Kuuma VF100 over temp alarm, but no high speed blower

Just had something interesting happen. We are in bed upstairs, when I hear a high pitched beep. Come downstairs to check it out, and the Kuuma is displaying "AL", and beeping. High temp alarm. I shut down the power, made sure doors were latched tight, adjusted the bd (Dwyer was showing a smidge over .06...maybe .062-.065), etc. Opened the door, looked inside, and the smoke flapper/guard as well as the very top of the firebox is glowing a dull red. We did have the heat setting on "high".

The part that concerns me, though is that the high speed blower did not kick on. Indeed, I took a piece of wire, and shorted the R and G connectors on the 24v control terminal, just to check. Sure enough, as soon as I shorted them, the blower kicks to high speed. I stood there for a minute or so, letting the high speed blower run. I then turned the control box back on, turned the heat dial down to "medium", and all seems to be well.

We do NOT have our thermostat hooked up to the Kuuma, so there is normally no high speed blower activity. However, it's my understanding that the high speed blower will kick on when the overtemp alarm is tripped. We were getting the ALarm, but no high speed blower.

Am I mistaken on the high-speed blower? Was I getting an alarm based on the thermocouple, and not on the plenum high limit switch? I'd like to understand what was going on, and more importantly, how to keep it from happening again.

Only "abnormal" things I can think of:

1) instead of putting in the usual large splits for the night, I put in several smaller, dense rounds (6-8"). I also filled the "holes" in the stack with smaller rounds (3" or so). The firebox was very full...couldn't fit anything else in. Usually, I only fill with big splits at night, but I still fill it as full as I can. I do have a habit of making the load into a game of Tetris, to get all the wood I can in there.

2) When I opened the stove, the smoke guard was at an angle (say, 45 degrees or so). It was caught on top of some of the wood, rather than hanging perfectly vertical. This is one reason it was so easy to see it glowing red.

Now, 10 minutes or so later, all seems to be normal. Running fine, air intake is in the pilot position, I've turned the heat control down to 1-notch-past-"medium", and looking in the firebox, all is good (black smoke guard is no longer glowing, etc), draft is pulling right at .05 according to the Dwyer, etc.

JRHAWK9 said:

The high temp alarm, the one you encountered, simply is a warning that firebox temps met whatever temperature that will set off that alarm. The thermocouple is what senses the firebox temperature and sends it to the computer. I get that myself on occasion when I do a re-load on a good sized bed of hot coals from all the off-gassing occurring. I have, in the past, opened the door briefly and there was a big bright blue ball of flames from all the gasses from the fresh load burning. Loading smaller splits will also make it easier for it to trip the alarm on a re-load over an abundance of hot coals. It's there just as a safety precaution to inform the operator that firebox temps are a bit hotter than "normal" and to draw attention to the possibility of too high of draft, open ash pan door, etc. In your case, and in my case, it's simply the abundance of off-gassing which is causing this. When that alarm goes off I just double check my draft and make sure all the doors are properly closed. The high speed blower will not turn on during this scenario. The high speed blower will turn on when the PLENUM TEMP reaches a certain temp when it trips the high limit switch (the switch installed in the supply plenum), not the firebox temp. Two completely different temperatures and safety checks. I have never had my high limit switch tripped.

Some things you can do when this happens is take a piece of tape and cover one or both of the pilot holes on the electronic damper box. This will limit the amount of air to just the three secondary holes on the front of the furnace. You can also momentarily decrease your draft down to like 0.03". The combination of these two will generally calm that huge flame ball from burning all those gasses from the fresh load.

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Good info. Thanks! I think a couple of flat "business card style" magnets will be real handy to keep near the air opening, and I'll just cover the holes if I get another flame up.

STIHLY DAN said:

With A front to back burn you are restricting the heat and off gassing from getting to the secondary heat exchanger with the smoke damper pulled in. How cold is it at your place? Kind of early to be loading it to the gills and having it on high. If you need that kind of heat why are you not using the thermostat?

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Good point. I wondered if the flap might have created a flow issue, once I saw it wasn't hanging freely. Possibly that, combined with the small rounds, created the issue. I normally don't load small rounds or splits for an overnight burn.

It was mid 20's here last night. I'm not using the T-stat because I haven't ran the wiring from the Kuuma to the upstairs location of the existing t-stat. The Kuuma, even with low blower only, has been doing a good job of keeping the house warm. Perhaps I'll go ahead and wire up the t-stat on a short lead for now. If I just skew the tstat setting high, it should be ok until I can get the full run in place.

JRHAWK9 said:

True, I blew right past that part. I actually removed my smoke flap, as it was annoying me....lol I can deal with a puff or two of smoke.

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I may do the same. The flapping is somewhat annoying when trying to load, but moreso is the noise it makes smacking around. I need to see if that noise reverberates outside the room. If so, I may just pull the flap altogether.

JRHAWK9 said:

IIRC, he is heating a rather large house....like 4,000SF.

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It definitely makes sense to get the t-stat/high speed blower hooked up. I hope to get caught up with work this week, and install the t-stat over the weekend. I do CNC plasma cutting, and Christmas present time is pretty busy...

STIHLY DAN said:

What size wood are you using? You don't want pieces to big, the wood needs to off gas to burn well. These epa a units don't like huge chunks like people may be used to on old stoves. Smaller pieces burn hotter. I don't like anything over 7 inches in the round form, Kuuma say's 6-7 largest also. I like 4 in the round and 5/6 as square.

