GARN over-fire/high temp limit control

[Jim K in PA]

The GARN controller supplied by Dectra does not incorporate a specific means to prevent over-firing of the unit. It seems to me that all that is needed to prevent an over-fire situation is to have a temperature controlled draft fan cut-out. As the unit approaches 200 or so degrees, the fan would be shut off, combustion would slow and temperature rise would be halted before reaching the boiling point.

I intend to implement this via a high-limit aquastat (like the Honeywell L4006E-1067) mounted to the front upper bung of the GARN, and wired in series with the low water cut out that is provided with the GARN. This is a break on rise, manual reset aquastat. I already have the temp well in the GARN for it.

Does this sound reasonable? Has anyone else incorporated an over-fire protection in their GARN controls?

 

[heaterman]

The GARN controller supplied by Dectra does not incorporate a specific means to prevent over-firing of the unit. It seems to me that all that is needed to prevent an over-fire situation is to have a temperature controlled draft fan cut-out. As the unit approaches 200 or so degrees, the fan would be shut off, combustion would slow and temperature rise would be halted before reaching the boiling point.

I intend to implement this via a high-limit aquastat (like the Honeywell L4006E-1067) mounted to the front upper bung of the GARN, and wired in series with the low water cut out that is provided with the GARN. This is a break on rise, manual reset aquastat. I already have the temp well in the GARN for it.

Does this sound reasonable? Has anyone else incorporated an over-fire protection in their GARN controls?

Please take my advise. You do not want to "slow" the combustion down on a Garn. You'll get used to determining how much wood to put in it at a given water temp before getting excessively high. Give it a couple months. The Garn doesn't need overfire protection in the first place because all it can do it steam a bit if too hot (>200*). Throttle that beast down or kill the air flow with a half load of wood in there and you'll bring new meaning to the word creosote.

 

[Jim K in PA]

Please take my advise. You do not want to "slow" the combustion down on a Garn. You'll get used to determining how much wood to put in it at a given water temp before getting excessively high. Give it a couple months. The Garn doesn't need overfire protection in the first place because all it can do it steam a bit if too hot (>200*). Throttle that beast down or kill the air flow with a half load of wood in there and you'll bring new meaning to the word creosote.

Thanks for the input heaterman. I expect to follow the learning curve like everyone else with respect to judgment on feeding my GARN. I was just concerned about an over-fire to boil scenario if someone else over fired it through benign ignorance. Say, if I am away and the wife decides "more is better" and loads the box with 175 showing on the temp gauge.

I suspected that creosote would form rapidly in the downstream part of the HX tubes on a shutdown. Boiling the GARN is something I REALLY hope I never experience, and this was a way I thought would prevent that.

So you think it would be better to let it boil until the timer shuts off than shut it down say at 210? (I am at 1000' elevation, so BP is 210 for me).

 

[BrownianHeatingTech]

If you do install an overheat aquastat (say, set to 205), I would suggest installing a really good heat dump zone (set for 195, for example). That way, you can still have the protection of the overheat aquastat, but nearly guarantee that you will never actually trigger it. If you go that route, I would suggest a manual-reset unit for the overheat aquastat.

Also, FYI, if you ever do boil the Garn, don't try to cool it down by dumping cold water in it. The manhole works like a giant check valve, and will let the steam out. As the unit cools, the vent will easily allow air back in. If you spray cold water in, you risk condensing the steam faster than the vent can allow air in, creating negative pressure in the Garn, and potentially collapsing it (it's not designed to withstand negative pressure).

Joe

 

[heaterman]

If you do install an overheat aquastat (say, set to 205), I would suggest installing a really good heat dump zone (set for 195, for example). That way, you can still have the protection of the overheat aquastat, but nearly guarantee that you will never actually trigger it. If you go that route, I would suggest a manual-reset unit for the overheat aquastat.

Also, FYI, if you ever do boil the Garn, don't try to cool it down by dumping cold water in it. The manhole works like a giant check valve, and will let the steam out. As the unit cools, the vent will easily allow air back in. If you spray cold water in, you risk condensing the steam faster than the vent can allow air in, creating negative pressure in the Garn, and potentially collapsing it (it's not designed to withstand negative pressure).

