87% Efficient Burnham 3-Pass Oil Burner versus a 94% Gas-Fired Mod-Con

We (four adults) live in a 50-year old ranch in northeastern MA. The total square footage of the ground floor is 1,700 ft2 and the basement is 1,500 ft2. The house is well insulated. We had treated cellulose blown into the wall cavities, as well as, 15" in the attic. We also have replaced all the windows, including the basement. The square footage of the ground floor is 1,700 ft2 and the basement area is 1,500 ft2. I am currently insulating the basement as well (the basement will have dri-core panels on the floor, R-13 foamboard on the walls, and R-30 fiberglass batts in the ceiling). We will add some additional baseboard heating elements to the basement as an additional zone.

The house is currently heated with a circa 1950's tankless American Standard oil boiler (Arcoliner 3B J3 series) and a forced hot water Mono Flo baseboard aluminum fin system on two zones. The total length of the baseboard emitters is 145 feet and they are all on the ground floor. There are currently no heating elements in the basement. The tankless coil boiler provides all the heat and hot water for the house. We currently run out of hot water quickly if two showers are taken back-to-back or if the forced hot water heating system kicks on when in the shower.

In 2010, we burned 720 gallons of oil. The total number of heating degree days in 2010 for our area totalled 5,778. If I did this correct, this yields a K-factor of 8 (5,778/720). I tried the NORA Fuel Savings Analysis (FSA) calculator and it gave me a design day heat load of about 30,000 btu/hr when I input Boston, MA as the location, design temperature of -2 F and a K-factor of 8.

I had a contractor come by and he told me that since we have a Mono-Flo baseboard heating system with aluminum fins (not cast iron), then he does not recommend a gas-fired high-efficiency mod-con boiler. The main reason being that the return water ends up being too hot and therefore the boiler will never have a chance to enter into condensing mode. If we had cast-iron fins, or a radiant heating system, then he said that is a much better situation for a mod-con boiler since the return water ends up being much cooler.

If we were to stick with oil, he recomended a Burnham MPO-IQ115 high-efficiency (87%) 3-pass oil-fired boiler. This is a cast iron water boiler with a maximum heating capacity of 98,000 btu/hr and a weather sensitive control that adjusts the boiler temperature up or down based on outdoor air temperature.

For an indirect water heater, he recommended the Burnham Alliance AL50SL. This is a 50-gallon, seamless hydrastone-lined tank that has a first hour rating of 225 gal/hr and continuous draw rating of 171 gal/hr with a 6 gpm flow rate. He said that would work fine to fill a 70-gal jacuzzi and still be able to supply hot water for other uses.

Given that the Design Day heat load for the house is about 30,000 btu/hr, does this sound like the correct boiler size? Burnham offers a smaller-sized 3-Pass oil boiler (MPO-IQ84) with a maximum heating capacity of 74,000 btu/hr. This is the smallest size they offer for the 87% efficient 3-pass boiler.

What about a 94% efficient gas-fired mod-con boiler, do you agree that gas-fired mod-con boilers would not work well with the Mono-Flo aluminum fin system because of the elevated return water temps?

Any and all comments are much appreciated.

I'm having trouble understanding why a monoflow system would return hotter water. It might, but I don't understand why. Yes, it's possible some possibly minimally diluted hot water could leak through, but a lot of the water passes through each convecter, so cools off as it passes each. Keep in mind also that those efficiency numbers are based on 'normal' cycles where the boiler minimizes its startup and cool down losses. At best, on the design day coldest temp, if your load is 30K, and your boiler is 74K, the boiler would run less than half of the time. A mod-con would be running 100% of the time, and thus have no startup cooldown losses, so even if it wasn't condensing (which I think it would), it would end up more efficient. Now, cost-wise, I don't know how those two compare to purchase and install, but considering a boiler can last decades, and fuel prices are only likely to go up, every little bit helps. The break even point may not be good, you'd have to calculate that.

Even with an outside reset controller that adjusts the boiler temp on the high mass unit, that would likely cause it to short-cycle even more since it can't modulate the burner. It would make the house more comfortable by reducing the cycling of max heat by letting it use a more moderate temp in mild conditions, but to supply that moderate heat, it would be turning on and off the boiler more (much more!) than a mod-con. Cycles affect not only the efficiency but the longevity.

Either would be a good choice. The 50 gal indirect would work fine on the smaller boiler providing you do not want to re-fill the tank right after using the tub to use a second time. A smaller boiler will take a little longer to recover and will reduce the IWH output slightly. Of course this all depends on the flow at the tub. I would definitely do the smaller boiler (either one) and think about a buffer tank for the heating if doing oil.