Where is my heat going?

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woodgeek

Minister of Fire
Jan 27, 2008
5,471
SE PA
So, after many years burning wood in an old coal insert (not a lot of wood, mostly ambiance and emergency backup), in 2020 I pulled the trigger on a new Fireplace-X Large, nextgen fyre, with a 2020 EPA cert and a very big pretty window. And secondary combustion tubes. LOL.

They installed it in May 2021, and I cured the paint on the last couple cool days before it got too warm to burn.

I have been learning my way around this thing for the last couple months, burning more than a half a face cord of well seasoned Ash (cut, split and stacked under a roof for 14 mos) on cold days, and trying different control settings.

The good news: The stove behaves as you expect, lights right up, I have figured out how to do reloads with only little/no smoke spill (my old stove had a lintel plate that dropped down when you opened the door, which made spills really unlikely). I can run it with the primary all in, and get a bright fire, pull it out and the stove gets darker and switches to secondary only (so the door doesn't obv leak too much). I get slightly higher temps (measured by IR temps off the window) and slightly longer burns with the air pulled out slightly (decreasing the primary air). Window temps on the IS are maxing out in the low 900s °F. (Assuming standard emissivity, temps of the glass may be higher, ofc).

My wood is mostly blocks, so my fires are generally of the 'short hot' variety, not densely packed, and going to coals in 3-4 hours.

The problem you say? I don't think I'm getting as much heat as my old stove!! The backstory is I have a sense that a box of wood in the old insert can heat my house 100% at 30°F outside for x hours. When I crunched numbers years ago, I worked out that so many lbs of wood per hour and the known BTU load of my house worked out to be 40-50% efficiency for the old stove. Nerd.

This stove should be BETTER than the old one, and has a bigger fire box and secondary combustion... it should easily heat my house using the same or LESS wood. Instead, my HP is working harder (still cycling versus shutting off) with the new stove.

Am I sure, comparing new to old over the course of years? Yeah, I'm sure.

My theory: MY old stove had a hard block off plate that I installed and sealed and leak checked myself. The NEW stove does not have a hard block off plate, the the installer swore the top of the chimney would be sealed airtight around the liner adapter (to a terra cotta chimney), so a blockoff plate would be unneeded. (the old system was a direct connect, which is why I did the blockoff plate).

Two bits of data:

1) if I burn for hours with the blower on med, my finished walls around the chase around the interior masonry chimney get barely warm. Can't feel it with my hand, but I can see the chimney flue with a FLIR, barely. If I then turn OFF the blower, an hour later the chase is very warm to the touch, even in the upper story where the insulated liner and flue are. My interpretation is that the chimney is drafting cool room air up around the outside of the liner, keeping the chase cool (the liner insulation is thin, its hard to imagine the chase being so totally cool otherwise. When I cut off the blower, the firebox now heats up (ofc) and now preheats the same air drawn up outside the liner.

2) When I am burning, and turn off the blower, there is air getting sucked in around the surround. Enough to draw in a match flame or smoke from a blown out match. Not a HUGE amount, but probably more than several cfm. This is not convection, the draw around the surround is inward all around its periphery. That is my smoking gun.

3) Part of my lost heat might be me running the blower on low, to reduce noise. The window is huge and radiating a lot of BTUs. I figure that the heat in the box has nowhere to go but into my house (tall interior chimney), so lower blower just means hotter air out of the blower and more radiant. But it is clear that lower blower gets me a lot less heat (more HP cycling). Cranking the blower (which is annoyingly loud on high) I get enough BTUs (finally) to get my HP to shut off, at least while the stove is cranking. Which never happens on med blower. I think this is suss.

Sound sensible to you guys?

Course of action with installer? Haven't been on my roof in years. Did he blow off sealing the adapter on top? Do a chitty job. Did the seal break?

What do I do now?
 
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IR readings off the glass are going to be misleading and not very helpful. Better to take them on the black metal of the stove body adjacent to the top of the door. The block off plate makes a notable difference in reducing heat loss from the insert. There's no point in heating outdoors. Put it back in with some roxul or kaowool insulation on top. In the meantime, run the blower on low. It will still improve convection. We very rarely run our blower on high.

What was the old stove? Did it project out onto the hearth or was it flush?
 
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I took the surround off the old stove. And there is no exposed firebox steel on the new stove, so I am using the window temp for relative measurements.

