Temperature-Controlled Rheostat?

[btuser]

I've got a Lopi Freedom insert with the blower. I've been tinkering with burn times/output, and have noticed I can adjust the burn just as easily with the fan speed as with the air intake. The blower has a in-line rheostat but you have to dial the fan speed manually. What I'm looking for is to adjust the fan speed on temperature, so as to not cool down the firebox too much when I shut the air down.


I've seen the Dial-a-temp, but that seems to be what I have already. I'm looking of for a motor control using a remote thermistor/sensor which will could then adjust fan speed. Any off-the-shelf suggestions, or am I on my way to the elctronics surplus store?

 

[Corey]

So if I understand correctly, you're saying turning on the fan changes (slows or speeds up?) the burn time as much as opening/closing the air control? Interesting perspective - any more specs on how much the burn rate changes, how big the fan is, etc.

As for the temp control, the simplest / cheapest thing I have seen would be:

(broken link removed to http://cgi.ebay.com/PID-SSR-TEMPERATURE-CONTROLLER-FURNACE-KILN-OVEN-oC-oF_W0QQitemZ230410057691QQcmdZViewItemQQptZLH_DefaultDomain_0?hash=item35a58277db)

This is a PID temp controller with relay and PWM output, plus type K thermocouple for about 36 bucks total. I have one on the beer fridge doing simple on/off switching, and have not tried out the PWM function specifically - but it should not be too far of a stretch to have the PWM output drive a triac or other type of switch gear to control the speed of the fan. Then you could really go crazy programming all the PID parameters to zero in on the exact control you want. I've been considering getting one for my stove - if for no other reason than to monitor the temp to 1º ! (or even 0.1ºF if you switch to a type T thermocouple, though that limits the max temp to ~660F) and have at least programmable on/off points for the fan if not full PWM control.

The standard caveats would apply to the fan, though - some fan motors don't respond well to speed control and can result in overheating, audible noise, shortened life, etc.

 

[Chargerman]

Interesting idea. I have a variable speed blower on my BBF but it is manual like you said.

If you figure something out please post your findings.

 

[LLigetfa]

So if I understand correctly, you're saying turning on the fan changes (slows or speeds up?) the burn time as much as opening/closing the air control?

I've found that fan speed can in fact take you out of the sweet spot. There is anecdotal evidence that too high a fan speed can reduce the efficiency of the stove as can too low a speed. I've noticed the same with the combustion air control whereby opening it up too much puts less heat into the room. Sometimes less is more.

I don't think the OP's intent was to regulate burn as much as keep it in the sweet spot for max efficiency. I too have contemplated a speed control that speeds up the fan as the stove temp increases.

 

[btuser]

So if I understand correctly, you're saying turning on the fan changes (slows or speeds up?) the burn time as much as opening/closing the air control? Interesting perspective - any more specs on how much the burn rate changes, how big the fan is, etc.

Thanks for the link! I just checked it out. I'm going to see if the temp behind my surround is above what it can handle. If not I'll flush-mount it right into the sheet metal.

I've got a big stove for the room (2.9 cf) and because its an insert a fan is more or less mandatory. I can put the fan on low and fiddle with the air control, or when it gets over the desired temp (more than 550f ) and I want the extra heat I raise the fan speed and it cools down the stove to a more reasonable/sustainable level. Stronger fan+hotter burn= more heat output for that 20-30 minutes before I start pulling too much heat from the stove/firebox, and I have to slow the fan down again. I find myself constantly fiddling with the stove in this way, either with the air control or the fan speed. Because there's no one here in the day by the time we get home the house is close to 60 degrees, so I like to burn it relatively hot, dump the heat into the living area, then let the coals go real slow before loading up again at 10pm. This means I'm messing with the fan and sometimes I forget, or I'm not here and no one else cares!

I understand that some of the stoves on the market have an temperature-actuated bypass that will scoop more air when needed or shut down when too hot, therefore wideing the sweet spot for secondary combustion. I don't believe my stove has that feature, but why not regulate the firebox temp by adjusting the fan speed? I think of the air control as the nozzle on my oil burner and the convection fan as the circulator on my radiation loop. Instead of a modulating burner (air control) you get a multi-speed circulator (fan control).

The current fan motor has a variable speed control, so I don't think it would have a problem. I'm looking for a plug-n-play setup, because even though I'm capable of soldering/splicing/building I'm not a UL listed shop.