Chase area around wood fireplace gets too hot

[Mr_D]

I have an RSF Focus 320 Wood fireplace installed by the local dealer. A carpenter and myself built the chase structure ahead of time as per the specs of the manual and talking with the dealer several times.

The fireplace install team seemed to do a good job but when it was all done I did some checking and found the chase area gets quite hot. I asked their technicians about it but they didn't really know if that's typical because they usually close up the front wall with tile, brick, etc and head out to the next job. In my case I don't have the tile or mantle installed yet so I still have access to the top of the fireplace and the chase area so I took some temperature readings.

After running a few hours the chase walls get up to 188F sometimes and that worries me. I think I need to redesign the chase venting, add a fan, or adjust how it's insulated. The RSF Focus 320 is on the main floor with cathedral ceilings. I bought the internal blower option and the basement heat dump kit with blower.

The chase is part of the house thermal envelope and is air sealed and insulated with 2x6 construction. It's 2ft deep, 6ft wide and 25ft tall. The Chase extends 7ft above the roofline and is insulated the entire way up. The insulated SS pipe terminates 2ft above that. It's quite tall overall.

The stovepipe is the proper dual wall, ICC brand 7" ID 9" OD stainless steel insulated chimney. It goes straight up with no bends of any kind. It has the factory spec vented flashing at the top along with radiation shields and so forth where it passes next to (through) the roofline. I also have far more than the minimum clearances of 2" to the chimney. The chimney actually has 7" clearance to the back and front walls and 27" to the side walls. That's far more than needed but the problem is the whole chase gradually gets really hot over the course of a few hours.

The chase walls are 2x6 studs insulated with spray foam to R20. It was sprayed from the outside onto the back of the drywall between the studs. Then covered with OSB, housewrap and vinyl on the outside The inside is fully lined with 5/8" type X drywall. The top of the chase and the chase liner next to living rooms wall has 1/2" Durock cement board for safety rather than drywall. Now I wish I had used Durock board on all the interior surfaces because drywall with paper facing and wood framing isn't really meant to take 180 to 190F degree's long term.

I read the temp with a fluke IR meter and also installed a J-type thermocouple up near the top of the chase and they agree on the temperature. I also have thermocouples on the firebox, chimney flue and the outer chimney pipe.
Firebox top = 250-330F depending on burn rate
Flue pipe 4" above firebox 400-680F
Chimney outer wall temp = 150-190F

The lower chase where it shares room air (due to no inner wall yet) stays cooler at 115F.
The upper area's of the chase run much hotter and the ceiling gets up to 188F. I'm sure having the house walls insulated is not helping because other than the small vent at the top the heat has no where to go. This used to be an outside wall so it's insulated. So the upper chase area ends up being insulated on all 4 sides.

I'm considering removing the living room drywall above the fireplace to access and remove the original insulation so the excess chase heat can flow into the house rather than being trapped in the chase.

Or, I could add passive vents or thermostatically operated fans in the chase to send the heat back into the house, effectively cooling the chase. It's certainly hot enough so I'm not worried about cooling the chimney itself too much. I read 300F+ coming out the top of the chimney.

I'll try and post a picture to make things more clear.
Basically... is it ok to vent the chase heat back into the house? especially when it's far hotter than it needs to be?

Lastly, the fireplace installers did not install a firebreak of any kind because I have cathedral ceilings even though I think it still needs one. The chimney didn't pass through any floors even though the house has a 2nd floor a few feet over from the fireplace. So I installed a Durock board firebreak about 7ft above the fireplace (11ft above the floor). I left a 1 -2" air gap around the chimney. The durock board cuts down the room air that was blowing out the top of the chase and also I didn't like the idea of having no firebreak at all. RSF sells an air tight radiation shield for high efficiency houses so that implies I could seal off the firebreak completely to stop the air loss but I didn't do that till I find out more. With the chase open to the vent at the top I was getting frost on all tiny air leaks in doors and windows since the top vent in a tall, hot chase depressurized the house a lot.