I get that the surround is keeping the heat in the masonry firebox, but I want that heat to leak out into my house later (interior chimney) (and get extracted by the blower later), not get bled off through the flue around the liner.
 
In a sense, this is comparing apples to oranges. Flush inserts work mostly by convection, that is, they are blower dependent. Removing the surround reduces this somewhat and a block-off plate can make a large difference. To even the score I would put one in and if there is room behind the insert, then I would put some insulation behind the insert too. This has made a day and night difference in some installs with a cold exterior wall fireplace.
 
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I’m learning a new insert. The only way I know of is to constantly monitor temps. Flue gas temps (I am measuring mine right at the adapter) are best ( nearly instant response). stove tops are ok. As are liner temps. Blower has to be running to be efficient I think. The more air the more efficient to a point of using to much electricity or cooling the firebox below clean burn temps.

I have run no blockoff , a loose blockoff and now an insulated but not air tight. Any heat going tip the chase or chimney will never make it into the house. I have an interior chimney and I can tell the difference. I am going to try and seal it better but it’s a pain as I it was installed after the the stove.

Being able to see flue temps and measure stove top temps have helped me dial in an efficient minimum burn rate. (At least what I think is an efficient one).
 
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In a sense, this is comparing apples to oranges. Flush inserts work mostly by convection, that is, they are blower dependent. Removing the surround reduces this somewhat and a block-off plate can make a large difference. To even the score I would put one in and if there is room behind the insert, then I would put some insulation behind the insert too. This has made a day and night difference in some installs with a cold exterior wall fireplace.
Your point is well taken, but the maker claims that the window is so large that a significant portion of the heat is radiant (compared to a flush with a smaller window). That plus interior tall chimney made me think I'd be OK with the blower on 'medium'. But I seem to be getting a lot less heat on 'medium' than 'max' blower setting.

And what about sucking air around the surround... anyone worried about that? A soft blockoff won't dent that.
 
Your point is well taken, but the maker claims that the window is so large that a significant portion of the heat is radiant (compared to a flush with a smaller window).
That's just marketing. A flush insert is mostly convective. With a blockoff plate, you will notice a marked increase in output temp coming from the top convective outlet.

I wonder if there is leakage through the masonry chimney to outdoors through tiny grout cracks? If so, that would explain the suction. Or it might just be leakage under the storm collar? A blockoff plate should also help reduce this.
 
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Leakage elsewhere seems more likely than the installer not putting HT silicone on the top adapter plate.
 
I noticed a huge difference when I installed my block off plate, I remember recording the temp difference, don't remember the actual number but it was significant and definately worth the effot to make it.
 
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Leakage elsewhere seems more likely than the installer not putting HT silicone on the top adapter plate.
Mine did. Then the liner got hot expand and pushed the top plate over and inch or so. Came back year later fixed it but did the same thing again. I fixed it with lots of silicone. This year.
 
Mine did. Then the liner got hot expand and pushed the top plate over and inch or so. Came back year later fixed it but did the same thing again. I fixed it with lots of silicone. This year.
Thanks, that is one thing I was worried about.
 
Did a few measurements today, and some calculations....

With the new hotness FPX ripping with a full load, and the blower turned up all the way, the temp (IR) of the metal air outlet is about 450°F, and the window (IR, emissivity = 0.8) is around 800°F.

So, BTU/h for hot air is simply = 1.08 * CFM * DeltaT(°F) but we need to know the CFM.

When the stove cooled enough, I used my anenometer to measure the air speed on max blower. Multiplied by the outlet area, I get about 80 cfm. The blowers are rated at 180 cfm, so this is a bit disappointing (the 180 cfm is at zero back pressure, unmounted).

So, 450°F output and 80 CFM is ~33,000 BTU/h.

As for the window, an IR thermo is a radiometer, not a thermometer. It assumes the emissivity is 0.8, and estimates the window temp to be about 800°F (warmer in the middle, cooler near the edges). I got the area of the window as 0.205 m^2.

Using Stefan's law, I get a net radiant emission of 3250 W = 11,000 BTU/h. (this takes into account the 0.8 emissivity and the input IR in the room).

So the way I am burning it (full, but not stuffed to the gills), I am getting a peak out the front of 44,000 BTUs/h. Which seems pretty reasonable.

The radiant fraction is high enough to be consistent with what I feel from it (it feels like 3kW of radiant), while also agreeing with @begreen's estimate that the radiant is a small portion of the total output (25%).

