Air temperature from fireplace insert

[Theo]

Recent post on glass temp inspires this one. Not sure how to measure "working" temperature of an insert, since there's no exposed top. Maybe I need to drill a probe into the flue.

I'm using a new VC Montpelier insert that doesn't seem to be working right - can only raise house temp by about 30F, which isn't going to be enough. While trying to figure the the problem (unsuccessfully, so far), I used a thermocouple gauge to measure temperature of blower air. Wire tip was positioned about 2" into the right-hand top duct, halfway between top and bottom plates. Fuel was seasoned oak baked a little extra in front of the insert.

With a deep bed of glowing coals and a full head of flames from several pieces of the re-dried cordwood, these were the temperatures in degrees F:

Air Open Closed
Fan
High 122 122
Low 136 134

Repeat some time later:

Air Open Closed
Fan
High 126 124
Low 141 138

(Sorry; can't seem to use tabs to make a neat table here.) Changing the secondary air flow didn't affect heat output significantly, even though the flame characteristics were very different. This was unexpected - might be related to why insert isn't delivering much heat.

After each change in combustion or output air flow, the temperature took only about 30 seconds to stabilize. Guess there isn't much thermal mass between the hot box and the ducts.

Anyone else have air temp measurements? Perhaps I'm expecting too much?

-Theo

 

[Corey]

Seems a little cold to me. I'm just starting another load now, so I may be able to post some numbers in another 1/2 hour or so. It's a bit curious that you mention seasoned wood, but then mention "seasoned oak baked a little extra in front of the insert" and "re-dried". This leads me to ask is this truly seasoned wood which is gray, has checks on the ends and the bark is falling off or something the wood guy said was seasoned? If it is the latter, this may affect your ability to get good heat out of the insert.

Ok - actually had a little power flash and had to go reset my router...the insert is now running 650F on the top inner surface (I have a vent that lets me measure the inner steel plate with an IR thermometer) and the air coming out the vent is 300F. Still have a ways to go to be under full fire.

 

[karl]

I've checked mine with a digital thermometer and its' about 190 degrees with fan not quite on high. That's hold the thermometer in the air stream about three or four inched in front of the outlet.

 

[RedRanger]

Well I have never measured that particular temperature of air. Can say though that I am careful about putting my face in direct line of that air flow,, it is coming out of there hot enough that the blast from that blower air is downright painful :shut:

 

[allhandsworking]

Recent post on glass temp inspires this one. Not sure how to measure "working" temperature of an insert, since there's no exposed top. Maybe I need to drill a probe into the flue.

I'm using a new VC Montpelier insert that doesn't seem to be working right - can only raise house temp by about 30F, which isn't going to be enough. While trying to figure the the problem (unsuccessfully, so far), I used a thermocouple gauge to measure temperature of blower air. Wire tip was positioned about 2" into the right-hand top duct, halfway between top and bottom plates. Fuel was seasoned oak baked a little extra in front of the insert.

With a deep bed of glowing coals and a full head of flames from several pieces of the re-dried cordwood, these were the temperatures in degrees F:

Air Open Closed
Fan
High 122 122
Low 136 134

Repeat some time later:



Air Open Closed
Fan
High 126 124
Low 141 138

(Sorry; can't seem to use tabs to make a neat table here.) Changing the secondary air flow didn't affect heat output significantly, even though the flame characteristics were very different. This was unexpected - might be related to why insert isn't delivering much heat.

After each change in combustion or output air flow, the temperature took only about 30 seconds to stabilize. Guess there isn't much thermal mass between the hot box and the ducts.

Anyone else have air temp measurements? Perhaps I'm expecting too much?

-Theo

Theo Split wood smaller around 4” season in the sun and wind at least a year. Smaller splits go faster! My Montpelier blows hot air that is uncomfortable to the hand when held infront! Buy the way Unseasoned wood burns like crape in all inserts also Its got to be the wood your trying to burn! Unless there is a problem with the insert or intall!

