This page is a record of some of the more interesting email questions that have come in from masons and clients. I hope that it will become a useful resource over time. Let me know if you have any feedback or comments ...................... Norbert Senf
Retrofitting into an Existing House
Heating a Long House
Intake Air, Slab on Grade
Outside Combustion Air
How do I get the ball rolling?
Q: What does an existing house have to have in order to accommodate a masonry heater? Can you build it atop an existing fireplace?
A: It depends. Usually with a retrofit, you have to look at each floor plan individually. Sometimes, using the chimney portion of an existing fireplace is possible. Often, the existing fireplace is torn down.
With a retrofit, you need to locate it in the floorplan so that it makes sense. It is better to have the chimney inside the house. If there is an existing interior chimney, it can often be used. If not, then the first consideration is the chimney routing and what constraints it might put on the location - you want to avoid cutting rafters, although cutting floor and ceiling joists is not as big a deal as it appears at first glance. Typically the heater needs a concrete block foundation in the basement, so location of existing beams, etc., has to be considered. For the chimney you need 16" x 20", plus 2" clearance to combustible framing, for a total of 20" x 24".
Q: What about having an oven in the kitchen?
A: The most common
layout is to use the heater as a space divider between kitchen
and living room or family room. The firebox door can be on the
living room side and the bake oven door on the kitchen side. This
is an excellent setup. We installed a core for a client last
summer in a new house that I particularly liked the layout of.
You can see the floorplan at:
Q: Do you have to build a chimney if one is not there already?
A: Yes. In some situations it makes sense to use a prefab chimney, or use masonry for the first 8 feet and then switch to metal.
Q: I have a house in that has a woodstove sitting on a brick hearth which sits on a concrete slab floor. No basement. The woodstove attaches to an existing chimney. Seems like an easy retrofit for masonry heater? I don't think you'd need any reinforcement because it sits right on the slab foundation. However, it's a 28 year old house and the slab is insulated around the perimeter only. In this situation, do you think the slab would steal away too much heat from the masonry heater?
A:It should work fine. With our system, we use a 4" basepad that is made from insulating concrete (vermiculite aggregate) and keeps most heat from bleeding into the slab. You could add additional insulation by using 2" of Dow Corning Foamglas, a cellular glass product with an R value of around 3.5 per inch.
Q: I have been told that a masonry heater would provide too much heat for a house as small as ours; the main house is two storeys, totalling less than 1200 sq. ft., with R50 insulation in the walls. In addition to wood heat, the entire south wall is glass to provide passive solar.
I like the advantages of a masonry heater, ie., hot clean burning fires and slow radiant heat. I have been told that it is possible to create a hybrid system by building a masonry surround for a high efficiency wood burning stove.
This supposedly provides the "best of both worlds"; the advantages of a masonry heater with the "immediate" heat that a wood stove provides. What do you think?
A: I don't agree that a masonry heater would overheat a small, efficient house. Masonry heaters are particularly well suited at providing low, steady heat output. You would simply burn a smaller fuel charge in the heater. You would still burn it fast, however, so that the burn remains clean. A heater fired with 30 lbs of wood once per day, for example, will give you a fairly steady output of about 0.75 kW, or about 6500 BTU/hr.
The main problem with using a metal stove in an efficient house is that it can only achieve a low heat output by cutting back the combustion air supply. This eventually leads to a smoldering fire, which is when the emissions rise dramatically and you get chimney deposits, etc. The new EPA certified stoves are much better at this, but it is still a difficult feat to perform, unless you have heat storage available, such as in a masonry heater.
You can add some heat storage to a metal stove, as you suggest. The first item to consider would be building code and insurance requirements. Typically, a metal stove is U.L. listed and has to be installed in accordance with the terms of its listing, in order to meet insurance company requirements.
Masonry heaters are able to get around this by being built under the masonry fireplace provisions of the building code. Typically, insurance companies regard them as masonry fireplaces and don't charge an additional premium as they do with wood stoves. It is not a good idea, however, to tell your insurance company that wood will be your main source of heat. In other words, you should have a backup system such as electric, etc.
A concern that is often raised, usually by engineers with no actual practical experience with masonry heaters, is that a heater will overheat a passive solar house because there are no controls on its heat output. However, with a passive solar house, masonry heater or not, you will still require adequate thermal mass or other heat storage ability in order to moderate the intermittent nature of solar gain. The other aspect that comes into play is the radiant-heating nature of a masonry heater. This inherently allows for a comfort level over a wider range of air temperatures than a hot air system, such as forced air or a typical metal woodstove.