Thanks for any insights on the best way to make this a safe install.
Note there are 4 JPG photo's attached, but the preview only displays the first one, so far.

Regards,
Dave

 

[Backwoods Savage]

I'm going to bump this to the top to get other opinions.

First, I think you are okay according to what I've read and when you get that finished off, then the temperatures inside the chase will go down. It might be interesting to get some better opinions though. Good luck.

 

[oconnor]

When I looked into ZC fireplaces, I understood that the chase was sealed above the stove as you mention. It may not be required, but makes sense, it would keep stove heat in the rooms. I don't think it would keep the chase cooler though.

Essentially, the stove would be inside the building envelope, abs the flute would be outside the envelope.

Perhaps a call to the manufacturer would be worth while.

 

[Backwoods Savage]

I definitely would.

 

[begreen]

Is the blower running when these temperatures were taken? It looks like the work is being done well and RSF is a quality system. Still, 190F continuous temps on a combustible make me a bit nervous. That is at the high end of the safety factor. The chase temps are already close to this and this is with it still mostly open. For sure the chase will get hotter at times once it is closed. Have you contacted RSF for guidance? I can see why you are considering adding an upper and lower vent to the chase to reelase some of this captive heat. Keep us posted with your findings from RSF.

 

[begreen]

One other thought would be to gravity vent the fireplace. Theoretically that should cool down the stove though there would be some heat loss in the chase from the ducts.

 

[Mr_D]

Good point from 'begreen'. The flue temp drops nearly 100 degree's if I turn up the internal blower to max. The heat dump blower to the basement also pulls heat but that one pulls air from the side of the firebox and doesn't blow as much heat or affect the flue temp much. The internal blower is optional so it should not be required for proper operation but I'm glad I have it. I use it 24x7 to try and lower the chase temps by lowering the flue temps but it's not enough to resolve the hot chase. With the blower off I see 170 -188F in the upper chase ceiling and with the blower on it drops 10-15 degrees at most.

Note that there is almost zero heat coming off the top or sides of the fireplace. It's literally at room temp on all sides which is kind of amazing in itself.
So all the stove output to the room comes out the front of the fireplace via the glass and the vents. That means all the heat making the chase too hot is coming from the insulated chimney.

What about wrapping the insulated chimney (which is R4 at best) with ceramic blankets or 1/2" ceramic boards to hold in the heat?
That should reduce the heat that goes into the chase and improve the situation. They want the chimney hot anyway right? So this would make it even hotter due to less heat loss into the chase area.

Yes, I've talked to RSF several times. They usually respond within a day via Email and were fairly knowledgeable. RSF Canada is International phone rates from here so I didn't call them. Regarding high chase temps and no firestop installed by their local installer, RSF said as follows:


In terms of the actual chase temperature, as long as the required 2” clearance to combustibles is in place, you do not have to worry about temperature. The chimney is tested and listed to be safe at closer clearance than your chimney is installed, so you do not have to worry.

I am surprised that they did not recommend installing a firestop in your chase. There was lots of time and effort that went into making that chase as insulated as possible, but the primary place for air to enter (the top) is not being buffered. It’s could be equated to building a home without an attic as a buffer for cold air.


I thought they would be more concerned considering how hot it gets and the chase is not even closed off yet.
I mentioned to RSF that the gypsum board manufacturer recommends 125F max continuous heat but he didn't reply on that part.

I would predict a fan system affecting the heat a lot. What about the Gravity Vent mentioned by 'begreen'. how would that be implemented?

