Introduction

I live in an 1890’s three-story brick home in south-eastern Pennsylvania. It has a one-pipe steam heating system that heats the building using a gas-fired steam boiler in the base-ment and cast iron radiators in each room. 

Since 2009, I have been developing the EcoSteam boiler control system to monitor and control conditions in and around the boiler. This website describes the current state of the system. 

EcoSteam reduces fuel consumption by correcting two issues that are fairly common with residential steam heating systems.

Indoor Temperature Control

It is often difficult to regulate indoor temperature using steam heat because of the thermal lag associated with the cast iron radiators. With a simple on/off thermostat control, a steam system will typically overshoot the target temperature because the radiators will continue to heat the space after the thermostat is satisfied, until the steam in the radiators has condensed back into water.

There are commercial steam boiler control systems that use a principle called "outdoor reset". The outdoor reset algorithm calculates how long the burner should operate, based on the relationship of the current outdoor temperature to the "design temperature". The design temperature is the outdoor temperature on the coldest day of the year at the home's location. With a properly tuned algorithm, the burner will shut down before the temperature setpoint is reached, and the temperature will "coast" to setpoint due to the radiator thermal lag.

EcoSteam takes the commercial systems one step further by including the calculated heat loss of the building in the outdoor reset algorithms. My outdoor reset algorithm calculates how much heat input (in btu) are required to compensate for the heat loss at the current outdoor temperature, and then determines the burner run time based on the firing rate of the burner. For instance, if the heat loss is 50 thousand btus per hour (MBtuH) and the burner produces 100 MBtuH, then the burner needs to run for 30 minutes to maintain the building temperature.

Steam Pressure Control

Most residential steam systems employ a fixed output burner fueled by either gas or oil. Since the boiler's steam production capacity is rarely matched to the steam consumption rate of the connected load (the radiators), this means that the system will produce either too little or too much steam. Too little steam, and the system may not be able to adequately heat the building on the coldest days of the year. Too much, and the system will build pressure beyond the point required to fill the radiators with steam, which wastes fuel.

In the spring of 2013 I replaced the single-stage oil burner in the boiler with a modulating gas burner from Midco International. This burner has a firing rate between 50 and 300 MBtuH, which allows the firing rate of the burner to be adjusted to maintain a desired steam pressure in the boiler. This allows the system to achieve a state of near-equilibrium, where the rate of steam production in the boiler matches the rate of steam condensation at the radiators.

Using this system I have been able to achieve low operating pressures of 2 ounces per square inch (1 kPa) or less, while still effectively heating the building.

Next Page: Environment