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My splits are larger than that. Some quite a bit larger. Sounds like I may need to adjust my split size. I'm still working off the split size we used as a kid, in my parent's old King stove. 4" round...wow. That's some of the stuff I considered "firestarter wood". lol If we need to go smaller like that, I will. Good news, is I should be able to pack more wood in the fire box.

STIHLY DAN said:

Truth be told, I do not use my T-stat other than as a thermometer. But my space is not your size. You need a lot of air to get everywhere in your place, more air lower temp may work for you. My unit stays on low fan and low temp on the controller until it drops below 0 outside.
Also, what temp are you keeping your place at? Stuffed full unit on high is a butt load of heat.

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I'd like it to stay above 68. 66 is where we normally kept the the LP furnace, to minimize LP usage, and my wife did NOT like that. The Kuuma thus far has been keeping it 71-72 while outside temps are mid-low 30's. Just now starting to hit weather cooler than 30's.

Got home late last night. House temp was 66. Filled the Kuuma, set heat knob to "high", and went to bed. Woke up around 3:30 am. House still felt chilly; checked LP tstat, and house was still at 66. Outdoor temp was 18. Woke up this morning, house still 66, outdoor temp 17. It looks like the Kuuma can keep up when the temps get low, but it isn't advancing.

Based on the info above, it sounds like I should be using smaller (than what I'm used to) splits, in order to maximize heat output. I just loaded a bunch of smaller rounds (some down to 3" or so), to see what happens. I'll hook up the tstat over the weekend, but for now I want to see what happens using the smaller splits, and changing no other variables.

If we can get down to 0 degrees, and the Kuuma can still maintain, I'll be content. We can use the LP "boost" up to the desired temp, then use the Kuuma to keep it there. I'd be much happier if the Kuuma could manage it all on it's own, however!

brenndatomu said:

Getting the blower to kick on high is going to make a huge difference in heat output!
Are you trying to wire into the existing LP tstat? If so, it'd be easier to wire in its own tstat...they're cheap enough...you can buy Honeywell FocusPro 5000 on the 'bay for $20 shipped...

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I ran the tstate wire and hooked it up yesterday. The Kuuma comes with a basic round tstat. I mounted it on the wall next to our existing LP tstat.

brenndatomu said:

And contrary to those old school stoves, smaller splits seem to affect burn time very little on secondary burn units like this...go ahead and re-split 'em

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This is surprising me. I was getting 12 hour burns using the larger splits, which I've traditionally used for overnight burns (due to growing up with the parent's smoke dragon King freestanding unit). In the Kuuma, I've started burning the small branches and splits that I kept around for "starter wood", after reading the input here. They're lasting 9 or so hours, and that's coupled with some tweaks to make the box hold a hotter temp (which is going to shorten burn time). Those are little loads like that would have been gone in just a few hours, in the fireplaces I'm used to from my past! They are tripping the high temp alarm as well. lol

brenndatomu said:

Want some more work?

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Ha! Tomorrow I should be caught up with everything...that needs to be done for Christmas. Then I get to jump on all the non-Christmas stuff I've been pushing back! That said...what do you need done?

Some time has passed. What I've found, is that the Kuuma alone seems like it can handle it down to low 20's. If temps are mid 30's, the Kuuma can get the house temp up to 73-74. Low 20's, it seems to hover around mid 60's. In short, it looks like the Kuuma is good for about a 40 degree differential.

I've made some adjustments, to allow for a higher firebox temp. I also spoke with Daryl at Kuuma and he sent a baffle set, which I installed in the heat exchanger. So, I've got the firebox temp set at max, baffles in the exchanger, temp set at "high", and I've started burning smaller splits. I also set the LP thermostat at 68, and just let it kick in when the Kuuma can't shoulder the load. The baffle set has been in for a few days now, and it seems that it has helped, some. Outside temps seem like they can be a little lower, and the Kuuma can still keep up. It's 21 outside right now, and it's been in the 30's all day today. It was 73 in the house earlier, and I believe it's still 71 now (11:23pm). I gave it a full load this morning, and 3 pieces "just to make sure I have coals at bedtime" around 6pm. Full reload happened about 20 minutes ago.

Burn times seem to have dropped, to about 9 hours (vs 12). I'm burning some of the larger splits at night again, just to get them to last a bit longer. Probably not putting out as much heat, though. I'm considering buying a small electric splitter, just so I can re-split the stuff as we put it in the "immediate use" wood stack. The small splits DEFINITELY put out more heat.

I've come to the conclusion that it may not be possible for the Kuuma to be our sole source of heat in the winter, when it's below 20 or so. However, it can still provide a large percentage of the heat required, and minimize our reliance on the LP furnace. Also, the Kuuma helps to maintain a more "even" heat than the LP furnace. It also keeps the tile flooring upstairs warm, which is great.

I have several more tricks to try (from other Kuuma users), as time permits. These tricks may increase the efficiency/output of the Kuuma -just- enough to break the 100% reliance threshold. Even if they don't, though, I'm still happy with the output. As mentioned before, the house is approx 4k sq feet, with much of the upstairs area (entrance, living room, one bedroom) having high, vaulted ceilings. The downstairs has 10' ceilings (only because it's unfinished...if we were to put drywall up, they'd be 9') as well. Not only are we about 500 sq ft above the "rated" size for the Kuuma, but the vaulted ceilings are adding an additional volume to heat.