EKO 60 low output

[Nofossil]

I have a friend with an EKO 60 that doesn't seem to be working right. I haven't been able to see it in person, but it isn't putting out anywhere near as much heat as it should be. Secondary combustion is described as strong, although one nozzle is better than the other. There's no visible smoke. Flue temperatures (measured with a thermocouple) are around 500. The circulator is a Grundfos 15-58 currently set on low speed. The boiler struggles to maintain a 10 delta T.

By my calculations, at 75% output (150kbtu/hr) it should be able to maintain better than 35 gain at 8gpm (about the most you could expect from a 15-58 on low).

I've sent links to threads here describing secondary tubes that need to be tack welded and secondary tubes that aren't aligned with the adjustment disc.

I've suggested a thorough cleaning of the HX tubes and lower chamber walls.

Any other thoughts about what could be happening?

 

[Nuker]

According to the charts on page 26 or 27 of this: http://www.ghp-books.com/snb98hgt7q6a1/StickwoodFurnace.pdf
a flue temperature of 500F means the efficiency is going through the tubes (pun?) and it could be due to excess air.

The excess air could make sense, because there would still be a good flame, just air moving too fast to allow the heat to transfer into the fluid in the exchanger.

 

[jebatty]

500F is certainly high, but I doubt in general not so high as to produce the low output which is observed. A Froling 170,000 btuh boiler can hit the mid 400's during high burn of small, dry splits. My Tarm (140,000 btuh) can do the same thing, and much higher before the chain turbulators were added. My readings indicate about a 2-3% drop in efficiency per 100F stack temp. That said, I have no personal knowledge or experience with an Eko 60.

 

[maple1]

My first guess would be dirty.

 

[Nofossil]

Yeah - I'm having a really hard time figuring out where the heat is going. As soon as the inlet temp starts to drop from 160, the outlet follows it down and pretty soon the outlet drops below 160, where the circ shuts off. Since there's a lag, it's hard to tell what the effective rise is, but it seems like it couldn't hold any more than ten or maybe fifteen degrees at most.

 

[kopeck]

I would think that if the tubes were really dirty you would see much higher stack temps.

Could it be a wood problem? If it's really wet wood though I would think he would have problems with secondary burn.

K

 

[Nofossil]

Dirty seems possible, though it was cleaned at the beginning of the season. He's trying to send me a video of the secondary burn. Here's what it looks like running. The wood boiler inlet is labeled 'wood boiler return'. There's a circulator set up for inlet protection - it starts when the inlet temp drops below 155 and shuts off at 160. The main wood boiler circ comes on at an outlet temp of about 175 and off at 160, though we were playing with these values during this period. The return from the primary loop is about 120. Almost as soon as the main circ comes on, the inlet starts to drop. Not long after, the outlet starts to drop. Pretty soon, the circ turns off.

 

[BoiledOver]

I had seen in my system what you described, although mine has but one circ through boiler to storage. While charging storage and there is no call for heat, it purrs along nicely. When a zone dumps cooler return water to storage, the boiler grabs it right away and the cooling begins. The theory to send only hot water out of the boiler was suggested by Ewdudley; Since installing an aquastat to control the on-off of the boiler circ, that issue has been minimized to an acceptable value. The RK controller keeps the circ running down to 170 max which is way shy of the 187 make on rise with 4 degree diff that my aquastat is set. This has made a big difference in the efficiency of this system. Maybe this does not apply but it could be considered.

 

[Nuker]

Is it possible the slow speed is too slow? Your 35 degree delta is correct for 8gpm and 150K BTU. My question of too slow is because if the water spent less time in the radiators, the return temp would be higher. Another thought is that the mixing pump for inlet protection might have too large of a flow rate and the boiler can't keep up with only that water contained in the boiler itself?

I don't have much boiler experience but am an engineer by trade and the sharp diverging slopes on your graphs after the circulator turns on are nagging my brain to remember something from my heat transfer and fluid flow classes.

 

[Nofossil]

It's fair to also mention that the recirculator is a Grundfos 15-58 running on high speed, while the main circulator is a 15-58 running on low speed. That should mean that while the recirc is running, it should be providing a higher rate of recirc flow than there is for flow to and from the primary loop.

The primary loop water is pretty cool, but it's mixed with enough recirc so that the inlet temp never drops below about 145.

The lines diverge because as soon as the main circ turns on, it starts drawing a percentage of inlet water from the primary loop instead of purely recirculated water. In about two minutes, the boiler outlet starts to drop.