Q: I am planning a one story home in the shape of a boomerang. The center room is about 600 sq. ft. and pentagon shaped and would be a great room (living room, kitchen, dining). I am considering putting a masonary heater near the center of this room. The house has two wings each extending about 40 out from the central pentagon and each about 17 feet wide. These wings would have bathrooms, bedrooms, utility room and studios. The plan is to enclose these rooms with walls. The masonary heater would certainly heat the central room.
However, it would be a great benefit if it also helped heat the rooms in the wings. I was wondering if there was any effective way to do this. One possibility that occurred to me is to install a circulating hot water system which would use the masonary heater to heat the water. Is this a practical possibility?
A: It is possible, depending on how complicated you want to get.
You can bury a heat exchanger assembly in the space between the core and the facing, and then thermosyphon to a ceiling mounted heat exchanger, or use a pumped system. We can manufacture the heat exchanger, but you need a local plumbing/heating guy who has experience in this area.
Q: Another wood stove specialist (not a masonary heater dealer) suggested that it might be possible to set up a convection pattern that would move the heat to the rooms in the wings. He suggested using cold air returns under the floor to bring cold air from the wings into contact with the heater where it would rise and draw more cold air after it. My mason thought that this would not likely work well with a masonary heater.
Perhaps you know some other method that might work even better.
A: I agree with your mason. I can't see much advantage over natural convection that would happen anyway.
The main variable these days is the efficiency of the house itself. In your case, the fact that your climate is mild also helps a lot. With current technology such as R-2000 (ie., airtight envelope, heat recovery ventilation, high performance windows), it is possible to drop the heat load of the house to very low levels. A side benefit is that there is much less temperature variation and stratification throughout the house. In other words, heat distribution becomes much less of an issue. Outlying bedrooms, for example, might not need heating to 70 degrees around the clock, but only require a small amount of backup heat that could be electric, etc. -- cheap to install, and the operating costs will be very low.
A backup system of some kind is recommended with a masonry heater, in any case. For example, if you leave on a trip you want to keep the house from freezing, and when you return, you want to be able to get it up to temperature. The masonry heater is not very fast at this, because of the time lag from the thermal mass.
Q: My plan is to have adobe floors on the main level and I am considering using radiant floor heat as a way to keep the floors from becoming uncomfortably cold. Do you have any experience with this? Will the hydronic heat be overkill? Even if the loops are fairly widely spaced?
A: Hard to say, definitively. Our house is has a concrete floor with an unheated basement underneath. The basement remains at about 55-60F during the winter, and the main level floor at about 65 - 70 F. The floor is not cold in the wintertime.
If you have centrally located vertical radiant panels, such as you can get with a masonry heater, there is very little stratification between floor and ceiling. The opposite situation would be a house with a leaky upstairs air barrier and a metal wood stove: the wood stove creates an air circulation whereby cold air that falls down the outside walls, particularly patio doors, etc., gets drawn across the floor, and then up and out when it hits the stove.
My own opinion is that a radiant floor is usually overkill, if a masonry heater will be the main heat source. Radiant floors are very nice, but expensive - particularly as a backup system.
Mitigating factors are:
As the house becomes larger and more spread out, it is harder for a central masonry heater to heat every corner. On the other hand, every corner doesn't necessarily have to be heated, as it would with a forced air system: the main space around the heater will remain comfortable, because it is a radiant heating system. This would be a similar effect to a hydronic floor - you could keep different zones of the house at different comfort levels. Then again, as a house becomes more efficient there is automatically less stratification, and heat distribution becomes less of an issue. Often, a very small amount of backup heat will take care of any problem areas such as remote bedrooms. For example, we have a masonry heater in a 2800 sq. ft. R-2000 house near Ottawa that heats the whole house, during 95% of the heating season, in an 8000 degree-day climate.
The maximum practical heat output from a masonry heater, fired twice per day with 60 lbs of wood, is about 7 kW, or 25,000 BTU/hr. You need to relate this number to the heat load of your house at a given outside temperature.
Q: The only way I would consider hydronic heat is if I could heat the water by some sort of heat exhange with the outer shell (perhaps copper tubing enclosed in the outer masonry) of the heater.
