Outdoor Wood Furnace Info
All-Purpose OWF Discussions => General Outdoor Furnace Discussion => Topic started by: greasemonkoid on June 04, 2018, 09:44:58 PM
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It's been a long road, not sure how things became so complex, but I owe a thanks to those of you who are always quick to respond and make the advice and info available.
Looking back things were pretty straight-cut and basic, but somewhere along the line features on top of other features were incorporated into the system driven by paranoia. At least it is possible to learn from others' mistakes, so nothing to worry about if the ash tray or firebox gets left open, I will know about it soon enough.
I've made three revisions having to shut it down each time, but this time I believe, is a winner. Having 170 degree water to wash the tractor is quite a luxury. Endless hot showers nearly free of cost are cherished.
Here is a glance at the manifold, probably ho-hum to you pro guys, but certainly a design challenge for the do-it-yourselfer. I expect someone will look at this saying - why is that like that? Well, there might be a reason... maybe...
(https://i.imgur.com/IDfrAhRl.jpg)
(https://i.imgur.com/9Oh7kQVl.jpg)
This is my solution to temperature creep during summer use - A controller operates the pair of fans forcing air into the cabinet and through the heat exchanger when boiler temp hits X degrees, it offers a slow and steady bleed off without torching the garage all of a sudden. I see that after a hot burn and the right kind of wood the creep-up is slow, but sure to happen even when every door seal is siliconed.
There is a hole cut into the upper cabinet, open the doors for normal function.
(https://i.imgur.com/mVHIYPNl.jpg)
I'll post a pic of the completed control console when I get the living room cleaned up a bit. Thus far I am pleased with everything, as it pretty much takes care of itself with redundant safeties for over and under temp conditions. It is impossible to put too much load on the system that would result in damaging return temps just as having a red hot stack is also impossible. And just for the record, every electronic component is wired with a bypass switch (except the Ranco thermostat) in case of failure.
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You're right, the hot water can be addictive.
What is the lower pump and crossover to the upper loop doing? I'm sure I'm not seeing the whole picture. I'd enjoy seeing more details of how the system all works.
Congratulations on getting fired up. You've put in a lot of work. That's a lot of threading. :)
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The lower loop and pump primarily serve as the circulator only for the highest gpm possible in low gear, but the crossover allows either pump to feed either loop in case of failure, it also allows for the heat exchanger loop to be stone cold in case of too much demand or clogged filters without sending cold water straight into the return inlet - i.e. after the HX loop has been off - it will be diluted with the circulation loop water, of course, total return temp is a factor of velocity/gpm of the HX loop so the sensors are located after the too loops converge and are mixed well. Should the total return temp drop too low the HX loop pump automatically shuts off. With the crossover, a trickle of flow can still enter the HX loop if necessary.
There is a large pressure gauge on the upper HX loop that is paramount for quickly determining filter condition and balancing the system - such as obtaining the desired flow across the DHW exchanger vs air to air heat exchangers. All this is in the name of avoiding dangerously low inlet temps in the event of an obsurdly high demand.
It is really difficult for me to explain. Power consumption and boiler longevity were high priority on the design constraints. I literally beat my head on the wall while designing this thing trying to run every scenario of heat demand and boiler limitations of the 100 different ways it could be designed. Much of it was utilizing what was already on hand to the best it could be.
And to know you pro guys build systems vastly more complex is quite humbling.
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Should have gotten a bigger enclosure box. Too late now...
(https://i.imgur.com/3KOWnETl.jpg)
Complete and functional control console.
I know thermocouple leads should not be extended, but I did an A-B-A test and saw only 5-10 degrees difference in the readings. All of the sensor lines have shielded cable properly grounded. It is interesting, after cramming a load of dry pine the flue temps will skyrocket, the control cuts the blower off, but leaves the damper open and temps slowly drop 100 degrees where it kicks on again. After cycling about 3 times it is pretty much a constant temp burn at 400-500 degrees. The first runup hit 750, all 3 gauges read the same thing. Don't think anything was damaged, but that's pretty darn hot.
(https://i.imgur.com/Ch4Jmiql.jpg)
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I'm not surprised at your stack temps, it doesn't have multiple passes to scrub off heat. I don't think it would hurt anything, but inefficient.
I think you've thought it through much more than I usually do. I still haven't wrapped my head around it all. It's one of those things where I can understand it better being there hands on. I typically do a simple Primary loop to the flat plate from the OWB/secondary to the forced air and not much else. Let the temps swing where they will within reason.
It's a shame you have to wait til winter now to test it all out. :thumbup:
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Yeah, this type setup is far beyond impractical for someone doing it in the for-profit industry. I've been known to engineer things to the extreme overkill. A hobby, challenge, tinkering, paranoia, I don't know, but they laugh when they see the boiler house. It wouldn't do for me to build a house, it would be made of I-beam and steel plate.
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;D
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looks great, very clean.
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Do you have a 2009 or never stove?
How clean in your chimney? I have always wondered how much difference it would make on those stove with clean vs normal buildup.
Also, what is your estimated flow rate? Nature's Comfort claimed that increasing the flow rate would pull more heat off but I never tested that either.
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I bought it 7 months ago - makes it a 2017 I guess.
The chimney should still be clean as a whistle, haven't had much wood through it yet, a weeks worth.
