# Garn wood consumption.



## wardk (Oct 29, 2013)

I installed the Garn last year , got into the mode of filling it right up with wood and used a lot. This year I added zone valves for more control and hopefully less wood. I made weird discovery this fall, being shoulder season I only filled the firebox half full and found that that the Garn puts out the same flu temp for the same amount of time as a full firebox.I can see this saving me a lot of wood.When you think about it a fan driven fire will only get so hot and last so long.Am I breaking the law of BTU?


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## jebatty (Oct 30, 2013)

Same flue temp for same amount of time for a full load and for a half load doesn't make a lot of sense to me. Ideally, except possibly for a very small load of wood, the flue should reach a maximum temperature regardless of the amount of wood in the load, as the fire is oxygen limited, controlled burn. But, a larger load of wood should maintain a range of flue temperatures longer than a smaller load of wood. 

The only Garn with which I have personal experience is a fixed speed draft fan, so the same amount of air is being pushed through the firebox regardless of the size of the wood load. If the draft fan on your Garn is the same (fixed, non-variable speed), then physics suggests to me that it will take longer to burn a large load than a small load of wood, the range of flue temperatures should be similar, and the variable would be the length of time the various flue temperatures are maintained during the burn, assuming that the water temp in the Garn is the same for both burns. However, as the tank of water increases in temp, there likely will be a somewhat higher and rising flue temperature as the water temperature rises.


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## woodsmaster (Oct 30, 2013)

Its going to be a little more efficient at heating cooler water temps. Mabey with the smaller load your only heating to 170 compared to 185 with a full load ?


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## heaterman (Oct 30, 2013)

What you're seeing is the simple fact that you can burn X amount of fuel with X amount of air. The air flow is fixed on a Garn so when more fuel is available than air to burn it you just wind up with a less efficient and dirty burn. Flue temp is simply a function of how much combustion is taking place and how much heat transfer is going on. 
What were your flue temps running on the 1500?


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## wardk (Oct 30, 2013)

All things being equal in this forced system I don't know if 8 splits would last any longer than 4. You guys are right , the main factor is the tank temp. increase between the two fires. Heaterman the flue is 400-450 at full burn.Thanks everyone for your input, I'll experiment with  the different fires and find out if the tank temps are similar.


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## TCaldwell (Oct 30, 2013)

Heaterman is correct, you will have more consistent  burns if you can weigh your wood. My experience with a model 1900 size firebox suggests 70 to 85lbs to be optimun for heat output, if the splits are smaller or very dry, opt for the lighter load, if you still need more heat wait atleast 45 minutes to reload. Remember there is alot of heat in the char phase.


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## Robert V (Oct 30, 2013)

I am in my first year with my garn 2000. I have found that the fire seems to burn better with a smaller load as well. I thinks this is due to the air/Fuel ratio. As an auto tech this make sense to me in comparison to an automotive engine. The leaner you can run an engine the more fuel efficient it will be. The difference between an internal combustion engine and a wood fire is the lean condition will damage the engine over time and create nox emissions above what the EPA would like to see. Does a lean wood burn cause damage to a wood burner?


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## heaterman (Oct 30, 2013)

Running lean (low fuel to air) in a wood burner does not hurt it at all. The things missing in a wood burner as compared to internal combustion are the issues of compression and timing. 
As the fuel is consumed and runs out, every wood burner will go to an extremely lean condition until air is shut off also.


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## Coal Reaper (Oct 31, 2013)

wardk said:


> All things being equal in this forced system I don't know if 8 splits would last any longer than 4. You guys are right , the main factor is the tank temp. increase between the two fires. Heaterman the flue is 400-450 at full burn.Thanks everyone for your input, I'll experiment with  the different fires and find out if the tank temps are similar.


 i thought you cold hold you hand on the flu of a garn?


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## jebatty (Oct 31, 2013)

The WHS3200 also has flue temps in the range of 400-450 at full burn (as does my Tarm). 



> heaterman: What you're seeing is the simple fact that you can burn X amount of fuel with X amount of air. The air flow is fixed on a Garn so when more fuel is available than air to burn it you just wind up with a less efficient and dirty burn. Flue temp is simply a function of how much combustion is taking place and how much heat transfer is going on.



There must be a significant difference between the Garn and some other gasification boilers that have a primary firebox and a nozzle leading to a secondary ceramic tunnel or burn chamber, with the draft fan dividing the combustion air between the two, because a full load of wood in my Tarm takes a lot longer to burn than a half load. In fact, in the primary firebox the wood on top often is barely charred while the combustion in the secondary chamber is roaring blue from the combustion gases being created at the bottom of the primary firebox just above the nozzle.


