Many hospitals operate their heating hot water systems at a fixed temperature. As the outdoor air temperature rises, the heating requirement goes down, so lower water temperatures can meet the load. As water temperatures rise, more flow is needed, but pumping energy is cheaper than the energy to heat the water. At a minimum, the temperature can be reset based on the outdoor temperature, but the more efficient way, if the whole building is on the Building Automation, is to reset based on what the actual hot water valves are doing throughout the building..

The savings of lowering boiler temperature are significant. For every 4°F the boiler water temperature is reduced, there is approximately 1% energy savings. This alone can result in a cost savings of 15% or more during the heating season.

Technical Description

Background on boiler controls

Prior to sophisticated controls, boilers were typically operated at a fixed discharge temperature of 180 - 200°F. With hospitals needing hot water for reheat typically all year round, this was very expensive to operate. It was also very inefficient. With the higher temperatures and low loads, the required flow of hot water is much lower. This creates numerous inefficiencies. Many hot water systems have constant speed pumps with three way valves diverting the water at the coils when it is not needed, so the systems were pumping water that was not needed. In addition, the low flow through the heating coils was also less effective as the heat transfer at the coils is most efficient at full flow.

By resetting the temperature down when the demand is not there, not only are you saving energy at the boiler, but the entire system runs more efficient by maintaining proper flow throughout the system and coils.

Design considerations

Boiler hot water temperature reset is accomplished either through the Building Automation System or the Boiler control panel. The heating hot water temperature is set at its maximum when the outdoor temperature is the coldest and minimum when the outdoor temperature gets to a warmer temperature (typically 60 F), or as valves start to close, the temperature setpoint can be lowered.

Control strategies

As mentioned above, the easiest way to accomplish this is to just reset the Heating Hot Water temperature based on the Outdoor Air temperature. An even more efficient way though, if the reheat valves are tied into the Building Automation System is to have the BAS look at all of the reheat positions and reset the Hot Water temperature based on the position of the valves so that a minimum of valves are open. The key to this is to make sure that the system does not include any constant volume boxes in the calculation.

Implementation

Commissioning is vital to implementation. The figure below shows the actual observations, hot water supply temperature (in purple and teal) for a boiler system compared to the idea (green line) of supply water temperature setback. (Beaverton MOB, 2/7/2022 to 5/7/2022)

Effective implementation of a setback control sequence requires, at a minimum:

• Effective supply water temperature control at all load conditions.
• Effective means of re-setting the temperature, which results in a reset signal during actual conditions.
  

How does this decarbonize?

By lowering the Hot Water Heating temperature, less heating energy is used.

Implementation

Barriers: Commissioning

Technically, any building designed in California since the mid-1970s should include supply water temperature setback, since it is a Title 24 requirement.

However, in data review and retro-comissioning, it is rarely working.

The results in existing buildings suggest it is possible (or likely) that this strategy can fail even if it is specified on the plans, included in the controls submittals, and programmed into the system. No project should assume that a temperature reset sequence is working after specifying it, seeing it on submittals, and programming it.

Always validate setback strategies by looking at data of outdoor temperature and supply water temperature, over a period of time (at least five days), to confirm that the sequence has been effectively implemented.

Barriers: Codes

The only thing to watch with resetting the boiler hot water temperature is that you don't lower it too much, but this can be easily fixed by changing the minimum reset temperature

Barriers: Culture

This strategy has been implemented many times and there are no real cultural barriers because space temperatures are properly maintained throughout.

Strategy

With a typical reset, the HW temperature would be set at its maximum (usually 180 F)when the outdoor temperature is around 30 F and then increases on a sliding scale until at 60 F it is lowered to its minimum, often around 140 F.

Financial analysis and business case

The cost to implement this is very minimal and only requires a technician to add the programming into the Building Automation System or Boiler controller, so the payback is almost immediate.

This is a typical strategy which is specified on many buildings throughout the country.