By Eric Moshier – Certified Heater Mason, Third-Generation Mason, MHA Technical Committee Member, ASTM E1602 Masonry Heater Group Member
This is the masonry heater use and care guide from Solid Rock Masonry LLC of Duluth, Minnesota. Eric Moshier is a Certified Heater Mason with more than 25 years in the trade, a member of the Masonry Heater Association Technical Committee, and a member of the ASTM E1602 Masonry Heater Group. A masonry heater is a radiant heat storage appliance. You burn a full load of dry cordwood fast and hot for 2 to 3 hours, the masonry absorbs that energy, and the heater radiates it back into the house over the next 12 to 24 hours. Operating one correctly is almost the opposite of operating a wood stove: you never damper it down while flames are present, never smolder it, and never try to make a single load last all night. Get the fuel, the load, the lighting, and the damper timing right and a masonry heater will run cleanly for generations with almost no service. Get them wrong and you will build creosote, foul the heat exchange channels, and shorten the life of the firebox. This guide covers what to burn, how to load and light, how long to burn, when to close the damper, firebox capacity ratings, ash and glass care, and seasonal start-up. It applies to site-built masonry heaters, SR Core heaters, kachelofen and kakelugn tile stoves, and soapstone heaters.
>> New to masonry heating? Start with What Is a Masonry Heater.
>> Download the Solid Rock Masonry Heater Owner’s Manual (PDF) – the complete document this page is drawn from.
Why Operation Matters More Than Maintenance
A masonry heater has no blower, no catalytic combustor, no baffle plate, and no electronics. There is very little to service. What there is instead is a firebox that reaches 1,600 to 1,800 degrees Fahrenheit and several tons of masonry that expand and contract with every firing. How you run the heater determines how long all of that lasts.
Nearly every repair call we take traces back to operation rather than to a defect: wet wood, smoldering fires, over-firing, a chimney damper closed too early, or a heater fired hard after sitting all summer. None of those cost anything to avoid.
Fuel: What to Burn in a Masonry Heater
Fuel is the single largest factor in how a masonry heater performs. Burn well-seasoned cordwood with a moisture content below 20 percent. Wood must be cut, split, and stored under cover and off the ground for 6 to 8 months minimum. Oak can take 2 years or more. A moisture meter costs around $100 and removes all the guesswork: split a piece and read the fresh face, not the outside of the log.
Split every piece to 3 to 6 inches in diameter and cut to approximately 16 to 18 inches long. Uniform size burns more evenly and lights faster. Large rounds and unsplit logs burn slowly, which is exactly what you do not want in a masonry heater.
Any species of hardwood is fine, and softwood works too as long as it is dry. Dry softwood beats damp hardwood every time. Properly dried wood shows end-grain cracks and makes a sharp cracking sound when two pieces are banged together. A dull thud means it is still wet.
Stack your wood off the ground on pallets or rails and put a roof over the top of the stack, leaving the sides open. A tarp thrown over a woodpile traps moisture against the wood and blocks air circulation, so the pile stays wet underneath. Most houses use 2 to 4 full logger cords per heating season, with 3 cords being about average. Run your wood supply on a 2-year cycle.
Never burn
- Wet, green, or unseasoned wood above 20 percent moisture.
- Treated, painted, stained, or glued lumber, including plywood, OSB, particle board, and pallets.
- Driftwood or any salt-exposed wood.
- Trash, household waste, cardboard in quantity, or paper products beyond a sheet or two for lighting.
- Coal, charcoal, or manufactured fire logs.
- Flammable liquids of any kind. Never use gasoline, kerosene, lighter fluid, or a starter gel to light a masonry heater.
Burning any of these voids the Solid Rock Masonry warranty.
Before You Light: Ash and Draft
Rake residual ash through the grate into the ash box so the firebox floor is open and the air inlet is not buried.
If you have not had a fire in the past 24 hours, verify draft before lighting. Light a match and crack the firebox door. If the flame is pulled into the firebox, you have draft. If flame or smoke pushes back into the room, the chimney is in downdraft and must be primed before you light a full fire.
To prime a cold chimney: open the cleanout door nearest the chimney, open any dampers, then light a half sheet of newspaper and place it inside the cleanout. Repeat until draft is established. A lit propane plumbers torch aimed into the cleanout will also work in most cases, though it does not work as well as flaming newspaper because there is less heat in the torch. Close the cleanout door before lighting the wood.
