What is the best gassifier return protection temperature?

[DaveBP]

I have read many times in different authoritative sources that boilers must be protected from return temperatures below 130F. Around this forum 140F is often stated as a safe minimum for wood boilers. Yet it seems most of the Termovar or Danfoss valves used for return protection by people on this forum are rated to open at something more like 160F. That is what Tarm used to supply with their boilers.

These thermostatic valves are made in models that open at 140F (or lower, for other uses I assume). So why aren't they more common? Seems to me the cooler the return temp the easier you can extract the heat from the boiler. And at lower pump speeds and less pump electricity.

Are there reasons that in real-world practice make it better to return water warmer than 140F? The only guess I can make is that it's safer for the many installations without heat storage that idle a lot.

If you have lots of storage and never idle under normal conditions wouldn't a 140F return be more efficient?

Any of you out there using 140F (55C) returns? Do you have storage? Problems with condensation or soot buildup in the firetubes?

I wouldn't expect it to make enough difference to make it worth the expense to swap out an existing setup but for an initial installation why not use the lower temp?

 

[leaddog]

I have found that my boiler runs better if I keep the temp up. I don't use a thermostatic valve but control my temp with a mixing valve at the outlet. I have it set so only 185* water goes out to storage and water is circulated inside the boiler until that temp is reached. When the temp. is reached and water starts to flow out it is mixed coming in and this keeps the temp up. I found that they will gasify at 140* but they work better at 160* and best at 180*. Thats because the fire box is surronded with water and that heats the wood so it starts to gasify better and also the combustion air is heated by the water and works better at higher temp. Thats my take on things
leaddog

 

[BrownianHeatingTech]

Termovar sent out a tech bulletin suggesting that the higher-temp valves be used whenever practical, as some failures of boilers using 140-degree valves have been reported.

Ideally, from a corrosion standpoint, the boiler would operate above the boiling point of atmospheric water, so that condensation would be impossible. From an efficiency standpoint, the boiler should operate at as low a temperature as possible. A system design needs to balance those factors to give efficient operation without risk of premature failure.

Joe

 

[Nofossil]

Joe is right on target, except that you only need to be above the dew point of your flue gas, not the boiling point of water. I have a hard time imagining that you'd ever get enough moisture in your flue gas so that the dew point would be above 130, though.

 

[BrownianHeatingTech]

Joe is right on target, except that you only need to be above the dew point of your flue gas, not the boiling point of water. I have a hard time imagining that you'd ever get enough moisture in your flue gas so that the dew point would be above 130, though.

Excepting if water pools after condensing on the inside of the flue pipe and running back into the boiler.

As I said that would be the anti-corrosion "ideal," but would by no means be practical for a real-life boiler. However, going from the standard 140-degree to the higher 160-degree return temp protection has been shown to reduce corrosion in some boilers.

Joe

 

[DaveBP]

However, going from the standard 140-degree to the higher 160-degree return temp protection has been shown to reduce corrosion in some boilers.

Joe, did any of this industry feedback come with details?

These boilers that corroded, were they gassifiers?

Were they being used with storage to minimize idling?

Was the corrosion in the fire tubes or just below them where water might pool? Water/tar mix in the upper chamber (firebox)?

Without details like that it's hard to know what's happening other than manufacturer risk aversion. And I'm not criticizing them for that. I just like to know details to help interpret what the problem really is. Where I work if one of our products fails (most often because of faulty installation) one of our crew might be on a plane to Algeria with parts and tools the next day.

 

[in hot water]

Check out the Taco delta T circulator. This would be a great way to flow and protect properly.

hr

 

[BrownianHeatingTech]

However, going from the standard 140-degree to the higher 160-degree return temp protection has been shown to reduce corrosion in some boilers.

Joe, did any of this industry feedback come with details?

These boilers that corroded, were they gassifiers?

Were they being used with storage to minimize idling?

Was the corrosion in the fire tubes or just below them where water might pool? Water/tar mix in the upper chamber (firebox)?