A: I was recently contacted by a stove builder from Denmark who uses a copper coil in the outer shell to heat domestic hot water. He estimates the output of the coil to be about 500 W, continuous. This would be a waste of time for a hydronic floor - you'd be lucky to get a temperature rise of a couple of degrees.
We have worked with Mark Klein of Gimme Shelter Construction in Almond, Wisconsin, to design a custom boiler top end for several heaters in order to provide enough output for hydronic floors. This requires a pump and a controller, as well as somebody to design the system, since the hot water from the heater would require tempering if you are using plastic tubing in the floor.
We can supply the coils, but the system integration has to be handled at your end.
To my mind, domestic hot water makes more sense. Our standard coil sits in the firebox. In a passive thermosyphon configuration with the tank, it is a simple and reasonably foolproof setup.
Q: How do you price heated benches?
A: The cost of a heated bench is really the masonry work involved in building the bench, and the bench top, which can vary quite a bit depending on materials (cut stone), and complexity (angled corners). Functionally, it is very similar to a piece of chimney lying on its side.
In pricing a bench, I would evaluate the cost of materials and labour. An extremely complicated bench (wraparound, angled corners, trimming and facing the stone top), would probably take me and a skilled helper about a day to build - if it's your first time you could probably double that.
Quite often we will "rough in" a bench for a client
when we install a core that someone else will finish. This
consists of laying it out, cutting the extra openings in the
bottom channels and chimney flue, and making all the cuts in the
liners and the short connecting pieces of liner to the bottom
channels. There is usually a cleanout or two, and if you need
access to an ashbox for a slab-on-grade heater, it adds a little
complication. I use a 7" Makita grinder with a 9"
cutoff conversion from Makita and a 9" dry diamond blade. It
usually takes me a couple of hours or so. If it is your first
time, it could take considerably longer. Here is an URL with some
Q:Is refractory mortar used between liners? What else do I need to know?
A: For heated benches, I use ordinary mortar between the liners. This is because it is hard to cut liners closer than about 1/4", sometimes 1/2". They often tend to be warped and out of square a bit, making them hard to mark very accurately. There is not much heat stress at the tail end of a contraflow heater, so using regular mortar is no problem. I also use ordinary mortar around any liner connections, and slush in mortar in those areas as well.
The mason then simply builds the bench as if it were a horizontal chimney, except for the benchtop. I tell them to slush all the connections solid with mortar. The benchtop is usually 2" stone, and it is mortared directly onto the flue liner. For long pieces of stone, you could create a slipjoint by laying a piece of fiberglass mat on the liner first.
The benchtop can get fairly hot and the joints between the benchtop stones have a tendency to hairline. Here is what I do: use a 1/4" joint, leave it open. Pump the joint full of type 1 silicone. Immediately, dust dry "mortar mix" powder onto the silicone and press it in slightly. This gives the appearance of a cement joint, but it is permanently flexible.
Q:What is Type I silicone?
A: Type 1 silicone is the ordinary kind that smells like vinegar, as opposed to the paintable Type 2 that doesn't have a vinegar smell. "GE" is the best brand, and we have tested it to 600 F. It is available at Home Depot and Walmart. I believe it is labelled as "household caulk" or something like that - it comes in a blue tube. Use the "clear".
Q:What do you recommend installing around the top and sides of the clay liner? In my case there is an air space of about 4-5 inches between the liner and the facing of the bench.
A: Normally there is about 1" or less of space. You can slush it solid with mortar to the front. Don't slush solid in behind.
Q:Can I fill the space with sand?
A: Definitely don't use sand. Slight movements will cause it to settle and create a wedge, which can crack the bench.
Q:There is only a small space between the top of the liner and the bench cover - what do I do here?
A: Our bench tops are usually 2" stone. These are mortared directly to the top of the flue liner. You can lay a strip of leftover fiberglass wrap on top of the liner, creating a slip joint. Usually there will be some hairline cracking in the joints between the stones if mortar is used for grouting. See the above note regarding silicone joints. We have had 100% success with this method.
Q: What holds the shut off damper frame in place? Am I supposed to mortar it onto the flue liner with heat stop?
A:The best way to hold it in place is to glue it on to the flue liner with silicone.