Joe

I'll go along with the dump zone idea. A person could use and aquastat to discard some heat into whatever one would choose. Be mindful of the fact that the dump zone would have to bleed off about 400,000 btu/hour which is a nominal firing rate for a 2000. I've seen them boil and while it may be a bit exciting, it really doesn't do anything but create a giant sauna in the building.

 

[BrownianHeatingTech]

I'll go along with the dump zone idea. A person could use and aquastat to discard some heat into whatever one would choose. Be mindful of the fact that the dump zone would have to bleed off about 400,000 btu/hour which is a nominal firing rate for a 2000. I've seen them boil and while it may be a bit exciting, it really doesn't do anything but create a giant sauna in the building.

Well, even if it can't absorb all of the output, it might be able to absorb enough to make the difference between boiling and not boiling, especially if someone just overloaded it a bit. If someone takes a full-hot boiler and loads it again, they deserve what they get...

Joe

 

[Jim K in PA]

I'm sorry I asked.

 

[brad068]

Another thing you can do if you are around when its boiling is to run the draft fan (not if the fire box is full though) for another 30 min. to an hour. pulling in cold air and not using it for combustion will cool down the unit pretty good too. When I was experimenting with my unit the timer switch stuck and ran overnight and it dropped @ 80*F in 9hrs.

 

[topofthehil]

as heaterman advised, " You do not want to “slow” the combustion down on a Garn."
it is not designed for this type of service. fast and hot burns is the way to go.
you don't want to shut down the induced draft on a Garn if there is "fire in the hole".
this happened to me, and heaterman is correct again, I brought new meaning to the word creosote and smoke.
I did not do this intentionally, it was that the fan overheated and stopped running.
turned out that the fan was not wired up properly from the factory.
on my initial fire, the fan worked long enough to get the wood going and then it over heated and shut down.
creosote and smoke started coming out of every flange and door on the Garn.
I cracked the door in an effort to create a forced draft and get the fire started again.
it helped, but I don't want that to happen again, so I do not load the Garn if the temp is > 185-190.

 

[heaterman]

I'm sorry I asked.

No need to be sorry It was a very good question and shows you've been thinking about the finer points of operating your Garn. The only dumb question is the one that doesn't get asked, so keep on asking. We'll be more than happy to shoot your ideas down in flames anytime! NOT!

As I recall you just started firing your Garn. Yes, No? You'll find that within a week or two of operation you'll develop a feel for the size of wood load based on outdoor temp and water temp very easily. My 9 year old grand daughter fires the 2000 we installed for her daddy. It's the easiest wood burner to operate that I have ever come across.

 

[brad068]

Heaterman,

What do you think about the switch in supply/return setup on the garn?

 

[heaterman]

Heaterman,

What do you think about the switch in supply/return setup on the garn?

I have always used the bottom port for supply to keep the maximum amount of head available at the impeller inlet.
Not 100% sure I buy the theory of using the top tapping and mounting the circ low of from that. I have to install a 2000 next week and I'm going to give Martin a jingle and quiz him on it. Seems to me that once you've exhausted the quart or two of water in a 3' piece of 1.5 or 2" pipe you're left with virtually no head on the suction side of the circ. The static pressure available is only what is in the pipe.

I usually try to engineer my piping so that I can flow 20-25GPM through the Garn at a minimum to keep it turned over in a reasonable amount of time. Primary/secondary works nice for that.

 

[Sawyer]

Heaterman, would you please let us know what Martin said? You are a much better person to ask him than I with your great understanding of the Garn system. Here I was just thinking of dropping the P/S system after talking to my Garn rep who said keep it simple, and then you come up with a good reason to install with P/S.

 

[brad068]

Heaterman,

What do you think about the switch in supply/return setup on the garn?