Keep in mind I have access to the tall living room walls and can vent into the chase from there. but actually there is still at least 12ft of chase above the living room walls up towards the roof that I can't squeeze myself into very well for servicing or adjustments. And that is where the most heat is, at the top. So one way to pull off the heat could be to open up an access hole high up on the living room wall and insert a 6' vent pipe up into the chase all the way to the top so it can draw hot air from the top area. Then put a blower or fan on the lower end installed behind a normal looking HVAC grill. The fan can draw heat from the top back into the living room through the new vent pipe. If needed there could be 2 variable speed DC fans in push-pull mode with each one set at optimal speeds that balance out the room and chase pressure as required. The idea is to move the heat into the living room without blowing excess conditioned air out the vent at the top of the chase or drawing in outside air.

Has anyone tried venting a chase with fans or perhaps has a simpler solution to a hot chase?

4 JPG's attached..

• Chase view above roofline
• Chimney view before chase top added
• View of top vent
• Radiation shield at the top

 

[oconnor]

Given the vent in the top of the chase, I think the chase should be considered outside the house envelope, and the air tight fire stop you mentioned installed above the fireplace. You could use spacers and durock in the chase to form a radiation shield around the flue. Unless the flue instructions specifically permit it, don't put anything around the flue for insulation. They usually require an air gap for cooling. Again, check the chimney instructions to confirm what if any air gap around it must be maintained.

 

[oconnor]

Here is a section from the Excel chimney manual that should apply to your install.

You may also completely cover the top of an FOCUS 320 as long as you maintain all fireplace standoff clearances and the 2" clearances around the chased chimney. The 2" clearance around the chimney must be open from the fireplace up to the ceiling. See Figure 13 for an example of a close clearance installation

 

[Mr_D]

The manufacturer RSF from Canada insisted I close up the interior walls around the fireplace before reading the chase temperature.
They said it would cool down and the open chase was the issue.
So to evaluate the heat conditions of the chase area I closed up the wall and ran some tests to send the data back to the factory so they can help figure out what to do.

I closed off the chase walls with Durock and monitored 3 thermocouples (chase top, middle, and flue temp) to record temperature data vs time.
As expected; with no room air to mix in the chase walls got even hotter than before. As a result I had to stop the test early before the chase reached equilibrium temperature. It went past 210°F near the top of the chase and was still going up when I stopped the test. I didn't catch the max temp on the attached IR photo but it's close, and it matches the contact thermocouple readings.

RSF had a short reply last month regarding hitting > 200°F on drywall with a 125°F limit. "As for the heat ...we don’t see anything wrong, we test at much higher temperatures."
They are either ignoring the science and related USG drywall maximum temperature ratings for drywall or not understanding the issue.
I requested what and how they tested but they didn't reply.


Chase temperature Test 1 setup:

• Chimney: ICC 7" Excel dual wall stainless steel installed vertically with no interruptions or bends.
• RSF Focus 320 Fireplace installed with the factory internal blower and upper vent as per the install manual.
• All factory standoffs and clearances are installed and in effect.
• The chimney has the factory radiation shield and vent at the top of the chase.
• USG 1/2" Durock panels are in place around the fireplace front opening forming a fairly effective air barrier
• Thermocouple meters in use were calibrated using an Ektron model 1120 thermocouple calibrator.
• Optional, fireplace variable speed internal blower and air dump kit are installed and turned off for the test.
• Fireplace air inlet is set to use outside combustion air.
• The intake air damper was set open to max for starting, then reduced to 50% position for the duration of the test.
• All temperature meter electronics are located outside of the chase area in conditions and operate at room temp.

TEST 1 setup: (see chart 1)
Fireplace was filled 60% full with dry, split hardwood maple and oak.
Fire was allowed to burn without refill except for 1 small split added at time 1:10.
Test was started and allowed to heat up and cool down in 5 hours without interuption.

Test 1 recorded peaked at 201°F and would have gone higher except the fire cooled off as the wood was used up.

