in-duct smoke detector

Do You Need 2 Separate Furnaces For Your 2-Unit House?

This week, I was at a client’s house while he was receiving a final inspection of the heating system for his legal secondary suite conversion. The required smoke detector located inside the return duct was blinking red, and had shut off.

We couldn’t figure out why.

It turns out that the detector simply did its job. Allow me to explain.

How They Did Them Before

In the past, if you were to convert a single family home into a duplex, you would have to put in two separate heating systems - one for each unit. If you have an air conditioning system, you'll likely have to add one of those too. And if it's using a forced-air system (most homes are), you'll have to put in a new set of supply and return ducts as well.

Additional ducts can take up a huge amount of space

Additional ducts can take up a huge amount of space

Not only is this potentially going to add $15,000 to your renovation, it's also going to eat up huge amounts of space in your new unit. It sucks (pardon the pun), but there's good reason for two furnaces. It has to do with fire safety.

When you add in a second unit, fire containment between the units is one of the most important aspects of doing a two-unit conversion. Even if you did a great job of ensuring all ceiling and wall separations were adequate, with proper fire-rated doors, there is still one area where fire could spread.

You guess it: The ductwork.

In both residential and commercial buildings, one of the biggest factors that can lead to the rapid spread of fire is within the ducts. This is especially true when a blower fan is used to distribute heating and cooling.

Ever try to manually fan a campfire to stoke it? Think of that times a thousand.

So what has the building code done? They’ve placed provisions to prohibit supply air from one suite to circulate into another suite ( Interconnection of Systems), or return air to re-circulate between units ( – Return-Air System), as well as “fire-stops” to separate ceiling assemblies between units ( Fire Stop Flaps).(1)

All this essentially means that you’ll need two separate heating systems.

But wait….

The Antidote

Buried deep in the building code is Table of Part 11, a compliance alternative for all of the items above for renovation of homes older than five years, and have been accepted by most municipalities.

Basically in lieu of all the items above, we could simply install an in-duct smoke detector that would shut down the power to the furnace if it detected smoke or fire in the ducts. This is a work-around that effectively provides a solution without having to install an extra heating or cooling system.

The one my client had installed

The one my client had installed

My guess on the reason for this would be an attempt to make it less onerous for homeowners to do a proper conversion.

These in-duct smoke detectors typically run $500 to $1000 installed. Not cheap, but much better than spending $15,000, and more ducts in your basement ceiling.

An Inadvertent Solution To Tenant Smoking

Back to the story about the in-duct smoke detector that was shut off.

The inspector reset the detector and informed us the system is properly functioning as it detected smoke and shut off the furnace. Yes….smoke. Not from a fire though, but rather cigarettes. The tenants upstairs had been smoking.

The fact that cigarette smoking would activate the detector and turn the furnace off was news to me. A light bulb popped over my head.

If you’re an investor landlord, what if you informed your tenants that if they smoked, they would risk turning the furnace off? Would that mitigate or eliminate smoking in the house?

Although I would never advocate shutting anyone’s heating system off because they are smoking, can informing them change their behavior, even if they had the ability to reset the detector and turn the system back on?

My guess is yes. Who wants to keep moseying over to the furnace room and resetting the furnace after every smoke? No one.

Thank you for hitting the pipe outside

Thank you for hitting the pipe outside

So if you’re doing a two-unit conversion, it might be worth informing the tenants of this. In addition to saving you from having to do a paint job for the next tenant, it’s also good that separate tenants in each unit won’t be blowing nasty cigarette smoke in each other’s units.

In The Literature

I was skeptical if this was in fact true. Maybe the detector was too sensitive? Maybe it was broken? So I did a bit of research on manufacturer’s literature of in-duct smoke detectors. Here’s some instructions I found on testing the system for a System Sensor D4120.(2)

[12.5.2] SMOKE RESPONSE TESTS To determine if smoke is capable of entering the sensing chamber, visually identify any obstructions. Plug the exhaust and sampling tube holes to prevent ducted air from carrying smoke away from the detector head, then blow smoke such as cigarette, cotton wick, or punk directly at the head to cause an alarm.

I assume that it might work as well for a variety of other smokable items that none of our tenants would even engage in. But what do I know?

When you do things properly, you minimize tenants like these living in your property

When you do things properly, you minimize tenants like these living in your property


1. Recent post Ontario Building Code - Basics To Cover Your Ass, where you can grab a free copy of the code online.

2. Link to the manufacturer's instructions for this in-duct smoke detector