How Much Water Should a Steam Heating System Need?

With steam heating systems, there’s one thing you can count on: They will always need feed water. How much water they need depends a lot on the system’s age and condition, but the feeding process never ends.Where does the water go? It leaves the system through evaporation, through leaky air vents on the radiators and mains. This type of leakage is especially aggravated by steam pressure that’s kept higher than necessary for the system (a condition we see all the time). And then there are the buried pipes. Even if there are just a few feet of buried return line on that job, there’s a good chance it’s leaking. 

Some home owners like to feed their steam boilers by hand, but the vast majority of home owners choose the convenience and backup safety advantages of an automatic water feeder. That’s because their heating contractors took the time to explain things to them. For instance, suppose there’s a leak in the system during the dead of winter when they aren’t home. An automatic feeder will keep the boiler running at its safe, minimum water line, and keep the house warm. A feeder can also protect a steam boiler by keeping it fed with water should the gas valve lock itself in the open position.

How much water a boiler needs to keep operating depends on its firing rate, and this is very easy to calculate. It works like this: All boilers, regardless of their size, lose water to steam at a constant rate. Ideally, they should be fed at 1 GPM per 250,000 Btu/hr, Gross Load (D.O.E. Heating Capacity). So, if a boiler is rated for, say, 500,000 Btu/hr, and the water level should drop to the “feed” line, you should be adding about 2 GPM to keep the burner on.

In residential steam heating, you can do this very effectively with McDonnell & Miller’s Uni-Match water feeder. The people at M&M came up with this feeder when the boiler manufacturers reduced the size of their replacement steam boilers. They designed the Uni-Match to protect those smaller boilers from nuisance, low water shutdowns.

The Uni-Match takes its signal from either a PS-800 probe-type, or a 67 float-type low-water cutoff. It has a timing circuit that makes it wait for a minute, has it feed for a minute, then wait for a second minute, and so on. This well-thought-out feed cycle gives the condensate a chance to return and greatly reduces the chance of a flooded boiler.

Here’s an important thing you should know, though. When you’re installing a Uni-Match feeder, you’re going to find two separate orifices inside the box. One of those orifices is for a feed rate of 1GPM, the other is for a feed rate of 4 GPM. There’s an orifice already installed in the Uni-Match at the factory, and this one is set to feed 2 GPM. This orifice will satisfy any steam heating boiler with a gross rating up to 500,000 Btu/hr.

If you’re working with a very small replacement steam boiler_say, one rated at 125,000 Btu/hr you should use the 1 GPM orifice, which is good for boilers up to 250,000 Btu/hr. This smaller orifice will feed at a slower rate and lessen the chance that returning condensate will flood the boiler.

If you have a larger steam boiler, one rated up to 1,000,000 Btu/hr, switch from the factory-installed 2 GPM orifice to the 4 GPM orifice you’ll find in the box. This larger orifice will let Uni-Match keep up with the needs of that bigger boiler and stop it from shutting down on low-water, should a leak develop in the system.

So how much water should a steam heating system need? It depends a lot on the system’s age and condition. But when the boiler needs water, it’s good to know the Uni-Match is there waiting with the right amount, and at the right time.

Ask your counterman to show you a Uni-Match water feeder, and tell your steam heating customers about the added security that automatic water feeders offer. They’ll be glad you did, and so will you!

Reprinted from CounterPoint October 1994, Vol. 1, Issue 4