What is Wrong With My ‘Old’ Furnace

Home Inspection Gas Burners

In order to heat our homes, we require two basic things: An energy source (like natural gas or electricity) and a distribution method (like air or water).

In Canada, most of our furnaces use natural gas to heat our homes. Natural gas is an amazing resource. It burns ‘clean’ relative to other fossil fuels (the major byproduct is water and CO2), it contains a large amount of stored energy in each unit of fuel, it looses little energy in way of distribution (unlike electricity that can lose 8-15% of source energy in transmission to your home), and it generates heat very quickly.

The down side of burning natural gas is any ‘incomplete combustion’ of natural gas can create nasty byproducts chief of which is carbon monoxide. Carbon monoxide is an odourless, colourless gas which builds in your blood steam and even with small doses can poison you as you sleep.

I thought this Article was About my ‘Old’ Furnace

It is. Now that we have established natural gas is great for home heating, but is deadly if burned poorly (and no furnace burns 100% perfectly), we have to talk about heat transfer in a furnace. You can’t directly send the air heated in a furnace with natural gas in the home or you could poison the occupants. To overcome this, furnaces use a ‘heat exchanger’ to transfer heat energy from the ‘combustion’ side of the furnace to the ‘household’ air.

Heat exchangers are pretty simple devices. For illustration purposes consider a heat exchanger like a bundle of short metal hollow tubes all in contact with each other. Half the tubes are going up-down, and the other half are going left-right. In this illustration, if we burn natural gas and put the heat through the up-down tubes, the metal tubes will get hot (like a muffler pipe on a car). The heat energy will also transfer into the left-right pipes. If we ‘blow’ air from the home through the left-right pipes, we can transfer that heat to the household air. Now, we have heat from combustion, and safe air in the home to breath.

Umm… are we going to talk about my ‘old’ furnace or what?

I’m getting to that… now that we have established that the heat exchanger is critical in our furnace to keep harmful gasses out of the household air, what do you think happens to your heat exchanger after 20, 30, or 40 years of use? That heat exchanger is exposed to very high heat and cooling cycles, risk of condensation and rusting, constant shaking from the blower fan, and some minor particles in natural gas make it an acidic environment.  In short, it is not a question of ‘if’ your heat exchanger is going to fail… it is ‘when’.

What Does a Home Inspector Look at Then?

The problem with cracks in heat exchangers is two fold. First, the cracks can be nearly microscopic. Second, in most furnaces you can only see 5-15% of the heat exchanger without completely taking it apart. 

What we do know is that the heat exchanger, with enough time, will fail. In year 1, the probability of a failure is likely less than 1% (not including manufacturing defects). However, by year 40, the failure rate in a given year could be as high as 10%. While that may not seem high, the ‘probability of failure’ (POF) escalates over time… for example…

  • Year 1 - POF 0.5%; Lifetime POF 0.5%
  • Year 2 - POF 1%; Lifetime POF 1.5%
  • Skip to Year 5 - POF 3%; Lifetime POF 8%
  • Skip to Year 15 - POF 5%; Lifetime POF 50%
  • Skip to Year 40 - POF 10%; Lifetime POF 99%

So basically, the older the furnace, the higher the risk it will start pumping harmful gasses like carbon monoxide into your breathing air. This is why when a home inspector looks at an ‘old’ (or ‘older’ furnace), we can give our clients the advice to consider replacement even if the old one is ‘perfectly good’ according to the seller. Because it is not a matter of ‘if’ that old furnace will try to kill you… it is ‘when’.

 By James Bell - Author | Owner/Operator of Solid State Inspections Inc