top of page

Frequently Asked Questions

Feel free to press Ctrl+F to find whichever topic it is you are looking for!

For further Assistance, Call 647-328-4212 or

Q1: Can air handlers be installed in attics?

Yes—air handlers can be installed in attics if the attic is conditioned or semi-conditioned and installation guidelines are followed. This includes providing proper insulation around the unit, service access, freeze protection, and drainage. Unconditioned attics may void warranties depending on manufacturer guidelines.

Q2: Can an undersized heat pump be paired with an electric duct heater to meet design heating loads?

Yes. This is a common strategy. The heat pump handles the majority of the heating load, while the electric duct heater provides supplemental heat during peak winter conditions. Proper controls must ensure the heater stages on only when needed.

Q3: How do cold-climate heat pumps perform at very low temperatures?

Cold-climate (or “hyper-heat”) heat pumps can maintain a significant portion of their capacity—often 70–100%—at sub-zero outdoor temperatures due to advanced inverter technology and optimized refrigerant control.

Q4: Do heat pumps support multi-zone configurations?

Yes. Many manufacturers offer multi-zone systems. Proper sizing is crucial to avoid excessive turndown issues or oversized indoor units that short-cycle.

Q5: How is heat loss calculated for small buildings or additions?

Heat loss depends on:

  • Building envelope R-values

  • Air tightness and infiltration

  • Ventilation requirements

  • Indoor vs. outdoor design temperatures

  • Window surfaces
    Small detached spaces tend to have high surface-area-to-volume ratios, increasing heat loss.

Q6: Can supplemental electric heat compensate for a heat pump that does not meet peak design load?

Yes. This hybrid approach is widely used. The heat pump covers the base load efficiently, while electric resistance heat handles temperature extremes.

Q7: How do you size an electric boiler for hydronic systems?

Electric boilers are typically sized to match the building’s design heat loss. A simple conversion of 1 kW = 3,412 BTU/h is used to select the nearest boiler capacity.

Q8: How can hydronic radiator systems be zoned effectively?

Common zoning methods include:

  • Individual thermostats with electric actuators or zone valves

  • Manifold-based distribution for multi-branch systems

  • Central boiler modulation based on demand from all zones

Q9: Where should boilers and major hydronic equipment be located?

Mechanical rooms should prioritize:

  • Short distribution piping runs

  • Drainage access

  • Ventilation

  • Service access
    Basements are the most common location in cold climates.

Q10: Can electric duct heaters be installed downstream of air handlers?

Yes. Electric duct heaters are commonly installed in the supply duct to provide supplemental heat. Requirements include:

  • Sufficient airflow to prevent overheating

  • Interlocked controls (blower must run before heater energizes)

  • Adequate electrical capacity

Q11: What airflow considerations apply when incorporating electric duct heaters?

Electric heaters typically require high airflow (often 1,000+ CFM for 15–20 kW heaters) and must be installed where static pressure and duct size can support these requirements.

Q12: How do you determine if an air handler can support a duct heater?

Confirm:

  • Maximum ESP the air handler can overcome

  • Available blower speed and airflow at the expected static pressure

  • Electrical service size

  • Minimum airflow requirements for the heater

Q13: How do you determine acceptable ESP levels for fans and air handlers?

Fans and AHUs are selected based on:

  • Required CFM

  • Duct static pressure

  • Accessories (coils, filters, heaters)

  • Manufacturer fan curves
    Most residential systems operate under 0.5 in.w.g., while commercial systems can exceed 1.0 in.w.g..

Q14: Can linear slot diffusers be supplied from one end only?

Yes. The plenum box behind the diffuser equalizes pressure, allowing even distribution along the diffuser length. Extremely long diffusers may require multiple feed points.

Q15: How much airflow can slot returns or diffusers handle?

Typical rules of thumb:

  • 1" slot: ~50 CFM per linear foot

  • ½" slot: ~25 CFM per linear foot

  • ¼" slot: ~12.5 CFM per linear foot
    Actual values depend on manufacturer free-area and acceptable noise levels.

Q16: What size HRV is appropriate for residential or light commercial spaces?

