Venting Question for Masonry Heater Owners & Fireplace Gurus

[honeysmomcooks]

Have to say that the masonry heater that we had constructed in our home is one of the best investments we could have made! Love-love-love it! Heater works and heats as it should.....as long as you never open the doors after the fire is lit. That being said, I have a question for those of you much more knowledgeable than I/we. We have a problem with smoke and ash billowing into the house when the door is opened to add wood. In investigating this, we have come to the tentative conclusion that the makeup air piping of 4" in diameter (which runs under the concrete slab and radiant heat tubing) is not large enough. According to the geothermal heating guys (who we talked to when it was suggested that there might be a negative pressure issue - house is definitely NOT sealed tight yet), a 4" pipe is only capable of moving something like 30cfm of air and what should have probably been installed was more like an 8" pipe which I understand can move an exponentially greater amount of air and would correct this problem. At this point, digging up the concrete slab and severing heat lines is not sounding practical, feasible, or appealing. Do any of you happen to know of some sort of fan or other solution that could be used to boost the airflow to send the smoke up and out?? We have a four socket electrical plug on one side of the fireplace that we could plug into if needed.
Have included some construction pics for reference. Cleanout above mantle and below heated bench on front, electrical on side without heated bench, two cleanouts on back. Top ring has since been attached to ceiling and doors are on.

Hope that all makes sense! Sincere thanks to you all in advance for any help that you are able to offer.

 

[woody&sons]

Masonry heaters aren't really meant to have wood added as the fire burns anyway.
Your solution is to build the fire, light it, close the door, and leave it closed until the fire has completely gone out.

Do you mean that all of the gasses from your firebox go through a 4" pipe at some point? That would be most of your problem. Masonry Heaters shouldn't smoke even with the door open. The smallest diameter opening in a heater is the top of the throat, which would be something like 3"x18". Is that an 8 inch round flue on top, or is it a smaller diameter insulated pipe? 8" is okay, 9 or 10 is recommended, or 8 by 12 clay flue (for about an 18" firebox). A masonry heater relies on liberal air supply and will not work well without a large enough chimney. If your chimney is smaller than 8"round, replacing it with 8"round or larger should help a lot.
Also, the shape of your firebox has a lot to do with your smoke problems. A masonry heater firebox is much deeper with a smaller door opening, 18x18 door for example or 18x18x22.5"deep compared to the 36"x 28"x about 10" deep firebox it looks like you have there. This is like a rumford firebox on a masonry heater. It looks like you will have a hard time filling your firebox full because of it's shallow shape, but if you have good glass in your door your should be able to put a good sized fire in there, big enough to heat the unit, Just keep the door closed. hope this helps

 

[honeysmomcooks]

Thank you so much for your thoughtful response, but no, the top flue is not the issue. It is plenty big (8 or 10" I think?) for what should be going out. Also, firebox measures approx 20" deep and about 34" across the doors. We generally load it with roughly 50 lbs of wood, twice a day. Plenty of heat. The issue we think that we are having is with the make up air coming in to the firebox from outside of the house. The smoke fills the firebox completely when you are trying to start the fire, forcing us to resort to firestarters (like a tiny Duraflame thing) to start the flame. If you attempt to start a fire with paper and it doesn't catch the first time, then you have to either wait until ALL of the smoke dissipates or suffer the consequences of smoke billowing into the house. Same goes for if a lit log rolls over and you wish to shove it back into the pile before going to bed - smoke and ash in the house.

So, here is the theory that we have. Inside the firebox there are two tiles, one on either side, with a hole in them. These are tied into a 4" pipe, which, as previously described, runs under the slab to a vent outside of the house. This way makeup air is brought into the firebox and allows the fire to have enough oxygen to burn well when the doors are closed. We suspect that the pipe is undersized for the amount of makeup air that the fire needs to draw, thus not allowing enough air movement to allow the fire to expell as efficiently as it should, and making it tough to start the fire properly (without it choking itself out). Does this make better sense??

So, our question again is.....is there something that we can do to aid/boost the air movement and help to make this unit more efficient without having to jackhammer up the slab, cut heating pipes and fork out alot of money?? Thanks again for your input and help!

 

[woody&sons]

Are you sure you have the dampers in the right position? You have a unique design there.

 

[Metal]

Find out what the inside diameter vent you have, how many feet there are, and what model/make of stove you have. That will give us some information to work with.

 

[KeithO]

Your masonry heater looks more like a fireplace than a true masonry heater. Normally the firebox is much deeper and narrower and usually the loading door is much smaller. Remember that the air temperature gets heated from ambient to over 1000F when pasing through the heater, so there is a substantial degree of expansion that takes place during heating. It could be that a 4" air feed may be OK. The heater should be designed to only recieve enough air for combustion, not tons of excess air like what happens with a fireplace. Excess air cools down the flue gasses which is no good because this reduces the amount of heat which can be transferred to the core.

Do you have construction photos of the heater ? Does it have a downdraft principle ? or is there a direct connection between the firebox and the chimney outlet above ?

 

[d.hebert]

A simple way to see if you have enough make up air in the house is to open a window in the room then open the door on the heater. If smoke still pours out then that is not your problem. Now if the 4" opening is referring to the combustion air intake to the heater I would think that is adequate. I have a Temp-Cast and we usually only have the intake door cracked open an inch, making for about a 4" opening. Your problem may be related to draft or it may simply be a shortcoming in the design. I don't recognize your heater design but appears to be of a russan design instead of a more common finnish contraflow. Perhaps you have more of a convoluted baffling system with results in a slower draft which could cause smoke to pour in the house when you open the door. As long as the heater heats well, does not produce creosote, and does not smoke with the door closed, and dosen't trigger CO2 detectors I would learn to live with it. Good luck, it's a beautiful heater.

D.Hebert
Temp-Cast 2000 Masonry Heater