Bucking Conventional Wisdom at 10,000 Feet

ccstelmo

Can I keep violating the law and get away with it.

I'm at 10,000 feet. Naturally aspirated boilers don't work so good up here, so I have a Riello power burner on my Buderus G-205 LP fired boiler. The air's not only thinner up here, it's a lot dryer, purer, and cleaner too. The boiler is on the 2nd floor and the flue is only 9'-long. The 2nd floor mechanical room distributes heat to the 2nd floor. There is another mechanical room in the basement 30 feet below. It provides for heat distribution to the basement and the 1st floor. It is a Reverse Return System.

The house is 5000 sf with about 4500 sf of it heated by radiant installed in concrete (storeroom buried in the back is unheated). A stairwell and a shower, each with one outside wall, have radiant in those walls. There's a 60'-long flat roof with two shallow "valleys" on the building and I ran a loop in each valley just in case I ever needed to melt them off in the winter (we get 12 to 14 feet of snow). Finally, because I wanted to build a new house that looks and feels old in this historic little ghost town, here and there in the house I installed cast iron radiators, 9 of them.

The radiant floors heat the home and are controlled by thermostats.

The cast iron radiators are in the same spaces that are heated with radiant floors and I run warm water through them because 1.) I want them to be operational 2.) I don't want them to burn anyone (this is a B&B) 3.) I never could figure out how to get them to agree to just sit there and be good looking and let the radiant do the heavy lifting.

The radiant walls are controlled manually on demand, in January and February when it gets cold up here.

The roof loops are controlled manually on demand, and I have never had to use them to melt snow up there when it gets too deep. Mostly the wind has cooperated and blown the snow off.

As mentioned, this is a B&B but only in the summer. In the winter we close off 80% of the house and live in three rooms. There is an 80-gallon hot water exchange tank in the system, but I bypass that in the winter too, using a 40-gallon electric water heater instead.

OK. That's the rundown. Here's the issue.

During the winter, with probably 80% of the potential heating load offline, the boiler hardly has any work to do. It gets lazy and doesn't fire right. Not only that, but it fires between 110 degrees and 130 degrees! I worry about that. Over the years I have been into that combustion chamber (literally, it's cavernous) yearly to see if it's condensing. BUT I DON'T THINK IT IS! Yes, it's a wee bit yellow. And yes, there is a very thin, almost imperceptible layer of "chalk" that discolors my hand when I wipe it across the surface. But really, it's been online for 20 years now and that's the worst it gets. There is no sign of degradation of the fire box. The interior surface of the 9-feet long flue has a slight roughness to it, but nothing you could call degradation. No dribbling condensation.

Whadayathink? Can I keep getting away with this? My motivation in doing so is lowered fuel expense running it at such a low temperature. (Delta T is still 20 degrees, after all.)

Chuck

hot_rod

Has it been running at partial load for 20 years?
Buderus has typically been okay with lower operating temperatures. 

Check the flue piping also, it could be condensing in there. It has to do with the dewpoint

If worry more about the on off cycling under small load operation?  Efficiency drops off with short run cycles you could be running in the 70% range, if that matters

EBEBRATT-Ed

With 20 years under its belt why worry now?

ccstelmo

Hot Rod - I only installed the 40-gallon electric water heater last year in response to energy use considerations. Before that, year round the 80-gallon exchange tank was on line, and I suspect that load actually disguised the problem I'm having now.
The flue shows no signs of degradation. I'm thinking one reason I'm getting away with this is the very short flue and the fact that it's a power burner. Surely the gases would condense if it were an atmospheric boiler? (I measured the flue gas temperature at 500 degrees).

Grallert

I think you're in good shape. I'd worry a bit about short cycling the burner, there are only so many cycles in it. So on a greatly reduced load I would imagine it firing often and for a short period? Though you're likely heavily insulated and that will spread out the on cycles. Doesn't look like you're condensing.
When you say "it doesn't fire right" what do you mean?

hot_rod

It's all about the dew point of the fuel, if or where condensation occurs. With oil you have a bit better dew point to work with, compared to LP or NG.

Cycle efficiency is basically a less than steady state condition. it is based on the burner on time, called run fraction.
Run fraction = burner on time ÷ total elapsed time. So a 5 minute run, followed by 20 minute off would be a 20% run time.

If you have a boiler with a fixed output of 85,000 btu/hr and a load of 40,000 you get a 47% run fraction.

Put the run fraction into this graph, run up to find cycle efficiency.

As you see below 30% run fraction you start to take a big hit on efficiency.

Spend some time in front of the unit to gather some numbers if this has any interest.

ccstelmo

Hey Hot Rod:
I solved this short cycling problem by installing a little adjustable time delay in the burner circuit. It's how I keep the delta T at 20 degrees. Those graphs you sent are informative. I'll check it out. LP Fuel up here costs about$2.87/gallon these days. I've got two 1000-gallon tanks buried outside that I top off in fall every year because the supplier won't come in the winter - snow's too deep. But even so, we keep this place closed up and only heat bout 20% of the house.