Question 6 of 6: Two-Pipe Fan-Coil System with Heat-Recovery Ventilation in a High-Rise Condominium

Question 6: Two-Pipe Fan-Coil System with Heat-Recovery Ventilation in a High-Rise Condominium (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Given. A high-rise residential building served by a two-pipe hydronic distribution serving one fan-coil unit per suite, with a water-cooled chiller as the summer source and a natural-gas condensing boiler as the winter source, and a separate heat-recovery ventilator in each suite. For the flow-rate illustration below, a suite peak cooling load of 3.0 kW with chilled water at 6 °C supply and 12 °C return.

Find. A system diagram; a description of the complete heat-transfer path from the occupied space to outdoors in the cooling season, naming every circulating medium and every key item of equipment; and a suite floor plan showing the FCU, the HRV, all their air, water and drainage connections, and which duct must be insulated.

(1) Overall structure of the system

The system has four distinct circuits, and the diagram is best read as a chain of heat exchangers linking them. The central plant holds a water-cooled electric chiller and a natural-gas condensing boiler; the chiller rejects its heat to a condenser-water loop that runs to a cooling tower on the roof, and the boiler burns gas and vents flue products. A seasonal changeover valve set selects which of the two sources feeds the distribution, and a distribution pump circulates it. Because the distribution is a two-pipe system, one supply main and one return main serve the whole building, and every suite gets the same water at the same temperature — which means the building is either in cooling or in heating, never both at once. Vertical risers carry the supply and return up the building, with a branch pair, isolating valves and a balancing or pressure-independent control valve at every floor. Within the suite, a single fan-coil unit with one coil, one fan and a thermostat conditions the recirculated room air, and a heat-recovery ventilator handles outdoor air independently, taking fresh air in and stale air out through the facade and exchanging heat between the two streams in its core.

The sizing of the water side follows from a simple energy balance and is worth showing. For a suite peak cooling load of 3.0 kW with a 6 K rise across the coil,

and on the air side, with the coil supplying air about 10 K below room temperature, $\dot m_a = 3.0/(1.006\times 10) = 0.298$ kg/s, roughly 250 L/s of air. The ventilation air handled separately by the HRV is far smaller — of the order of 30 L/s continuous for a one-bedroom suite under NBCC Subsection 9.32 and CSA F326 — which is precisely why the two functions are separated: sizing the FCU for ventilation as well would make it eight times larger than it needs to be.

The two-pipe arrangement is the system's defining compromise. It is cheap, compact and easy to run in a residential tower where floor-to-floor height is at a premium, and for most of the year it works. Its weakness is the shoulder seasons: on a sunny October day a south-facing suite needs cooling while a north-facing one needs heat, and a two-pipe system cannot deliver both. Buildings operated this way typically run a manual or outdoor-temperature-based changeover with a deliberate dead band, and occupants on the wrong facade open windows. A four-pipe system, with separate chilled- and hot-water mains and either two coils or a single coil per suite, removes the limitation at the cost of a second pair of risers.

(2) The path of heat from the suite to the outdoor air in the cooling season

In summer, the heat that people, lighting, appliances, solar gain and conduction through the envelope deposit in a suite must travel through four circulating media in series before it reaches the outdoor air. Each transfer happens in a named heat exchanger, and each requires a temperature difference in the right direction, which is why the chain works only in that order.

Circuit 1 — indoor air, within the suite. The FCU fan draws warm, humid room air across a return grille into the unit and pushes it through the cooling coil, where it is cooled and dehumidified, typically from about 24 °C to about 14 °C. The cooled air is delivered back into the living space through a supply grille or a short duct, mixes with the room air, and picks up the room's heat gain again. Moisture that condenses on the coil fins collects in a drain pan and leaves as liquid water — that latent heat has also been removed from the room and now travels in the water circuit. The suite thermostat cycles the fan or modulates the coil control valve to hold setpoint. Key equipment: fan-coil unit (fan, cooling coil, filter, drain pan), supply and return grilles, room thermostat, control valve.

Circuit 2 — chilled water, from the suite to the plant room. The heat picked up by the coil raises the temperature of the water flowing through it, typically from 6 °C supply to 12 °C return. That warmed water leaves the suite through the branch return, joins the return riser, and is pumped down to the chiller's evaporator, where it is cooled back to 6 °C and sent up the supply riser again. Nothing is consumed in this loop; it is purely a transport mechanism, and the pump work it takes ends up in the water as additional heat. Key equipment: two-pipe supply and return mains and risers, distribution pump, balancing and control valves, expansion tank, air separator, and the chiller evaporator.