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## wardk (Oct 31, 2013)

jebatty said:


> The WHS3200 also has flue temps in the range of 400-450 at full burn (as does my Tarm).
> 
> 
> 
> There must be a significant difference between the Garn and some other gasification boilers that have a primary firebox and a nozzle leading to a secondary ceramic tunnel or burn chamber, with the draft fan dividing the combustion air between the two, because a full load of wood in my Tarm takes a lot longer to burn than a half load. In fact, in the primary firebox the wood on top often is barely charred while the combustion in the secondary chamber is roaring blue from the combustion gases being created at the bottom of the primary firebox just above the nozzle.


The  Garn has a fan downstream in the flu and pulls the make up air through the firebox and a ceramic ventury. The flu passes back and forth in the tank and the flu temp is measured before the last pass through the tank to the chimney , I would guess that the chimney would still be a least 350. If you look at my avatar you   can see that all the wood in the firebox is burning equally.


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## Coal Reaper (Oct 31, 2013)

jebatty said:


> The WHS3200 also has flue temps in the range of 400-450 at full burn (as does my Tarm).
> 
> 
> 
> There must be a significant difference between the Garn and some other gasification boilers that have a primary firebox and a nozzle leading to a secondary ceramic tunnel or burn chamber, with the draft fan dividing the combustion air between the two, because a full load of wood in my Tarm takes a lot longer to burn than a half load. In fact, in the primary firebox the wood on top often is barely charred while the combustion in the secondary chamber is roaring blue from the combustion gases being created at the bottom of the primary firebox just above the nozzle.


 my varm concurs. consistantly 200-220*C.  i brush the tubes when it jumps to 240.


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## TCaldwell (Oct 31, 2013)

the flue temp at the thermometer at the beginning of the last pass is not the same as the temp leaving the appliance, if you monitor the temp in the first piece of flue pipe attached to the garn it is about 75deg lower than the thermometer. Also the hx design allows alot of residence time in a large body of cool water, enabling the ability to extract alot of temp, espically at the beginning of a burn.


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## maple1 (Oct 31, 2013)

I'm not so sure it extracts much more than other boilers? At least from the flu temps posted in this thread - if I was understanding those right. Mine also runs between 200-250c, although more often closer to 250 when it's really humping. I brush when it gets over that - mine does run a bit hotter than the fan Varms, apparently the nature of the natural draft units. I try to make a point of brushing before that, but I have my lazy periods. Would be interesting to see everyone's stack temps from all different units.

On the lower boiler temps, I have started changing up my routine a bit - I don't think I'll try to charge up my storage as high as I was last year. I was pushing for something like 190/180 at bedtime, I think I'll try stopping 10 or so degrees short of that & just start a bit earlier the next day if needed - thinking cooler water around the HX tubes will get more heat out of them & up efficiency a bit. Might be a marginal effect though.


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## TCaldwell (Oct 31, 2013)

maple, it takes less energy=higher effiency when starting at a low storage temp, than trying to get that last 5 deg into storage at the end. Since most of us have to work, higher storage temps afford a extra time period that probably outweighs a few extra splits, as nofo says the waf needs to stay high at all costs.


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## heaterman (Oct 31, 2013)

Routine flue gas temps at the outlet of a Garn are in the 275-325*F range at full burn.


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## wardk (Oct 31, 2013)

hobbyheater said:


> My guess is you are burning  beetle kill wood ? Most Gasification boilers including the Garn are designed for a wood moisture content of 17% to 25 % .  I believe you are in the Kamloops area , and your Beetle kill wood could easily be under 7%  . What is happening with a full charge of wood there is more smoke than your boiler can completely burn , there will be no visible smoke but because the burn is not complete you are losing efficiency big time .If you are burning the dry beetle kill wood , try mixing in a little green aspen or poplar to slow the fire down just a bit .
> With the old Jetstream I burn wood that is under 7% fuel moisture content . So it can only burn a 75% charge of wood without destroying the oxygen to fuel ratio that the boiler was designed for .Boiler output is still the same with the 75% fuel load available for combustion , but  storage gets up to temperature in the same time  but on a little less wood .
> My $.2 cents worth


I need a moisture meter, I dought the pine is at 7% that being said my usual load is a combination of Fir and pine. I have always been under the impression that fir was better  but lately the pine seems to be hotter and easier to get.


