Basement Suite Heating – Your Questions Answered

The Weiss-Johnson Blog

Heating A Legal Basement Suite

With the increased need for affordable housing as well as a desire to pay part of mortgages though rent, there is an increased need for basement legal suites. Weiss-Johnson Sheet Metal has vast experience in doing such projects in both new housing as well as renovations and have these helpful hints to help you with such projects.

Each legal suite requires an independent heating system. Below are the 3 most common ways in which this can be achieved.

  1. Forced Air Furnace 
  • The addition of a separate forced air furnace, and associated ductwork, can achieve this goal.
  • The Mechanical room will need to be fire taped and drywalled to ensure proper “fire separation” before ductwork is installed, or can be completed after by way of a dropped ceiling below the ductwork and then finishing with drywall around the venting, flexes, pipes, and plenum (should you chose to drywall after the ductwork install, the permit holder will need to call the “Concealed Ductwork Inspection” prior to drywall). All penetrations through this fire barrier will need to be sealed properly with approved product by your drywall contractor.
  • If you chose to drywall prior to the ductwork (most common) a “Legal Suite Rough in” will need to be done by all your mechanical trades to ensure that when the room is fire rated that nothing is buried above the drywall.
  • Once this is complete we can return to install our ductwork.
  • After the ductwork is complete and our QC is done a “Concealed Duct inspection” will need to be called by the permit holder.


Most of the time when we install a new furnace, sized for the basements heating load, the existing furnace can be oversized for the remainder of the home. If the existing furnace is oversized by more than 20% it is good trade practice to replace it with the appropriate sized model. Please consult WJ for more information.

Space. Most existing mechanical rooms don’t have the space for the addition of another furnace system.

Cost varies depending on what alterations need to be done but an average furnace system for a legal suite will be between $4800-6400.00. In instances where people add this system in a completed home, the existing furnace will often be oversized and will need to be replaced as well, regardless of how new it is.

This is the only type of legal suite heating system where you can get proper humidification and filtration.

  1. Electric Baseboard Heat paired with an HRV system for Ventilation
  • Electric Baseboard heat is sized and installed according to the heat loss of the suite.
  • An HRV is installed and ducted to exhaust from the bathrooms and to supply Fresh Air to the living areas of the suite. The HRV will be the Primary Exhaust system for the suite.


Initial cost is typically less than a Forced Air System. An HRV system with electric baseboard heat will run between $3800 -$5000

Operating costs will be higher for the homeowner. Electrical costs for the baseboard electric heaters will far exceed the cost for gas of a basement furnace. No humidification or air filtration is available with this option and this could be an issue for some.

Warranty. Electric space heaters don’t have great warranties when compared to a furnace.

  1. Hydronic Heating paired with an HRV system for Ventilation
  • Hydronic, or infloor, heating is installed using a series of pipes located below a poured concrete slab. Heat transfers from the heated water/glycol and heats the concrete.
  • An HRV can be installed and ducted to exhaust from the Bathrooms and to supply Fresh Air to the living areas of the suite. The HRV will be the Primary Exhaust system for the suite.


This system is most expensive and virtually impossible to do in a renovation without incurring huge costs. Also cannot have any humidification/filtration added. It is however the most comfortable, even heat for the occupant.

General Information:

One furnace that has some distinct advantages that we have installed in some basement suites is the Dettson fully modulating CV-15 furnace. With its compact design, it can be virtually installed anywhere. The overall size of the furnace is 10” wide by 22” high by 27” deep, not much bigger than a computer tower!

With a 15,000 BTU output and fan with just over 300 CFM, it often isn’t oversized for a basement suite like many other furnaces where we have to run excessive outlets and return air grilles to move the necessary air and heat. The return air and supply ducts can be ran with as little as two 9” round pipes. This means less ductwork and smaller bulkheads which helps offset the increased cost of this furnace over a conventional furnace.


The Weiss-Johnson Blog Team

We are the Weiss-Johnson Blog Team! All blog posts are written by a collective of WJ employees. Each one of our articles is designed to inform our customers of all the happenings in the world of home comfort! Thanks for reading!

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