Flue Temp Boiler Shutdown or ?

[Tennman]

Hopefully solving the last piece of the storage control system. After the wood is gone I need something to shut down the boiler to prevent the loading unit from circulating between charged tanks and a cooling boiler. Surprisingly with my boiler set at 190-195F, I'm seeing brief periods of idling as storage temp gets above ~160F on the bottom. Don't know if this is normal. In my mind it implies the loading unit pump is too small to keep up with the boiler's capacity as storage gets near boiler output temp, but using the Laddo sized for a 60 class.

Since I now see brief periods of idling at the end of charging, looks like I need some additional logic, some type of time delay, to make sure all the wood is completely consumed before shutdown. I know some use just a timer, but I want the flexibility of sizing the load and charging storage based on weather and my schedule. So just a timer would require constant timer resetting.

I read the current post. Seems like TC's/Garn's solution involving water temp and flue temp makes sense. Or I could be just moving towards Nofossil's controls. I read Jim's solution for Deep... something, but I really don't have time or talent to develop something. There must be a simple solution for folks using EKO and BioMass boilers with storage that monitors flue temp and shuts down after some adjustable time limit at a low temp setpoint. Used Search but striking out.

Thanks in advance.

 

[jebatty]

Tenman, it's all about the math. Assume maximum rated output of 205,000 btuh, top of tank 190F, bottom of storage and storage return to boiler at 160F, and also assume no idling at this point and no system draws or losses. This means that your flow rate from boiler to storage and return is 13.67 gpm (13.67 x 30 delta-T x 500 = 205,000). And for the purposes of this example, assume your Laddo has a maximum flow rate of 13.67 gpm.

Next assume that output continues at 205,000 btuh but bottom of tank is now 165F. Delta-T now is 25, and your Laddo can only move 13.67 x 25 x 500 = 170,875 btu's. See the problem? Your output is greater than the capacity of your system to absorb, and your boiler temperature will rise and the boiler will go into idle at the high limit operating point. And if instead bottom of tank was 170F, your Laddo can only move 136,700 btu's, and your problem is even worse.

The problem is that you're loading too much wood into your boiler, and the easy solution is to reduce your wood load so that as it burns down btu output will fall to match the capacity of your system to absorb the btu's s as delta-T closes. It is easier than you think to load properly to prevent idling if you do weighed wood burns. You've probably seen my posts and the posts of others on this, and if not, search it out.

One solution that will work, but only to the point where you are not in a wood overload situation again, is to add a parallel circulator between the boiler and storage that will kick in and boost flow as delta-T closes. This can get somewhat complex and is not necessarily cheap.

Another solution is not to try to load your storage to the maximum. With weighed wood burns it is possible to load storage to the maximum every time without idling, but unless really, really needed it may not be worth the time to try to do that.

Incidentally, my solution for DP had nothing to do with your problem. My solution deals with the situation where the wood loads burns out (0 btu's), all boiler output has been absorbed by the system, and the draft fan continues to run wasting electricity. The WG has a 1.5 hp draft fan. Therefore, I devised a way to shut the draft fan off when it was no longer needed, along with a simple way for the draft fan to come on at the start of a burn without using a timer.

Don't forget that for wood burns there are only limited strategies to control wood boiler output and maintain high efficiency and low emissions. Those strategies are complex and usually involve Lamda sensors, variable speed draft fans, and a computer control system, along with dampers, solenoids, sensors, etc.

 

[Tennman]

I agree Jim about burning too much. A habit left over from those pre-storage years of feeding the boiler every time you walked by it. I'll learn to burn smaller loads (but it's addictive to see those top and bottom temps after all these years!). So by burning smaller loads to eliminate occasional idling I guess just a simple flue sensor/relay would work for 99% of the time.

I've gone to Pexsupply and googled flue temp sensors/relays/controllers. Looking for suggestions of models/brands from experienced folks.

Thanks for that math Jim. Based on top and bottom temps of my 1000 gal and outside temp can begin to estimate roughly # hours before another burn needed.

 

[jebatty]

A quick and simple shutdown would involve a temperature controller of any kind that takes a K-type probe and has a low limit relay or switch. Insert the probe where desired and set the low limit. The controller is wired to your draft fan (or a relay) that makes/breaks the circuit to your draft fan. When the probe temperature drops to the low limit, the draft fan circuit breaks and the fan shuts off. Then also install a bypass switch for the temperature controller to re-make the draft fan circuit. On startup, start the fire, hit the bypass switch to turn on the draft fan, and as soon as the temperature is higher than the low limit, flip the bypass switch to "off," the draft fan will stay on because the temp is higher than the low limit, and you are good for another burn cycle.

 

[jebatty]

Even simpler would be a snap disc low limit thermostat on your flue with a bypass switch.

 

[ewdudley]

A couple links:

PID controller with alarm suppression so shutdown is disabled at startup until after flue temperature is hot:

https://www.hearth.com/talk/threads/snap-disk-to-cut-off-fan.135530/

Snap disc or PID controller with startup timer to disable shutdown until after flue temperature is hot:

https://www.hearth.com/talk/threads/end-of-burn-draft-fan-shutdown-control.73405/

 

[Tennman]

Thanks. I've been following your first referenced thread EW. Didn't want to barge in on it. I've never messed with a PID, barely know how to spell it. Was hoping for some type of programmable controller where I could set the flue gas temp to shutdown things. That second thread appears to hit the nail on the head. Already have one K-type in my flue for the exhaust gas temp sensor that's a battery powered display.

Looks like I'll be drug kicking in screaming into learning about controls. But, did recently get a couple of RIB relays working. Big milestone for me. Thx