Balancing high limit & cycle time with Ecobee

2468ben

Hi, I'm in a split-level with a Buderus and Slantfins, two zones on the big top floor and one on the bottom floor.

I did as much insulating & weatherizing as I could, and added mini splits on the top floor that handle 10-11 months a year. But the bottom floor is like an advent calendar and made no $$ sense to put splits down there.

So besides the coldest 30-45 days when I use the boiler for everything, it's only heating the downstairs while mini splits handle the top. Between that and the insulating it's clearly oversized and running shorter cycles. I want to know if I can adjust things to burn less oil, and I know with an ecobee I can set a minimum run time.

The cycles were usually 5-7 minutes on a 30F day, and I just noticed the boiler limit was 190F, but it never gets close to 190F before the end of the cycle. When I set the min runtime to 15 minutes for a while, it finally reached 190F at ~11 minutes, then 4 minutes just circulating). Later I tried a couple days at 175F, the burner only needed ~9 min a cycle + 6 circulation, and the pipes were still ~160F at the end. It's been about the same # of cycles per day at either temperature, and the 15 min cycles are usually 60-90 minutes apart so the boiler and rads always look totally cooled down by the time the next cycle starts.

It seems like I can use the Ecobee to run my boiler lower and slower when it's spending months as an oversized single zone, but is this a good idea? And what high limit + return temp is safe for a cast iron buderus? I could try going down to 165, and/or an even longer cycle, but I don't want to cause any issues. I also don't mind kicking the limit up to 180-190 or whatever is right for the deep winter stretch; this is mainly about what would help the rest of the time when I want the upstairs mini splits to handle as much heat load as possible.

Jamie Hall

You don't want the return temperature to be below 160 F for any length of time, as that boiler is not designed to operate as a condensing boiler. So that sets a lower limit.

The boiler is going to cycle on its high limit -- or the thermostat -- when only the downstairs is in use -- it's much too big for the job, and cycling is the only good way to control that. It is most efficient to run a longer cycle -- the loss in efficiency is mostly from starting cold -- and you can play with thermostat and aquastat settings a little to see what helps.

However... if the boiler is mostly operating about 160, changing the cycle timings is not going to make much if any difference in fuel usage. You need a certain number of BTUs to keep the space warm, and that will take a certain number of gallons of oil, no matter whether you run it in short cycles or long ones...

2468ben

Thanks for the detailed answer. Just to clarify, when I'm seeing these recent cycles, the boiler's starting around 130, getting up to 175 after 9 minutes of burning, then down to 160 after 6 minutes of circ. When I put a heat camera on the baseboard pipes, they're almost always within a few degrees of the boiler, until the pump stops of course.

So should I not circulate through the pipes while the boiler cools down below 160F, even if the burner's off at that point? And does the initial burn time (when it's ramping up from cold 160F) count as bad for the boiler?

Jamie Hall

No. You should be OK with those temperatures and timings.

EdTheHeaterMan

Think of it like this. The flue gas is going to condense as long as the water in there is keeping the flue gas temperature below the dew point. 130°F to 140°F return temperature is going to make that flue gas temperature high enough to stop the condensation and evaporate the condensation that is already there from start up. The faster you get to your 160° or higher the better. (oversizing will do that)

The condensation problem only happens when a boiler never gets hot enough flue gas to evaporate the condensation away. If a non condensing boiler operates for a 30 minute ON cycle, with 120° water in it, then shuts off, that condensation never leaves the cast iron surface. It will mix with trace elements of the byproducts of combustion like carbon and sulfur to make carbonic acid and sulphuric acid that will sit here and eat away at the ferrous metals.

If your boiler is operating at over the dew point for more time than not... you have nothing to worry about. That water is all gone when the burner stops. When the burner stops, there is no more flue gas being produced.

2468ben

Thanks, I appreciate learning the science behind the guideline.

I checked a couple more runs and they keep matching, made a chart for my own sanity:


Last question:
I've got a L7248 aquastat, which says a hard-baked 15F high limit differential so if I'm set at 175F it'll kick back on ~160F. Is there a better aquastat, economizer, ODR, etc for my case, instead of me trying to force runtimes with Ecobee? I have an indirect too, low water demand though.