Loading the Firebox
Burn directly on the firebrick floor. Do not use andirons or a fire basket. The firebox geometry was designed around a fire sitting on the floor, and lifting the fuel changes the airflow the design depends on.
Load anywhere from 30 percent up to 100 percent of the rated design fuel load for your model. Wood can be stacked log cabin style, straight in, or tepee style. Place the kindling on top of the wood pile, or along the side of the pile on the larger 22 to 32 inch wide fireboxes. Keep the load below the top of the firebox opening and clear of the door glass.
If your firebox is designed for a 60 lb load, you may burn up to 60 lb every 8 hours. Burning more than the rated load in 8 hours is over-firing, causes premature damage, and is not covered under warranty.
Ambiance and extended burns
If you want a longer fire for ambiance or company, do not load 100 percent of the wood at once. Start with 30 percent of the fuel load, then add 1 to 2 logs every hour or so, up to the maximum rated load. Keep the damper 100 percent open the entire time. Do not damper down the way you would with a wood stove. Dampering down chokes combustion, causes dirty burning, and deposits creosote in heat exchange channels and bells that are not easy to clean.
Lighting: Top Down, Every Time
Light a masonry heater from the top of the load, not the bottom. Place kindling and newspaper or a natural fire starter on top of the stacked splits and light there. The fire burns downward through the load, the flue warms early, and smoke from the wood below passes up through an already burning flame front. A top-down burn has been tested to be both the fastest way to get a fire going and the cleanest.
>> Do Masonry Heaters Need Outside Combustion Air?
The Burn: 2 to 3 Hours, Dampers Wide Open
Open the combustion air damper on the firebox door fully. If a chimney damper is installed, open it fully. If you have a heated bench damper, open that too. Both the chimney damper and the door air intake must stay 100 percent open any time flames are present.
A full load burns down in roughly 2 to 3 hours. During that time the exterior of the heater will barely feel warm. That is normal. The heat is going into the mass, and the surface will not reach its peak until several hours after the fire is out. Judging a masonry heater by how warm it feels during the burn is the most common misunderstanding new owners have.
Masonry heaters burn very clean when operated correctly on dry wood. If you ever see creosote, either the procedure is wrong or the wood is above 20 percent moisture.
Rake the coals
When the large flames have died down and you see primarily small blue flames, rake the coals to the front of the firebox. That concentrates the remaining coals and burns them down faster so you can close the dampers sooner. At this coal stage you may close the air intake damper on the door by 50 percent.
Closing the Damper: The One Real Safety Rule
How you close down depends on whether a chimney damper is installed.
No chimney damper installed. You may close the air intake damper on the firebox door when a small amount of coals is still burning. Any residual carbon monoxide will still vent up the chimney.
Chimney damper installed. When only coals remain and there are no large flames, close both the chimney damper and the door air intake by about 50 percent. Wait to close the chimney damper fully until all coals have finished burning. Closing the chimney damper too early traps carbon monoxide inside the house. That is dangerous.
- Chimney damper and firebox air intake must be 100 percent open whenever flames are present.
- Dampers may be closed by 50 percent when only coals remain.
- Dampers may be fully closed only after all coals have burned out completely.
- If you smell wood smoke in the room, you closed the damper too early. Open it back up.
A damper left open, or a door gasket that no longer seals, lets room air wash through the heater and carry your stored heat up the chimney for the entire 12 to 24 hour discharge cycle. Always have working carbon monoxide and smoke alarms installed to code, and install new batteries at the start of every heating season.
How Often to Fire
One firing every 24 hours covers most homes through shoulder season. In deep cold, fire twice a day, roughly 12 hours apart. Two fires per day is the maximum on every model. Adjust output by load size and firing frequency, never by restricting air. A smaller load burned hot is correct. A full load burned slow is not.
Firebox Capacity Ratings
Heat output below is calculated at 7,000 Btu per pound of seasoned cordwood under 20 percent moisture content, at 78 percent average system efficiency. Remember that a masonry heater is a radiant heat storage appliance and this output is spread across 12 to 24 hours. Do not compare these figures directly against the peak BTU per hour number from a Manual J heat loss calculation.