Q: Am I supposed to omit the mortar joint in front of the damper so that it can slide in and out? I don't see any other way to span that opening, unless I leave out a whole row of bricks and put a chimney block across the opening. Also, if I am supposed to use steel to span the opening, am I supposed to use one or two? One doesn't appear to have enough surface area to hold the bricks.
A: If you are spanning the opening with blocks, the easiest way is to hit a mortar joint, and simply leave it out. ie., the block is supported by mortar on either side of the damper. If you are spanning the opening with bricks, you can use two steel bars to lift the brick up enough to create the slot. Will the bricks show? The above assumes that they won't.
Q: There will be 20+ feet of chimney over the damper. If I corbel the block and rest the chimney on that, will there be too much weight on those blocks?
A: Only the flue liners should be resting on the corbelled ledge. I would do the corbelling out of bricks, ie., insert a course of bricks here to create the corbel.
Q: Your written directions suggest an alternate method of running steel bars above the damper for the flue liners to sit on. Would this be a better method for me since I am using the precast chimney blocks? It seems like if I do it this way, I can only support the flue liner on two sides. That brings up the question again that it seems like a lot of weight on those two small steel bars. Mortaring solid for eight inches doesn't seem to be enough.
A: I've never used the steel bars to do this myself, but suggested it as a method for those who want an alternative to corbelling. You could get extra strength by having them welded together. Unless a welder is handy, corbelling is probably the best method. Ten flue liners (20 feet) is only about 400 lbs, not enought to shear off a 3-sided brick ledge, particularly with an 8" solid mortar connection to the blocks.
Q: If I use the corbeled blocks, should I coat the inside of them with Heat-Stop? For that matter, if I use the steel should I coat that with Heat-Stop?
A: I'd use clay bricks, which will only give you 2" of exposed brick inside the flue. They are the same material as the flue liner (terra cotta, or earthenware clay). Don't parge them with Heat-Stop, as it will only flake off. However, you could use Heat-Stop to set the bricks, so that the mortar joints exposed inside the flue will be made from Heat-Stop.
Q: I just read your paper on masonry heaters and noted the paragraph where you mention the importance of expansion joints between the stove and the veneer. My stone mason is not proficient at veneering fireplaces and I would like to ask what we need to know about doing it right and preventing cracking.
For instance, does he need to keep an air space all around the heater? Is there a big transfer of heat around the doors where the masonry veneer is actually exposed to the fire heat.
A: With our system, all of the expansion joints are taken care of internally. You need to follow the assembly instruction to the letter, particularly about the fiberglass wrap.
As stated in the finishing instructions, there should be a solid mortar slush between the veneer bricks and the core. This is for heat transfer, and to provide a seal. Otherwise, the heater will leak. Do not use an airspace.
Around the doors, there is a firebrick protector on the sides, so that the venner bricks are not directly exposed. This is described in the finishing instructions under "door installation". The main problem area is heat transfer through the steel lintel that supports the face brick above the door. This is why we have the heat shield bricks on the firebox lintel arranged so that they can slide forward and protect the outer lintel as well. It is important that the outer lintel clears the heat shield bricks.
Q: Our masonry heater will sit on a slab-on-grade. Where will the air intake ports be?
A: Unless your code requires it, outside air is not required for the heater to function properly, even in an airtight house. Normally, for a slab on grade there is an ash space under the firebox, with an ashbox door that has an adjustable air supply.
If your code requires outside air directly to the firebox, then you have to run a 6" insulated flexduct in your slab, and put a damper in it. It creates some complication in hooking up the heater, as well. Most jurisdictions will allow makeup air instead. In other words, the outlet can be near the heater, it doesn't have to be right to the firebox.
Q: Is outside combustion air required?
A: Yes and no. Your local code may require it. It serves no useful function, otherwise. This may seem like a radical assertion, however it is based on best available science. For more information, check the seminar on outside combustion air.
Q: I plan to purchase the kit and install it myself this summer. What is the fist step to get the ball rolling? What kind of lead time do you need for delivery? What questions do I not know enough to ask?
A: Normally, clients sent us a floorplan with the proposed heater layout, and I can comment on it and provide a quote for the components. Usually, the first thing to look at is chimney location, and where it will go through the roof, sit on the upstairs floorplan, etc.
We take a 50% deposit to book manufacturing, with the balance due upon receipt of the components. In the summer, about 2 months lead time is good.
This page last updated on January 15, 2001
This page was created on August 9, 1999