Author Topic: Interlocking furnace with HRV need explanations  (Read 5191 times)

Offline Alexhvac66

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Interlocking furnace with HRV need explanations
« on: April 07, 2015, 08:09:59 PM »
Hey guys I'm new in the trade and I need some clear explanations on why interlocking the furnace with the HRV using NO , NC , COM , I understand that when the HRV will be working the furnace will start , my question is why ? What are the benefits ? What is the purpose of interlocking the HRV and the furnace ? I hope you guys can give me clear explanations ! This is my first post looking foward to talk with you guys ! Have a good day ! Alex

Offline walker

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Re: Interlocking furnace with HRV need explanations
« Reply #1 on: April 07, 2015, 08:42:25 PM »
you need the fan on the furnace to be running with the HRV to effectively draw in the fresh air and distribute it throughout the building.  If you don't interlock it with the furnace fan you're just drawing in fresh air to sit in the duct work.

Offline Admin

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Re: Interlocking furnace with HRV need explanations
« Reply #2 on: April 08, 2015, 01:01:36 PM »
Walker is right, the purpose of interlocking the furnace fan and HRV is to improve efficiency and, in some installations, prevent cross-contamination.

There are other factors to consider when installing an HRV.  When it comes to new construction we follow the Building Code and in some cases Energy Star guidelines.

If you were installing an HRV in an older home then you can follow the installation manual.  I attached the Vanee 41600 HRV manual.  The HRV can be installed as a fully ducted, simplified or central draw point system.  In the case of a fully ducted HRV no interlock would be used.  I find that most HRV's are installed as simplified systems and, with Vanee, the indoor connections can be on the supply and return or return and return ducts.  The return and return method would require that the HRV and furnace fan be synchronized, but with the supply and return method interlocking is optional.

When it comes to new construction we cannot use the supply and return method that the manual allows.  Only the return and return method can be used as per OBC Articles 9.32.3.4(7)(a) and 9.32.3.11(3),

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Article 9.32.3.4(7) - Where an exhaust air intake for the principal exhaust fan is connected directly to the duct system of a forced air heating system or other central air circulating system, it shall,
(a) be connected to the return air side of the system, and

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Article 9.32.3.11(3) - Where a heat recovery ventilator is connected to a forced air heating system, the supply side of the ventilator shall be directly connected to the return air side of the forced air heating system.

If the house is an Energy Star home then we follow the 2012 ENERGY STAR ® for New Homes Standard.

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4.7.2 Distribution Systems
4.7.2.1 - Interconnection to Forced-Air Distribution System
Where a central forced air system is utilized either fully or in part to distribute ventilation air, the principal exhaust fan control shall be interconnected with the forced air distribution system such that switching on the principal exhaust fan operates the forced air distribution fan.

No ventilation supply inlet is required in Type I homes and sometimes the total ventilation capacity is achieved by using only the standard exhaust fans found in bathrooms and the laundry room, but the principal exhaust fan (bathroom fan) would have to be interlocked with the furnace fan.  We use a relay and install a 3 way switch near the thermostat, and it turns the bathroom fan and furnace fan on.  I use the same 3 wire method using C, NC and NO.  There is a 2 wire method but on some thermostats, energizing the “R” and “G” terminals at the furnace has the effect of energizing “Y” at the thermostat and can turn on the AC.  If you identify this type of thermostat, you must use the 3 wire method and use a NC wire.

The OBC requires the HRV be interlocked (coupled) in a Type II home.  A Type II home would have a solid-fuel burning appliance (wood fireplace).

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9.32.3.1 General
(2) For the purposes of this Subsection a dwelling unit shall be categorized as,
(a) Type I when,
  (i) all fuel-fired combustion appliances located in the dwelling unit are direct vented or, except for fireplaces, are mechanically vented induced draft, and
  (ii) the dwelling unit does not contain a solid fuel-fired combustion appliance,
(b) Type II when a solid fuel-fired combustion appliance is installed in a Type I dwelling unit,
(c) Type III when a mechanically vented induced draft non-solid fuel-fired fireplace or a natural draft appliance is present, or
(d) Type IV when electric space heating is present.

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9.32.3.6 Ventilation Systems Coupled with Forced Air Heating Systems
(1) This Article applies to a mechanical ventilation system in a dwelling unit that contains a forced air heating system which is used for delivery of ventilation air.
(2) In a Type I dwelling unit, a ventilation supply inlet is not required.
(3) In a Type II dwelling unit, the mechanical ventilation system shall include a heat recovery ventilator, coupled to the forced air heating system, installed in accordance with Article 9.32.3.11.

With wood burning fireplaces the HRV must also be balanced so that the flow of exhaust air does not exceed the flow of intake air.

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9.32.3.8. Protection Against Depressurization
(2) Where a solid fuel-fired combustion appliance is installed, the ventilation system shall include a heat recovery ventilator that is designed to operate so that the flow of exhaust air does not exceed the flow of intake air in any operating mode, and that complies with the requirements of Article 9.32.3.11.