Replacement boiler sizing

Mage60

Hello,



I am a new homeowner looking to replace a 50+ year old oil-fired Arcoliner boiler rated at 73 MBH output. The home is a 1400 sq ft cape (unfinished basement not included) with forced hot water convectors in the Boston area.



My question is on sizing. During the last winter, the house would heat up fine but relatively slowly, say 2-3 °F/hour on cold days. I set the thermostat back a good 7 °F or so during the day, so the warm up time was farily significant. I noticed the boiler would fire for a good 30-40 min, turn off for say 10 min while the pump circulated, and fire on again for another 20-30 min. This would repeat 2-4 times while the house got up to temperature.



I have since learned that the first floor was uninsulated, so I had cellulose blown in which should cut down on the heat loss significantly.



One contractor recommended a boiler rated at 84 MBH. Would this now be oversizing? I am unsure because the house took so long to heat up before (which would dictate a larger boiler needed), but with this new insulation and a low-mass, low water content boiler (which would dictate a smaller boiler), I imagine it will be much faster this year. I wonder if I would want to stay at the 73 MBH range or even go smaller. I was hoping to install the new boiler this summer, and not go through another winter to find out how much the insulation helped.



Any advice would be most appreciated!

Rich_49

You should

certainly contact a reputable local contractor to perform a heat loss calculation for the house in it's present state before committing to any sizing .

Jamie Hall

Certainly contact a reputable contractor --

there are several in the Boston area (use Find a Contractor, searching by State, not Zip).



However, there are some other considerations.  There has been considerable debate on the subject of setbacks on the Wall, mostly in connection with steam -- but some of the same principles apply, with some quirks.



You can get more heat out of your convectors if you run hotter water into them.  This would help recover from a setback faster.  The boiler capacity can be determined -- besides looking at heat loss -- by looking at the total available convection and figuring out how much heat it can absorb at a given temperature.



OK -- so we can get faster recovery with a big enough boiler running at a high enough temperature.  But at what expense?  The modern "mod-con" boiler achieve their best (and quoted!) efficiencies only when running with relatively low temperatures, so that they can condense the moisture in the stack gases.  This is where they get that efficiency from.  If they are run at higher temperatures, you lose that efficiency (and burn more fuel) -- which may be a consideration.



I have to admit that I am not a big fan of setbacks which take longer than an hour to recover from, regardless of the type of heat (steam, hot water, air, makes no difference).  At least in my experience, they actually cost money...  My suggestion for your house would be a setback of no more than about 3 degrees.

Zman

Boiler

A heat loss will tell you how much energy your house is losing to the outdoors on the coldest design day for you climate. This is the starting point for sizing your boiler. Given what you have said, your max heat loss is in the mid 30kbtu range.

Another consideration, particularly since you are interested in fast recovery from a temp setback is how much available radiation you have (size of your radiators). A 73 kbtu boiler that cycles off 30% of the time at full load is likely about 30% oversized to the radiation. Even if you were to put in a million btu boiler, it would not heat the house any faster than a 50kbtu model.

I would suggest a smaller firetube modcon boiler. They have onboard  outdoor reset which maximizes efficiency. I also would minimize your setbacks.

  I like triangle tube and lochinvar. Dan has been impressed with the new Utica SSC. I think it is worth a look.

Carl