Insert/Masonry Chim./Block-off-plate

[tfdchief]

I want to ask everyone's opinion here. Here is the example: Insert in a masonry fire place with a surround sealed to the face of the brick. SS liner connected to the insert, to a top plate - sealed to the top of the brick chimney and tile. SS liner not insulated, just in the clay liner that is in good condition. I know that the preferred way to seal the old chimney to stop inside air from drafting up and out, is a block-off plate and not rely on the surround to brick face seal. Here is my question. With a block-off plate, is there any chance that radiated heat from the insert, that now is trapped in the old fireplace fire box, would heat up the old brick fire box too much and in turn be a possible threat to framing around it? The clearance of framing to the existing masonry fireplace is unknown since it is hidden behind walls, ceilings, etc. What do you all think? I am guessing that without the block-off plate, at least there is room for the heat from the insert to dissipate somewhat up and around the SS liner, inside the existing chimney. Or do you think that the difference would be negligible?

 

[cycloxer]

That's really a tough one to judge. It really depends on the design of the fireplace and chimney. Do you know if they were they built to code (or do they at least look that way)? How much exposed wood and framing are we talking about here? Any pics?

 

[tfdchief]

That's really a tough one to judge. It really depends on the design of the fireplace and chimney. Do you know if they were they built to code (or do they at least look that way)? How much exposed wood and framing are we talking about here? Any pics?

A 1940ish Cape Cod style 1 1/2 story. No way to know if it was inspected for code compliance. Masonry fireplace seems to be well built...goes from the basement with ash clean out to the roof, fireplace in the living room on the first floor. the furnace and HW heater flue is within the same brick chimney, but completely separate flue from the fire place. The only framing that can be seen is in the attic around the brick chimney and there is about a 2 in space visible there. Some electrical wiring has been added on the second floor and the romex goes from the attic to the basement in the space around the chimney.....so chances are the 2 in clearance visible in the attic is maintained the entire height of the chimney. The thing I am getting at is that many insert installs are done in existing masonry fireplaces and there simply is no way to know exactly how it was built. Yet the block off plate seems to always be recommended. I just started thinking about it one day and it has been bugging me.

 

[begreen]

Block off plates are most effective with exterior flues where the heat is going to be dissipated outdoors without one. If this is an interior chimney, then the block off plate may not be as beneficial.

That said, in this case, all is speculation due to the unknown clearances from the brick to combustibles. A clue might be to investigating how many remodels the house has had since the fireplace was built. When we tore out our fireplace, it was clear that subsequent remodels over the life of the fireplace (and there were several) didn't give a hoot about clearances.

Conclusion... assume the worst case scenario for clearances in an older fireplace.

 

[tfdchief]

Block off plates are most effective with exterior flues where the heat is going to be dissipated outdoors without one. If this is an interior chimney, then the block off plate may not be as beneficial.

That said, in this case, all is speculation due to the unknown clearances from the brick to combustibles. A clue might be to investigating how many remodels the house has had since the fireplace was built. When we tore out our fireplace, it was clear that subsequent remodels over the life of the fireplace (and there were several) didn't give a hoot about clearances.

Conclusion... assume the worst case scenario for clearances in an older fireplace.

Near as I can tell, there have been no remodels. No one has any knowledge of the history of the house. "Conclusion... assume the worst case scenario for clearances in an older fireplace"I agree with that statement completely. I am just not sure how that computes into whether or not to put in the block off plate. My thought is that it would be safer to not put it in and let the heat from the insert have more room to dissipate. I don't think there is much air getting out of the house through the old chimney anyway. By the way, this is my son's house and I have been constantly monitoring since we installed it. I am paranoid with my family! You expert installers and forum members chime in here! I want ALL opinions. It just seems to me that this aspect of an insert install in existing fireplaces may be something that has not really been considered much.

 

[begreen]

I don't remember the thread, but this was discussed a while ago. If the insert is jacketed, I don't think this is much of an issue. That adds another layer of protection and the blower will cool it off a lot. But I can appreciate wanting to be extra sure, especially when family is involved.

