Draft Fan Speed Control - Simple 2 speed

[jebatty]

I don't recall seeing much if anything on draft fan speed control, but if a good thread exists, please let me know. I have a manual motor speed control connected to my draft fan. I use it to slow the fan down only when flue temps get much higher than I want, usually when the temp starts to get close to 500F. My boiler works really well with flue temps around 450-475F.

I located a K-type temperature controller for $25, and maybe someone knows one that is even less expensive. What I thought is to connect the motor speed control on a "slow" setting so that when flue temp reaches the desired maximum temp, the fan slows down, and then speeds up again as the flue temp drops. A DPST relay would do this nicely. The coil could be 24vac or line voltage, whatever is desired, and would be controlled by the temperature controller.

Are there better ways to provide automatic draft fan speed control?

EDIT: Relay should be DPDT.

 

[ewdudley]

This may or may not be helpful, but Dean put up some info about a variable speed single phase motor control:

https://www.hearth.com/econtent/index.php/forums/viewthread/51010/

IIRC the unit can be configured to run off a 4-20ma input.

On EBay there are various PID temperature controllers for $40 or so that can be configured for thermocouple input. In the datasheets for these controllers they document versions that will provide a 4-20ma control output, as opposed to a fixed-period-duty-cycle-on-off relay control output. But I haven't found the 4-20ma control output versions for sale yet, not that I've looked very hard either. (It's possible they all provide 4-20ma outputs and I've mis-read the datasheets, I couldn't figure out what the deal was.)

Putting the two together you might have more exactly what you want maybe.

Cheers --ewd

 

[Tarmsolo60]

I installed the latest(last year) eko controller on my tarm(rk2100u? maybe) after a part on the tarm control went bad. My tarm runs better now than it ever did. I can set the fan speed i want, it decreases when it gets close to the set point. it also has other configurable options on it.

In my mind the electronic controls are the way to go. However it won't adjust on your flue temp alone and is pricier (around $240) if i recall correctly.

 

[dirttracker]

Jim,

What are you using for a motor speed control? I already have a temp controller on my Solo 40 to turn the pump on, I could use another output to switch in/out the speed control. This would sure be nice as my boiler likes to run with fan on high unitl the fire is going good, then I could cut the speed as the stack temps will run around 600F under full burn It would be nice to cut the flue temps back to 450 or so during this time and hopefully get a little more efficiency.

Thanks

Eric

 

[DaveBP]

Eric, do you have turbulators in your fire tubes? 600F does seem higher than you might want.

 

[jebatty]

Eric - two questions: 1) agree with are you using turbulators and 2) are you using the Condor-type probe thermometer or somethng else? Before adding turbulators, my Condor probe thermometer was reading 600-800F burning very dry pine. After the "chain turbulator" of my own design the probe read 575-650F. Then I got a K-type probe thermocouple and digital thermometer and the temp dropped to about 465F where before it was 600F. I think the Condor was quite inaccurate, the saving grace being if you get a more accurate digital reading you can turn the nut on the Condor and set it to read accurately.

The motor speed control I picked up at the hardware store years ago as a "fan speed control." It was in my electrical junk box. By turning a knob the speed can be 100% to off, and it has a set screw to put a minimum speed stop on the knob.

Good luck if you pursue this. I am likely to install this design unless something better shows up that doesn't cost very much.

 

[dirttracker]

I have turbulators in the tubes, I made them out of sheet metal, but I need to make something better out of chain. I did notice an improvement when I installed the sheet metal turbulators, it cut the stack temps down about 75 degrees. Now if the tubes are getting dirty I get close to 600F, on average at peak burn I get around 550F with clean tubes.

I'm measuring the flue gas with a K thermocouple and one of the cheap temp controllers. It seems to be accurate at room temp, and I believe it to be fairly accurate at operating temperature. I have several other temp controllers with K thermocouples at other locations, they all seem to be close to each other and about where I would expect them to be.

I may just have to break down and buy some chain and give that a shot. I burn mostly elm with a bit of hard maple and ironwood mixed in. The wood could be a little drier, I don't have a moisture meter but I suspect it's just on the edge of too high. I get a bit of bridging very infrequently. I figured wood with too much moisture would give lower stack temperatures not higher.

Jim, does your fan control vary the output voltage or output frequency? I wonder if it's a ceiling fan controller or the like.