According to the Grundfos 15-58 spec tables it should be right between 8 and 9 gpm in low gear with one pump running. The dynamic head pressure (static pressure deducted) is about .6 psi - equates to 8-9gpm according to the numbers. I never checked it with a bucket, figured a gauge would get me close enough. Yes there is bernoulli's equation at work possibly skewing the numbers, but trivial really.
At least the top side is simple and unremarkable -
(https://i.imgur.com/c4j8Nf4l.jpg)
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What size and length of pipe do you have? That seems like a lot of flow on low.
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27 feet one way, but those are tandem 1" lines.
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The way I'm seeing by the picture the pumps can only pull in through their own 1" line? I haven't done the math but it seems like 8-9 gpm is high but I dunno.
Obviously it makes good hot water for you. :)
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Looks excellent! :thumbup:
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The way I'm seeing by the picture the pumps can only pull in through their own 1" line? I haven't done the math but it seems like 8-9 gpm is high but I dunno.
Obviously it makes good hot water for you. :)
Aah yes, good catch. The original plans were different from the actual result, have to remind myself of that.
Yeah, should have done the bucket test, don't guess flow can be calculated from head pressure on a gauge then.
Edit - Wait a minute, with the crossover pipe one pump pushes through two different loops and two different return lines. Although, both pumps are fed by their own single line only. I don't even know how my own dang system works, might figure it out someday.
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Yeah, I think you're right, one feed, through both loops, two returns.
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Yeah, should have done the bucket test, don't guess flow can be calculated from head pressure on a gauge then.
There are guys smarter than me on this but I think you would need a gauge on each side of the pump. Then it should be accurate.
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I don't think a second pressure gauge would do any good. Just have to check with pump off and then with it running.
I am guessing when you are at such a low pressure, it will be hard to find flow that way.
With only 27' of pipe, you should have pretty good flow rate. I just wouldn't have guessed that much.
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I guess the inlet would be negative pressure, outlet would be positive, the difference should be where you land on the pump curve?
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For your entertainment:
Units = PSI
Static head pressure - 2.1
Circulator pump low, both loops - 2.6
Circulator pump med, both loops - 2.8
Circulator pump high, both loops - 3.0
Circulator pump low, circulation loop only - 3.0
Circulator pump med, circulation loop only - 3.7
Circulator pump high, circulation loop only - 4.0
Both pumps low, both loops - 3.4
Both pumps high, both loops - 4.5
Both pumps low, circulation loop only - 4.5
Both pumps high, circulation loop only - 6.5
So the question is - at what gpm does the static pressure of 2.1psi return to 0? Might be where that extra gauge comes into play.
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I am confused. Isn't the pressure sensor after the pump? If so, why would it ever return to 0? If it is 2.1 psi then that means the gauge is a little under 5' from the top of the water in the boiler.
I didn't look at the pics enough to figure out exactly what is going on there. What valves were open for the pressures listed above? It sounds like the pumps are working at least somewhat in parallel from those numbers.
Where is the pressure sensor located?
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With having two returns I can see how the pressure wouldn't climb as quickly. I think (and I'm not super smart on these things) that having the pump halfway around the loop from the expansion point (point of no pressure change), then the majority of the restriction (when both returns are being used) is upstream before the pump and not showing up on the gauge. It's moving the loop mostly by suction.
I realize some of the scenarios listed were with one return and the pressure climbs more quickly on higher speeds.
If the pump was at the supply outlet on the stove one gauge on the outlet would be fairly accurate (I think). I wonder if you subtracted the static pressure off the total, then doubled the number left to account for the drag of the supply line, you could be fairly close to an accurate number for accounting for total pressure drop for the whole loop. Convert to feet of head and you could find gpm on a pump curve.
I'm probably off by a mile.. :)
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Mentioning a flow rate which equals zero pressure is in regard to the feed line to the pump - the pressure on that line only. At 2.1psi you'll have, up until a certain flow rate, positive pressure. Yes? Beyond that flow rate the inlet pressure goes from positive to negative pressure, then you have suction and the pump is basically pushing uphill at that point.
The gauge is on the heat exchanger loop with the crossover line connected, the valves are closed on the exit of the exchanger loop manifold end, so nothing actually flows through there, but pressures can act on the gauge. This scenario is for using the circulation loop only.
Both loops typically remain open and the circulator loop pump runs 24/7, the heat exchanger loop pump runs about every several hours to reheat the hot water tank.
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Using the method I described above using one return I come out around 5.5 - 6 gpm, single pump low speed.
But I could be way off.
Edit: I was writing this during your response so it may not make sense. You're right, on lower speeds the inlet of the pump would possibly still have positive pressure, but not much.
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The pressure should never drop to 0 on the inlet line. If it does or goes negative then you will have short pump life. (gravity needs to supply more GPM to the pump than you are pumping)
I was thinking about the pump mounted at the boiler before when I said no return sensor would be needed where there would be no pressure drop.
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Makes sense.
My guess factoring in the pressure drop of the single incoming line using one return line you'd be at 5.5 or 6 gpm, two return lines at 6.5 or 7 gpm. Low speed on a 15-58. A bucket test at the height of the fill/overflow pipe would be interesting!
The short loop really helps get flow rates up and reduce electric consumption. I cringe when folks talk of circulating 300+' and wonder why the pipe costs so much and they have to burn a lot of juice keeping that loop turning.
BTW, thanks for posting this stuff greasemonkoid.
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You're quite welcome. Thank you guys for tossing around ideas, it's great to hear what the gurus have to say. :)