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Existing (water) radiators, existing newer boiler, old building and building load.... High gas$$HMMM

I looked at a two flat that probably dates to the late 1920's, based upon the American "Corto" Radiators that are in the building.

I physically looked at and measured each radiator, cross referenced the style in Dan Holohan's "E.D.R" book, and came up with 357 total square feet of EDR for all 6 radiators (one of them is huge, at 97 inches long- I'd hate to move it :) ).

The existing boiler is a Dunkirk model PVW1125 with an input rating of 112,500 BTUH. I couldn't find any info on that particular model, but based on a current production model with the same input rating, it has an 82,000 BTUH net AHRI rating.

The floor space being heated is 1,200 SF.

Original construction of structural brick walls (probably 10-12" thick), one inch air space, lath and plaster. Windows have been updated.

If I pull out the rules of thumb...

Assuming maximum 140*F water in the radiators, that is 32,130 BTUH out of the radiators (90 x 357).

Assumed heat load would be 30 BTU/SF x 1200 = 36,000 BTU. In my experience with forced air, this number is always high for average construction. Brick buildings (structural, not veneer) lack most of the extra air infiltration places that stick built buildings have.

This assumed heat load calculation is really close to the assumed ideal running water temperature BTUH out of the radiators.

Measured temperature at the boiler return with a digital thermometer and pipe clamp was 107*F and the supply was only a little more at 112*F (yes, this is a problem).

So, the thermostat is cycling at 69 degrees with an average water temperature likely around 110*F and an outdoor temperature of 21*F at the time.

An average water temperature of 110*F is 10,710 BTUH out of the radiators (30 x 357).

Nowhere near 140*F...

Customer is complaining of high gas costs, not poor heating. Homeowner states that the system cycled through the recent cold snap, maintaining a 70*F indoor setpoint.

So, in theory, the existing boiler should be more than adequate to heat the space, correct? It almost seems that the case for being oversized can be made based upon feedback.

I have never personally witnessed one of these systems actually running 140*F water through the radiators, unless the system literally runs for hours, and then it is like 80*F or more inside...

I am thinking that the high energy use is caused by the lack of a temperature differential between the supply and return. There are no valves in the system to regulate water flow (other than what is at the radiators), so it is running wide open. The installed circulator is a B&G NRF-22. Estimated feet of head is 3.6 (60 x 1.5 x 0.04).

Whatever was put there for balancing valves when the system was originally installed was removed when the boilers were replaced last time.


Thank you.


  • Steamhead
    Steamhead Member Posts: 14,842
    Have you done a combustion test on the boiler? If so, what were the results?
    All Steamed Up, Inc.
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
  • danski0224
    danski0224 Member Posts: 7
    I did.
    Reading below draft hood:
    576*F flue
    12% O2
    4ppm CO
    Calculated Efficiency of 69.4%

    There isn't a motorized vent damper and it is standing pilot.

    The plan is to pull the draft hood and install a barometric damper, add a vent damper and convert it to intermittent ignition.
  • Steamhead
    Steamhead Member Posts: 14,842
    Sounds like too much excess air. What was the CO2 percentage?
    All Steamed Up, Inc.
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
  • danski0224
    danski0224 Member Posts: 7
    I didn't write that down.
    EBEBRATT-Ed Member Posts: 10,324
    Stack temperature seems to high especially with that big load on the boiler and low water temperature. Need a better combustion test.

    Also low water temp will cause condensation in the flue. I would pipe in a 3 way valve and a bypass their are many ways to do this.

    I would 0 in on the combustion first. The boiler should be large enough for the radiation and I doubt the radiation is undersized.

    Stack temp and too much excess air=heat is going up the flue as @Steamhead alluded to