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# Replacing a gravity boiler #2

Member Posts: 6,688
a copy of Dan's book "E.D.R.". It has ratings for almost any old radiator out there. Measuring square feet is much more than just length times width. The system circulator is sized to the radiation- go here for details:

http://www.heatinghelp.com/newsletter.cfm?Id=125

Size the new boiler to the building's heat loss, then you will know how much heat you need to move. This will determine the pipe size needed. If you pipe primary-secondary (recommended) the boiler circ is sized to the heat load.

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## Comments

• Member Posts: 540

These systems are few in my neck of the woods, but I do have the opportunity to replace an 1920 American Radiator boiler with standing radiators and would love to tackle this one.

I have a couple of questions: When measuring the sq.ft. of the existing radiators it's length x width to get the output of the radiator itself correct? #2 The existing near boiler piping is made up of 4" blk pipe and goes down from there any thing to be concerned about when redoing the near boiler piping in smaller diameter pipe?

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• Member Posts: 540

The house is 1920's vintage no thermal upgrades have been addressed, my thought was to size the boiler to the max. output of the radiation available.

Here's the scoop:3400 sq/ft heated The system as is, isn't gravity it is a pumped system ,4 zones. I assumed a converted gravity system, due to age and size of existing piping. Measured 478 sq/ft of standing rad's (each rad is 8" deep) after some surfing, assumed 170 btu sq/ft brings me to 81260 btu needed. System will be pri/sec.

Unfortunately this is a time sensitive job, don't want to make any grave errors in sizing near boiler piping.

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• Member Posts: 6,688
The standard output

was 150 BTU per square foot, with the boiler at 180 and a 20-degree delta-T. This would have resulted in an average rad temp of 170. But I'd still do the heat loss, you never know what rule of thumb was used to size those things.

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