 

[ControlFreak]

Hi Theo,

Everybody here has their brains calibrated to talk in terms of the temperature of the stove, not the temperature of the air being blown out. Get one of those magnetic thermometers and put it on the stove somewhere that's closest to the hot spot of the stove.

It makes me wonder... Maybe I need to put a probe on my stove and see what the temp of the output air is... Could be pretty interesting!

Dan

 

[gpcollen1]

You need to know the temp of that thing. Is this the stove where you put the temp gauge on the front of the stove somewhere up top?

For starters, with a low or medium fire, the fan should usually be running on low. For hotter fires, you can run it up to high.

Your issue here indicates that you are not burning the stove hot enough to get the airflow hotter. You need to tell us more about this 'seasoned oak' because 'baking' it in front of the stove is not really going to do too much if there is still enough moisture inside. You should try, as mentioned previously, to split your pieces smaller to burn it hotter. In addition, if you want to get the stove really hot, you want to get it going and pack that firebox full of good wood.

Bottom line is I think there is a bit of user error here that needs some fine tuning...not the stove.

 

[Theo]

You need to know the temp of that thing. Is this the stove where you put the temp gauge on the front of the stove somewhere up top? ... Bottom line is I think there is a bit of user error here that needs some fine tuning...not the stove.

I have been measuring the forced-air temperature because that is how much of the heat gets into the room, and because the thermal path between firebox and air ducts is shorter, and has less inertia, than for any of the cast-iron externals. Blown air responds in a few seconds to changes in fire conditions, where external surfaces take many minutes. I have also used a wire-probe digital thermocouple gauge to follow the temperature of various metal parts and spaces inside the insert.

I make fires with fully seasoned splits in the 2" range to build a coal bed quickly, then move up to 4-5" for the longer burn.

The problem turned out to be not user error, but stove defects. My Montpelier had arrived with a badly damaged air gate, cracked liners and a detached thermo-switch. (Some of this damage took a while to uncover.) It took an absurd amount of time and effort to get service from an unhelpful dealer. Even with evidence of a hard drop or other mis-handling, dealer absolutely refused to believe that there could be anything else wrong. He asserted that it was definitely under-firing, or definitely over-firing, or definitely needed a chimney baffle, then etc. - anything but actually take look at the unit in operation.

After multiple ineffective visits from service agents and regional distributor, I eventually managed to get VC involved. Their guy quickly determined that there was no secondary burn, and that the primary air channels underneath were not working. They swapped in a demo unit, which apparently works to spec; blower-air temperatures can be well over 200F.

If anyone wants to know how the insert behaves without that small but critical bottom air feed, you can open the two grilles, reach under and cover the two round holes with fingertips. (The metal there doesn't get too hot.) In normal operation, this feed drives a gas-generating fire in the packed fuel; secondary combustion of these gases makes the main flame above. The primary air jets, though gentle, drill neat holes in the wood/coal bed. With the narrow internal channel blocked (in my case, possibly by broken firebrick), the flame collapses immediately and a distinct gentle whistle disappears. In the unlikely event that this ever happens again, I'll just blow the lines clear.

Dealer and distributor were apparently ignorant of how the stove is supposed to work, and were not bothered that a big fire would shut right down as soon as the slider was closed. They did everything possible to keep me from talking directly with VC. With their help alone, I would still have an insert useful only for decoration. I wonder how many other customers are disappointed by poor stove performance due to hidden damage, and because dealers take this level of interest and responsibility?

-Theo

 

[allhandsworking]

You need to know the temp of that thing. Is this the stove where you put the temp gauge on the front of the stove somewhere up top? ... Bottom line is I think there is a bit of user error here that needs some fine tuning...not the stove.

I have been measuring the forced-air temperature because that is how much of the heat gets into the room, and because the thermal path between firebox and air ducts is shorter, and has less inertia, than for any of the cast-iron externals. Blown air responds in a few seconds to changes in fire conditions, where external surfaces take many minutes. I have also used a wire-probe digital thermocouple gauge to follow the temperature of various metal parts and spaces inside the insert.