I have always used the bottom port for supply to keep the maximum amount of head available at the impeller inlet.
Not 100% sure I buy the theory of using the top tapping and mounting the circ low of from that. I have to install a 2000 next week and I'm going to give Martin a jingle and quiz him on it. Seems to me that once you've exhausted the quart or two of water in a 3' piece of 1.5 or 2" pipe you're left with virtually no head on the suction side of the circ. The static pressure available is only what is in the pipe.

I usually try to engineer my piping so that I can flow 20-25GPM through the Garn at a minimum to keep it turned over in a reasonable amount of time. Primary/secondary works nice for that.

Well, I think it would have a least 6" of cover over the top inlet port. ft. head pressure is no different in a 2" column than in a full tank at same water level only the total weight of the water-Newton's Laws I think as long as the water keeps entering the inlet pipe there should be no problem.

I can see gaining on the stratification principle and if there are check valves installed in the right places I suppose if you ever had a leak the pump would only be able to exhaust what water is available until cavitation.

But what gets me is during firing I think the water would boil over sooner but on the other hand convection should maintain stability,right? It will be interesting to see what our "Cult Leader" has to say. Amen!

 

[Rick Stanley]

I've been checking into Garns and other wood burners, here and at other sources, for over a year. The vast majority of info I've been able to dig up has said that Garns are big and expensive but they are the best thing since sliced bread especially if (like me) you have multiple buildings to heat and have your own woodlot. Guys that have them, guys that don't have them, guys that sell them, guys that want one, guys that have never installed one, guys that do install them and every other kind of guy you can think of, have all unaminously said that they are by far the simplest gasser in the universe. Simple, simple, simple, easy to hook up, easy to run, easy, simple, piece of cake, nothing to it. So I have bought one. It's sitting out in my old shop. Been there for weeks. I have talked with Martin Lunde, Garn dealers, Garn owners, system design pros, licensed plumbers, pipe fitters, boiler installers and dealers. I have seen a new Garn owner pay good money to have an install started only to be told it was done wrong and have to tear stuff out and start over because his pumps were too low. Now, it appears there's even a debate on supply and return ports. Every time I look up, there is a debate about this pipe and that size and this piping scheme or that.

I'm not going to pay 6grand for an install that may or may not include wiring and firing. So I am going to make a stab at DIY. This forum has been a super info source, but I'm thinking that sometimes you can get too much info. I had one guy say "you have a Garn, it'll work great no matter how you hook it up".

So I guess that next year after Sawyer, Tat, myself and others have been running these rigs for awhile, we'll have more answers. But right now, when I seek answers, I get more questions. HA, it keeps life interesting :coolsmile:

 

[heaterman]

This is the main reason I like using the bottom port for suction/supply. If you connect your circ to the bottom port you have about 5' of water on top of the impeller inlet. That same 5' of water is also available to keep your piping flooded and in a somewhat positive pressure situation. 5' of water column equates to a bit over 2 PSIG. This is usually enough to keep all but very high head pumps purring along happily.
I have noticed very little difference in the tank temp, (5-8*) top to bottom when using that design and maybe part of that is due to the fact that my installs are usually primary secondary with 15GPM+ flowing through the Garn constantly. In that scenario you have to bear in mind that the return water is now blending at the top which is the hottest water. That fact in and of itself is enough to keep at least some thermal turnover going in the Garn.
What little difference there is in the available supply water temp is of far less importance to me than keeping the circ alive and well. I think another way of looking at it is if you absolutely need the maximum supply water temp that can be delivered, you may be better off rethinking the heat delivery side of the equation to allow yourself more range. The Garn is like a big flywheel that gets recharged with energy once maybe twice a day. The more you can let that temperature swing the less times you have to fire it. Designing a system that needs 180* water all the time is something that I try to avoid like the plague itself. Our most recent install is in a home with a combination of panel rads and in floor (Viega Climate Panel). The system will heat well all the way down to 110* if it needs to. (I don't recommend firing below 120-130 unless the fuel is exceedingly dry) The point is that the owner can fire his unit once a day in the evening when he's home from work and forget about it overnight and all the next day. Design the heat emission side of the system with that in mind and the small difference in the top to bottom temps becomes inconsequential.

Just food for thought.