More Heat With Less Fuel?

[jrsdws]

Calling all heat masters, engineers, braniacs, and anyone.

On my biomass furnace, I have the option of running just one auger/burn pot or two.

The manufacturere shared the programmed feed table with me. I've been tweaking and playing to get more heat like we all do. I've come across a scenario that I'd just like to have some validation from somebody on as I'm not sure of the logic here.

Please keep in mind the following feedrates are calculations based off of using medium sized kernal corn. I burn various mixed ratios of corn and pellets.

One pot on feed #4 equals 8.6lbs per hour

Two pots of feed #3 equals 10.9lbs per hour TOTAL or 5.45lbs per hour per pot

I keep a k-type thermocouple fixed about 6" in front of the exchange tubes inside the plenum. This is fixed and never moves.

With one pot on #4 I see air temperatures averaging 192*F with distribution fan on high.

With two pots on #3 I see air temperatures averaging 187*F with distribution fan on high.

So there is where I'm looking for the logic. Why do I see essentially the same kind of heat output burning more than 2lbs per hour less in one pot?

 

[Harvey Schneider]

A question first. Are you sure the air temperature is uniform across the plenum with both one pot burning and with two pots burning? I suspect you are monitoring a point that favors the heat output of the second pot.
The other possibility that comes to mind is that there is a change in efficiency with a change in feed rate per pot.

 

[Pelleting In NJ]

Does the combustion blower run at a higher speed if both burn pots are being used?

 

[jrsdws]

Probe is centrally located in front of the tubes in the plenum. Results are duplicated regardless of which pot is used when burning just one.

The combustion blower is just one speed no matter what.

I do, however, close the intake of non-burning pot so burning pot gets full comb air. When running both that air is split, of course.

 

[Harvey Schneider]

Hypothesis:
With one port closed you are restricting air flow of the combustion blower, so you are moving less air through the vent. That results in less heat out the vent and more heat available to the heat exchanger. That increases efficiency of the furnace.

 

[mepellet]

How about the convection blower? Does that run at a higher speed when both burn pots are fired?

 

[jrsdws]

How about the convection blower? Does that run at a higher speed when both burn pots are fired?

Convection blower is on a rheostat and user controllable. Both readings are with blower at max.

New Hypothesis:
With one port closed you are restricting air flow of the combustion blower, so you are moving less air through the vent. That results in less heat out the vent and more heat available to the heat exchanger. That increases efficiency of the furnace.

Not the way this system works. Each pot is allowed intake air from independent and separate air intakes. If anything, blocking the out of service pot might increase CFM to the pot in use because it gets the full draft from the combustion blower instead of sharing it with the other side.

I've always speculated air flow to be the reason, whether it pure volume or actual speed in which it's drawn through.

 

[Mr. Spock]

Not the way this system works. Each pot is allowed intake air from independent and separate air intakes. If anything, blocking the out of service pot might increase CFM to the pot in use because it gets the full draft from the combustion blower instead of sharing it with the other side.

I could not find a user manual on your stove but I think Harvey's hypothesis might be onto it. The increased CFM to a single pot may increase heat output due to higher rate of burn. Dividing the airflow between two pots may reduce burn temp for each pot.

The two main factors to maintain temp at the heat exchanger is burn rate and the convection air moving over the heat exchanger. Since the convection air is the same in both scenarios the only variable I can infer is the burn rate.

Possibly a defective thermocouple?

 

[Harvey Schneider]

Closing one intake increases the air to the other, yes, but the total air flow will decrease because there is more resistance to air flow. So I think you get two effects. The single pot burns hotter and the total air up the vent is reduced.

 

[Mr. Spock]

A reasonable hypothesis me thinks. If there is a restriction running a single pot it might be verified measuring the amperage draw on the combustion motor.

 

[Harvey Schneider]

A reasonable hypothesis me thinks. If there is a restriction running a single pot it might be verified measuring the voltage on the combustion motor.

The applied Voltage should be the same unless there is more sophisticated control hardware than usual. There might be a change in combustion blower current, but it might be small,
You might get a better picture of what's going on by also looking at vent temperature and pressure differences.
Perhaps you can tap into the vac switch on your blower to look at chamber pressure. A drop in chamber pressure would confirm that the blower is moving less air.

 

[Mr. Spock]

Ya I meant to say amperage but you replied before my edit. Your fast! Thought I would suggest the easy route first.

Jrsdws, I think you got some good info to go on. Hopefully you can confirm the hypothesis.

 

[becasunshine]

No wait. Wait. You blew my mind with the original post. YOU HAVE TWO BURN POTS? AND TWO AUGERS? I may recover by tomorrow. Now what were you saying? Did somebody mention applied voltage?

 

[jrsdws]

I'll just have to apply that explanation as it seems logical and it does warrant some more playing around with air intake when burning both pots.

This unit has no vacuum switch to test and it's far from "sophisticated". *s* I could get into measuring exhaust temps but that's about it.

And yes...two augers each feeding it's own burnpot. On full feed it's rated to burn corn at 22lbs/hour and produce 180,000 btu. That's why we call it "the beast".

 

[DexterDay]

No wait. Wait. You blew my mind with the original post. YOU HAVE TWO BURN POTS? AND TWO AUGERS? I may recover by tomorrow. Now what were you saying? Did somebody mention applied voltage?

I think some pics are in order here... I have seen numerous pics of the Beast.. Blown away every time.

Yes. Two pots, two augers, and a total of 180,000 BTU Yes. It's a BEAST

I agree with Harvey. Although they may have independent intakes. The combustion blower is running at the same speed? No?

This results in more air. More oxygen = Hotter flame

How's the Mt. Vernon runnin? Treating you good?

 

[jrsdws]

I'll get some new pictures of the beast with both pots blazing the next time I fire her up. I added burn pot gaskets recently and it really makes a difference.

The combustion blower is fixed speed regardless of any settings adjustments......310cfm.

How's the Mt. Vernon runnin? Treating you good?

I'm loving the Mt Vernon! It's a workhorse and heats the whole house on far less fuel than the beast. We've used it down to about 10degrees outside overnight and never came off of medium. First night in low teens I ran it on high and woke up sweating. I think the Quad will handle about 90% of my heating duties now.

The Beast will be ready for the sub zero and/or high wind times that are inevitable out here.

 

[Harvey Schneider]

I'm loving the Mt Vernon!

Quadrafire Mt Vernon Non AE (Dining Room) with Skytech 3301P Remote Thermostat

Maybe this should be a different thread. How did you hook a remote thermostat to an AE? I didn't know that could be done.

 

[jrsdws]

Maybe this should be a different thread. How did you hook a remote thermostat to an AE? I didn't know that could be done.

I have the NON Ae....the old model. You can hook any thermostat up to it.