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Piping an indirect or baseboard off a steam boiler

Piping an indirect water heater into a steam boiler is not an uncommon application with gas or oil fired steam boilers. There are a few bits of information we need to talk about before we do the piping.

1. We can return the water from the indirect to a boiler tapping normally in the opposite end of the boiler from the supply.   Preferably the opposite corner of the boiler.

2. The boiler should have at least one 1" or larger tapping higher in the boiler than the return tapping. Two would be better although one can be as small as 1/2".

3. It is best to use a bronze circulator to pump this water. Although many have used the smaller wet rotor circulators it is best to use a three piece circulator. The circulator must pump away from the boiler. Think of the boiler as the expansion tank connection.

4. The piping from the supply tapping must elbow down toward the floor and mount the circulator close to the floor to keep the live steam away from the inlet of the circulator in case of surging water.

5. Is is a good idea to install a strainer before the circulator as some dirt my get disturbed in the bottom of the steam boiler with the circulation of water. 

6. The use of a check valve is recommended.

7. Use an aquastat to keep the boiler from heating above 212º so we do not produce steam in the off seasons or when the thermostat is not calling for heat.

8. Verify the boiler is a cold start boiler. If not wire it so it does not maintain any temperature.

9. If there is an automatic water feeder it may maintain a lower water level than the tapping. The homeowner should always check the water level and manually feed the water even if he has an automatic water feeder. See Steam FAQ.

Correct Piping

Incorrect Piping

Do not supply and return to the bottom of a steam boiler. There is no control of water temperature in the boiler. The circulated water will become about 120º quickly and there is no way to fire the boiler. If you try a strap on aquastat the boiler will create steam. The control should mount directly into the boiler near the water level (not shown).

Piping Baseboard above the water level

All the rules above apply. Make sure you pipe in the bypass from the return to supply. Throttle the valve down flowing through the boiler and allow more return water to blend into the supply to cool it down some. The pressure in the baseboard is low pressure and we are concerned about flashing to steam. If we pump away from the boiler and size the baseboard for 160º - 170º we will not have a problem.

Simple wiring diagram for Indirect or heat zone piping