HRVs are sized based on:

  • Occupant load

  • Floor area

  • Local code requirements

  • Required continuous ventilation rate
    Commercial applications often require units in the 300–500 CFM range or higher.

Q17: What are typical ASHRAE 62.1 outdoor air ventilation rates?

ASHRAE 62.1 divides spaces by occupancy type. For most small commercial spaces:

  • People outdoor air rate: ~5–10 CFM per person

  • Area outdoor air rate: ~0.06–0.18 CFM per ft²
    Specific values depend on the occupancy category.

Q18: What is a fire blanket wrap for commercial kitchen exhaust ducts?

It is a fire-rated duct insulation system used to ensure that grease-laden kitchen exhaust ducts maintain the required fire resistance when traveling through combustible spaces.

Q19: How do you size makeup air for large exhaust systems?

Makeup air should equal or slightly exceed exhaust volume.
For example:

  • 1,000 CFM kitchen exhaust → ~1,000 CFM makeup air

  • 3,600 CFM industrial exhaust → similar makeup air required
    Tempering makeup air is strongly recommended in cold climates.

Q20: Can HVAC systems be installed in attics in cold climates?

Yes—if freeze protection, insulation, and service access are properly addressed. The attic may require partial conditioning or enclosure.

Q21: How do you design HVAC for second-storey additions?

Common approaches:

  • Dedicated heat pump or air handler for the new level

  • Extending existing ductwork if system capacity and pressure allow

  • Multi-zone systems
    Air balancing and return placement are critical.

Q22: How do you design HVAC when the furnace or AHU is located in the attic?

Typical layout:

  • Short branch ducts serving upper floor ceiling diffusers

  • A main supply trunk descending to lower levels

  • Vapor barrier control around ductwork

  • Properly insulated platform and drainage

Q23: How is HVAC sized for large-volume commercial spaces like garages or workshops?

Key factors include:

  • Large infiltration loads

  • Frequent door openings

  • High ventilation or exhaust requirements

  • Ceiling height and stratification

  • Equipment and occupant heat gains

These spaces typically require oversized heating capacity and properly designed destratification.

Q24: How are mixed-use buildings typically ventilated and conditioned?

A common setup involves:

  • A central HRV or ERV for continuous ventilation

  • Independent HVAC systems for each residential unit

  • Dedicated makeup air and exhaust for commercial spaces
    Separation of air streams is essential for fire and odor control.

Q25: How do you determine cooling capacity for indoor units?

Cooling capacity depends on:

  • Indoor design conditions

  • Outdoor design conditions

  • ESP

  • Blower speed

  • Refrigerant charge
    Equipment should be selected using certified AHRI ratings.

Q26: What factors determine furnace/air handler sizing?

  • Heating or cooling load

  • Static pressure capability

  • CFM requirements

  • Ductwork design

  • Local code (e.g., minimum ventilation airflow)

Q27: What is required to perform energy modelling for high-efficiency programs?

Energy models generally require:

  • Certified energy modeling software

  • Documentation of envelope, HVAC, ventilation, domestic hot water systems

  • Verification by an accredited energy professional

Q28: Does electrical wiring type matter for HVAC equipment?

Yes. Large electrical loads (e.g., electric boilers, duct heaters, heat pumps) require:

  • Adequate breaker sizing

  • Proper wire gauge

  • Copper wiring preferred for high-amp loads
    Aluminum may require special connectors or upgrades.

Q29: What is the purpose of fire protection around ductwork?

To prevent fire spread along duct runs, especially in kitchens, garages, or multi-unit buildings. Fire wrap or rated shafts ensure compliance.

Q30: Where are duct smoke detectors typically required?

Typical locations include:

  • Air handling systems that serve multiple suites or the whole building (interconnected systems).

  • Systems that could transport smoke from one suite/space to another (recirculating systems).

  • Where a code or a smoke control strategy requires one (e.g., systems tied to smoke control or fire separations).
    Exact triggers depend on code, occupancy and whether the HVAC system serves more than one fire compartment.

Q31: Why do some fireplaces require make-up air?

Fireplaces exhaust large amounts of indoor air through the chimney or vent system. If the building is tight or other exhaust devices are operating, the home can become depressurized, causing the fireplace to struggle for combustion air. Make-up air prevents backdrafting, smoke spillage, and combustion safety issues.