I make fires with fully seasoned splits in the 2" range to build a coal bed quickly, then move up to 4-5" for the longer burn.

The problem turned out to be not user error, but stove defects. My Montpelier had arrived with a badly damaged air gate, cracked liners and a detached thermo-switch. (Some of this damage took a while to uncover.) It took an absurd amount of time and effort to get service from an unhelpful dealer. Even with evidence of a hard drop or other mis-handling, dealer absolutely refused to believe that there could be anything else wrong. He asserted that it was definitely under-firing, or definitely over-firing, or definitely needed a chimney baffle, then etc. - anything but actually take look at the unit in operation.

After multiple ineffective visits from service agents and regional distributor, I eventually managed to get VC involved. Their guy quickly determined that there was no secondary burn, and that the primary air channels underneath were not working. They swapped in a demo unit, which apparently works to spec; blower-air temperatures can be well over 200F.

If anyone wants to know how the insert behaves without that small but critical bottom air feed, you can open the two grilles, reach under and cover the two round holes with fingertips. (The metal there doesn't get too hot.) In normal operation, this feed drives a gas-generating fire in the packed fuel; secondary combustion of these gases makes the main flame above. The primary air jets, though gentle, drill neat holes in the wood/coal bed. With the narrow internal channel blocked (in my case, possibly by broken firebrick), the flame collapses immediately and a distinct gentle whistle disappears. In the unlikely event that this ever happens again, I'll just blow the lines clear.

Dealer and distributor were apparently ignorant of how the stove is supposed to work, and were not bothered that a big fire would shut right down as soon as the slider was closed. They did everything possible to keep me from talking directly with VC. With their help alone, I would still have an insert useful only for decoration. I wonder how many other customers are disappointed by poor stove performance due to hidden damage, and because dealers take this level of interest and responsibility?

-Theo

Wow Im sorry. I have read your post over the past few months. Im so glade they finally were able to help you! VC did the right thing once they got involved. You are going to enjoy it now! I really like the unit and had a great last winter! You must have been going crazy trying to get that fire started with blocked air intakes! How did they unblock the air channels. or did they just replace the unit with that demo model?

 

[Theo]

... VC finally did the right thing once they got involved. You are going to enjoy it now! ... You must have been going crazy trying to get that fire started with blocked air intakes! How did they unblock the air channels. or did they just replace the unit with that demo model?

Thanks - going crazy was right. It didn't help that dealer and distributor apparently didn't know how the stove is supposed to operate, and called it OK when they could make a big fire with the slider open. The fact that it snuffed completely with the gate shut didn't bother them. The VC guy did a partial disassembly and clean-out, but the underneath air channels must still have been blocked somehow. (I'm guessing debris, but who knows.) So they replaced it, only five months (and 90% of the heating season) after I first tried to get help from the dealer.

It is a very pretty thing to watch, especially with a small wood charge and the firebox full of gently swirling flame.

-Theo

 

[allhandsworking]

... VC finally did the right thing once they got involved. You are going to enjoy it now! ... You must have been going crazy trying to get that fire started with blocked air intakes! How did they unblock the air channels. or did they just replace the unit with that demo model?

Thanks - going crazy was right. It didn't help that dealer and distributor apparently didn't know how the stove is supposed to operate, and called it OK when they could make a big fire with the slider open. The fact that it snuffed completely with the gate shut didn't bother them. The VC guy did a partial disassembly and clean-out, but the underneath air channels must still have been blocked somehow. (I'm guessing debris, but who knows.) So they replaced it, only five months (and 90% of the heating season) after I first tried to get help from the dealer.

It is a very pretty thing to watch, especially with a small wood charge and the firebox full of gently swirling flame.

-Theo

Very Nice indeed! Good luck and keep warm! Mark