| Model | Firebox | Fuel load per fire | Max load per 24 hours | 1 fire per day | 2 fires per day |
|---|---|---|---|---|---|
| SR-13 | 13 in w x 18 in d | 35 lb | 70 lb | ~191,100 Btu | ~382,200 Btu |
| SR-18 | 17 in w x 18 in d | 60 lb | 120 lb | ~327,600 Btu | ~655,200 Btu |
| SR-22 | 22 in w x 18 in d | 65 lb | 130 lb | ~354,900 Btu | ~709,800 Btu |
| SR-24 | 24 in w x 18 in d | 70 lb | 140 lb | ~382,200 Btu | ~764,400 Btu |
| SR-32 | 32 in w x 18 in d | 75 lb | 150 lb | ~409,500 Btu | ~819,000 Btu |
>> SR Core DIY Masonry Heater Kits – models, configurations, and pricing
Ash, Glass, and Everyday Care
Ash
Rake ashes through the grate into the ash box daily. Empty the ash box about once per week if you burn 2 fires per day, or every other week at 1 fire per day or less. Lift the grate, scoop the ashes into a metal bucket with a tight lid, and place cool ashes in a compost heap, garden, or lawn. Never store ash in a combustible container, and never set an ash pail on a wood deck. Assume the ashes are live for several days.
Glass
Clean the glass with newspaper dampened in water. For stubborn spots, spray an orange-oil-based wood stove glass cleaner. Do not use abrasive pads, scrapers, or oven cleaner. Normal hot fires keep the glass clear apart from some darkening at the outer edges, which happens on almost any masonry heater door and is expected. If the entire pane is going black regularly, check your wood moisture, your load size, and your door gasket first. If none of those explain it, the firebox design or a blocked channel may be the cause and it is worth a call. Bake oven door glass does not stay clean because oven temperatures never get hot enough to clear soot. That is normal.
Door and gasket
Test the seal once a season. Close the door on a dollar bill and pull. If it slides out with no resistance, the gasket rope has compressed and needs replacing. Gasket is a wear item, and replacing it is the cheapest heat recovery available on an older heater.
>> Solid Rock Masonry Heater Doors – replacement doors, glass, and gaskets
Starting the Season: Warm Up Before You Fire Hard
A heater is cured once, when it is built. You do not repeat the curing schedule every year. What we do recommend each fall is two or three smaller fires at roughly one third of your rated fuel load, spaced 8 hours apart, before you go to full design load. An already-cured heater does not take on enough moisture over a summer to be at real risk. This is a cheap precaution, not a repeat cure. It costs you a day and it removes any question.
>> Masonry Heater Curing Schedule – the full 14 day cure and 15 fire sequence for a new heater
Annual Care: What Happens Once a Year
Before each heating season the heat exchange channels get vacuumed out through the cleanout doors and the chimney gets swept, along with a short inspection of the firebox, the gaskets, and the exterior masonry. Solid Rock Masonry does not perform chimney sweeping or channel cleaning. A certified chimney sweep is the right call for annual cleaning.
>> Masonry Heater Maintenance – the full annual channel vacuuming and chimney cleaning routine
Using a Bake Oven or Cook Surface
If your heater has a black oven, the oven shares the firebox and its temperature rises and falls with the burn. Bake after the fire has burned down, working the falling curve: bread and pizza first while it is hottest, then roasts, then slow items and drying as the oven cools. A white oven, heated indirectly, holds a steadier temperature and is easier to schedule around. Both oven types use the same daily heating fire with no separate firing required. Keep the oven floor swept and use an oven thermometer rather than guessing. The oven door gasket wears the same way the firebox gasket does, and it is replaceable.
Masonry Heater Use and Care Quick Reference
| Item | Correct practice |
|---|---|
| Fuel | Seasoned cordwood below 20 percent moisture, split 3 to 6 inches, cut 16 to 18 inches, dried 6 to 8 months minimum |
| Fire location | Directly on the firebrick floor. No andirons, no fire basket |
| Load | 30 to 100 percent of the rated design fuel load for your model |
| Lighting | Top down, kindling on top of the load. Prime a cold chimney first |
| Air control | 100 percent open whenever flames are present |
| Burn time | 2 to 3 hours |
| Heat delivery | 12 to 24 hours of radiant heat after the fire is out |
| Coal stage | Rake coals forward. Dampers may close 50 percent when only coals remain |
| Damper | Fully closed only after all coals have burned out completely |
| Firing frequency | Once per 24 hours typical, maximum 2 fires per day |
| Ash | Rake through the grate daily. Empty the ash box to a covered metal container on a noncombustible surface |
| Glass | Damp newspaper, or an orange-oil-based stove glass cleaner. No abrasives |
| Door gasket | Dollar bill test once a season. Replace when it slides free |
| Season start | Two or three fires at about one third of rated load, 8 hours apart, before full firing |
| Annual service | Channels vacuumed, chimney swept, firebox and exterior inspected |
What Never to Do
- Never close the chimney damper while flame is visible or fuel is unburned.
