Another sizing question

Iceman

Please forgive me if this question has been answered many times before.

The local boiler guys want to replace my old boiler (116K input, 100.5K IBR, 82% AFUE) with another 100K+ BTU unit. The old boiler had no trouble heating the house before we blew in insulation, replaced all the doors and put in insulated windows w/storms. I’ve performed rudimentary heat-loss calc’s that give me 50-53K BTU depending on assumptions (because of odd orientation of house, mixed construction, etc.). And that’s based on a design temp of –10F which is below what is normally used for our area (0F).

I’ve also performed a usage test when I came back from vacation. With the house at 60 I jacked up the thermostat to 70 and timed the cycles. After the initial burn I averaged 7 minutes firing every 17 minutes (41%) until I closed in on the set temp. This would lead me to believe that my actual heat loss may be in the neighborhood of 41K BTU (41% x 100.5K BTU).

Either way I’m well below the 100K+ they are talking about. With one of the modulating, condensing, low-mass boilers I’m considering, my understanding is that you’d like to match the capacity to the heat loss (with some safety factor).

Help me out. Am I way off base?

Doug_7

Boiler sizing in existing house

You are right on. Another way to get an accurate fix on building heat losses and boiler sizing for an existing house is your natural gas meter.

Read your gas meter for several days in a row in cold weather and get the daily gas consumption and convert to Btu / hour. Get the Heating Degree Days (based on average outdoor temperature) for those same days from your local utility or weather station.

Plot a graph of gas use vs. Heating Degree Days, It is a pretty straight line. Pick off the gas use for the coldest day you expect and add a small safety factor. Correct for the efficiency difference between the old boiler and new boiler.

See attached. This graph is for a 50,000 sq ft building and the degree days are in degrees celsius. A 45 Heating Degree Day corresponds to an average outdoot temperature of -30 C. The graph line intercepts the vertical axis at the domestic hot water heater gas consumption.

This method is accurate and provides an excellent cross-check on any other methods of calculating or estimating building heat loss for boiler sizing.

Iceman

Thanks,

I'd seen that method before, but my current boiler burns oil. I'll have to get bifocals if I want to try to read the tank guage that accurately.

Doug_7

Boiler sizing in existing house

The other easy method is to record the total hours per day that the burner actually fires. Knowing burner rating this gives you the BTU / day.

I use a Red Lion timer attached to the burner which records cumulative burn hours. Red Lion Model CUB7T100 Programmable Timer. Records time when any voltage between 10 and 300 volts is applied.

I have Red Lion timers on all gas appliances. Has allowed me to determing actual heat loads and efficiency.