Question Regarding Draft Fan

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Dec 8, 2017
New Hampshire
Hello, I am starting my second season heating my home with a Tarm Solo Plus MK II. I struggled with it last year, but thanks to all of you, this summer I installed cellular window shades, insulated my house using cost sharing from my utility company, insulated my thermal storage, and collected and cached kindling like a squirrel gathering nuts. I also learned how to isolate my three miles of plumbing so if I wanted to burn oil, I didn't have to run the hot water hrough the wood boiler. I never did find the time to learn to knit.

Anyway, I am starting Season 2. I notice the draft fan stays on until the water temperature reaches 200 degrees, then shuts off, which is fine; that's what it is supposed to do and I get that. But once it kicks back on, and I'm done loading it with wood for the night, it never shuts off.

One would think the draft fan would shut off when the water temperature drops to 170 degrees (the temperature the circulator kicks on and off). Or, in the alternative, the draft fan shuts off when there is no longer a call for heat.

What I've been doing, and I don't look forward to doing this again, is to get up in the middle of the night and stumble down a flight of stairs to turn off the draft fan long after the fire has gone out. There has to be a better way.

For what it's worth, the manual states that the fan shuts off once the temperature drops below 140 degrees. That could take hours, and seems like a waste of electricity to me.

Any advice? Other than to call Tarm?

MandoLinda, you've made some real progress on heating your home. I might be able to help you with the problem you described, having had a similar issue when I first started using my Tarm Solo Innova.

When you burn at night, if the water temperature is raised past your boiler's high temperature limit of 200 degrees, and your fan turns off, you have over loaded your boiler with too much wood. If there is unburned wood left in your firebox when the fan stops, the boiler idles while the remaining wood smolders and smokes. After the boiler has cooled down a bit and the temp falls below the 200 degree mark, the fan will kick back on and the smoldering wood will start to burn again. The fan will keep running off and on all night that way, as the boiler alternately idles and burns, until the wood finally burns away completely. Then the temperature is finally able to drop to the boilers low limit setting of 140 degrees. Only then will both the fan and circulator shut off automatically as designed.

The solution is to load only enough wood to completely burn out just before the boiler gets to 200 degrees. With the fire burnt out, the boiler will readily cool off to the lower limit set point of 140 degrees and the fan and circulator will both shut off automatically as the should. At 200 degrees there should be no left over wood in the firebox to restart burning, and keep the fan and circulator running off and on all night. Tonight when you get up to turn the fan off, check and see if there is still wood in the firebox. If there is load up less wood tomorrow night.

There is a way to determine exactly how much wood to add to your firebox so you don't overheat the boiler past your 200 degree upper limit. You can weigh exactly how much wood you need to load in the firebox to bring your storage temp back up to 200 degrees and no more. There are threads by Jebatty on wood weighing that will help you make up a cheat sheet to tell you exactly how many pounds of wood you need to burn to bring your storage up to no more than 200 degrees from whatever temperature it's at when you load.

This is how I do it with a wheelbarrow and three inexpensive bathroom scales. I have four temperature gauges on my 1000 gallon storage tank, arrayed from top to bottom with a single readout. Tonight the average temperature of the four gauges was 140 degrees. I looked at my cheat sheet. It told me I needed to burn 69 pounds of wood to reheat the tank 40 degrees to bring my 1000 gallon storage tank back to180 degrees, which is the upper limit of my Solo Innova.

I next loaded up my wheelbarrow with wood. In the boiler room I weighed the wheelbarrow full of wood on three inexpensive bathroom scales, one under each of the wheelbarrow's two back legs, the other under the front wheel. After removing some wood from the wheelbarrow, the three scales added together weighed 111 pounds. Since I'm not going to burn the wheelbarrow, I subtracted the wheelbarrow's weight of 42 pounds from the 111 pounds. 111 pounds minus 42 pounds = 69 pounds, the exact weight of wood wood I needed to load into the boiler. (I usually throw in a couple extra splits to compensate for any heat (btu's) used by the house while the fire is burning).

I routinely go through this procedure, every time I burn. After tonight's burn, storage is back up to 179 degrees. The fire burned itself out just as storage reached 179 with no left over wood in the firebox, only hot ashes. Then the boiler cooled off to the low limit set point temperature, and the fan and circulator automatically turned themselves off.

This is a simple procedure works perfectly every time with wood with of about 20% moisture content. My first week or so of burning I was doing exactly what you're doing now until I figured this out. I haven't had to shut the fan off manually since. There's no reason you should have to get up in the middle of the night to shut yours off either. I learned about wood weighing from Jebatty's threads here. He could probably help make out a cheat sheet for you if you pm'ed him. He's great at these calculations and also has the same boiler as you. Ask him to read this thread and get his input.

Idling and smoking will cause creosote to build up, which is the last thing you want. Our boilers were designed to avoid idling and creosote build-up by burning full out while recharging our storage. Shutting the boiler off manually to get the fan to stop, when there is still wood in your firebox will cause smoking and creosote buildup, same as when the fan stops at the 200 degree high limit mark. Look at the instructions for burning in your Solo Plus manual. They will help clarify how to load your boiler.

Once you are burning correctly and not overloading, if the fan is still on little too long with only a few coals left in the fire box, call Travis at Tarm. He'll explain to you how to fine tune your low limit setting using the low limit control on the front panel, so that the fan shuts off a little bit sooner.

One other thing, if you've been idling your boiler for a year or so, check your chimney stack to make sure creosote hasn't built up that needs to be cleaned out. You wouldn't want to chance a chimney fire, which is what happens when creosote build-up ignites. I usually use chimney brushes to clean mine out once a year at the end of each heating season. Thus far, no creosote build up to speak of, but my boiler rarely idles unless I mistakenly add a little bit too much wood to it.

Good luck MandoLinda. I hope this helps.

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Hi Mike,
Thanks for the comprehensive explanation, I appreciate it. Happily, I don't overload the boiler to the point of reaching 200 degrees very often. I had the chimney cleaned a few weeks ago and the flues in both the wood stove and the wood boiler didn't have sticky creosote build up. But I will try your method to determine how much wood to load. Thanks again!

MandoLinda, if your fire is out, but the fan is still on, and the boiler has cooled down enough, with no wood left in the firebox to reignite, all you should have to do is raise the low limit setting a small amount. You can do this with the Lo Limit Thermostat dial. The Lo Limit Thermostat dial, should be the lower knob to the far right of the front panel. Check your manual to be sure.

If your Lo-Limit Thermostat dial is currently set too low, the fan won't shut off as soon as it should. You can adjust that thermostat when you get up tonight to turn the boiler off if the fan is still running, as long as the wood is all burned up, the fire has been out a while, and the boiler has cooled down enough. Turn the dial clockwise until the fan turns off. It should't take much of a turn. By doing that, you've reset the low limit set point a little higher. Leave the dial set to where the fan turned off. The fan will turn off sooner from now on at that new set point.

You don't want to set the low limit too high though, so the boiler can't complete its burn. You mentioned 170 degrees. That would be way too high. Once you've adjusted it, before your next burn, double check with Travis at Tarm, that you have the dial turned to the correct spot and not too high a temperature. If the fan won't shut off using the Lo-Limit Thermostat dial when the fire has been out a while and the boiler has cooled down enough, ask him why not.

My Solo Innova's lo-limit is currently set to shut off the fan at 120 degrees. I think you mentioned yours should shut off at 140 degrees. Check with Travis.

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