Solid Rock Masonry LLC in Duluth, Minnesota builds custom masonry heaters across the Upper Midwest and nationwide, and one detail comes up on almost every job: building a masonry heater against a wall. When a masonry heater goes against a wall and the back will not be seen, we lay 4-inch solid concrete block (CMU) behind it for mass and to meet building code. Working in only 4 inches of clearance has always been the hard part – keeping that back wall plumb and level, and keeping the block from pushing out when you backfill behind it. Over 25-plus years we have settled on a simple field method: a two-piece rigid foam spacer that solves both problems at once and ties cleanly into the stone front and the side wing walls. Here is exactly how we do it.
By Eric Moshier – Certified Heater Mason, Third-Generation Mason, MHA Technical Committee Member, ASTM E1602 Masonry Heater Group Member
The problem with building a masonry heater against a wall
Two things fight you when you lay a 4-inch block back wall in a tight space. First, keeping the wall plumb and level when you can barely reach behind it. Second, the block wants to push out and go crooked the moment you start backfilling behind it with the core expansion material. For years those two problems made the back wall the slowest, most frustrating part of the whole heater. The fix turned out to be simple.
The 4-inch rigid foam spacer
We place two 2-inch pieces of rigid pink or blue foam board between the sheetrock and the block. Stacked together they measure exactly 4 inches, which is the required clearance behind the heater, so the block is automatically set at the right distance on every course. Because the block bears against a solid, straight surface, it stays plumb on its own and you only have to level each course. The foam also backs up the block so it cannot blow out when you backfill behind it.
>> Masonry heater building codes and clearances
How the spacer moves up the wall
We stand three 2-inch strips of 2″ ridgid styrofoam, about 6 inches wide by 48 inches tall, against the sheetrock, then set a 48-inch by 48-inch sheet in front of the strips, tight against the 4-inch CMU. Lay a couple of courses of block, and when you reach the top of the foam, pull the three strips out, wash the foam with a sponge and water and then slide the whole spacer up to the bottom of the last course laid. It is fast, it keeps the wall straight and plumb the whole way up, and it takes the blowout problem off the table, which is the whole challenge of building a masonry heater against a wall.
Tying the back wall to the stone and wing walls
If you are putting full stone on the front, or building 4-inch wing walls on the sides so you cannot see behind the heater, the back wall has to be tied to that stonework or it can work loose over time. We cut #9 or #11 wire into pieces about 18 inches long. One end is looped into the block and embedded in the horizontal bed joint of each course, and we leave a tag end sticking out past the block. When the side stone or wing wall goes up, that tag end is looped into the stonework, tying the back wall and the sides together into one solid unit. We prefer this to standard metal brick ties. Either way, always tooth the back wall into the front walls at the corners so everything locks together.
Whenever we enclose the back side with wing walls, we always leave a 4″ x 4″ opening at the bottom of the wing wall on both sides so air can move behind the heater. On a standard-height heater we leave an open air space at the top. It you are worried about a cat or other critters getting behind the heater, you can install metal grates to cover the opening. On a tall heater that runs to the ceiling, 8 or 9 feet, we instead leave a 1-inch gap across the top of the entire heater, so the heat can come out the back side and off the top of the cap rather than being trapped behind the stone.
On the finished heater, the wing wall often carries air flow vents – one low and one high. Room air is pulled in through the lower vent, rises through the channel behind the stone as it warms against the mass, and returns to the room through the upper vent. It is a simple convective loop that pushes gentle, even heat into the room while the core radiates. From the outside you would never know the 4-inch block back wall, the foam spacer, and the wire ties are all doing their work behind the stone. Alternatively, if you wanted to heat a room behind the wall of the heater. You could also install a cold air return grate at the floor in that room along with a grate at the top of the heater to allow the warm air to circulate in that room. The room could reach temperatures over 70º, so make sure there is a louvre on the grates to be able to shut them off or turn them on if the heat was wanted. Grates on the heater would also be needed for this method.
Why it matters for a masonry heater against a wall
A masonry heater is a radiant heat storage appliance. Its mass is what soaks up the heat from a short, hot fire and radiates it back into the home over 12 to 24 hours, so every part of that mass, including the back wall, has to be built solid, plumb, and tied together. Getting the back wall right on a masonry heater against a wall also keeps your required clearance exact and consistent, which matters for both performance and code. This is one of the small, unglamorous details that separates a heater that lasts generations from one that gives you trouble.
>> What is a masonry heater?
>> Masonry heater planning guide
>> Building your own masonry heater (DIY)
>> Our custom masonry heaters
>> Masonry heater gallery
>> More Solid Rock Masonry resources
Talk to Solid Rock Masonry
Solid Rock Masonry LLC
5347 Howard Gnesen Road, Duluth, MN 55803
218-343-2978
eric@solidrockmasonry.com
www.solidrockmasonry.com