shop cabin stove, I can empirically assure you that a 4” clearance to the back with a 4” masonry backsplash would be safe under extreme firing conditions. I think the backsplash is an important part of the design in that it protects the wall from direct radiant heat from the metal. I’m not sure what the best suggested clearances would be to the sides. This is why I had to clearly state on the cooktop plans that no testing on clearances had been done.
I am also wondering about modifying the Shop Cabin Stove design to incorporate an air heating wall on the back of it, as per your suggestion. Here’s a quick diagram (below) showing what I think that it may look like, but I am unclear as to what the best design is for this. My questions are:
How many holes, and what size do they need to be, on the top and bottom of the wall?
How many wythes wide should the space inside the wall be?
As far as I know, there is no specification for this anywhere in the written code. In fact, in the ASTM 1602, which is the presiding code for masonry heaters, there is specification for wing walls when abutting combustible walls but it does not address the need to specifically build for convection in the space. I have found this to be good common sense but is an additional detail.
The ASTM 1602 prescribes a 4” wing wall when coming off the corner and 8” wing walls when coming off any other part of a masonry heater wall.
I can’t answer this for sure. We are exploring the conceptual requirements of adequate protection and clearances between a cooktop and a combustible wall. I would imagine that at just a few courses of brick, the protection from the radiant heat from the cooktop would be sufficient. As an architectural shape/pattern, I would imagine bringing it up to 5 1/2 ft. or more.
The application of “wing walls” as an appropriate way to link the back of this heater to combustible walls only applies if the unit could be considered a masonry heater. The cooktop, and the higher temperatures it generates, is definitely a challenge to this definition. In my mind, the approach we are working out here addresses the clearances to the back of the cooktop but the clearances to the sides still need to be considered. The generic 4 ft. is extremely extreme. Here we are coming up the challenges of custom work where units are not products and not tested and insured by labaratories and therefore are subject to generic clearances which are very conservative.
I know that this reply does not answer conclusions but hopefully it at least sheds additional light on the topic.
Here in Alberta I would need 4’ of clearance on ALL sides of this appliance. It’s a bit ridiculous, but that’s what I have to work with. That said, I was thinking of doing something more like I show in the drawings below.
The ASTM isn’t recognized here in Canada.
I was wondering how large the holes need to be in the wall mainly for function, not for code. Is there an optimal size for those holes?
I don’t understand the whole wing wall thing. They’re 4" off the corners but 8" off the other parts? If I built this convection wall as I show in the drawings below, I wouldn’t have any wing walls, correct?
I’m mainly wondering how tall the wall needs to be for optimal functioning purposes. I can make this wall as tall as 8’. Would that work, or would that be too much space for the convection process to function properly?
You have sketched some kind of cover (metal, I assume) for the air inlet and outlet. Is that some kind of standard hardware, or custom? Would the hole need to be covered, or can it be left open? I may prefer to leave it open to get more air moving through the wall. Your thoughts?
Scenario: Existing 4" slab on grade construction with in-floor hydronic heat. In your informed opinion, can this shop cabin stove be built right on the slab above the in-floor hydronic heating tubes without ill effect to the tubes or the hydronic system?
Interesting, I am not sure how the specific code references go in Canada. It would be good to track this down. We don’t yet have an experienced Canadian heater builder in our ranks but I will work on recruiting a couple (of amazing ones) I know! Do you know if the NFPA 211 applies in Canada? This is where the default clearances when no other reference applies would likely come from for the US context. It might be good to split this specific Canadian code chain of reference into a new thread.
It depends on both quantity and size. I would imagine they need to be at least 2”x2” to be effective.
Yes, you are correct on all accounts.
This is the most interesting question. What is your goal with creating the convection cell, anyways?
Because of the way masonry heaters generally produce radiant heat, they generally don’t tend to need convection strategies. With a cooktop, you will tend to get some convection going as the room air washes across the metal surface (aside: metal wood stoves generally heat by convection created by the heat exchange between the air and the metal surface of the stove whereas masonry heaters predominantly transmit their heat more directly in a radiant manner). This can be accentuated by placing a thermoelectric fan on the surface with terrific results.
I had originally suggested creating a convection cell behind the unit because I assumed you were planning on placing it against a wall. If it is in the middle of a room, you may prefer a simple elevated backsplash or even including some containment/trim in your cooktop itself as @Danocon did on his neat recent build.
If you like the volume the natural convection cell offers from an architectural standpoint, then it will be interesting to experience its result. I would recommend having some kind of vent cover on the openings so that you don’t create an opportune place for cobwebs and such. We have been considering adding these to our catalog, but have not yet found the demand to justify a minimum order…
One reason to create a convection cell is to actively try to move heat to another area. A good example of this would be piping the heat to an upstairs and trying to draw the colder air down the stairway. In this case, you might want to have operable grills so that you can turn this functionality on and off.
Another very interesting strategy is to actually create a “plenum” which can be connected to a more sophisticated HVAC system to move the warm air generated to more remote areas of the house. In this case, I believe it is best practice to create a sealed heat exchange box and have it connected to the duct work so there is not question of mixing products of combustion with the heated room air.
It is unlikely that the heat generated at the bottom of a masonry heater, particularly if there is one solid course of refractory material before any channels, would be sufficient to overheat hydronic heating tubes that are sufficiently embedded in the slab.
Hello all, I just want to complete Norbert’s response (wich is very on point). As a heater builder in Canada, I’ve been confronted to this type of situation frequently. Like Norbert said, the ASTM standard is not recognized here. A standard masonry heater falls into the masonry fireplace category in the building code. The basic clearances are very similar to what is expressed in the ASTM. The problem arises when the unit as a heated metal part (such as a cooktop or a barrel for a rocket stove). The metal makes it a custom stove in some regard and then, things get a lot more complicated. Like Norbert stated, you then have to follow the rules for an uncertified stove… In Quebec, where I practice, uncertified stoves are not accepted anymore by most municipalities, which complicates furthermore this process. The way out of this conundrum is to keep the heater in the masonry fireplace category at all costs to avoid the uncertified stove label.
We built a heater for a judge over a decade ago and I will never forget when, over dinner, the judge said… “The last thing to change in a changing society are its laws.” I mainly add this to provide motivation that even if a regulatory body doesn’t currently easily accept a certain category of design, that it is not completely out of the question that this could someday change… and perhaps even that we might be able to be a part of that change. Someone like @mheat knows that it may only come through painstaking documentation and work… but I would like to encourage us to think that it is possible.
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