Read Hot Water Zone on a Steam Boiler text version

Hot Water Zone on a Steam Boiler

Text, illustrations and photography copyright Noel Murdough

It is often desirable to add a hot water heating zone or an indirect water heater to a steam heating system. This can be done, but several concerns need to be addressed.

The first concern is that there's no pressure in the system at the water line. At about 28 inches below the water line, there will be 1 PSI due to the weight of the water. Many residential boilers have a lower water line than that, so we aren't dealing with much pressure. This is a concern chiefly at the inlet to a pump, where low pressure combined with high temperature can cause the pump to cavitate. You can hear this happening, and it destroys pump impellers. In designing a pumped zone, you need to keep this from happening. Many people believe that pumps don't last long on steam systems because the water is gritty, so they add a strainer to the pump inlet. This only makes cavitation worse. Usually, the water gets blamed.

Adding a strainer to a pump inlet, where the static pressure is less than a pound and the water temperature is at or near boiling, will almost certainly cause cavitation in the pump. Piping to avoid cavitation is important.

A pump strainer can be added, but not right at the pump inlet. It should be installed at the boiler outlet, followed by the bypass tees, connecting the supply to the return of the zone around the boiler. By using this configuration, the water coming from the boiler will be strained, but the bypass water will be fed right to the pump inlet piping. A balancing valve should be installed in the bypass piping AND on the boiler piping.

You should make the bypass full sized. Once the new zone is filled and ready to put into operation, start the pump with the valve on the bypass wide open. Adjust the boiler balance valve or the bypass valve, to maintain 180 degree F. supply temperature in the hot water zone with the boiler steaming.

As the pressure at the inlet is held steady by the expansion tank (the boiler itself, on steam, is the expansion tank), the water in the bypass pipe will be at about ½ PSI. The water at the pump discharge will be at a positive pressure. The strainer would severely restrict the inlet pressure of the pump if it were piped after the bypass tee, right at the pump.

The indication that the strainer is becoming restricted will be a gradual reduction of heat output. If it were at the pump inlet directly, the indication would be a destroyed impeller in the pump.

The circulator must be installed as low as possible below the water line of the boiler. The only thing that keeps the static pressure at the pump positive is the weight of the water above it. It is imperative to pump away from the boiler, to maintain positive pressure at the circulator inlet. Sometimes it is necessary to throttle the return into the boiler AND the bypass valve to maintain a little head (resistance), so that the inlet pressure stays positive. Maintain a delta-T across the zone that indicates a good flow rate.

Another advantage of designing it with a 180 degree supply temperature to the zone is that you can actually deliver twice the heat than if you designed for 200 degree supply temperature. If you design for a 20 degree delta-T in your new zone, at 200 degrees, return water temperature would be 180 degrees, and not much water would go through the bypass. The ¾" boiler piping would provide 4 gpm, or 40,000 BTUH to the zone.

Now consider this.... if you design for 180 degree water, and 160 degree return water, you can design up to twice the BTUH load. Run 80,000 BTUH of 1-inch baseboard and run a 1-inch bypass around the boiler. Use the same ¾-inch piping in and out of the boiler.

Run 8 GPM through the zone to get the 20 degree delta-T. Run the same 4 GPM through the boiler, except now it is 160 degree return water. That 4 gpm through the

boiler is now at a 40 degree rise. That gives us 80,000 BTUH, which promptly mixes with the 4 GPM coming through the bypass at 160 degrees.

Average supply temp is now 180 degrees, with 80,000 BTUH added on the way by. All without pump cavitation.

The limit to the size of the pumped zone is the boiler pickup factor. Even this might not be what it seems. A target pickup factor is between one third and one half of the actual steam load connected to the boiler. In reality it can be much more, or even a little less. It is really important to know the amount of radiation connected to the system, and subtract that amount from the gross steam output of the boiler (usually about 80% of the firing rate). If the boiler is oversized to begin with, there might be quite a bit of load available for the water zone. NEVER use more than this number when sizing the new zone, or the steam side won't heat properly when both run together.

The practical limit for height of the zone above the boiler is 30 feet. Since the pressure at the water line of the boiler is zero, the rest of the piping above runs in a vacuum. There must not be any vents or valves above the water line of the boiler. Any packing leaks would be an air leak INTO the zone, which will stop circulation. The shut-off valves and the purge/drain valves should be near, but under, the waterline of the boiler. A purge valve set-up should be on BOTH ends of the zone, so that it can be filled and purged with a hose. The boiler has no way to push water through to initially fill and purge the zone. Brass hose bib caps installed on the drain valves add security.

Wire the thermostat from this zone to bring on a pump relay that starts the pump and brings on the burner through a dry set of contacts, as the boiler control is likely to be 24 volts AC. Wire the burner thermostat contacts from the relay, but break one lead through a "breaks on rise" aquastat, installed in the boiler outlet piping to the new zone. It would be installed in the same general location as the strainer; before the bypass tee. Its purpose is to stop the boiler from making steam, while allowing the water zone circulator to continue to run until the thermostat is satisfied. If the steam heat thermostat calls at the same time, both zones will heat together, as the steam heat thermostat won't be interrupted by the aquastat. Set the aquastat as high as you can without making steam. Around 200° works well.

Indirect water heaters and hot-water heating zones can be very effective on a steam system if all of the details are taken care of.

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