Experienced Garn Jr. users?

Not to be a smart a** but...

1. Depends on the wood
2. Depends on the heating load

That being said, I find nearly all users reporting that the coals after a burn are good for at least 16-24 hours. Exception being those who burn 100% soft wood like poplar, aspen or other light species.

Looking at loading intervals, a Jr will give you about 500,000 btu from storage so a 50,000 btu load will deplete the stored heat in 10 hours.

There is no coal bed in my Garn, especially with poplar or pine in it. It;s burnt out & gone within two hours. 12 hours? You have to restart, I do, it's fast & easy. Propane torch instead of matches & paper, the fire is pushing up the stack temp within minutes, & I'm back in the house. Because there's no natural draft on these, shutting the fan off early is going to leave you a smelly smoking bunch of unburnt wood sooting up the inside of the Garn, kind of defeats the purpose of a gasser.

I don't have a Garn - but do have a gassifier that leaves no coals for the next fire. No problem to start the next one with a bit of newspaper & some kindling & a torch. So don't get hung up on the coals part - I went 17 years using a boiler that I made one fire for the winter in, in October, and I wouldn't go back to using that thing for anything.

You can also increase your time between fires, maybe, by bumping up your stats a couple degrees when you are burning, and bumping back down when not. Use your house as storage, so to speak. But that requires that the boiler can pump out a lot more BTUs than your house would use - so it still comes back to heatload. So maybe a burn, plus a re-load before the fire goes totally out, when you are home. I do that here in the evenings - get storage topped up & the house lots warm. Fire is about out when I go to bed & I don't make another one on until late the next afternoon. Have programmable stats that bump up around the same time I light the fire, and bump down a bit at bed time.

mike van said:

There is no coal bed in my Garn, especially with poplar or pine in it. It;s burnt out & gone within two hours. 12 hours? You have to restart, I do, it's fast & easy. Propane torch instead of matches & paper, the fire is pushing up the stack temp within minutes, & I'm back in the house. Because there's no natural draft on these, shutting the fan off early is going to leave you a smelly smoking bunch of unburnt wood sooting up the inside of the Garn, kind of defeats the purpose of a gasser.

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Which control do you have with your Garn? The one with the wind up timer or the new electronic control?

Customers with the new electronic control (after 2012), which automatically shuts off the combustion blower based on flue temp have nearly universally reported a coal bed remaining after firing.
Simply rake them around a little to get the "live ones" up to the top, throw on a little kindling, start the combustion blower, add your fuel, shut the door and walk away.

The older style control with the wind up timer was not capable of doing this.

I have the newer auto one - I keep the ashes next to nothing, what ash there is.

what is the flue/water temp differential that the new garn controller opens the contactor at? I have been fooling around with a vfd, reducing the hz from 60 to 45 when the sec burn temp drops below 1200degf, maintaining that speed till a 5degf diff is reached opening the contacts to the vfd and tank mix loop. The overall inducer on time is longer but the reduction in excess air and increased heat transfered to water is noticeable. Also at that point my primary air is at 95% and secondary at 5%.

Hi Tom - According to my book, the controller will stop the blower when flue gas temp is within 5 d. of the tank temp for 10 minutes. The 10 min. delay allows the operator time to add additional fuel is so desired. I've been going to get ahold of Chris & see if this[time] could be modified as I feel the blower runs too long as it is. You're not pulling any more heat from the wood when it gets that close, it's just wasting electricity [my opinion]

Im sure martin tested different temp differentials and times, not sure of the results but it does run longer.

Bear in mind that the flue temp reading is taken at the end of the 4th heat exchanger tube. That being the case, there is still a fair amount of heat being generated in the firebox.

I don't think it matters where the flue temp is measured, when the tank temp stops going up but the blower runs for another half hour, I believe that to be wasted electricity.

mike van said:

I don't think it matters where the flue temp is measured, when the tank temp stops going up but the blower runs for another half hour, I believe that to be wasted electricity.

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No it does matter.
Bear in mind that there is substantial temp drop between the fire box and that 4th pass. You have roughly 40-50 sq ft of heat exchanger surface in between the 2 points so that heat is going into the water.
If the flue gas is still measuring 180-210* at shut off, the fire box temp is still fairly high. Probably in the area of 400-600*. That means there is still heat that can be extracted and also that there is still combustion going on and obviously some coals left.

When I no longer see the tank temp going up, and for a half hour that blower is still running sucking cold outside air into the firebox and sending it through those pipes, it's wasted electricity. If there was [useful] heat being extracted, it would show on the tank temp.