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Oil Boiler Sizing Problems

I have been trying to replace an 80-year old oil-fired snowman boiler with a new heating system. The oil heating dealers I have spoken to don't seem to have any idea of how to correctly size a heating system for our house.

Now that I've done the figures myself, I find they are all trying
to sell oversized systems. They base system size on the amount
of surface area of the radiators in the house, which was determined by the builder over 80 years ago, when houses were less insulated, more drafty and oil was cheaper.

The area we are looking to heat is about 1800 s/f - the 2nd and 3rd floor of a 2-family home.

The contractors suggested boilers in the 130,000 BTUH range. When I calculated using the actual surface area of all the radiators in the house, I came up with 85,000 BTUH..

According to our state's energy savings program, the proper way to do this is to calculate total heat loss from the house, and this points to an even smaller boiler.

I still would like an oil-fired system. How do I find an oil heat contractor in the Boston area who will do this correctly, and not just rely on a rule of thumb?

Re: Oil Boiler Sizing Problems


Yes, I agree; the only accurate way to calculate heat loss is to do a HEAT LOSS CALCULATION.

A HLC is a computer-driven program that considers an elaborate number of factors such as geographical location, type of construction material used in the walls, floors, basement, amount of insulation, square footage of west-facing walls, height of the ceilings, # & type & sq.footage of windows & doors, to name a few.

Some installers are reluctant to take the half hour or hour that it takes to enter all this data into the computer to get the results.

Keep searching until you find an installer who owns a HLC as a standard piece of their equipment & knows how to use it.

Depending on the amount of insulation in the exterior walls (R19 recommended) and the attic (R40 recommended) and tightness of the windows (double pane or storms recommended), and 8' ceiling heights, a "rule of thumb" often used is to assign 30 btu/hr to 40 btu/hr to each sq.ft. of floor space.

In your case that would factor out to between 54k btu/hr to 72k btu/hr for new boiler size; any lack of insulation or draftiness of windows, etc. would require assigning up to 50 or 60 btu/sq.ft., thus raising the required boiler size to from 90k btu/hr to 108,000 btu/hr.

As noted, these "rough estimates" are exactly that, rough estimates, and a HLC is the only way to get a definitive answer.

It should be also noted that most oil-fired boilers used today have an output range that varies by approx. 30%, & can be changed by simply changing the size of the fuel nozzle ($2 item)---typically, boilers for domestic application have nozzles that burn fuel at from .50 gal per hour to 1.20 gal. per hour.

A responsible installer will try several nozzles by different size & mfgrs at the time of installation to get one that will give a nice flame pattern, with minimal noise; he will then use a combustion analyzer to optimize the fuel, air & fuel pressure mix to get the most efficient flame that burns all of the fuel in the flame economically---a soot test is required by law & must be a low soot number of one or zero; this number is attached to a label on the burner housing; low soot means the flame is burning all the fuel that enters the combustion chamber.

Re: Oil Boiler Sizing Problems

Thanks, Nashuatech, for that very clear explanation.

I also forgot to mention that there is a heated first floor apartment (separate heating system)under our apartment, so we get a bit of our tenant's heat coming up through the floor, so the heating required may actually be less than the rule-of-thumb you stated.

Here's my problem: I'm trying to get a state Oil Heating Efficiency Grant. The state requires that this calculation be done by the installer, with all the calculations shown. The work must also be done by a member of the local oil heat trade association.

I've been going down the list of members, and no one seems to know how to do these calculations, or at least that's what they're telling me. So, I'm at an impasse, the state wants the calculations, and none of the eligible installers want to do it.

It looks as if I may have no choice but to switch to gas, since there are gas installers willing to do it right.

Thanks again for your wonderful answer.

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