 

[michaelryba]

Personally, I would only worry about the situation you describe if the insert is a really tight fit in the fireplace. The stonework around my fireplace is only warm (my IR gun at this moment says 105 °F right above it). Also, I run the fan 100% of the time when burning to put that heat into the room. If there were a power outage & you were cranking the stove for hours it could be a potential hazard.

 

[tfdchief]

I don't remember the thread, but this was discussed a while ago. If the insert is jacketed, I don't think this is much of an issue. That adds another layer of protection and the blower will cool it off a lot. But I can appreciate wanting to be extra sure, especially when family is involved.

It is jacketed on all sides. That is what I have always thought as well....just don't like guessing as there is no way to know what the outside temp. of the stove is behind the surround. But thanks, I am getting more comfortable hearing the same things I have thought.

 

[tfdchief]

Personally, I would only worry about the situation you describe if the insert is a really tight fit in the fireplace. The stonework around my fireplace is only warm (my IR gun at this moment says 105 °F right above it). Also, I run the fan 100% of the time when burning to put that heat into the room. If there were a power outage & you were cranking the stove for hours it could be a potential hazard.

Pondman, that is exactly what I have found with the one in question. Thanks for the colaboration. He runs his fan all the time as well and it thermostatically increases speed with the temp of the fire. I think that will keep the outside from getting to hot and radiating much heat. I still think the best in this situation is to not have the block of plate and that seems to be what you are indicationg I think.

 

[cycloxer]

I don't think the block off plate is going to have much of a negative effect w/ regards to safety. Actually it acts as a firedoor to the chimney should you experience a chimney fire. The area to be most concerned with is the liner. This is the hottest component. Here is where you really want to know your distance to combustibles. I know that you know this, but with a block off plate at the top of the chimney, you drastically reduce any upward airflow out of the chimney.

I vote for the block off plate as the surrounds on the inserts do not really seal up very good with the face of the fireplace. However, a properly constructed block off plate can be made virtually air tight. There is a performance benefit as well.

 

[tfdchief]

I don't think the block off plate is going to have much of a negative effect w/ regards to safety. Actually it acts as a firedoor to the chimney should you experience a chimney fire. The area to be most concerned with is the liner. This is the hottest component. Here is where you really want to know your distance to combustibles. I know that you know this, but with a block off plate at the top of the chimney, you drastically reduce any upward airflow out of the chimney.

I vote for the block off plate as the surrounds on the inserts do not really seal up very good with the face of the fireplace. However, a properly constructed block off plate can be made virtually air tight.

cycloxer, Thanks. Good thoughts. "Actually it acts as a firedoor to the chimney should you experience a chimney fire." I am not sure what you are saying here. Can you explain your theory of the "firedoor". If there is a block off plate and top plate, then the air space around the liner (where the flue fire wood be, inside of it) is a dead air space theoretically, isn't it? How would that help during a flue fire?

 

[cycloxer]

...should anything burn inside the liner or chimney, it can't get inside the fireplace and into the room unless it melts through the metal. Okay, this is more important w/ a free standing stove in a hearth. I suppose w/ an insert w/ a tight fitting metal surround it has less importance.

There was a house fire in December in Northampton, MA and the pictures were posted on the forum. It was with an insert in an improperly built fireplace. The area that caught fire was some wood bracing a few feet up from the fireplace opening. The mortar had broken away and exposed wood. The liner was too close to this exposed wood and eventually, after some time of drying out, the wood it ignited.

 

[tfdchief]

...should anything burn inside the liner or chimney, it can't get inside the fireplace and into the room unless it melts through the metal. Okay, this is more important w/ a free standing stove in a hearth. I suppose w/ an insert w/ a tight fitting metal surround it has less importance.

There was a house fire in December in Northampton, MA and the pictures were posted on the forum. It was with an insert in an improperly built fireplace. The area that caught fire was some wood bracing a few feet up from the fireplace opening. The mortar had broken away and exposed wood. The liner was too close to this exposed wood and eventually, after some time of drying out, the wood it ignited.

I saw those pics! OK I understand now. I would hope, even if there was a chimney fire in the SS liner that the creosote left in the old masonry chimney would not catch fire because it certainly wouldn't have anywhere to go. We cleaned the old chimney before putting in the SS liner as good as we could but of course there was some, very little, reisdual left. It just wasn't possible to get it all. Thanks for the explaination.