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## Robert V (Oct 31, 2013)

While we are on the topic of storage temps, what is a good goal temp for the garn? I know it says not to fire over 200 deg due to water loss, steam, ect...  What are you garn owners using for a high end temp goal? I have only been running mine about a month or so and so far am happy with it. My flu temp on the controller seems to run around 320-380 on average. Is this normal?


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## heaterman (Oct 31, 2013)

I usually tell people to try and stay 180-190* if they can. Too much risk of pump cavitation when water temps get to crowding 200* or above.
The flue temp you are seeing is at the beginning of the last pass on the heat exchanger and the temp at the end of it will be 60-80* less than what you are reading.
I'd say you're in normal territory. Reduce your load size or clean your flues if you see sustained temps of 400* plus for a good part of the burn.


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## jebatty (Nov 1, 2013)

> Routine flue gas temps at the outlet of a Garn are in the 275-325*F range at full burn.





> The flue temp you are seeing is at the beginning of the last pass on the heat exchanger and the temp at the end of it will be 60-80* less than what you are reading.


The math doesn't work. Garn controller readings of 400-450 at full burn translates to 330-380 at the outlet, which still is good (using 70 as the average). Most users will rely on the controller gauge instead of installing their own, so they shouldn't be surprised to see 400-450 on the controller gauge at full burn. I think it is important to take into account that user hx tubes are not whistle clean with every burn, that they are burning a variety of woods at variable moisture content, that they have variable wood loadings, and that they have variable tank temperatures. A hot tank temperature, say 170F will result in higher flue temp than 120F tank temp, other things being equal.


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## heaterman (Nov 2, 2013)

You may see 400 - 450* on the readout/gauge for 15 minutes or so at peak burn but the average is about 100* less than that over the course of a normally loaded Garn.
Very true that the flue temp will be affected by the water temperature. This is why any boiler is more efficient when it can directly fire into low temps.


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## ewdudley (Nov 2, 2013)

heaterman said:


> Very true that the flue temp will be affected by the water temperature. This is why any boiler is more efficient when it can directly fire into low temps.


Again, this is only true under certain conditions for certain boilers that are not sophisticated enough to control flue temperature to the minimum non-condensing flue gas temperature throughout the burn cycle. 

Any boiler is more _effective_ when firing into low temperatures, but as you well know _efficiency_ is measured according to the temperature and composition of the flue gases, not the temperature of the water at the heat exchange surfaces.


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## jebatty (Nov 2, 2013)

Various sources state that a 100F change in flue temperature represents about a 2% change in efficiency.

My Tarm has constant draft which does not vary, much like the Garn; however, I can manually adjust the draft by an adjustable damper. 

The Froling with lambda and variable speed draft is interesting to watch in operation. The data sheet on the Froling says that the FHGL-50 can vary efficient output between 85,000 and 170,000 btuh. For example, as storage water temperature rises and delta-T closes, the Froling draft fan slows down to reduce output, reduce the likelihood of idling episodes, and I presume maintain efficiency.


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## Gasifier (Nov 2, 2013)

How much difference would you be talking on efficiency if the water temps were lower, say a tank sitting at 130 when firing begins, compared to a tank that is sitting at 160 or 170 when firing begins? Wouldn't it be a very small difference? I know there are a lot of variables involved.


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## maple1 (Nov 2, 2013)

Some things in this wood heating thing I still have a hard time getting my head around. You just brought up one. If the draft fan in a lambda boiler is slowed down - based not on burn conditions or fuel load but rather rising water temps & closing delta T's - wouldn't that lower the combustion efficiency? It is cutting down the air supply while the fuel supply might be remaining constant - which seems to me at that point to be setting up for a less efficient burn.

EDIT: That was replying to jebatty's post - gasifier got in ahead of me on the reply.


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## Gasifier (Nov 2, 2013)

maple1 said:


> Some things in this wood heating thing I still have a hard time getting my head around. You just brought up one. If the draft fan in a lambda boiler is slowed down - based not on burn conditions or fuel load but rather rising water temps & closing delta T's - wouldn't that lower the combustion efficiency? It is cutting down the air supply while the fuel supply might be remaining constant - which seems to me at that point to be setting up for a less efficient burn.
> 
> EDIT: That was replying to jebatty's post - gasifier got in ahead of me on the reply.


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## ewdudley (Nov 2, 2013)

maple1 said:


> If the draft fan in a lambda boiler is slowed down - based not on burn conditions or fuel load but rather rising water temps & closing delta T's - wouldn't that lower the combustion efficiency? It is cutting down the air supply while the fuel supply might be remaining constant - which seems to me at that point to be setting up for a less efficient burn.