 

[Nuker]

I understand that the lines diverge once the pump(s) come on, the thing that strikes me as odd is the angle of the inlet line temperature after the first few minutes. The first few minutes of drop in inlet temp is less than the sharp drop of the second angled part. The relatively straight rise on the outlet says the burner input is constant, but the change in angle of the return line temp seems to show a step change in increased load that overcomes the heat input.
Whether that net heat loss is due to reduced output of the boiler (in general) or over-draw of heat (momentarily due to flow change) is hard to tell since the recirculators turn off, and the cycle repeats.
If the temperature settings were changed to allow a longer cycle time, it might give more information.

A balanced heat exchanger should have mirrored, divergent lines when graphing those temperatures over startup, and if they reached a thermal equilibrium after a time, they should be parallel but maintain the delta required to input heat into the rooms.

 

[Nuker]

After re-reading the earlier posts, I think your important numbers are the 175 out and 120 return. This is a delta of 55F for some amount of time. If the main circuit is losing that 55F for some time before the recirc kicks on, the furnace is playing catch up and becoming overwhelmed, and the drop in temperature is shutting off the circulator. I think you might try setting the main and recirc so they turn on at the same time or at least overlap in their temperatures so the excessive loss of heat through the main circuit can be anticipated or mitigated.

 

[Nofossil]

After re-reading the earlier posts, I think your important numbers are the 175 out and 120 return. This is a delta of 55F for some amount of time. If the main circuit is losing that 55F for some time before the recirc kicks on, the furnace is playing catch up and becoming overwhelmed, and the drop in temperature is shutting off the circulator. I think you might try setting the main and recirc so they turn on at the same time or at least overlap in their temperatures so the excessive loss of heat through the main circuit can be anticipated or mitigated.

The recirc prevents the boiler from ever seeing 120. In fact, the recirc is running almost 100% of the time - any time the inlet is below 155 it turns on, and stays on until the inlet hits 160. If this thing were cranking out 150kBTU, it would have no problem heating 140 degree water to 170. Instead, as soon as the inlet sees water below 160, the outlet starts to drop almost immediately from 175.

 

[Nuker]

Well, I am getting too tired to think on this much more tonight. I work with 2 mechanical engineers that I will ask these questions to tomorrow, they might have better ideas than me (and we all like a good challenge). I expect they would want to know the other variables since boiler output is in question. They would be volume of water in the boiler, volume in the primary loop, piping sizes of each section, pump gpm of recirc (I can look it up if you don't have it handy), and either type and length of the heaters or BTU output. If you don't know, any value or approximation would help.

 

[Nofossil]

Well, I am getting too tired to think on this much more tonight. I work with 2 mechanical engineers that I will ask these questions to tomorrow, they might have better ideas than me (and we all like a good challenge). I expect they would want to know the other variables since boiler output is in question. They would be volume of water in the boiler, volume in the primary loop, piping sizes of each section, pump gpm of recirc (I can look it up if you don't have it handy), and either type and length of the heaters or BTU output. If you don't know, any value or approximation would help.

Assuming pretty low head loss, the recirc should be around 15 gpm and the main wood boiler circ around 8. Don't know the boiler volume. The primary loop volume is effectively infinite as any time that the primary loop is running, heat is being extracted in one or more secondary loops. I think the plumbing is 1 1/4" but it might be only 1" between the boiler and the primary loop. If that's the case, I'd expect the boiler to overheat due to inadequate flow rate. What we're seeing looks more like an extraordinarily high flow rate.

 

[ewdudley]

The heat output of the boiler to the system is gpm times deltaT. It looks like supply temperature to system averages a little less than 170 degF, call it 167, while the injector pump is running, and return temperature from system is given to be 120 degF. Duty cycle of the injector pump looks like it is about 50%, maybe a little more. So 8 gpm times 50% times 57 times 500 is (conservatively ?) 114000 btu per hour, or 33 kW. That seem low to me, but maybe not alarmingly low as a bona fide net output rate under steady burn conditions.

I can't tell from the discussion whether the injection flow is additive to the recirc flow, but I assume it is. Also it's hard to tell as accurately from the graph what the duty cycle of the recirc pump is. But in any event the heat output of the boiler is gpm though the boiler times deltaT through the boiler, which will be a higher gpm due to recirc and a lower deltaT due to return protection mixing. These two number are harder to estimate looking at the graph, but the point is that deltaT through the boiler should be much lower due to recirc flow.

For the purposes of measuring boiler output you could try running the recirc on low or medium speed constantly, and then set the hysteresis of the injector pump tighter, so the supply temperature will remain nearly constant. With 8 or 10 gpm of 170 degF recirc mixed with 8 gpm of 120 degF injection, return temperature to boiler won't go below 145 degF while the injection pump is running, and the somewhat lower return temperature may help lower stack temperature.

Then you can measure injection pump duty cycle with a stop watch (or with a computerized control system), multiply by system deltaT as measured while injection pump is running, and then integrate over multiple injection pump cycles to get a measure of boiler heat output. For comparison, my DP 45 puts out about 65000 btu per hour steady state with stack temperature of 340 degF, as measured by the same methodology, since coincidentally my injection pump is a 15-58 on low speed pumping into low head.