TEST 2 (see Chart 2)
Test 2 used a fuller load of wood at 75% full and saw higher temperatures.
Chase temps near the fireplace itself in the lower area's of the chase ranged from 70-90°F and eventually maxed out at 130°F near the end of the test. Test readings were viewed through a temporary 5x10" access hole in the Durock board for purposes of meter readings. The meter access hole was air sealed during the test.

Chase temps further up the chase were much hotter than the lower area's and ran higher after the chase area was enclosed.
The higher up the chimney pipe I go the hotter the temps are. Chase temps hit 150F in 1 hour, 205°F at 1.5 hours and continued up to 210°F at which point I stopped the test by closing the intake air damper and turning on the fans to try and cool it down.

Conclusion:
The chase design and material selection need significant changes to provide a safe fireplace installation since it runs far hotter than the installed drywall is rated for.

Here is my list of possible fixes. What do the pro's suggest?

• Install better chase ventilation to expel more of the heat coming off the chimney. Add an intake vent half way up the chase to assist.
  Chimney pipe temps runs 400-700°F inside anyway and > 200°F on the outside. These temps are actually hotter as you go further up the chase due to the chase air being 120F hotter up there. Maybe RSF - ICC chimney makes a dual wall chimney pipe with 2 -3" of insulation instead of 1" that it has now? that might cure it right there.
  
  
• Use high temperature thermal barrier for the chase walls instead of low temp drywall rated at only 125°F.
  
  
• Vent excess chase heat back into the house with passive vents or fan assisted air flow.
  
  
• Enclose the chimney itself in 1200°F ceramic insulation (12-16" insulation pipe or use flat sheets) while respecting the 2" chimney clearance rules. I'd give it 4-6" clearance around the pipe so it's like a high temp chase around the chimney. The heat getting to the outer drywall would be cut by half or more and then I could vent the chase with cool outside air and the chimney would still stay hot for no creosote buildup.
  
  
• Install a firebreak with a radiation shield 8-10ft above the floor to address safety concerns and limit the loss of conditioned room air out the chase top vent This should have been in the original design. With a firestop /air block in place then the really hot section at the top could vent with cooler outside air.

I've been waiting for RSF and the local Fireplace installer to have a real reply withi a useful answer for 2 months now. I wrote many times and called as well but I think they don't want to hear about these cases if there is a nationwide issue using low temp drywall. The dealer said they normally just use 1/2" drywall, close 'em up and move on. They have no idea what the interior temperatures really are.

 

[oconnor]

...Maybe RSF - ICC chimney makes a dual wall chimney pipe with 2 -3" of insulation instead of 1" that it has now? that might cure it right there. ...

What is the model of chimney you are using? I'm surprised you mentioned a 1" insulated chimney. I thought their wood burning chimney was a 2 " unit. 1" is usually used for gas and oil venting.

 

[oconnor]

I gave the manuals a read. I see the ICC 1" is approved for their RSF fireplaces. I did note that they require a full length radiation shield when that same chimney is used with other stoves. That may be your solution.

Oddly, the USA instructions for excel 7" chimney show it having two inches of insulation.

 

[orionrising]

I think your problem is the spray foam, urethane foam is hugely thermally reflective rather then absorbant and prohibits all air movement. When it first came into use they called it self extinguishing until they had some bad fires on unfinished structures, as small fires were insualted and reflected so well they flashed. It now requires a thermal barrier over installations. I believe it is 1/2 drywall minimum.

 

[begreen]

The chase is well insulated so it doesn't surprise me that the upper chase temp is getting close to the chimney surface temp. A fireblock is required at the first story level. With one in place a screened vent at the top of the chase should alleviate concerns. If not, remove the sheetrock and replace with cement board.

 

[Mr_D]

O'Conner, Thanks for reading all the way through this. The chimney is the ICC 7' EXCEL, model XC-7EL48.
It's 7" ID and 9" OD, so a 7' pipe with 1" insulation on each side. Probably R-value 3.5 or so. I took a photo of the pipes before it was installed to see what type of insulation it had, as shown below. It looks like rock wool to me.

