Heating hot water system design

Heating hot water systems are the backbone of space heating in most Canadian commercial and institutional buildings. A central plant, traditionally boilers and increasingly heat pumps, heats water that is pumped to air handler coils, perimeter radiation, fan coils, radiant floors, VAV reheat coils and domestic water heat exchangers.

The single most important design decision today is supply water temperature. Older systems were designed around 82 C (180 F) supply water. Modern systems, built to work with condensing boilers and heat pumps, are designed for much lower temperatures. This guide covers temperature selection, terminal units, pumping, piping and controls.

Why supply temperature matters

Condensing boilers

Condensing boilers achieve their high rated efficiency only when return water is cool enough to condense water vapour in the flue gases. For natural gas, that means return water below roughly 54 C (130 F), and efficiency continues to improve as return temperature drops. A condensing boiler connected to a system returning 70 C (158 F) water will perform little better than a non-condensing boiler. See condensing boilers: getting the efficiency you paid for.

Heat pumps

Air-to-water and water-to-water heat pumps lose capacity and efficiency as the supply water temperature rises. Many commercial heat pumps have a practical maximum supply temperature well below traditional boiler temperatures, and their coefficient of performance is markedly better at lower supply temperatures. Designing for low-temperature heating is essential to electrification. See air-to-water heat pumps for commercial heating.

Distribution losses

Lower water temperatures reduce heat losses from piping, especially in long distribution runs and older, poorly insulated buildings.

Typical design temperatures

These are typical ranges, not code values. Systems are usually designed so that supply temperature is reset lower as outdoor temperature rises, so actual operating temperatures are lower than design for most of the season.

Terminal units at lower temperatures

Lower supply temperatures and wider delta T reduce the output of a given heating coil or radiator. Designers compensate by:

• Selecting heating coils with more rows or more surface area.
• Using larger or longer perimeter radiation, or fan-assisted convectors.
• Using radiant floors or panels, which work well at low temperatures.
• Improving the envelope, which reduces required output.
• Checking VAV reheat coils for capacity at the lower water temperature, since reheat coils sized for 82 C water may not deliver enough heat at 55 C.

In retrofits, it is common to test how existing terminals perform at lower temperatures. Many buildings were oversized and can operate at lower temperatures for most of the year, with higher temperatures needed only on the coldest days.

Delta T and flow

Flow is set by load and delta T. A wider delta T reduces flow, pipe size and pump energy, and lowers return temperature for condensing boilers. Traditional designs used about 11 K (20 F). Many modern designs use 17 to 22 K (30 to 40 F) or more for boiler systems. Heat pump systems are often designed with narrower delta T to keep supply temperature down, depending on the equipment.

The same flow relationship as chilled water applies: L/s = kW / (4.19 x delta T), adjusted for glycol where used.

Pumping arrangements

As with chilled water, variable flow with two-way valves and variable speed pumps is the norm for new systems. Common arrangements:

• Primary-secondary: boiler pumps on a primary loop, variable speed distribution pumps on the secondary. A common choice with older boilers that need constant flow.
• Variable primary: variable speed pumps circulate through boilers and the building, where boilers allow variable flow.
• Boiler-mounted pumps with hydraulic separator: each boiler has its own pump, and a hydraulic separator decouples them from the distribution pumps.

Three-way valves blend supply and return water, raising return temperatures and wasting pump energy. They work against condensing boilers and should be avoided except where needed, such as minimum flow at the end of a run.

Piping and system components

1. Size piping for velocity and friction limits suited to the pipe size and noise sensitivity.
2. Provide air separation at the point of lowest solubility, typically the hottest point in the system, near the boiler outlet.
3. Install dirt separation, which is important for condensing boilers with small water passages.
4. Size expansion tanks for system volume and temperature range. See expansion tanks and system pressurization.
5. Insulate piping to meet NECB and provincial energy code requirements.
6. Provide balancing provisions, or pressure-independent control valves, so each circuit gets design flow.

Glycol

Glycol is used where heating coils or piping are exposed to freezing, such as makeup air units, outdoor air preheat coils, snow melting and outdoor piping. Glycol reduces heat transfer and increases pumping energy, so it is often confined to the loops that need it, separated from the main water system by a heat exchanger.

Controls

Good controls are essential to keep return temperatures low and efficiency high:

• Outdoor reset: supply temperature is lowered as outdoor temperature rises. This is the most important control for condensing boilers and heat pumps.
• Demand-based reset: supply temperature is adjusted based on valve positions, so it stays as low as possible while satisfying the most demanding zone.
• Differential pressure control: variable speed pumps maintain a differential pressure setpoint, reset based on valve positions.
• Warm weather shutdown: heating plant shuts down above an outdoor temperature where no heating is needed, subject to reheat needs.
• Staging: boiler and heat pump staging to keep equipment in efficient operating ranges.

Many buildings fail to achieve condensing efficiency because reset schedules are too high or disabled, three-way valves bypass hot water to return, or a few zones require high temperatures and drive the whole plant.

Transition to heat pumps

Many Canadian building owners are planning a transition from gas boilers to heat pumps to reduce emissions. Common strategies include:

1. Lowering supply temperatures in the existing system and measuring how terminals respond.
2. Identifying and upgrading the few zones that require high temperatures.
3. Adding heat pumps as the lead heat source, with existing boilers for backup and peak loads.
4. Reducing heating load through envelope and ventilation heat recovery improvements.

See electrification of commercial heating for more on this transition.

Frequently asked questions

Can existing buildings run at lower temperatures?

Often, for much of the year. Most heating systems are sized for the coldest design day, and outdoor reset can reduce supply temperature significantly in milder weather. Testing and trending reveal how low the system can go.

Do low-temperature systems need more piping?

They need more terminal surface area, and narrower delta T designs need more flow. Wider delta T with low supply temperature is possible with properly selected terminals.

Key takeaways

• Supply and return temperature selection is the key design decision for modern heating systems.
• Condensing boilers need low return temperatures; heat pumps need low supply temperatures.
• Terminal units must be selected for the lower temperatures, especially reheat coils.
• Variable flow, wide delta T and outdoor or demand-based reset keep efficiency high.
• Plan the path to heat pumps when designing or renovating. Water balancing verifies that each circuit receives design flow.