The rate of fuel supply is the rate of gasification, which is for the most part the rate of pyrolization sustained according to the rate of primary air supply.  So if primary and secondary air supplies are decreased together then combustion efficiency can remain high.  Lambda controlled gasifiers can take it a step further by fine tuning the ratio of primary to secondary air supply as total air supply is reduced.

Overall efficiency (for non-condensing solid fuel units) is determined by how complete the combustion process is, how little excess oxygen (and nitrogen) is introduced (and heated), and how low the (non-condensing) exhaust temperature is.  So if combustion is complete, excess oxygen is minimized, and flue temperature is minimized; then efficiency is maximized. 

This implies than optimum efficiency can be maintained by lowering combustion rate as water temperature increases, assuming constant heat exchange area.


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## heaterman (Nov 2, 2013)

ewdudley said:


> Again, this is only true under certain conditions for certain boilers that are not sophisticated enough to control flue temperature to the minimum non-condensing flue gas temperature throughout the burn cycle.
> 
> Any boiler is more _effective_ when firing into low temperatures, but as you well know _efficiency_ is measured according to the temperature and composition of the flue gases, not the temperature of the water at the heat exchange surfaces.




That is true.  
Efficiency is measured by analyzing the flue gas of any fuel burning appliance at a given point in time and under prevailing conditions. Efficiency does however rise and fall in a boiler in tandem with the water temperature in the vessel. If you look at a chart or graph of efficiency vs water temperature for any boiler you can see that even the best gas boilers will be around 85-87% when working into water temps in the 180* range. 
If you take the same boiler, under the same circumstances and fire it into 110* water the graph will show efficiencies over 94-95%  because the vapor in the flue gas is now collapsing back into liquid and releasing its latent heat.

The same thing, albeit to a lesser extent, happens in a wood fired boiler that can tolerate low return water temps. But alas....we are splitting hairs here. A far larger factor is the moisture content of the fuel in terms of overall thermal efficiency.


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## jebatty (Nov 2, 2013)

> ... based not on burn conditions or fuel load but rather rising water temps & closing delta T's


 EW answered this better than I could, and I did not say that draft fan was regulated only by rising water temp and closing delta-T's. Those factors likely are involved, and consistent with EW's statement, the Froling also measures oxygen and varies the primary and secondary to maintain high efficiency.


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## maple1 (Nov 2, 2013)

Think I got it.

After a year with mine, I have no doubt a lambda can get more out of the same wood. There are times with mine that I have to crack open the fire door to let more air in, on startup. Just too much off-gasing all at once if I load too much in, for the conditions in the boiler (everything cold) - and the secondary burn comes & goes as the oxygen gets used up & then rushes in again. The so-called 'puffing', I believe.


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## TCaldwell (Nov 3, 2013)

keep it going, this is a great thread


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## Robert V (Nov 3, 2013)

So, in simple terms, the greater the temp difference, aka Delta T, the better heat transfer from the HX to the water being heated.  As the two become closer in temp, the more difficult it is to transfer heat. So, in a real life application, what would be the ideal temps, both high and low, for an unpressurized system?  At what point are you spinning your wheels by getting the water too hot or letting it go too cold?  What is the harm in allowing the water to cool too much?


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## wardk (Nov 4, 2013)

Robert V said:


> So, in simple terms, the greater the temp difference, aka Delta T, the better heat transfer from the HX to the water being heated.  As the two become closer in temp, the more difficult it is to transfer heat. So, in a real life application, what would be the ideal temps, both high and low, for an unpressurized system?  At what point are you spinning your wheels by getting the water too hot or letting it go too cold?  What is the harm in allowing the water to cool too much?


Hi Robert; I don't have the answers ,I hope the experts will chime in . I have a couple questions about your system, how hot do you run it and what temp do you refire say after 24hrs? I haven't used panel rads but was considering them for some cold areas, do you find them efficient and how hot do you run them?Thanks


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## heaterman (Nov 4, 2013)

You are correct Rob. It's a basic law of physics that the greater the temp difference is, the "faster" the higher temperature will transfer to the lower. 
It's also why a person has to upsize a plate type heat exchanger by a much larger degree than you would think to get approach temperatures within 5* or less.

The chart I posted tells a couple stories if you look at it. It's easy to see how heat transfer accelerates as the differential between flame or heat exchanger surface temp and water become greater.  The other factor you see in play there is modulation, which is not common in wood burners but interesting to note anyhow. I think, that efficiency goes up at lower burn rates because the ratio between flue gas volume and sq ft of heat exchanger surface increases.