FTR, I also measured the CFM at different settings. Max (loud): 80, Med (just below loud): 67 cfm, Med (quiet-ish): 41, L (silent): 30.

Looking at that, running the blower low enough to be quiet cuts the heat output in half. Turning it down a little bit (to not be screaming) only costs 20% on the heat output.

The retrofit blower I put on my old stove was 125 CFM on high unmounted, but 90 CFM through the stove. So how the bigger (180 cfm nominal) blower on the FPX is only giving 80 cfm is a mystery to be investigated further.....
 
Hoping you find the answer soon..terrible--you did all that to decrease your bill and now the bill is so much more higher after your efforts to keep it down...sad..clancey
 
Sorry to reply so late. I have no clue about your stove, but WOW, that math blew me away! I understood some of it. Reminded me of Math classes in school.
 
Sorry to reply so late. I have no clue about your stove, but WOW, that math blew me away! I understood some of it. Reminded me of Math classes in school.
He is a teacher and a good one.
 
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I only have a piece of crap stove, but there is only a door on the front, thats it. Plus my damper to control her...other wise its a cast iron rectangle with fire it in. Do I know its not the same as a new one? Yes. Its just so simple....open the door, put wood in, close door.....thats it.
 
Sorry if I missed this point in the posts, but does the stove have an outside air kit? If not, you may be feeling the combustion air flowing around the surround. $.02
 
I've got the new FPX ripping again with the cold, on some rather punky (but dry) oak I got from a friend.

Its popping out the heat, keeping my $$$ electrical strips from running, and just needs a reload every 2-3 hours bc of the punky wood.

I am happy with the stove, its doing its job, easy to run, and looks good at the same time. It might not be getting the efficiency it is rated for (the point of this thread), but that is something I can always try to fix later. In the meantime, the secondaries are keeping the emissions down relative to the old boat anchor it replaced.

:)
 
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Old boat anchor, lol....thats a good one! I just feel if youre warm, and not burning your house down, you are doing ok. I am nervous every day kinda.....but its this, or oil for 1000 bucks....no way man!
 
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I have a top plate that is sealed to the terra cotta with high temp silicone. No block off plate. Insulated liner in a terra cotta lined glue. My stove seems to run fine. But I’ve often been curious, does all that heat generated by the liner being in the terra cotta, and it being sealed at the top with no where to go, create a lot of pressure? Where does all that heat go? Absorb into the masonry? Dissipate towards the bottom where the stove is? One would thing it would possibly blow the top plate right off at some point
 
I have a top plate that is sealed to the terra cotta with high temp silicone. No block off plate. Insulated liner in a terra cotta lined glue. My stove seems to run fine. But I’ve often been curious, does all that heat generated by the liner being in the terra cotta, and it being sealed at the top with no where to go, create a lot of pressure? Where does all that heat go? Absorb into the masonry? Dissipate towards the bottom where the stove is? One would thing it would possibly blow the top plate right off at some point
No. It will not blow the plate out. And yes all that heat dissipates into the masonry
 
The wind applies a lot more force to that plate than any warm air rising.
 
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I have a top plate that is sealed to the terra cotta with high temp silicone. No block off plate. Insulated liner in a terra cotta lined glue. My stove seems to run fine. But I’ve often been curious, does all that heat generated by the liner being in the terra cotta, and it being sealed at the top with no where to go, create a lot of pressure? Where does all that heat go? Absorb into the masonry? Dissipate towards the bottom where the stove is? One would thing it would possibly blow the top plate right off at some point
I would pull the surround and take a look at the install and see if you can insulate the fireplace and install a block off plate, you will see a difference in heat output, especially if you have an exterior chimney.
 
I would pull the surround and take a look at the install and see if you can insulate the fireplace and install a block off plate, you will see a difference in heat output, especially if you have an exterior chimney.
I’ve considered it, but honestly, I have zero complaints about heat output. My regency heats my 1800 square feet house with no problems at all.
 
If the chimney is internal, the heat going up there is (mostly, depending on the fraction of chimney inside and outside) not lost to the home, mostly. If it's fully external, a block-off plate would allow you to heat the same, but using (somewhat) less wood.
I'm not sure that (wood usage) would be meaurable (others might have experience), but it might increase room comfort by preventing cold air from coming out of the vents when the insert is not in use.