Hot water radiators

Don McIntyre

How can you tell what the capacity is on a hot water radiator? I know they have info on steam radiators but how about hydronic? I have to size a replacement hydronic boiler with radiators on both floors and I would like to compare the existing radiation to the heat loss of the sturcture.

Brad White_44

Sized the same way as steam (EDR)

but the outputs differ with average temperature.

What you are doing is a good essential exercise to assess your heating capability and what temperature water will do the job at a given outdoor temperature.

With 2 psi steam (215F) you well know that the output is 240 BTUH per SF in a 70 degree room. Gosh is that easy to heat with!

This drops to 150 BTUH per SF when the average water temperature is 170 F. Basically the output drops by 20 BTUH for every 00 degrees drop below that all in a 70 degree room.

Naturally at 70 degree average water and a 70 degree room the output is zero (no temperature difference of course!)

So with AVERAGE water temperature at Degrees F:

100 = 10 BTUH/SF

110 = 30 BTUH/SF

120 = 50 BTUH/SF

130 = 70 BTUH/SF

140 = 90 BTUH/SF

150 = 110 BTUH/SF

160 = 130 BTUH/SF

170 = 150 BTUH/SF

Mike T., Swampeast MO

With water standing iron radiators you can forget about their heating ability when sizing a replacement boiler. The only thing that matters for sizing is the heat loss of the structure.

As long as the boiler supplies the necessary heat in the worst conditions, those hot water rads will do their job regardless of their temperature. In most cases the temp required will be WAY below the "standard" 180 degree temperature. Such systems are frequently among the best candidates for exceptional energy savings if you use a condensing/modulating boiler.

The main thing to watch out for are later modifications where the piping or radiation seem comparatively small or of a different type.

Brad White_44

Absolutely, Mike-

That ever-important distinction!

We make a good tag-team there!

Brad