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## Robert V (Nov 4, 2013)

e="wardk, post: 1563237, member: 23023"]Hi Robert; I don't have the answers ,I hope the experts will chime in . I have a couple questions about your system, how hot do you run it and what temp do you refire say after 24hrs? I haven't used panel rads but was considering them for some cold areas, do you find them efficient and how hot do you run them?Thanks[/quote]

wardk,
I am not very experienced with my system yet. I have only been running it about 2 months. So far I have been heating the water to around 190 deg and allowing it to cool to 135 ish.  I am running a water to air hx in the basement, 30 plate hx for dhw, and 10 panel radiators of various sizes for the main level. Heater man did the heat loss calculation and help me design the system.  The panel radiators are working really great and I am glad I went with them. The temp to them varies based on whatever the water temp is. They are capable of putting off a lot of heat but most of the time they are just a touch warmer then the air in the room. They are silent, self governing, and seem to be very efficient. The house never felt this warm with our propane forced air furnace. How long have you had your garn? What kind of system do you have?


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## Robert V (Nov 4, 2013)

heaterman said:


> You are correct Rob. It's a basic law of physics that the greater the temp difference is, the "faster" the higher temperature will transfer to the lower.
> It's also why a person has to upsize a plate type heat exchanger by a much larger degree than you would think to get approach temperatures within 5* or less.
> 
> The chart I posted tells a couple stories if you look at it. It's easy to see how heat transfer accelerates as the differential between flame or heat exchanger surface temp and water become greater.  The other factor you see in play there is modulation, which is not common in wood burners but interesting to note anyhow. I think, that efficiency goes up at lower burn rates because the ratio between flue gas volume and sq ft of heat exchanger surface increases.


 
I have a long way to go but I am catching on to this stuff. Never thought I would find home heating so interesting. Hard to explain my interest to other people. Anyone relate to that?


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## heaterman (Nov 4, 2013)

Robert V said:


> e="wardk, post: 1563237, member: 23023"]Hi Robert; I don't have the answers ,I hope the experts will chime in . I have a couple questions about your system, how hot do you run it and what temp do you refire say after 24hrs? I haven't used panel rads but was considering them for some cold areas, do you find them efficient and how hot do you run them?Thanks



wardk,
I am not very experienced with my system yet. I have only been running it about 2 months. So far I have been heating the water to around 190 deg and allowing it to cool to 135 ish.  I am running a water to air hx in the basement, 30 plate hx for dhw, and 10 panel radiators of various sizes for the main level. Heater man did the heat loss calculation and help me design the system.  The panel radiators are working really great and I am glad I went with them. The temp to them varies based on whatever the water temp is. They are capable of putting off a lot of heat but most of the time they are just a touch warmer then the air in the room. _*They are silent, self governing, and seem to be very efficient. The house never felt this warm with our propane forced air furnace. *_How long have you had your garn? What kind of system do you have?[/quote]



Love to hear that. It's really difficult to describe the difference in the "feel" of heat from a rad vs the same room temp with forced air. It's as close as you can get to having a wood stove in each room.


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## wardk (Nov 5, 2013)

heaterman said:


> wardk,
> I am not very experienced with my system yet. I have only been running it about 2 months. So far I have been heating the water to around 190 deg and allowing it to cool to 135 ish.  I am running a water to air hx in the basement, 30 plate hx for dhw, and 10 panel radiators of various sizes for the main level. Heater man did the heat loss calculation and help me design the system.  The panel radiators are working really great and I am glad I went with them. The temp to them varies based on whatever the water temp is. They are capable of putting off a lot of heat but most of the time they are just a touch warmer then the air in the room. _*They are silent, self governing, and seem to be very efficient. The house never felt this warm with our propane forced air furnace. *_How long have you had your garn? What kind of system do you have?





Love to hear that. It's really difficult to describe the difference in the "feel" of heat from a rad vs the same room temp with forced air. It's as close as you can get to having a wood stove in each room.[/quote]
My system has the garn supplying a 70 plate hx to pressurized secondary, the loads are hydronic basement floor,water to air hx forced air, 2 cast iron rads, 2 baseboard and a sidearm for dhw. I have just put a Burnham lp boiler inline waiting on a gas fitter to check out my install and do a first fire. This system heats a 3900 sq ft house and warms a 1800sq ft  shop. Still fine tuning with the help of the experts on this forum.


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