Circuit 3 — refrigerant, inside the chiller. This is the only stage that moves heat "uphill", from cold water to warmer condenser water, and it is the only one that consumes significant energy. Inside the chiller, refrigerant evaporates at a temperature a few degrees below the chilled-water return, absorbing heat from it; the compressor raises the refrigerant's pressure and temperature so that it can now condense at a temperature above that of the condenser water; the refrigerant condenses in the condenser, giving up both the heat it collected in the evaporator and the compressor work; and an expansion valve throttles it back to evaporator pressure. This is exactly the cycle analysed in Question 3, and its COP is what determines the electrical cost of the whole chain. Key equipment: chiller (evaporator, compressor, condenser, expansion device).

Circuit 4 — condenser water, from the chiller to the roof. Because the chiller is water cooled, its condenser is a water-to-refrigerant exchanger, and the heat rejected there — the suite load plus the compressor work, so roughly 25–35% more than the cooling load — is carried by a second water loop, typically 30 °C leaving the condenser and 35 °C arriving at the tower. Key equipment: chiller condenser, condenser-water pump, piping to the roof.

The final transfer — to the outdoor air, at the cooling tower. In the tower the condenser water is broken into droplets over fill and falls through an upward stream of outdoor air driven by a fan. Some sensible heat passes directly to the air, but most of the rejection is evaporative: a small fraction of the water evaporates, taking its latent heat of vaporisation from the water that remains, which is why a cooling tower can cool water to within a few degrees of the outdoor wet-bulb temperature rather than the dry-bulb. That evaporated water is made up continuously, and a bleed or blowdown stream is drawn off to stop dissolved solids concentrating. Key equipment: cooling tower (fill, distribution basin, drift eliminators, fan), make-up and blowdown, water treatment.

The ventilation path, running in parallel. Independently of all of the above, the HRV draws stale, humid air from the kitchen and washroom and exhausts it to the facade, while drawing an equal quantity of outdoor air in and delivering it to the living room and bedroom. In summer the core pre-cools the incoming hot outdoor air against the cooler outgoing air, reducing but not eliminating the ventilation load; whatever remains is picked up by the FCU and joins the chain above. The HRV therefore does not reject heat to outdoors on its own account — it reduces the load the rest of the system has to move.

Two features of the chain are worth stating explicitly because they are what the marks are for. First, every transfer is down a temperature gradient except the one inside the chiller, which is the only place work is done and the only place the direction of heat flow is reversed relative to temperature. Second, the quantity of heat grows as it travels: the tower rejects the suite load plus compressor work plus both pumps' work, so an inefficient chiller does not merely cost electricity, it enlarges every downstream component.

(3) Suite floor plan with FCU, HRV and all connections

The layout is driven by four constraints. The FCU goes where the risers are — in a bulkhead or a service closet on the corridor wall of the living room — so that the branch pipework is short, the coil is accessible for filter changes from inside the suite, and the fan noise is remote from the bedroom. Its connections are the chilled/hot water supply and return from the riser (shown solid and dashed in purple), a supply-air grille discharging into the living room, a return-air grille drawing room air back, and a condensate drain. The HRV goes near the wet rooms, above the kitchen ceiling, because its exhaust must be picked up from the kitchen and the washroom, where the moisture and odour are; from there it needs the shortest possible run to the facade for its two outdoor connections.

The condensate drain is the connection most often forgotten and the one that causes most of the water damage in buildings of this type. It runs from the FCU drain pan, through a trap deep enough to hold the fan's negative pressure, and falls continuously — a minimum grade of about 1 in 50 — to an indirect connection at the kitchen stack. It must never run uphill, and it should be provided with a secondary pan or a condensate overflow switch, because a blocked drain in a suite on the twentieth floor damages every suite below it.

Which duct must be insulated. Any duct carrying air at a temperature far from that of the space it passes through must be insulated and, where it is below the surrounding dew point, vapour sealed. In this suite that means:

• The fresh-air (outdoor-air) duct from the HRV to the living room and bedroom — this is the section marked on the plan. In winter it carries air at close to outdoor temperature between the facade and the HRV, and downstream of the HRV it still carries air well below room temperature; without insulation it would sweat in summer, lose its tempering in winter, and be a cold, condensation-prone surface running through a warm ceiling.
• The outdoor-air intake and the exhaust duct between the HRV and the facade, for the same reason — the exhaust duct carries warm, saturated air out through a cold facade zone in winter and will condense inside if it is not insulated and sealed.
• Any supply-air duct from the FCU that leaves the conditioned space — if the supply run crosses a bulkhead adjacent to the facade or an unconditioned corridor, it carries 14 °C air past a warm humid surface in summer and must be insulated with a vapour barrier.

The return-air path inside the suite needs no insulation: it is at room temperature by definition. Nor does the short supply run wholly within the conditioned living space, though it is often lined for acoustic reasons rather than thermal ones.