Details were very sketchy, as I'm pretty sure that Acaso was trying to avoid identifying the boiler in question. It was a European wood boiler, so I expect it was a gasifier with storage.

Joe

 

[Nofossil]

Is it fair to say that you'd want the boiler jacket temp to be above the dew point and the flue gas temp to be above boiling? That seems doable and should completely eliminate the risk of corrosion. If you're running very low flue temps, you'd probably want a condensate trap in any event.

It's notable that EKO does their efficiency tests with a water inlet temp of less than 135 degrees (at about 35gpm on the EKO 80, if I remember right).

 

[Fred61]

It's notable that EKO does their efficiency tests with a water inlet temp of less than 135 degrees (at about 35gpm on the EKO 80, if I remember right).

Do you mean 3.5 gpm? A fire hose would be needed for 35 gpm.

 

[Nofossil]

It's notable that EKO does their efficiency tests with a water inlet temp of less than 135 degrees (at about 35gpm on the EKO 80, if I remember right).

35 gpm. The EKO 80 has a 2.0" inlet and outlet, and they're pushing it to the max. My EKO 80 typically runs at about 6gpm.

I've made the comment in the past that the test setups don't necessarily reflect likely operating conditions.

 

[sdrobertson]

On the EKO, where would you notice condensation? I've seen Seton style boilers with black goo from where the chimney hooks up, would this be the same as this would be where the exhause would hit the cooler chimney?

 

[heaterman]

Personally, I like a circ/piping design that bypasses the system and shunts hot water back to the boiler when temps drop below a given setpoint. (I like 150* for wood)

Like Hot rod mentioned above, Taco's circ that works off water temp setpoint is the berries for this application.

 

[Nofossil]

On the EKO, where would you notice condensation? I've seen Seton style boilers with black goo from where the chimney hooks up, would this be the same as this would be where the exhause would hit the cooler chimney?

I think the danger zone would be the bottom of the HX tubes. That's where the cold water comes in.

Condensation in the flue pipe is a completely different issue. In normal operation, none of these boilers have a flue temp anywhere near cold enough to have flue condensation, and if they did ten degrees difference in inlet water temp probably wouldn't make a difference.

The concern is that if the inlet water is too cold, then there could be an area in the water jacket that's below the dew point of the flue gas. The thin boundary layer of flue gas that's actually in contact with the cold metal could cool enough to condense in that area. If that happened for extended periods of time, you could get corrosion on the firebox side of the water jacket. This would be much more likely to be a problem if you're burning wood with high MC and/or idling a lot.

 

[jebatty]

From the data tables I have provided previously and other experience, it is evident that a 160F Termovar allows return water much lower than 160F based on the balancing valve setting and boiler output temp to the Termovar. A high boiler output temp will cause the Termovar to start to open, and stay open (because it is still sensing very hot water), thus allowing system return to the boiler, and with a "very little" setting on the balancing valve, not enough boiler output will be allowed to mix with system return to maintain a 140F + boiler return.

Said another way, set balancing valve at "very little" flow to Termovar, at boiler output somewhere around 150-160F the Termovar will begin to open. If system return is "cold," the mixing of system cold with the minimal hot water to the Termovar will result in system return of 120-130, while still allowing boiler output to system of 160F and higher. The Termovar is sensing 160F + water and staying partially open. It seems that the Termovar is doing a better job of maintaining at least 160F output to system (even if at very low flow, 0+ to 3 gpm) than it does at maintaining return water protection.

In the same example, if the balancing valve is opened "more," thus allowing more feedback through the Termovar to the boiler, return water temp will rise, and it is quite possible, by adjusting the balancing valve to more "open" to provide return water temp of 140-160 with cold system return.

Two issues arises with a more "open" setting. First, as system return rises, say to 140 or so and higher, a relative high feedback is maintained, boiler return temp can rise well above 160, because not "enough" hot water is being shunted to the system. And of course, if system return is 160 or higher, the Termovar balancing valve could be shut completely. The Termovar appears never to shut itself completely, and always some flow continues through the Termovar, even when it is not needed.