I was not aware there is a full length radiation shield available. That's very interesting.
They may not spec the stove to use it but I can certainly inquire about that. That's basically what I'm trying to do, come up with a way to insulate the pipe better to keep the radiated energy away from the chase walls while not getting the interior pipe too hot either, if that is a concern. I suspect the 2" air gap requirement supplies a bit of chimney cooling via air convection while protecting nearby combustibles at the same time.

I measured 235°F on the outer wall of the metal chimney while it was running and the pipe was in 70°F room air at the time. I used high temp flat black paint to make a square shape Infrared target on the pipe with an emissivity of approx .95. It's painted directly on the chimney surface to get a decent IR reading. Krylon's 1602 is supposed to provides a surface emissivity fairly close to .95 and that works with most IR meters.

Each flat black IR target looks like a hot red emitter to the IR gun since that's the only place it can get a reading. Otherwise you can't measure stainless steel due to the reflections and you end up reading the wall adjacent to it. Other items I've painted all measure correctly compared to direct contact thermocouple temps so I think this method with prepared targets is close enough for basic calculations.

The chimney temp appears to be cumulative as you go up. I was able to measure chimney temps up to 8ft high so far but I wonder what it reads 27ft up near the top where it's exposed to 200°F chase air instead of the cool 70°F air down below.

 

[orionrising]

you could run a bit of a duct up to the top and a small ventalation fan sucking the air back in the the living space maybe?

 

[oconnor]

The part in the image about the Canadian Install instructions is what I'm referring too. Not sure how the chimney clearances and temps would be different in the same chase with different wood burners attached - I'd seriously look into the RST and RSF insulation shield.

I would not recommend the idea from earlier of wrapping the chimney in insulation, ceramic or otherwise. It needs the air space to cool.

I assume you have installed a block off above the fireplace, essentially making the chimney cavity a dead air space? If so, the venting it to the outside or to the room would seem easier.

 

[Mr_D]

The chase is well insulated so it doesn't surprise me that the upper chase temp is getting close to the chimney surface temp. A fireblock is required at the first story level. With one in place a screened vent at the top of the chase should alleviate concerns. If not, remove the sheetrock and replace with cement board.

Yes, I agree about the fireblock. The local installers insisted I didn't need one due to having a cathedral ceiling and the chase didn't pass through any floors. I should have insisted but it was my first fireplace and the company that has installed hundreds of units "should" know what to do. It's a lot harder to retrofit now but possible.

The chase goes past the living room ceiling and then about 5ft of attic space so I agree it needs a firestop. That would also limit air loss out the top vented flashing that makes the whole house negative pressure at this time. I'm trying to get the dealer to install the official factory radiation shield with a fire stop about 10ft up but they keep waiting for the factory to have a reply.

> If not, remove the sheetrock and replace with cement board.
Removing 1000lbs of drywall and replacing it with something else inside a narrow, confined, 27ft tall finished chase is not very practical. I'd rather insulate the pipe or walls with high temp insulation of some kind and figure out better venting. Also, cement board of itself such as Durock may not be adequate. The USG rep told me it can only be used up to 200°F because it has a low R-value of .39 in 1/2" panels. The low R-value doesn't offer much thermal protection for materials or insulation behind the panel so it has to be installed with a thermal insulation board of some type in certain installations.

 

[begreen]

I think a firestop and venting may be sufficient. That seems like the most practical approach.

 

[Mr_D]

you could run a bit of a duct up to the top and a small ventalation fan sucking the air back in the the living space maybe?

Excellent idea. I did that actually as an experiment. I ran 24ft of 4" flex metal duct with a 4" 60cfm fan located at room level and it draws air from the top of the chase.
The air comes out of the duct into the room at 140°F. The flex pipe isn't insulated so it cools a fair amount as it passes through the lower, cooler portions of the chase but it does help at least.

