EDR stands for EQUIVALENT DIRECT RADIATION.  That is an architect’s term to express the heat load on a steam heating system in a building and is expressed in so many thousands of square feet.

The specific definition is: “The surface area on a radiator that will give off 240 BTUs per hour of heat when surrounded by 70 degree F air and filled with 215 degree F steam”.

This means that when the building was originally designed, the radiation was sized for a 215 degree F steam temperature.

There is a direct relationship between steam temperature and steam pressure.  As steam pressure rises, so does it’s temperature.

If we look at a chart showing the pressure/temperature relationship of steam, we find that at 1 PSI, steam has 216 degrees F of temperature.  What this means is, if we have 216 degrees F (1 PSI) of steam out at the farthest point in the system, then all we need at the boiler is enough extra pressure to make up for any pressure losses through the system.

In practical terms, we can run our boiler to shut off at 2 PSI and come on at 1/2 PSI.  You can raise it little if you need to but that should be enough.

Now, what about pumps?  A vacuum or condensate pump is a machine designed to move a fluid (air or condensate) at a certain flow rate (CFM or GPM) at a certain pressure (″Hg. vacuum or PSI, pressure).

Most pump manufacturers’ catalogs will show their offerings for certain EDR ratings but it is important to understand that PUMPS DON’T HAVE EDR, BUILDINGS DO!

The various pump manufacturers will use flow rates which in their judgement are adequate for a given EDR load, but these will vary between manufacturers.  Many times in the case of vacuum pumps, these ratings are marginal and may not prove satisfactory.  This is especially true in an older building that has developed a few air and trap leaks.

The important thing to consider when choosing a vacuum or condensate pump is not the EDR rating but the flow capacity.  This is especially true in the case of vacuum pumps because there is no direct formula you can use to size the vacuum pump on a steam system.  Experience and judgement must be used.  ONE THING IS CERTAIN, IT IS DIFFICULT TO ERR IN PROVIDING TOO MUCH AIR CAPACITY.

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