Second, with a more "open" setting, and system return being cold, boiler output can rise into the high 180's and possible to idle, with low flow to the system because of high feedback to the boiler.

My gut tells me that the Termovar is not optimally designed for "cold" system return, which I will define as less than about 120F. If system return is 130-140F or above, the Termovar quite easily will maintain a 160F boiler return by adjusting the balancing valve and leaving it alone. If system return is less than this, a "more open" balancing valve setting is needed, and if system return then rises, a "less open" balancing valve setting is needed. The ideal may be a relatively consistent system return temp, rather than variation over a wide range, as this should allow a balancing valve setting on a set it and leave it basis. Otherwise, it may be desirable to have the balancing valve set more "open" than optimal at higher system return temp, but thereby provide greater return water protection at lower system return temp, at a "cost" of some loss of boiler efficiency by maintaining higher boiler temp and somewhat lower gpm output to system.

After reading this, I'm not sure that I have clearly stated what I see.

BTW, I have digital temp sensors on boiler output to system and system return to boiler (before Termovar), and an analog thermometer on boiler return (after Termovar). The digital temp sensors are for a differential controller for a Grundfos 15-58 (currently experimenting by leaving this set on LO all the time) which turns "on" when boiler output is +16F over system return. This allows the boiler to quickly heat with very little output to system. I have a second circ (Taco 007) controlled by an aquastat, which turns "on" when boiler output is 185F (off at about 180F). This second circ provides a gpm boost at high output temps. This is a simple, if not a bit crude, variable speed circ setup based on boiler output.

 

[boilermanjr]

Our partners in Sweden and the men in the testing labs at the boiler manufacturers are adamant that wood burning boilers should not receive return water below 140 degrees. This temperature not only protects the boiler from condensation in the firebox and HX tubes and prevents thermal shock, but also elevates firebox temperatures high enough to make a significant difference in the combustion process. Wood does not burn well in a relatively cold, heat quenching environment. Pellet burning boilers can use slightly lower return temperatures as the moisture content in pelletized fuel is generally much lower than is found in the average stick of wood.

 

[heaterman]

Our partners in Sweden and the men in the testing labs at the boiler manufacturers are adamant that wood burning boilers should not receive return water below 140 degrees. This temperature not only protects the boiler from condensation in the firebox and HX tubes and prevents thermal shock, but also elevates firebox temperatures high enough to make a significant difference in the combustion process. Wood does not burn well in a relatively cold, heat quenching environment. Pellet burning boilers can use slightly lower return temperatures as the moisture content in pelletized fuel is generally much lower than is found in the average stick of wood.

I concur. This quenching effect is also readily apparent and easily seen when you throw a few sticks of frozen wood on an already burning fire. One more reason that OWB's will have a very difficult time hitting anything resembling a clean burn outside of lab conditions. Makes one wonder if the EPA testing criteria for the current crop of "certified" OWB's includes frozen wood for those units slated for outdoor use. I'm betting a Franklin that it doesn't. I'll wager another Franklin that none of the manufacturers of those units has brought this fact to the attention of those doing the testing.

Anyone can see this in their own boiler by observing how much quicker gasification occurs when the water temp is at 150+. The same holds true with the frozen wood scenario. Get a fire going and while in gasification mode, add a few chunks of frozen wood and watch what happens. Now repeat the same test using wood that is at room temperature. You'll notice a marked difference in the effect it has on the fire.

As a mechanical contractor I have long held the opinion that any and all heating machinery should be in an enclosed and heated space. Placing a heating appliance outdoors just doesn't make sense. One has only to work on a few gas fired rooftop units to appreciate what I am saying. The effects of cold inlet air on a burner....any kind of burner, will always lead to reduced efficiency.

just my $.02

 

[hkillam]

This is an interesting thread. I assume that this doesn't apply to a Garn, as you aren't returning water to the boiler, but rather to storage. I sure hope that is the case, as it isn't uncommon for me to let my storage get down to 140 before refiring, and in those cases, the return water is often in the 100 - 125 F. range, at least for awhile, until the thermal mass of concrete or gypcrete get back up to a more reasonable temp.