Q32: What is backdrafting and why is it dangerous?

Backdrafting occurs when exhaust appliances pull air down the chimney instead of allowing smoke to exit upward. This can bring smoke, carbon monoxide, and combustion gases into the living space. It is most common in airtight or negative-pressure homes.

Q33: What types of fireplaces are most sensitive to house depressurization?

Most sensitive → least sensitive:

  1. Open wood-burning fireplaces (very high air consumption, easily affected)

  2. Wood fireplaces with fire-screen/door-open operation

  3. Decorative gas fireplaces with open fronts

  4. Sealed-combustion direct vent gas fireplaces (little to no impact from house pressure)
    Sealed combustion units draw all combustion air from outdoors, greatly reducing make-up air needs.

Q34: How much air does a typical open wood-burning fireplace consume?

An open wood-burning fireplace can use 200–600+ CFM of indoor air while operating. In a tight home, this is enough to:

  • depressurize the house,

  • cause smoke spillage,

  • starve combustion,

  • trigger backdrafting of other fuel-burning appliances.

Q35: What factors increase the risk of fireplace spillage?

  • Running kitchen exhaust hoods

  • Running clothes dryers

  • Running bathroom exhaust fans

  • Airtight construction (low natural leakage)

  • Closed interior doors that restrict air supply

  • Large chimneys in cold weather (poor draft until heated)

  • Basement locations (negative pressure zone in stack effect)

Q36: How do you ensure a fireplace has enough combustion air?

Common methods:

  • Dedicated outdoor combustion air duct (for wood fireplaces)

  • Passive make-up air duct with motorized damper

  • Mechanical make-up air system tied to exhaust/appliance operation

  • Conditioned make-up air units in very tight homes or large residences

  • For gas fireplaces: using sealed combustion/direct vent units

Q37: Are outdoor combustion air kits effective for fireplaces?

They help, but they do NOT eliminate indoor air consumption entirely.
Even with an outdoor air kit, open fireplaces still draw some air from the conditioned space due to the nature of open-flame drafting.

Q38: Why do some fireplaces smoke when doors are opened?

Opening the doors on a wood fireplace disrupts draft and allows room pressure to overcome the chimney.
This is common in:

  • cold chimneys,

  • when the fire is weak,

  • when the home is depressurized,

  • or when the chimney is oversized compared to the firebox.

Q39: Is a large open-faced fireplace more likely to spill smoke?

Yes. The larger the opening area, the more indoor air is required for draft and the greater the risk of spillage.

Q40: HRVs/ERVs help with fireplace pressure issues?

HRVs/ERVs help balance airflows but do not replace the need for dedicated combustion air or make-up air.
An HRV running on high exhaust mode may even increase depressurization.

Q41: What is the safest type of fireplace for modern airtight homes?

A sealed combustion direct vent gas fireplace:

  • does not use indoor air

  • is unaffected by negative pressure

  • cannot backdraft

  • is highly efficient

  • satisfies most code requirements without additional make-up air

Q42: What is make-up air and why do homes need it?

Make-up air is outdoor air supplied into the building to replace exhausted air.
Homes need it to prevent:

  • backdrafting of fireplaces/furnaces

  • poor indoor air quality

  • smoke spillage

  • cold air infiltration

  • pressure imbalance affecting door operation

  • reduced HRV/ERV performance

Q43: When is make-up air required in a home?

Generally required whenever exhaust devices or combustion appliances can depressurize the home, such as:

  • large kitchen range hoods (over 300 CFM in some jurisdictions)

  • open wood-burning fireplaces

  • large dryers

  • whole-house exhaust systems

  • multiple simultaneous exhaust appliances

  • very airtight homes (new Canadian builds often are <= 3 ACH50)

Q44: What happens if a home does NOT have adequate make-up air?

You may see:

  • smoke spillage from fireplaces

  • poor chimney draft

  • backdrafting of water heaters

  • whistling at windows and doors

  • cold infiltration through leaks

  • negative pressure in basements

  • uncomfortable drafts

Q45: How much make-up air is needed?

