Hydronic Heating Systems Flashcards

What is the primary function of an expansion tank in a closed-loop hydronic heating system?

Answer: To accommodate the volume increase of water as it heats and expands

As water heats it expands in volume. The expansion tank absorbs this expanded volume, preventing excessive pressure buildup.

What is the purpose of a 'primary-secondary' piping configuration?

Answer: To allow each zone to operate at different temperatures and flow rates without affecting boiler flow

Primary-secondary piping uses closely spaced tees to hydraulically decouple the boiler loop from distribution loops, allowing independent flow rates.

What is 'thermal shock' in a cast iron boiler?

Answer: Rapid temperature differential causing cracking when cold water hits hot boiler sections

Thermal shock occurs when cold return water suddenly contacts hot cast iron boiler sections, creating rapid thermal expansion differentials that can crack the cast iron.

What is the function of a 'differential pressure bypass valve' in a variable flow hydronic system?

Answer: To bypass excess water flow when zone valves close off, protecting the pump and boiler

When zone valves close simultaneously, pressure increases. The differential pressure bypass valve opens to allow excess flow to bypass, maintaining minimum boiler flow.

In radiant floor heating, why is water temperature limited to 85-120°F?

Answer: Floor surface temperatures above ~80°F are uncomfortable for occupants

Floor surface temperatures above approximately 80-85°F are uncomfortable for standing occupants, so supply water temperature is limited to maintain comfortable floor temperatures.

What does 'Delta T' (ΔT) mean in a hydronic heating system?

Answer: The temperature difference between supply and return water

Delta T (ΔT) is the temperature difference between supply water and return water as it circulates through the heat emitters, representing how much heat is extracted.

What is 'hydraulic separation' and which device achieves it?

Answer: Decoupling boiler flow from distribution flow; low-loss header or hydraulic separator

Hydraulic separation decouples the boiler loop from distribution loops using a low-loss header or hydraulic separator, allowing independent flow rates.

What is 'cold start' operation in a boiler?

Answer: Starting a boiler from ambient temperature; causes thermal shock from cold water contacting hot surfaces

Cold start means firing a boiler from ambient temperature with cold system water. For cast iron boilers, cold return water initially contacts hot iron sections, risking thermal shock and cracking.

How does a three-way mixing valve work in a hydronic radiant system?

Answer: It blends hot boiler supply water with cooler return water to produce a lower supply temperature for distribution

A three-way mixing valve blends hot supply water with cooler return water to produce a mixed supply temperature suitable for lower-temperature distribution systems.

What problems arise from excessively high water velocity in hydronic piping?

Answer: High velocity causes erosion-corrosion, noise, and air entrainment

Excessively high water velocity causes erosion-corrosion at fittings and elbows, generates flow noise, and can cause air bubbles to be entrained.

In a multi-zone hydronic system using zone valves, when does the circulator pump operate?

Answer: When any zone valve opens and calls for heat

In a zone valve system, a single circulator operates whenever any zone valve is open and calling for heat.

What does 'condensing mode' mean for a gas-fired hydronic boiler?

Answer: The boiler recovers latent heat from flue gases; activates when return water drops below approximately 130°F

Condensing mode occurs when return water temperature is low enough to cool flue gases below their dew point, causing water vapor to condense and release latent heat—significantly improving efficiency.

What is the purpose of a 'low-water cutoff' (LWCO) device on a hot water boiler?

Answer: To shut off the burner when water level drops below a safe minimum

The low-water cutoff is a critical safety device that shuts off the burner if water pressure drops below the minimum safe operating level, preventing operation without adequate water.

What is 'air binding' in a hydronic system?

Answer: Air trapped in radiators or piping that blocks water flow; corrected by bleeding or installing air eliminators

Air binding occurs when air pockets trap in radiators or piping, preventing water circulation and causing cold spots or no-heat complaints.

What is the recommended approach for treating water chemistry in a closed-loop hydronic system?

Answer: Add inhibited glycol solution and use corrosion inhibitors to prevent scale, corrosion, and microbiological growth

Closed hydronic systems require inhibited glycol plus corrosion inhibitors to protect metals from galvanic corrosion, prevent scale deposition, and control pH.

What is 'balancing' a hydronic system?

Answer: Adjusting flow rates to each zone to ensure design heat delivery throughout the system

System balancing adjusts flow through circuit balancing valves so each terminal unit receives its design flow rate, ensuring even heat distribution throughout the building.

How does an indirect water heater work in a hydronic system?

Answer: It uses a coil submerged in the storage tank to transfer heat from boiler water to domestic water without mixing

An indirect water heater contains a submerged coil through which hot boiler water flows, transferring heat to domestic water without the two water streams mixing.

What causes 'kettling' noises in a hydronic boiler?

Answer: Scale or lime deposits on heat exchanger causing localized boiling; resolved by descaling

Kettling is caused by mineral scale deposits on the boiler heat exchanger. Scale acts as an insulator, causing water adjacent to the hot surface to flash to steam locally.

What is the purpose of the 'aquastat' control on a hot water boiler?

Answer: To maintain boiler water temperature within a set range by controlling burner operation

The aquastat monitors boiler water temperature and cycles the burner on/off to maintain water temperature within the high-limit and low-limit setpoints.

What is 'outdoor reset control' in a hydronic heating system?

Answer: Varying boiler water temperature inversely with outdoor temperature to provide only as much heat as needed

Outdoor reset adjusts boiler supply water temperature downward as outdoor temperature rises, so the system delivers only the heat needed—avoiding overheating and enabling more condensing boiler operation.