24-Bld-A4 Building Environmental Control Systems · May 2017

Question 2 of 14: Cooling Load, Water-Source Heat Pump + HRV Integration

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Notes on this paper

National Exams May 2017 — 07-Bld-A4, Building Environmental Control Systems (Building Engineering). 3-hour closed-book exam: Section 1 (four 20-mark essay/calculation questions) + Section 2 (ten 2-mark multiple-choice questions).

Reference texts: ASHRAE Handbook — Fundamentals (2021 ed.); ASHRAE Standard 62.1, Ventilation for Acceptable Indoor Air Quality (2022 ed.); McQuiston, Parker & Spitler, Heating, Ventilating, and Air Conditioning: Analysis and Design (6th ed.); National Building Code of Canada 2020 (NBCC); National Energy Code of Canada for Buildings 2020 (NECB).

Question 1.2: Cooling Load, Water-Source Heat Pump + HRV Integration (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.

(1) Heat gains vs. cooling load. Heat gain is the instantaneous rate at which heat enters (or is generated within) a space — from solar radiation through glazing, conduction through the envelope, occupants, lights, and equipment — at the moment it occurs. Cooling load is the rate at which heat must actually be removed by the cooling system to hold the space at its design condition. The two are not equal instant-for-instant because radiant heat gain (solar and to a lesser extent conduction) first gets absorbed into the thermal mass of the room's walls, floor, and contents, and is only released to the room air — and therefore into the cooling load — over the following hours. This storage-and-release process is why the peak cooling load lags and is damped relative to the peak heat gain, as shown below.

Fig. 1.2a — Typical summer-day heat gain vs. cooling load. Heat gain peaks near mid-afternoon (~14:00, solar + occupancy + equipment); the cooling load peak is delayed (~17:00) and flattened because thermal mass stores part of the radiant gain and releases it later.

The building envelope's thermal performance (insulation R-value, glazing SHGC, air-tightness) sets the MAGNITUDE of the heat gain curve — a poorly insulated, highly glazed envelope produces a taller gain curve. The thermal mass of the envelope and interior contents (concrete floors, masonry, furniture) sets the SHAPE: high mass damps and delays the cooling-load peak relative to the gain peak (as drawn above), which is why heavy buildings can shift part of their peak cooling demand later into lower-price or lower-ambient-temperature hours, while a lightweight, low-mass space tracks its heat gain almost instantaneously, giving a taller, earlier, and less-forgiving cooling-load peak that the mechanical system must be sized to meet directly.

(2) Water-source heat pump (WSHP) structure and mode changeover. A packaged WSHP for a condominium suite has four core components: a refrigerant compressor, a four-way reversing valve, a water-side (coaxial or plate) heat exchanger connected to the building's common condenser-water/geothermal loop, and an air-side heat exchanger (finned coil + blower) that conditions the supply air to the suite.

Fig. 1.2b — WSHP structure: compressor → reversing valve → two heat exchangers (water-side tied to the building loop, air-side serving the suite).

Cooling mode: the reversing valve directs hot, high-pressure refrigerant from the compressor to the WATER-side exchanger, which acts as the condenser — refrigerant rejects heat into the (cooler) building loop water. The refrigerant then expands and evaporates in the AIR-side coil, absorbing heat from the room air and delivering cool, dehumidified supply air. Heating mode: the reversing valve swaps the refrigerant path so the AIR-side coil becomes the condenser (rejecting heat to the room air) and the WATER-side exchanger becomes the evaporator, extracting heat from the (typically warmer, tempered) building loop water. Changeover is done electrically: a solenoid on the four-way reversing valve is energized/de-energized by the unit's thermostat/controller in response to a call for heating vs. cooling, instantly swapping which exchanger is condenser and which is evaporator without any mechanical piping change.

(3) Air-side integration of HRV and WSHP for one residential space. The HRV pre-treats the ventilation (outdoor) air using heat recovered from the room's own exhaust air stream before that pre-conditioned outdoor air is handed to the WSHP's air-side coil for final sensible (and, in cooling mode, latent) conditioning, then delivered to the room. This two-stage arrangement lets the small, code-mandated ventilation air be tempered efficiently (by heat recovery) rather than by raw compressor capacity, while the WSHP handles only the final trim and the room's actual thermal load, and the HRV core simultaneously exhausts stale room air to maintain indoor air quality.

Fig. 1.2c — Air-side integration: outdoor air is pre-conditioned by the HRV core (using heat recovered from room exhaust) before the WSHP does the final trim, controlling both indoor thermal comfort and indoor air quality from a single supply-air stream.

Topic: Residential HVAC systems — thermal-mass lag, heat-pump reversal, and ventilation heat recovery.

Key relations: cooling load = heat gain convolved with the space's thermal-mass response (a damped, time-lagged version of the gain profile); WSHP COP is governed by the refrigeration cycle operating between the loop-water temperature and the room air temperature; HRV effectiveness η = (t_OA−t_OA′)/(t_OA−t_IA) (see Question 2.6).

Why this works: Separating ventilation heat-recovery (HRV) from space conditioning (WSHP) lets each device do the job it is best at — the HRV core recovers a large fraction of the sensible (and, in enthalpy-core versions, latent) energy in outgoing exhaust air essentially for free, so the WSHP's compressor need only trim the already-tempered ventilation air plus meet the room's own envelope/internal loads, rather than conditioning raw, extreme-temperature outdoor air from scratch.

Common pitfall: Treating heat gain and cooling load as interchangeable, or sizing HVAC equipment to the peak heat-gain value instead of the (lower, time-shifted) peak cooling load, which oversizes equipment and worsens part-load performance.

Source: ASHRAE Handbook — Fundamentals, Ch. 18 (Nonresidential Cooling and Heating Load Calculations) & Ch. 9 (Residential Systems); McQuiston, Parker & Spitler, Ch. 3 (heat pump cycles) & Ch. 11 (ventilation and heat recovery).

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