Make-up air should equal or slightly exceed total exhaust airflow.
Rules of thumb:

  • Size MUA to 90–110% of the largest exhaust appliance

  • For open fireplaces: assume 300–600 CFM as a starting point unless manufacturer specifies otherwise
    Mechanical engineers often calculate the worst-case depressurization scenario.

Q46: What types of make-up air systems are used in residential settings?

  1. Passive MUA (simplest)

    • A duct with a backdraft damper or motorized damper

    • Opens when house pressure goes negative

    • Works best in leakier homes

  2. Active MUA

    • A fan brings in outdoor air

    • Can be interlocked with range hoods, dryers, fireplaces

  3. Conditioned mechanical MUA

    • Includes a heat source (electric heater, hydronic coil, or furnace)

    • Used when large MUA volumes require tempering in cold climates

    • Common in high-end residences or commercial-grade kitchens

Q47: Should make-up air be heated in cold climates like Ontario?

Yes.
Unheated MUA can:

  1. dump cold air on occupants

  2. freeze pipes

  3. disrupt HVAC balancing

  4. reduce comfort
    Mechanical codes often require MUA heating when delivered directly to occupied spaces.

Q48: How should MUA be interlocked with exhaust devices?

Typical interlocks:

  • Range hood ON → MUA fan + damper ON

  • Dryer ON → MUA damper ON

  • Fireplace in use → pressure switch opens MUA damper

  • Whole-house depressurization → MUA opens automatically
    Control strategies vary with local code requirements.

Q49: Can HRVs/ERVs serve as a source of make-up air?

No.
They are balanced ventilation systems.
They are not designed to deliver large volumes of air to offset high exhaust loads or to mitigate fireplace safety concerns.

Q50: What testing verifies if a home needs MUA?

  • Blower door test (ACH50 and natural leakage)

  • Pressure testing between floors

  • Worst-case depressurization test (required for combustion safety)
    These tests quantify how much exhaust the house can handle before pressure becomes unsafe.

Q51: Does a very tight home require mechanical MUA even for small exhaust appliances?

Often yes.
Homes built to modern standards (<3–2 ACH50) may experience fireplace spillage or water heater backdrafting even with moderate exhaust loads (e.g., 200 CFM).

Q52: What is the purpose of pressure sensors in HVAC systems?

Pressure sensors measure static, dynamic, or differential pressure in air handling systems. They are used to ensure correct airflow, prove fan operation, monitor filter loading, control dampers, maintain safe combustion, and prevent negative-pressure hazards.

Q53: What are the main types of HVAC pressure sensors?

  • Static pressure sensors (measure pressure in still air)

  • Differential pressure sensors (measure pressure difference between two points)

  • Pressure switches (on/off control based on setpoint)

  • Draft pressure sensors (for chimneys/flues)

  • Building pressure sensors (monitor indoor vs outdoor pressure)

  • Duct pressure transducers (send signals to controls/BMS)

Q54: Why is differential pressure monitoring important?

Differential pressure is used to monitor:

  • Filters (dirty filter alarms)

  • Coils (clogging)

  • Fans (airflow proving)

  • Pressure across rooms (air balance)

  • Fire/smoke dampers

  • Door opening forces in pressurized stairwells and corridors

Q55: Where are static pressure sensors typically installed?

Common locations:

  • Supply plenum after the blower

  • Return duct before the coil

  • Branch ducts to verify proper airflow

  • Ductwork served by VAV boxes

  • MUA ducts to monitor delivered pressure

  • Fan inlet/outlet regions

Q56: How do pressure sensors prevent dangerous negative pressure?

A pressure sensor can trigger:

  • Opening of a motorized damper

  • Activation of a MUA fan

  • Shutdown of exhaust fans
    This is often used with:

  • Fireplaces

  • Range hoods

  • Large exhaust fans

  • Tight homes

  • Multi-unit residential buildings

Feel free to press Ctrl+F to find whichever topic it is you are looking for!

For further Assistance, Call 647-328-4212 or

Aeris Consulting

Design sustainable. Design efficient.

HVAC Design

Electrical Design

Plumbing Design

Life & Safety (Plan) Design

info@aerisconsulting.ca

647-328-4212

  • Facebook
  • Instagram
google.png

©2025 Aeris Consulting

bottom of page