Estimate boiler sizing

Rich_41

I own an eight-unit building in the Pittsburgh, PA area and I am planning on replacing the existing hot water boiler this summer with two smaller high efficiency boilers. Prior to speaking specifics with my boiler guy who will be doing the install, I am hoping to get an estimate from the knowledgeable folks here on the approx boiler size(s) required.


Here are the facts regarding the building:


1) Built 1930, all brick, 4 stories (2 apts per floor).

2) Eight 1-bedroom units, each approx 850 sq ft.

3) Approx total living area (not including the basement, stairwells) is 6800 sq ft.

4) Apt ceilings are 8 feet.

5) Average 9 windows per apartment (approx 28x54 inch). All are new double hung windows.

6) The units are comfortably warm in the winter and not drafty.

6) Each unit has 4 radiators - one in the livingroom, divingroom, bath (smaller rad unit)and bedroom.

7) There is some blown insulation in the attic - I would guess 4 inches. I'm not certain if there is any insulation behind the plaster walls - I'm guessing not.

8) Current boiler is probably original to the building and was converted to natural gas at some point.

9) Current boiler is 1 million Btu - I think - and keeps the apartments plenty warm in the winter. I control the heat with one thermostat, which is located in the bedroom of one of the 1st floor apartments.

10) Hot water for showers, etc is run off of a separate hot water system.


Thanks again for your input.

Brad White_9

Take your information

and perform a real heat loss calculation. Take a look at the "Heat Loss Calcs" tab above on this site.

You have most of the parameters down. I am not going out on too much of a limb to say that a 1 million input boiler (800 MBH output or so) is over-sized.

At 6800 SF as a check figure only, that is about 117 BTUH per SF. I would think that is double or more of your real number when you get done, for essentially uninsulated masonry.

Do a take-off estimate of your connected radiation. This will allow you to determine if a lower water temperature will serve you on the coldest day. Maybe you send out 180 degree water today when even 150 may do the trick. You could even consider a condensing boiler if you have gas available.

Let us know how it works out. You are on the verge of an excellent energy savings opportunity.