Currently have a 65 yr old 4 section kohler oil fired boiier. Used 1300 gallons for heat and domestic hot water each year Am converting to natural gas. House is 1700 sq ft 1st level with 125 feet of CAST IRON baseboard. Also have radiant in a 200 sq ft room. 700 sq ft finished attic with 50 ft of slant fin baseboard. Newly reinsulated with fiberglass in exterior walls. 6" fiberglass in roof. Replacement windows and new doors installed. My question is should I use a conventional 80 to 85% efficiency boiler such as a burnham 205 nil - tei2 or a high efficiency single by pass condensing boiler such as the burnham alpine 150w-2L02? Also am I sizing it correctly?

A btu is a btu. So with that in mind if the original boiler heated your home then something close in btu output will also do the same.What should really take place is a heat load calculation of your home and a room by room analysis.This will insure you are not oversizing your boiler and also will determine lineal feet of baseboard and flowrate needed. Then after knowing this a boiler selection could be established based on return water temperature.The return temperature will determine if a condensing boiler can be justified for your project.

Thank you for your reply. A heat load calculation was done with a determination that an output of 90,000 btu would be sufficient for heating. I'm not sure if this includes the domestic hot water for a separate 50 gallon water heater. Since the old boiler is not functioning it is not possible to determine the return water temperature. The Alpine 150 has a 150 btu "input". The next one down is 105 btu "input". I don't want to oversize but am concerned the 105 is insufficient. Also is a "condensing" boiler best for my situation of cast iron base board?

I'm not sure if this includes the domestic hot water for a separate 50 gallon water heater.

You need to sort that out. A 50 gal gas water heater will be something on the order of 40,000 BTU (per hour), so that will make a difference in how it is divvied up.

A calculated heat load of 90,000 BTU in a 2500 sf home with upgraded insulation would seem high, but you haven't mentioned what your climate is.

Your past usage of 1300 gal of heating oil per year for heat and water represents usable heat of about 1300 X 115,000 X 70% eff = approx 100 MMBTU or 100 million BTU total for the year. Divide that up however you want, but if you conveniently assume 70% went to heat and 30% went to hot water, you have about 70 MMBTU and 30 MMBTU respectively. 30 million BTU yearly represents 80,000 BTU per day year round, which might be a good approximation for hot water, but you'd have to figure out how your particular heating season divides up the 70 MMBTU for heating.

6" fiberglass in roof

Was that code minimum?

You can take your heating degree days for last year (or sometimes even last night) and work backwards to get what is needed to heat on a design day. Usually more accurate than a heat load calc - but in your case, the insulation has changed and your old boiler isn't available.

I don't know where you are located, but I suggest before you spend thousands you put together a wish list (that may include cooling, air purification etc.) and make a total strategy of efficiency (perhaps envelope improvements) op and purchase cost. While nat gas is usually cheapest fuel, sometimes geo or ashps can give you a good payback for money spent.
IMHO the trade off on high efficiency boilers is often offset by purchase price and repair possibilities. Like geo against nat gas the gap is narrowed by rebates and tax credits.
good luck,
joe

Posted By harv11733 on 24 Nov 2011 08:35 AM
Thank you for your reply. A heat load calculation was done with a determination that an output of 90,000 btu would be sufficient for heating. I'm not sure if this includes the domestic hot water for a separate 50 gallon water heater. Since the old boiler is not functioning it is not possible to determine the return water temperature. The Alpine 150 has a 150 btu "input". The next one down is 105 btu "input". I don't want to oversize but am concerned the 105 is insufficient. Also is a "condensing" boiler best for my situation of cast iron base board?

What's your location? (so we can look up your design condion outdoor temp and annual heating degree days.) That 90KBTU heat load number seems high by at least a factor of 2, maybe as high as three, based on your  fuel use.

eg:  A 65 year old cast-iron boiler is probably only running 75% efficiency, at best, which means you're getting (0.75 x 138,000=)~100KBTU out of each gallon.  Let's say you live in Worcester MA, (my current home town. :-) ) with about a 6800 heating degree day (base 65F) and a 99th percentile design temp of 0F.  In 6800HDD you used 1300 gallons which is ~0.91 gallons/HDD, or (100,000 x 0.19=) 19000BTU/HDD.  There are 24 hours in a day, so that's 19000/24= ~800BTUs per degree- hour.

Your design temp is 0F, your HDD base temp is 65F, so the difference is 65F degrees. At 800BTU/degree-hour and 65F heating degrees at design temp, you need at most something with an output of (65F x 800BTU/F-hr=) 52,000BTU/hour.

Given that in an old boiler standing by all summer at elevated temp to deliver the domestic hot water probably burns 200gallons/year on hot water heating, the odds are your 0F heat load is under 45KBTU/hour.

If your old boiler is 4-5x oversized for the load, poorly insulated and has never had the heat exchangers refurbished your actual efficiency is going to be under 60%, and your design condition heat load correspondingly lower.  But tell us where you are, and we can use real weather data & design temps. 

It's important to get the SMALLEST unit that actually meets your design condition heat load, even with a modulating-condensing boiler, even more so if the system is divided into separate zones, since the smallest zone will have an average heat load well below the maximum output of the boiler, and probably below the minimum modulated output even on design day.  A boiler that short-cycles on zone calls will have lower operating efficiency and will wear out sooner. 

