Harman Boiler Burn Rate Question

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.

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.

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?

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.

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.

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.