A 60cfm fan drops the upper chase temp by 20F. It lowers the temps from 200F down to 180F.
That's a step in the right direction but it appears I need quite a bit more air flow to make much difference using this approach.

I'd prefer less heat getting into the chase from the chimney but 1" of rockwool in a dual wall chimney can only do so much considering there's 400-700°F flue temps on the inside of the pipe.

 

[Mr_D]

I think your problem is the spray foam, urethane foam is hugely thermally reflective rather then absorbant and prohibits all air movement. When it first came into use they called it self extinguishing until they had some bad fires on unfinished structures, as small fires were insulated and reflected so well they flashed. It now requires a thermal barrier over installations. I believe it is 1/2 drywall minimum.

It has 5/8" USG type X drywall on the chase interior and 1/2" Durock cement board on the top and upper section where it connects with the house. At the time I thought Durock was the best we could do for fire rating but the low R-value of cement board is a concern.

 

[Mr_D]

I would not recommend the idea from earlier of wrapping the chimney in insulation, ceramic or otherwise. It needs the air space to cool. I assume you have installed a block off above the fireplace, essentially making the chimney cavity a dead air space? If so, the venting it to the outside or to the room would seem easier.

Regarding wrapping the chimney... I recognize the heat buildup concern. Since it requires a 2" clearance what if I used a 20" ID insulated pipe or just a metal heat shield? That would give it about 5.5" of clearance on all sides like a small chase and I could vent that out the top with a larger vent or even an an air intake up high 7ft below the top if needed.

I tried blocking off the fireplace at the 10ft level with a temporary Durock firestop and left a 1" air gap around the pipe. Without the influx of room air it ran significantly hotter so I had to remove it.

Ultimately I think it will have a firestop that splits the chase into upper and lower cavities. The lower level vents into the room (with accessible fans if need be) and the lower level doesn't run very hot. The upper level then vents to the outside with as much vent as needed to cool it down. The vented flashing at the top around the pipe may need to be changed out for something with more air flow. The increased flow up high shouldn't affect the lower chase or living space too much due to the added fire stop.

 

[oconnor]

I expected to see the instructions mention an airtight radiation shield where the chimney passes through the vapour barrier, like you would see in a ceiling connection box. That would be the firestop.

The heat shield is mentioned in the pic above. Excel sells one to install over the full length of the chimney when required.

 

[begreen]

Before considering wrapping the chimney be sure the mfg agrees and provides a list of suitable insulation.

 

[Mr_D]

Before considering wrapping the chimney be sure the mfg agrees and provides a list of suitable insulation.

I'm trying to work with them and a list of suitable insulation would be great but I'm not counting on it because it's not likely they test alternate designs in detail.

They run a standard UL test for each model of stove and chimney mounted in a predefined test environment and it has to meet certain conditions.
They probably aren't going to recommend anything other than the specific combinations and materials they tested even if the results out in the field are quite inadequate regarding safe wall temperatures.

Each mfg should specify a maximum R-value for insulated, enclosed wall assemblies around fireplaces.
Each product has it's own output characteristics and the wall /vent system needs to dissipate the heat coming off the chimney and fireplace chassis while remaining within safe operating temperatures.
Saying we should insulate the chase with no specs of any kind is too broad. I'd follow a recommendation if there was one.

A high temp product of ceramic, glass or rock wool insulation panels around the chimney itself with proper spacing (or lining the chase walls) along with appropriate ventilation should be close to ideal.
We'd have a warm chimney with reduced creosote buildup and reliable, fire rated, relatively cool chase walls. They don't go that route due to the material expense and cheaper products such as drywall gets by in many cases. Although they don't check temperatures after the install so how would they really know if it's adequate or not?

The rep I talked to today has been in the business for 20 years and has never actually measured how hot their products get.
Maybe exceeding US Gypsum max drywall temperature by nearly 100°F is a National sized can of worms they hope will go away.