- Never damper the air down to stretch a fire while flames are present.
- Never burn wet, treated, painted, or salvaged construction wood.
- Never use andirons or a fire basket. Burn on the firebrick floor.
- Never use flammable liquids or gel starters.
- Never exceed the rated fuel load in any 8-hour period, or fire more than twice in 24 hours.
- Never fire a heater hard after the summer without warm-up fires.
- Never use the heater to cure drywall mud, or fire it while drywall mud is curing.
- Never store wood, kindling, or anything combustible against the heater or inside its required clearances.
- Never keep firing a heater that is spilling smoke into the room, has a crack you can see through, or has loose firebrick. Stop and have it inspected.
Masonry Heater Use and Care FAQ
How long should a fire burn in a masonry heater?
2 to 3 hours. A full load burns fast and hot with the dampers wide open, and then the fire is done. The masonry releases that stored energy over the following 12 to 24 hours.
Why does my masonry heater feel cold while the fire is burning?
Because the heat is going into the mass, not into the room. Starting from a cold heater, with the brick or stone sitting at about 70 degrees, expect 2 to 4 hours minimum from lighting before you feel warmth on the outside. That is roughly when the fire finishes, and the surface keeps climbing for hours after that. The lag is normal and it is the whole design. If you fired the night before, the heater should still be warm when you light the next one, so you will not notice the delay at all.
Can I add wood partway through the burn?
Yes, if you are running an ambiance or extended burn. Start at 30 percent of the fuel load and add 1 to 2 logs every hour or so up to the maximum rated load, keeping the damper 100 percent open the whole time. What you must not do is exceed the rated load in an 8-hour window, or damper down to stretch the fire.
When exactly do I close the chimney damper?
Close it about 50 percent when only coals remain and there are no large flames. Close it fully only after all coals have finished burning. Closing it early traps carbon monoxide in the house.
How many fires per day can I run?
Two, maximum, on every model. Each fire may be up to the rated fuel load for your firebox, and the two fires should be roughly 12 hours apart. Exceeding the rated load in any 8-hour period is over-firing and is not covered under warranty.
Do I have to re-cure my heater every fall?
No. A heater is cured once, when it is built. Two or three smaller fires at about one third of rated load before you go to full firing is all we ask for at the start of the season, and it is a precaution rather than a necessity.
How much wood does a masonry heater use in a season?
Most houses use 2 to 4 full logger cords per heating season, with 3 cords being about average. Your number depends on your house, your climate, and how well your wood is seasoned.
My glass keeps going black. What is wrong?
Some darkening at the outer edges of the glass is normal on almost any masonry heater door. It comes from the way heat circulates in front of the glass. Our doors have air wash built in at the top and bottom, which helps considerably but does not eliminate it entirely. The center of the pane should stay largely clear because the fire runs hot. If the whole pane is going black regularly, that is different. Wet wood above 20 percent moisture, a load too small to reach temperature, or a gasket that has stopped sealing will all do it, and so will an incorrect firebox design or a blocked heat exchange channel. The last two need to be looked at rather than burned around. Send photographs of the door and the firebox. Bake oven glass is a separate case and will not stay clean, because oven temperatures never get hot enough to burn soot off.
Is it safe to leave the house while the heater is burning?
Treat it like any solid fuel appliance. Someone should be home through the active burn, both because the damper has to be closed at the right moment and because no fire should be left unattended.
Questions About Your Heater
If your heater is not behaving the way this guide describes, or you are not sure whether what you are seeing is normal, send photographs of the firebox, the outside of the heater, and the chimney along with a description of what it is doing. Most questions can be answered without a site visit.
Solid Rock Masonry LLC
5347 Howard Gnesen Road, Duluth, MN 55803
Phone: 218-343-2978
Email: eric@solidrockmasonry.com
Web: www.solidrockmasonry.com
>> Masonry Heater Planning Guide – design, sizing, finishing, and construction
>> Additional Masonry Heater Resources – manuals, guides, and downloads
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>> Are Masonry Heaters Worth It? Complete cost analysis
Eric Moshier is a Certified Heater Mason, a member of the Masonry Heater Association Technical Committee, and a member of the ASTM E1602 Masonry Heater Group. Solid Rock Masonry LLC has been building, repairing, and restoring masonry heaters, fireplaces, and stonework from Duluth, Minnesota for over 25 years.