How to Run a Hot Water Zone Off a Steam Boiler

Here’s a simple way to add a hot water zone to a steam-heated building without using a heat exchanger. Your new zone can serve an indirect domestic water heater, or a baseboard zone, but that zone must be not more than 30 feet above the boiler water line. At higher altitudes, it will have to be even closer to the boiler, because of the drop in atmospheric pressure.All you’ll need will be an all-bronze Bell & Gossett Series l00 circulator, two BSA-3/4 Flo Control valves, an angle thermometer, three full-port 3/4″ ball valves, a switching relay, an Aquastat and a few feet of 3/4″ copper tubing. 

Here’s how you do it. First, you have to remember that this is an open system. The water above the boiler water line can easily be hotter than 212 degrees when the boiler is steaming. The only thing that keeps the water in a liquid state is the pump’s pressure. The trouble is, when the pump is shut off, its pressure vanishes. When that happens, the hot water in the radiator can flash to steam, creating a racket in the radiator and driving the water back down to the boiler.

But that won’t be a problem for you because one of our ITT Bell &Gossett Representatives came up with a simple trick years ago to solve this problem. They used a bypass line (through the bottom of the two SA-3/4″ Flo-Control valves) to blend the cool return water from the zone with the hot supply water from the boiler. The Flo-Control valves also do a great job of preventing gravity circulation into the zone once the thermostat is satisfied.

Pipe it as we’ve sketched it, and then fill your new zone with a hose attached to Boiler Drain #l. Purge the zone and the bypass piping back to Boiler Drain #2. Make sure you have Ball Valves “A” and “B” closed, but leave Ball Valve “C” open. Now, don’t use automatic air vents in your new zone because they can let air in as well as out. If air gets into the top of the zone, the water will fall back into the boiler, but if you leave out the vents, the water will stay up in the piping. You know why? Because of atmospheric pressure. It’s the same force that keeps water in a straw when you hold your finger over the end and lift it from a glass. We think simple ways are the best, don’t you?

Next, steam the boiler and start your all-bronze Series l00. Use your two ball valves (“A” and “C”) to blend the water between the boiler and the bypass until you get a 180 degree reading on your thermometer. (Valve “B” is for service only and normally stays open.) Then, take the handles off the ball valves and you’re set; you’ll never have to touch the system again.

You’ll control your new zone by cycling the Series 100 and the burner with a room thermostat (through the switching relay and the Aquastat). If you’re not making steam for the rest of the house or building, the burner and the circulator will come on at the same time, and the boiler will run up to the aquastat’s high limit. If the water is hotter than 180 degrees (as it would be if the steaming cycle had just ended), only the circulator will run to satisfy the hot water zone. Beautifully simple, isn’t it.

We recommend you use an all-bronze Series 100 for this service because condensate is usually very acidic and tough on ordinary pumps. The all bronze Series 100 is a workhorse that will last for many years, even on this rough-and-tumble duty. Another plus is that the Series 100 has a much wider opening through its impeller and waterway than, say, our smaller Red Fox circulator. Because of its wide waterway, a Series 100 is much less likely to clog with steam-system sediment than other pumps. That translates to happy customers, and no) callbacks..

Reprinted from CounterPoint April 1994, Vol. 1, Issue 2