The classic heat purging economizers are the

__Becket Heat Manager__ (manufactured by Intellicon) and the

__Intellicon 3250 HW+__, which are pretty easy for DIY retrofit on many boilers, but there are others. Newer boilers often come with smarter controls included. The

__Beckett Aqua Smart__, and

__Hydrostat 3250 Plus__ are sometimes OEMed into new boilers, but can also be retrofitted.

Short of replacing sash weights with coil spring units,

__retrofit pulley seals__ can dramatically reduce the air leakage at the sash weight pockets for low money if the window trim is already tightly caulked:

(I ended up installing a bunch of these in my 1923 vintage bungalow.)

Build a spreadsheet using a U-factor of U0.20 (=R5) for the above-grade exterior wall area, and U0.35 for the window area, run the room by room heat load numbers I=B=R style, assuming a 60F temperature difference (69F indoors, 9F outdoors) and see what you come up with for conducted heat loss. Assuming you have 6-9" of fluff in the attic, use U0.04 for the ceiling losses of top floor rooms. Add a foot of height to the wall area to account for the joists between rooms. Use U0.5 for solid wood doors, U0.8 for paneled doors and single-pane wood sashed windows (no storms).

The basic formula is:

Area x U-factor x 60F= heat loss

eg: If a top floor room is say, 13 x 15', and has 9' ceilings, and two 12 square foot windows (measure the full area inside the casing trim, not just the glass), and just one 15' of exterior wall, the conducted heat loss is:

Ceiling:

195 square feet x U0.04 x 60F= 468 BTU/hr

Windows:

24' x U0.35 x 60F= 504 BTU/hr

Walls: The gross area (including joist depth) is (9'+1') x 15' = 150', less 24' of window is 126' of wall area.

126' x U0.20 x 60F= 1512 BTU/hr

Add it all up and you're at 2484 BTU/hr, a ratio of about 13 BTU per square foot of conditioned space for that room, before air infiltration factors, and before subtracting off the heat of mammalian bodies (230BTU/hr per sleeping human, etc.)

It's useful to track it room by room then add up the whole house numbers. With the room by room numbers you can also look at the load per

__square foot EDR of the radiators__, which would tell you how well balanced the heating system design is, and to get a handle on the water temperatures necessary to meet the 99% load (and colder.) A typical cast iron boiler set up for 190F output and a 20F in-to-out delta-T for a 170F return puts out ~170BTU per EDR' from the radiators. If you crank the boiler up to 215F you can get a bit more out of the rads, but not a lot more. Don't expect more than 200 BTU /ft-EDR out of them with any hot water boiler, and not more than 160 BTU/ft-EDR with most condensing boilers, if considering that option. Knowing the approximate load/radiation ratio can be a critical point in that decision tree.

For the basement load, just count the square feet of foundation above grade. If insulated with 2" of roofing polyiso with half-inch wallboard on the interior you're looking at about U0.065- U0.075, if it's the terra-cotta + brick veneer construction. So if you have 200' of perimeter with 2' of exposure, no windows, the above grade losses prior to insulation would be

400' x U0.2o x 60F= 4800 BTU/hr, and probably another 2500-4000 BTU/hr of below grade loss (including the slab), potentially ~7000-9000 BTU/hr.

After insulation it would be about:

400' x U0.075 x 60F= 1800 BTU/hr of above grade losses, and probably less than 2500 BTU/hr for the rest, something like 4000 BTU/hr total, possibly only 3000 BTU/hr depending on the R-value of the soil below the slab.

But run the conducted loss numbers, see where they fall before including any infiltration/ventilation loss estimates.