Another Mod/Con Boiler Question(s) Please?

derekw

Hello Heating Help,
I have a question similar to ones asked recently about installing a mod/con gas (propane) boiler with baseboard space heating. I am a homeowner, not a pro, and have been reading trying to educate myself to make a good decision.

The Situation: I am considering replacing an older oil-fired boiler (Burnham V7 approx. 191 btu) with a new one using propane (no natural gas here). The house was built in 1996, 3 bed, 2 ½ bath, approx. 3100 sq.ft., located in the western mountain region of Maine. It is 3 floors, 5 zones, but with a very open design and few doors separating rooms or floors. I used a boiler replacement worksheet from Weil McLain to calculate total heat loss. Using 3 different assumptions about construction, I came up with a range from 75,000 – 81,000 btu/hr. The total amount of baseboard (Slant/Fin line 30) is approx. 170 ft. In addition to the baseboard there is a floor heater in the entryway that puts out approx. 620 btu/hr. Using data from Slant/Fin, running 180 degree H2O @580btu/hr./ft., plus the in-floor heater should produce approx. 104,863 btu/hr. Going to 150 degree H2O @380btu/hr./ft. would give approx. 70,815btu/hr. Dropping to 130 degrees gives approx. 50,385 btu/hr. So even at 150 degree H2O temp., output appears to dip below heat loss at design temperature. One more piece to the puzzle: we have a fireplace insert and burn wood in the evenings during the coldest part of the heating season. It kicks out a lot of heat into the 1st floor living area, but I don’t know how to account for that (it’s enough so that the boiler shuts off after the fan gets going). Also, I understand that no boiler would be running at design temperature a lot of the time. However, it appears that during the peak season, the condensing boiler would not have a low enough return water temp. to operate as efficiently as it could/should (correct?)

The propane company that services our town suggested installing a Bradford White “Brute” FT 199, wall hung, “combi” for space heating and domestic hot water both.

My Questions: 1) Does the BW 199 seem a bit oversized? They have a 140 model also (at 140K btu). 2) Related to question 1, the BW 199 says it can automatically adjust how much fuel is used depending on the demand – from 20% to 100%. Would this offset any concern about being oversized and possibly “short-cycling” (as I understand it!)? 3) The product description says it has/uses outdoor reset. I assume this is how it adjusts it’s output from 20% to 100%? 4) How does the output adjustment work…does it actually vary the amount of heat/fuel used as it burns to control the water temperature (vs. burning at max. for shorter or longer periods of time)? 5) We currently have “on demand” hot domestic water with the old Burnham and it supplies enough for our needs (although I suspect very inefficiently!). Are there any reasons I should consider a separate tank vs. the “combi”? 6) Overall, what am I missing in this decision and do you have any suggestions?

Thank you so much for reading this long post! I tried to give as much info. as possible because sometimes I see your experts having to ask a lot of questions back and forth.
Thanks again….Derek in Maine