Harman Boiler Burn Rate Question

[wpessin]

I recently had a Harman PB105 Pellet Boiler installed (posted questions earlier on the install and received some terrific tips). I am looking to compare notes on whether my burn rate is too high or perhaps I just needed to re-calibrate my own estimates on how many pellets I would burn.

I have a fairly large house (about 6,000 sq ft), and have the system running in series with my oil boiler. It powers 3 air-handler zones, 1 base-board zone, and an 80 gallon super store/water tank. Each heat zone has a programmable thermastat that varies between 70 deg and 64 deg depending on the time. Since installation, the oil boiler hasn’t kicked in and I haven’t burned a drop of oil. The max temp is set to 180, min 170, and "3" on the feeder.

I started out using Premium Ultra brand pellets from Home Depot, but was getting odd behavior out of the auto ignition (flash-bang starts where the chamber would fill with smoke then "flash-bang" on ignition kicking a huge billow of smoke out the stack) . I cleaned the pot, found lots of fines in there, and mixed in a bag of Cubex with each bag of the Fireside. Since then, have been getting normal auto starts, however, I’m average about 5 1/3 bags/day...much more than what I thought I would need. Granted it’s a big house, but that seems excessive to me.

Other notes...it’s a direct vent (through the foundation), and the installer hooked up outside air. I’ve noticed a small amount unburned pellets in the ash pan. I think the unburned pellets are from slow startups with the old brand. The installer will be back in to check the draft and other aspects.

Thanks in advance for your thoughts on this.

 

[Eric Johnson]

I don't know anything about pellet burning, but welcome to the Boiler Room, wpessin.

Just one simple question: How many gallons of oil a day did you burn, on average, before you got the pellet boiler?

We should be able to do a fuel comparison calculation and see how your oil consumption compares to your pellet use, on a btu-basis, and take the troubleshooting from there.

 

[wpessin]

I burned about 8.5 gallons/day of oil during the first part of the winter.

 

[Eric Johnson]

Alright, first, you have to understand that I'm not a math guy. So I might have screwed this analysis up, and I would welcome any corrections to my muddled calculations. And I also don't have a solution or an answer to your question, other than pointing to some numerical data that may or may not prove useful in doping out whether or not your boiler is behaving the way it should.

There are just less than 139,000 theoretical btus contained in a gallon of oil.
A ton of pellets contains about 16.4 million theoretical btus.

Since you burned 8.5 gallons of oil, (and not factoring efficiency of the burners, since I suspect they're both about the same), your house required 1,181,500 btus on a average early winter day when heating with oil. By contrast, you are averaging 5.33 bags per day with pellets, which works out to 213.2 pounds of pellets per day @ 8,200 btu/lb or 1,748,240 btus per day. That's a big difference. As to cost, assuming $250 per ton for pellets and $3 per gallon for fuel oil, you come out spending about $25 a day either way.

One question I have is, do you keep the house a lot warmer when you're running pellets? Also, do you know the rated efficiencies for each boiler? If they're substantially different, that might explain the disparity. And, it's colder now than it was earlier in the year.

So that's the deal. You're losing theoretical efficiency somewhere. I suspect you'd have some luck trying to make your pellet boiler more efficient, primarily because you can measure your exact fuel consumption fairly easily, so it will be easy to make note of any gains. Those of us who heat with chunkwood have to rely on less precise indicators when trying to tune up our systems.

Anyway, as I said, it's great to have a member with a pellet boiler. I know we'd all be interested in learning more about your system and your progress on this issue. It always takes me at least a year with a new wood-burning appliance to get really comfortable, and a couple of years to completely work out the bugs and make them more efficient.

 

[Eric Johnson]

Alright, here's another question: Does the hot water from your pellet boiler go directly into the oil boiler to satisify the zones, or is it connected to the zones independently? Are you heating up the oil boiler pressure vessel with pellet-heated water, in other words?

 

[wpessin]

Eric, great analysis, thanks. To answer your questions, the Harman has an 85% efficiency, and the oil boiler an 83% roughly equal. The last week (since I've been running the pellet boiler) has been more mild than the early part of December when we had an extended cold snap--meaning I've needed less BTUs---but as you point out, clearly I'm expending significantly more. The programmable thermostats have not been changed at all--so that part of the equation is constant. All the zones are controlled by the oil boiler aquastat--the pellet boiler pushes hot water into the return of the oil boiler (in series).

My operating theory is that I have too much combustible air (the too much draft) causing the pellets to burn faster and the heat being sucked out the vent "too fast", but then again, I am far from an expert on this stuff. The install put in an outside air line, but was not able to do a draft test since the plumbing and electric work still needed to be completed. I'm hoping to get him out tomorrow to run a draft test and do a diagnostic check on the board.

 

[Eric Johnson]

Ah ha! That's what I was looking for.

If you're pumping hot water into the return of the oil boiler (I go into the top of my gas boiler), then I'd wager that a good portion of your heat is going right up the oil boiler's stack. Unless you have an automatic damper of some sort, there's nothing to stop it. It's literally a big hole in your heating system, shooting your precious pellet heat right up the stack of an appliance that isn't even firing.

If that's the case, that's where the rest of your btus are going.

 

[wpessin]

Wow, that could be the case, but wouldn't that pipe be the slightest bit warm if heat was coming up it? It's as cool as other metal items I have down there that aren't part of the system (i.e., room temp). The pellet vent, on the other hand, is blistering hot. If your theory is right, is there an automatic damper I could use that knows when to close the vent when the oil boiler isn't firing, but then opens it when it fires?

 

[Eric Johnson]

If you have an automatic damper, then it probably only opens when the burner fires, which shoots my theory down. But the stove pipe wouldn't have to be hot to still lose a lot of heat. If there's a way to test the draft or check the position of the damper when the pellet boiler is working, that would be worth doing. I have a 1958-vintage Weil-Mclain gas boiler, and there's no damper, so that was a big problem for me until I figured it out. Depending on your equipment, you may or may not be in the same boat. Clearly something to be checked.

 

[wpessin]

Well the Tech just came and determined the draft was too high...he turned it down to the lowest level, so now we'll see if that will reduce the overall burn rate and improve the efficiency. Of course, we're about to get hit a new cold-snap here in the NE...

 

[Bartman]

Temp loss through a series boiler I think would vary greatly on the design of the oil/gas boiler. Over the years I've never noticed any great loss of heat through my oil boiler, but the only time it's going to lose it, if it does, is when the zones are running. My original oil unit was a cheap dry base steel New Yorker builder's grade boiler, (cost me $462 without controls or burner in 1985). The "new" boiler is a triple pass cast iron, and I do seem to notice that it doesn't radiate as much heat through itself as the older one did, so temp losses are probably less. What I do notice is that if my wood fire goes out and I'm running on oil, I have have to heat up a lot more water through the wood boiler, and that make for one big radiator in the boiler room.