Sorry franknbaum -- I didn't see your post...
... but, now that the real cold weather is here, I have a pretty good handle on what these coils can & can't do.
Over the last few days, it has not been above freezing. We've had lows down to 15* & highs of 24*, & some high winds thrown in to boot. What I've found is that the coils do an excellent job of heating my two zones (one zone is the top floor, where our bedroom is, & the other is our TV room with drafty windows & unheated crawl space underneath -- it has cathedral ceilings & was added on to the house within the last 20 yrs or so), but it does need help from the oil burner occasionally. I have the thermostats set @ 70* & the rooms will maintain about 68* on the coldest nights. Boiler temp seems to stay around 155*-165* with the zones running. The oil burner will kick on after you load up the stove with coal, or the indirect boiler gets a call for hot water. I turned the aquastats hi limit down from 190* to 170* with a 20* differential, & lowered the low limit from 160* to 140* with a 10* diff. This minimizes burner run time -- it will only run during a high load period.
2 weeks ago, they were forcasting temps that following weekend to be up to 65*, so I let the stove go out & turned the 2 zones down to 65* to save oil. In just 4 days (before that warm weekend) of temps in the upper 20's at night to mid 40's during day, I burned through a solid 1/8th of a tank of oil!!
During these last 4 super-cold & windy days, I can't even tell if the oil gauge has moved at all -- that's how much of a difference the coal makes using the coils. AND the thermostats are set 5* HIGHER to boot! My house never got below 66* during a 15* morning, which never happened with the Mark I unless it was in the mid 30's outside. It would have been in the upper 50's to low 60's inside (with the bedroom at 54* & the TV room @ 52*) if I still had the coil-less Mark I !
I lit the stove last Sunday, & I've gone through 12, 50# bags already! The Mark III is a hungry bastard!
I'm wondering if I'll run out of coal before the season is over -- at this rate I'll need 7.5 tons!!
We had some warmer weather when I first lit the stove ( I think that Monday it got up in the mid 40's ), & when I got home from work , I went to check the boiler temp, seeing that no zones had called for heat in a while. The digital temp on the aquastat was reading 263*!! CORE MELTDOWN
That was the highest I've ever seen --- & it probably got even higher when I was at work!
So to sum up, these coils have made the rest of the house warm with very little help from the oil burner. It's much nicer to be at home now! I used to use the burner for the TV room & when the upstairs got unbearably cold ( close to upper 40's! ) & of course for DHW, so with the run time it's seeing now, I'll still be using less oil.