The resulting measurement is only as accurate as the estimate of the injection pump flow, but since injection pump flow is constant while it is running, measuring the output from one set of conditions to another should give a precise basis for comparison to see if any changes are having the desired effect.

 

[Nuker]

Some comments from the other engineers I work with; see the marked up, attached graph.

From point 1 to point 2, it appears main pump circuit is off, but the recirc is on since there is still a delta, and the temps are climbing in a relatively parallel nature.

From point 2 to point 3, it looks like the main pump turns on, causing the larger delta, which is increasing in nature. This indicates the heat lost is greater than what the boiler is supplying.

Between point 2 and point 3 (at the purple arrow), the sharper decline of the slope indicates an additional heat load which could be a cold slug of water returning from the circuit, or a secondary loop actuating. Since the other cycles on the graph don't have the same slope change, that would indicate a variable load which would be a secondary loop, not a fixed load which would be the cold slug of water.

Also, it seems odd that the main loop (which should be your only heat sink) doesn't turn on until 175F at point 3, but your large heat loss starts earlier at point 2. This shows that the system is losing a lot of heat before the main circ has turned on.

The one cycle I marked up looks like your best cycle of that set, and the temperature settings could be changed to Main circ ON at 185, keep OFF at 160. Keep recirc ON at 155, but keep it on until 165 or 170F.

If the total divergence from point 2 and point 3 is less, and the time between the 2 points is longer, the settings are going in the right direction.

 

[Nofossil]

Nice analysis. The main circ is actually on a bit longer - right to the point where the return starts to climb again. For sure the drop in inlet temp is due to cool water from the primary loop mixing with recirc water. Note that the inlet temp is measured right at the boiler, so the boiler is never seeing inlet water below about 145.

What's really odd to me is the steep drop in outlet temp after your point 3. The main circ is still running, and if we assume that there's any lag at all, we're seeing water that was well above 150 when it entered the boiler a minute or two before. That 150 degree water starts the rapid drop from 175 at the outlet. That tells me that 150 degree water passing through the boiler is not being heated to 175 - probably quite a bit less than that, based on the steepness of the drop.

The inlet to outlet time lag appears really short for a small circ running slow. Could there be a 'short circuit' through the boiler?

 

[Nofossil]

Here's a different time period showing both the main circ and recirc pumps.

 

[Nuker]

What are the temperature settings on this, still 175/160 & 155/160?

 

[ewdudley]

The inlet to outlet time lag appears really short for a small circ running slow.

The injection flow is mixed with the recirc flow. If water jacket is, for instance 30 gallons, you should expect to see the full impact of the injector flow after less than a minute and a half, assuming recirc plus injector flow is around 23 gpm.

 

[Nuker]

Looking at the starts of both pumps, they almost start at the same time for the second, third and fourth cycles. On the first cycle, the Main precedes the Recirc by a short time.

The first run looks to have worse curves than the later 3, and that was when the Main turned on first. When they turned on at the same time, the curves look better. This tells me that if the Recirc could precede the Main, the curves may look better yet.

(1) The longer run of the main pump on cycle 1 looks to be due to the higher starting temp, so starting the main at 180 or 185 may help.
(2) Also, the flatter curve and lower total drop of inlet temp on curves 2-4 looks to be due to the pumps starting at the same time. If you kicked the recirc on at 165, and off at 160 it could turn on before the main, that may allow some anticipation of the oncoming temperature drop.

If possible, options 1 & 2 might be tried in succession, and it might give more information.

 

[ewdudley]

Again, the recirc flow is not really relevant to measuring the heat output of the boiler. Draw your thermodynamic box big enough so all you have is one pipe going in and one pipe going out. Then determine the flow through the thermodynamic box, which is the injection pump flow and only the injection pump flow. Then measure out much heat is added, on average, to the injection pump flow per unit time.

 

[stee6043]

So many graphs and charts. I have what may be a silly suggestion - ask him to put both circs on low and see what happens.

Second silly suggestion, turn his fan speed down. Way down. If it's at 50% already, shutter those things.

Disclaimer: I didn't read the stuff above me. In my very simple world high stack temps usually mean the fan speed is too high (assuming clean tubes, which are rarely dirty if using nice seasoned wood in my experience) and low output temps mean too much water flow.

Just my two cents.

 

[ewdudley]

I have what may be a silly suggestion - ask him to put both circs on low and see what happens.

For the purposes of measuring boiler output you could try running the recirc on low or medium speed constantly, and then set the hysteresis of the injector pump tighter, so the supply temperature will remain nearly constant.

That wouldn't be any fun.