Controlling an existing hydronic boiler
Use a little more care when repairing or maintaining this equipment.
I like working with the control techs in our industry. They have such a passion for their craft. If left alone, they will control or monitor everything that moves, slides, shakes or switches.
However, I do rein them in a bit when controlling a hydronic heating system. Several hard limits and a couple danger zones should be heeded. Of course, these should be verified with the boiler manufacturer.
- Design temperature. Most hydronic systems were engineered to supply 180° F water at the outdoor heating design temperature.
- Condensing temperature. Operating a standard noncondensing boiler below 140° will allow the flue gases to condense. This could destroy the boiler, flue and chimney. It also could void the warranty.
- Delta T. This is the temperature rise across a boiler. Most boilers are designed for a 20° to 25° temperature rise. If the system return water temperature is 150°, the supply water temperature to the system from the boiler should be 20° to 25° higher.
- A temperature span greater than the boiler design temperature rise could cause thermal shock inside the boiler. Thermal shock can literally shake a boiler apart due to rapid expansion and contraction. Think of what happens to a paperclip as you bend it back and forth.
- Reset ratio. This is the ratio at which the temperature of the supply water will change when the outside air temperature changes. A typical reset ratio in the industry is a 1-to-1 ratio. That means the supply water will change 1° for every degree the outside air temperature changes.
I prefer to monitor the return water temperature as the driving temperature as it leads to less boiler cycling. Some of the newer boilers heat so quickly it could rapidly meet the temperature of the control, shutting off the boiler. The table below is an illustration of a 1-to-1 reset schedule.
- Three-way valve. This is a control valve with three pipe connections, usually mixing or diverting. A mixing valve has two inlets and one outlet. It is usually installed when a blended temperature is required.
A diverting valve has one inlet and two outlets. This is most likely used for bypassing, such as a boiler in a two-pipe system during the summer. Most diverting valves I have seen have two positions, either fully open or closed, but could be modulating.
- Outdoor heating design temperature. This is the design temperature that engineers use when sizing a heating system. It used to be the coldest temperature that the locale experienced. It is now the temperature that occurs 2% of the time. This means that in a typical winter, the outside temperature will be at or cooler than the design temperature 2% of the time, or about 175 hours per year.
When we are controlling a hydronic boiler, these temperature limits must be followed. For example, when a reset control is installed on a boiler and the supply temperature is below 160°, the return temperature will typically be 20° lower or 140°. At that temperature, the flue gases start to condense. Extended operation below that temperature could cause damage to the boiler. Most standard-efficiency boiler manufacturers will void their warranty if boilers are operated below 140°.
In an effort to reduce heating costs and limit flue gas condensation, three-way valves are sometimes installed. The three-way valve will allow lower supply temperatures to the building by bypassing the boiler. The boiler can still operate between 160° to 180° but the supply water temperature will be allowed to drift lower in direct proportion to the outside air temperature. If incorrectly installed or controlled, the three-way valve could allow thermal shock to occur, destroying the boiler.
- Blend pump. To limit the chances of thermal shock, some boiler manufacturers require that blend pumps be installed in conjunction with the three-way valves. The blend pump will take hot water from the supply piping of the boiler and feed it into the return piping of the boiler. This supply water will increase the temperature of the return water, limiting the delta T.
One of the drawbacks to a three-way valve is that the boiler may require extensive re-piping as the manufacturer may stipulate the blend pump connection to be 10 to 20 pipe diameters upstream of the return boiler connection. If the return pipe size is 4 in., the blend piping will have to be 40 in. to 80 in. upstream. This may be almost impossible in an existing boiler room.
- Thermal purging. On small hydronic systems with large-mass boilers, thermal purging has been a very cost-effective solution. Use either a two-stage thermostat or a time-delay relay. On a call for heat, the system circulator starts. If after a certain amount of time the thermostat is still calling for heat, the burner starts.
This allows the system to use the residual heat in the boiler to heat the building. If there is not enough heat, the burner will start.
- Connect circulator to fan terminal. Most commercial thermostats allow continual fan operation during the occupied time. On smaller commercial buildings, we will sometimes control the pump using the fan terminal of the thermostat. During the occupied setting on the thermostat, the circulator will operate continuously. During the unoccupied time, the pump will only operate when there is a call for heat.
- Lead-lag controls. These are used to change which boiler is the lead boiler in a series of boilers. This is meant to even the wear of the boilers. When using boilers piped in a primary/secondary system, I do not like to switch the lead boilers continuously. If you do, you are always firing into a cold boiler, allowing the flue gases to condense.
I prefer to switch the lead boilers yearly. If you use the same boiler throughout a month or season, you will always be firing into a warm boiler. It will prolong the life of the boilers. If you have multiple boilers in a long row, be careful when allowing the one furthest from the chimney to be the lead boiler as the flue gases may condense before reaching the chimney in light load conditions.
Outdoor Temperature Hot Water Supply Temperature Return Water Temperature
0° 180° 160°
30° 150°* 130°*
60° 120°* 100°*
*Standard boiler could be condensing at this temperature.