 

[sdrobertson]

My gut tells me that the Termovar is not optimally designed for "cold" system return, which I will define as less than about 120F. If system return is 130-140F or above, the Termovar quite easily will maintain a 160F boiler return by adjusting the balancing valve and leaving it alone. If system return is less than this, a "more open" balancing valve setting is needed, and if system return then rises, a "less open" balancing valve setting is needed. The ideal may be a relatively consistent system return temp, rather than variation over a wide range, as this should allow a balancing valve setting on a set it and leave it basis. Otherwise, it may be desirable to have the balancing valve set more "open" than optimal at higher system return temp, but thereby provide greater return water protection at lower system return temp, at a "cost" of some loss of boiler efficiency by maintaining higher boiler temp and somewhat lower gpm output to system.

After reading this, I'm not sure that I have clearly stated what I see.

I'm protecting the boiler by limiting the temp that is able to leave the boiler to a set temp (mine is 185 to 190). I have a two way motorized valve hooked to a controller that modulates from closed to opened when I reach my set temp. Before this valve, I have a 3/4 inch line running from the boiler "out" to just before the boiler "in" which circulates water all the time. With the valve closed (under 185) the water has to circulate in the boiler. When the boiler water has reached the set point, the valve starts to open and flow water to storage but some water still returns to the boiler "in". I have a ball valve in the 3/4 inch line that is closed about halfway as at this setting, when the 2 way valve is opened all the way, my boiler "in" mixes hot water from the boiler and cold water from the bottom of storage and it is normally around 140 degrees. The one observation I have that is different from using a mixing valve is that I'm not flowing water to the storage at a constant rate, but instead I open the valve at 185 and when the water temp from the boiler "out" goes below 185 it closes. The water that has entered the boiler is down to the 140's and it takes a couple minutes to raise back to 185. By the time "fresh" water reaches the boiler "out", its up to 175 or so, so this is the couple minutes to raise it to 185.

Wow, I think I just confused myself with this explanation!!
Shannon

 

[Gooserider]

Dumb question perhaps, but is there any concern about the boiler temperature when the fire is out? Presumably when the fire has died down to the point where it is no longer able to keep the boiler output above the storage temperature you will stop circulating to heat water, but what about keeping the boiler warm? What if it's summer time and you are doing a once a week fire for DHW, do you need to circulate storage water through the boiler to keep it warm, or is it OK to let the boiler cool to room temperature as the fire burns out?

How about restarts? Assuming you still have 120* or so in storage, is there any advantage in starting to circulate that water before lighting the fire in order to "pre-heat" the boiler?

Gooserider

 

[sdrobertson]

Dumb question perhaps, but is there any concern about the boiler temperature when the fire is out? Presumably when the fire has died down to the point where it is no longer able to keep the boiler output above the storage temperature you will stop circulating to heat water, but what about keeping the boiler warm? What if it's summer time and you are doing a once a week fire for DHW, do you need to circulate storage water through the boiler to keep it warm, or is it OK to let the boiler cool to room temperature as the fire burns out?

How about restarts? Assuming you still have 120* or so in storage, is there any advantage in starting to circulate that water before lighting the fire in order to "pre-heat" the boiler?

Gooserider

There has been several threads with the first point you bring up. Several different ways to accomplish it - I'm using the two way valve. Your question on the firing once a week is a excellent one. In my case it goes directly with the restarts as my boiler is usually cold. I think Nofossil runs hot storage water into the boiler to warm it up when cold starting - this will be one improvement that I'll plumb in over the summer as I think its gentler on the boiler when starting and the boiler sure does burn allot better when hot. I really don't see full heat output till I fill with a second load as then the boiler and ceramic are fully up to temps.