It would take a very large, underinsulated and drafty-breezy house need the full output of the Alpine 105 in my neighborhood, and it's min-modulation might be more than half your design condition load, which would mean it can't be tweaked to modulate perfectly with load- it'll cycle on/off more often than it otherwise might need to if it were "right sized" for the load.  The Alpine-80 modulates down to 16K-in but the -05 only goes down to 20K, and the -150's min-mod is 30K (which is more than 80% of my design day heat load, let alone my average.)  If you upsize to the 150 and your actual design condition load is 35-45K, it's bound to always cycle on zone calls rather than continuously modulate except on the coldest days of the season.

Cast iron baseboard is GREAT for mod-cons, compared to fin-tube since it has many times more thermal mass, which makes it less prone to short-cycling the boiler, and a more predicatable output at lower water temps- you can get far more condensing efficiency out of it than you could with fin-tube, and it's far less likely to run into short-cycling problems even at low operating temps, unless it's been micro-zoned, with a separate thermostat in every room.

Thank you for your kind and detailed reply.
My home is on the north shore of long island, 60 miles east of NYC in Port Jefferson.
I hope to determine which gas boiler would be the most efficient for my situation.
Cedar shake home with newly added 3 1/2 inch fiberglass insulation in walls, 9" in eaves and 6" in roof.
House has maximum number of large newly replaced anderson windows and sliding/french doors.
125 feet of cast iron baseboard, 50 ft slant finn and a 200 sq ft room with radiant heat on a slab.
Overall square footage is 1700 sq ft 1st level, 700 sq ft upper level, 1200 sq ft unheated basement/garage.
There are 3 zones.
I will install a 50 gallon hot water heater to allow for sufficient hot water when we have guests.
I used 1300 gallons in heating seasons ending in 2010 and 2011.
Again thank you for your responses.

If you have concerns about short cycling or small zones, add a water buffer tank.

On Long Island the 97.th percentile design-temps are in the mid-teens, 99th percentile in the high single digiits, with ~5600 HDD. Assuming 75% efficiency (~100KBTU/gallon) if you use 1300/5600= 0.23 gallons/HDD, that's ~23,000BTU/degree-day which ~960BTU per degree-hour. With a an outside design temp of +10F, that's (65-10=) 55 heating degrees, so your heat load at design temperature is a maximum of (55 x 960=) ~38KBTU/hr.

It's probably really only 35K, discounting for the oil used for hot water heating. You'd do just fine with an ALP-80 or a Triangle Tube Solo-60 or similar, and going to the ALP105 would be a mistake. If your design load at +10F is 38K, it would have to be colder than 25-below zero to overstretch the 63K non-condensing output of the ALP80- you have some real margin there! If your +10F heat load is actually ~35K or less (likely) you'd be good down to -35F. (Last time it was that cold in Port Jeff L.I. was toward the end of the last ice age!)

These are realistic numbers- to have a 90K design condition heat load you'd have to have used 3000 gallons or more.

I'm betting the 90K number came from...

"Lessee, 1700' plus 700' plus 1200' is 3600', so times 25BTU a foot gives ya 90K."

...which is a typical old-schooler's methodology that oversizes it EVERY TIME, usually by more than 2x (as in your case), but the contractor never has to get the 5AM call from the irate and freezing client. If you upsize from there you're paying more to get even less efficiency and a shorter boiler life, and even going with the ALP-105 would be a mistake, from a short-cycling potential point of view:

The only zone of significant short-cycling potential is the attic, with 50' of fin-tube (assuming it's just a separate zone, not tied to any of the higher-mass baseboard or radiant floor), and it may ultimately define the low-end of the outdoor reset curve on the boiler. At 120F average water temp it'll get good condensing efficiency, but delivers only 10-11KBTU, which is only ~2/3 minimum modulated output of even the ALP80 at low-fire (but it's half the min-mod of the -105, which is why upsizing would be an issue.) If you bump the average water temp up to ~130-135F the fin-tube will deliver enough heat to keep the boiler from cycling, but you'd be at the bare edge of condensing. Ideally you'd like to be able to operate at a temp under 120F to max out the condensing efficiency, but as long as the burn cycles are longer than 2-3 minutes even on that low-mass smaller zone you'll be fine. It may take some tweaking of the outdoor reset curves and hysteresis on the boiler controls to keep the burn times up while serving that load, but it's likely you'd be able to get there. Worst case you might replace some or all of the fin tube with baseboard of higher thermal mass (Runtal, or cast-iron baseboards), if it just doesn't want to behave. Alternatively, adding 10 gallons or so of buffer tank (an electric HW heater, not wired up) plumbed in series with the zone would get you there.

For the domestic hot water, use an "indirect" tank heated by the boiler, controlled as the "priority zone". The size of the tank would only need to be as big as the biggest bathtub, unless you have some luxury-shower with a gazillion sidesprays or something, but int most cases a 40 gallon indirect would be enough, and would provide more hot water per hour than a 50 gallon standalone. The output of even the smallest mod-con at maximum fire is substantially bigger than the burners on most 50 gallon standalone tanks (the ALP-80 can put out 2x the heat of some 50 gallon tank burners), and will be significantly more efficient.