Posted By jeepermat on 03 Sep 2011 09:45 PM
I am in the process of replacing a 1955 Weil Mclain oil fired boiler with an LP unit, I am not sure of the size of the boiler, as most tags/info is long gone.
I have 96' of cast iron baseboard throughout the house that is all on 1 loop.
My plan right now has been to size the boiler based on the existing baseboard, I have no plans to add to the baseboards, or do any in floor radiant heat.
Using some info I found online I determined that Cast Iron baseboard can reject 600 btu/hr at 180F, so that would put me at ~58000 btu. Is that an acceptable way of sizing a replacement boiler?
The house is 1300 sq ft, has minimal insulation in the walls, at some point foam was added to the walls. The attic has R40 in it. The basement is uninsulated. All windows have been updated.
I am interested in using a Condensing boiler, however I am concerned that I wont see any benefits from it, using cast iron baseboards.
Would I be better off replacing the boiler with another standard 83% unit, or will I see any benefit from a condensing unit?
Every good heating system design starts with a room by room heat loss calculation. But in the absence of that, if you have good documentation of fuel use over a heating season and haven't been using other supplemental heat such as wood stoves or space heaters you can put reasonable upper bounds on the whole-house heat load at outdoor design temps using fuel use and weather data.
What's your location?
How much fuel did the beast burn in a year? (Or if your oil supplier stamps a "K-factor" on the billing, what's that number?_
Was it also heating potable hot water, or just space heating?
ANY cast iron baseboard can benefit from a condensing boiler, and at current propane prices the payoff would be pretty quick on the upcharge from an 83" cast iron unit in most places that have a significant heating season (US climate zone 5 or higher, or even the cool edge of zone 4.) Cast iron baseboards are superior to fin-tube, in that they deliver heat more predictably than the latter, even with 95F or 100F heating water, which is WELL into the condensing zone, and it has signficant thermal mass which can help avoid efficiency robbing short-cycling the boiler when it's running cool temps. How much of a benefit that is will depend on your actual heat loads and the minimum burn rate of the modulating boiler (smaller==more efficient) but I suspect the benefit will be substantial:
At 96 feet with 100F average water temp you'd get about 12,000BTU/hr (125BTU/foot) out of the Burnham BaseRay or Weil McLain RC cast iron baseboard, which is near the min-mod of some of the smallest condensing boilers out there. At 120F average water temps (well into the beginning of condensing) it'll put out ~20KBTU/hr (210BTU/foot) which COULD be your entire design-day heat load if the place is pretty tight.
If 58KBTU/hr was the heat load at design temp back when the house was built, BEFORE you the foam was added to the walls, the new windows went in, and the attic got bumped to R40. (Odds are that the original was actually considerably less than 58K,- given that 2x or even 3x system oversizing was the rule back in 1955.) With air sealing & insulation improvements it's highly likely that you've cut the heat load in half, maybe more, and it's at most 30K. More likely than not your real design condition heat load is under 25K, or could be with a modest amount of air sealing &/or basement insulation (both of which are worth while, if you're heating with propane or oil.) If your design condition load is 25K, the baseboard can deliver that with 130F average water temps, which would still be at the beginning of condensing and getting ~90% thermal efficiency. (Any time the water returning from the baseboard entering the boiler is under ~125F it's condensing in a propane burner.) So if during the least efficient hours of the season it's still getting 90%, the average efficiency will likely be mid-90s.