Can a stove be too efficient?

[Caleb Joens]

So I recently bought and installed a US Stove 6041 insert. I have been burning for 1.5 months now and thus far have enjoyed the heat, learning experience (many thanks to all of the information that everyone shares on this site) and keeping the LP man at bay. What I am experiencing now is something very different, and I am not too sure that it is a problem necessarily. In an attempt to reduce air leakage around the firepot I crimped the tracks that the ash cleanouts slide in. The makes them air tight and eliminates air being pulled out of the stove that did not move through the firepot; and I did not realize how much air must have been moving through them, because now I have to relearn how to use this stove now.


Previous to crimping the ash clean outs I could run the stove on level 5 (max) feeding 5lbs/hr all other settings are auto and room fan max always. The heat coming out was at 265F and was heating the room up very nicely. After crimping the ash clean outs to seal them, I am running on level 4 and the stove is shutting down because of high temperature. Shutting down is bad, but it seems like I am spitting out much more heat than I was before and have even begun trimming pellet feed down and lowering the draft fan speeds. Unfortunately I cannot check the output temp because I dropped my thermometer and broke it, but I am going to buy a new one and see what it says. I am still trying to get the pellet burn balanced again, but the flames are much more active and it seems like I am capturing much more heat from this stove.


It was -17 last night with a wind chill at around -35, my basement was 73F running on level 4of5 and the stove was still dropping to level 1 periodically because the thermostat was satisfied. So unless I am mistakenly chalking up the high temp issues to increased heat output and capture instead of a faulty snap disc I think that I have saved myself some money in pellets.


Has anyone else experienced a paramount increase in heat output like this, especially with this stove?

 

[hyfire]

Until you measure outlet temps at the same blower speed, I would not draw any conclusions. The manufacturer usually does stuff for a reason, becareful with your mods, you still want the stove to be safe.

 

[iceguy4]

becareful with your mods, you still want the stove to be safe.

I read your post 2 times and I have an idea what you did....seems harmless enough BUT what if the manufacturer uses that leaked air to cool a certain part of the stove...maybe you have created a potentially dangerous situation Make no mistake, the company who built this unit WANTS to get as much heat as is possible (to sell stoves) Any modification to pellet stove without prior approval is taking chances with your life (if you sleep with it running)
I cant think of it now but I have worked on a piece of equipment where I was impressed with the cleverness of the engineer who designed it, It was also use of air to cool a part.. My point is.... chances are good that people WAY smarter then you (pellet/ wood burning knowledge no insult intended) have looked it over...IMHO yer playing wit fire...

 

[IHATEPROPANE]

I wouldn't assume the manufacturers got it right. There are many examples of poorly designed burn pots or convection blowers that are too close to the fire wall.

That being said, you need to be extremely careful with any modification, there can be unforeseen effects that may be dangerous.

 

[iceguy4]

That being said, you need to be extremely careful with any modification, there can be unforeseen effects that may be dangerous.

Yea...what he said!

 

[Pelleting In NJ]

The output heat (BTUs per hour) of a stove is a result of a temperature rise of a given volume of air. If you lower the air volume (or CFM), the air temperature goes up, but the total heat output in BTUs is the same because the volume is less. It actually makes the stove less efficient because the heat exchanger transfer efficiency drops as the temperature difference across the exchanger is reduced. (blowing more cold air across the exchanger extracts more heat from the combustion gases)

 

[hyfire]

However if he has the same volume of air on his tests and his vent temps are higher, I would say it is putting out more heat than before, provided his room temperature is the same for the tests also.

 

[Caleb Joens]

Thanks to everyone for the comments. While I understand that the manufacture of the stove wants it to be as efficient as possible, they also want the build to be as cost effective as possible. It’s a matter of price point for sale of the unit vs. amount of time/material that can be placed into the unit and still cut a profit. Thus it is reasonable to believe that there are some small areas in the build where precision was sacrificed for throughput and cost, although safety and regulations are hopefully never sacrificed in the name of throughput and cost. It is with this in mind that looking at the stove and how it operates you can objectively determine possible areas for improvement.

What I did to the stove was to eliminate a source of room air being pulled into the burn chamber but not being used for combustion, which has a twofold impact upon efficiency by decreasing the temperature differential across the heat exchanger and increasing the flow of the hot air across the exchanger. Because the stove operates in a negative pressure system the same amount of air is going to be pulled out of the exhaust, so by eliminating air leaks more air was passing through the firepot increasing the burn rate, which is why I needed to drop the draft fan speeds but still supplied the same amount of combustion air as I was before crimping the ash cleanouts. Additionally, slowing the air down as it passes by the heat exchanger allows for more of the heat to be transferred to the room air increasing the efficiency even more with the same amount of pellets being burnt.

To measure the efficiency of transfer you have to maintain the same flow of room air (ideally the same temperature prior to the exchanger as well, but that is difficult to control) and measure the output temperature of the room air. Now take the difference in output temperature and input temperature and the cubic feet per minute of the room air to determine the total BTU transferred to the room, now divide that by the actual fuel burnt and you can tell how much heat you extracted per pound from the pellets, simply assuming 8000BTU/lb would not accurately represent real cost as not all pellets are created equal. Take this by the cost per pound of the pellet and you will determine actual cost per BTU from the stove.Now if you use this same method to determine heat capture across multiple pellet feed/burn rates, you can determine where the greatest efficiency range is. This will likely be on a low heat / high room fan setting, but nobody wants to transfer small amounts of heat into the room. The stove manufacturer efficiency claims are at one very specific setting, and they are not likely the ones that you would run at in your house. Finally, buy the next brand of pellets and repeat the process.

I dropped the feed rate for the pellets and I have not had an overheat incident since, but the room is still plenty warm. I have not bought a new thermometer yet, but it is at the top of my to-do list.

I assure you that I am making controlled modifications with safety being the #1 priority….although starting your house on fire would probably get you the most heat for the least amount of pellets but only for a short period of time.

 

[hyfire]

Great lets get some data with your thermometer and see if it made a small or big change in output..