Replacing Your Furnace
There are usually two major reasons why you are
choosing another forced-air furnace. The first is that your furnace does not
function. It has just broken down, irrevocably, or it has been “red-tagged” or
condemned by gas inspectors. If it is winter, and your house is getting colder
quickly, you may not have the luxury of making a reasoned choice on what to buy
next. The other situation is that your furnace is getting old, or your fuel
bills are becoming too excessive to tolerate. In this case, you have the time to
shop around and get the best furnace and fuel for your situation.
This
About Your House is written to address both situations. If you have a
dead furnace and a chilly house, you will probably take some shortcuts in your
selection process.
Choice of Fuels
For many years, CMHC and others could offer sound advice
on what fuel choice would be the most economical. During that period, heating
systems based on electricity or propane cost the most to operate. Heating oil
was somewhat more economical, and natural gas (if available in your community)
was the least expensive choice.
Since 2000, the prices of these commodities
have been fluctuating, and it is difficult to offer reliable advice on pricing.
At one point in 2001 – 2002, heating with electricity in Manitoba was as
economical as heating with natural gas. Predicting these prices over the next
two decades (a common life span for a furnace) is nearly impossible. The best
advice is to make a calculation based on the current prices quoted to you in
your locality. See the text box entitled “Calculating fuel costs.”
Calculating Fuel Costs
Here is a rough comparison of the relative costs
of heating an older house in Ottawa. You can put in your own fuel prices and the
efficiencies of the appliance that you are choosing to compare relative
costs.
Note: It is often difficult to isolate the cost per
unit of fuel, be it gas or electricity. Include all the costs that relate to the
m³ of consumption for gas (for example, gas supply charge, gas delivery charges,
gas surcharges). Electric utilities often also have a bewildering range of
charges. Apply all the charges except fixed charges (for example, $10/month
connection charge).
For oil appliances, use an energy content of 38.2
MJ/litre of oil. For electricity, use 3.6 MJ/kWh and 100-per-cent
efficiency.
Note: 80 GJ (or 80 gigajoules) is the energy required for heating
the example house over the winter (heat load). Your own house will likely be
different. However, the relative costs calculated for alternative fuels and
furnaces in the example house should help you make a selection for your
house.
Furnace Sizing
You probably do not
need a furnace with the output of your current furnace. Most furnaces in
Canadian houses can provide far more heat than the house requires. A properly
sized furnace will be running almost continuously during the coldest day of the
winter. Having a furnace of a correct size will result in efficient operation
during the whole heating season. A grossly oversized furnace will run only for a
short period, never coming up to peak efficiency. Note, however, that sizing may
not be a big issue with high-efficiency, condensing gas furnaces. Due to the
design of condensing appliances, they are efficient even when oversized.
So,
how do you size your furnace? You can have the contractor use a home heat loss
calculation that is available from Canadian Standards Association (CAN/CSA F280)
or a sizing procedure from the Heating, Refrigeration and Air Conditioning
Institute of Canada (HRAI). Having a proper sizing will cost you $150 – $300
from a qualified contractor.
Those who keep their heating bills, and who are
mathematically inclined, can try the calculation in the text box entitled
“Calculating house heat loss from utility bills.”
Calculating House Heat Loss from Utility Bills
Here is a sample
calculation, using a three-month meter reading for a typical house. You can use
any period (but at least two weeks of winter weather is necessary). You can read
the meter yourself for the information, look at your furnace bills or phone your
utility to see if they have appropriate records. The natural gas usage of other
gas-fired appliances in the house is estimated from gas utility data and
subtracted from the total for the period in question, so that the gas
requirement for heating can be isolated. (Oil furnaces are harder to size using
this method, but it may be possible using oil fill-up intervals and the number
of litres delivered.)
The goal is to find a relationship between the gas
consumed and the heating degree days (HDD). A heating degree day is essentially
the number of degrees of heating required over the course of 24 hours, compared
to a reference temperature of 18°C. For example, if the average daily outside
temperature is 10°C, then the number of heating degree days for that day is 18°C
- 10°C = 8 HDD. You can get the approximate HDD for your calculation period from
the
Environment
Canada website. Use the data from the “Degree Days: Below 18°C” row.
Once
the relationship of the HDD and gas consumption is established, then you can
calculate gas consumption for the design temperature in your area. This
temperature is usually available from a mechanical contractor or your local
building officials. It is not the extreme minimum temperature; it can be
estimated from the average temperature over 24 hours on the coldest day of the
winter. To approximate the design temperature: go to the historical weather data
for your community on the
Environment Canada website;
find the coldest January over the last several years; then pick out the lowest
daily average temperature in that month; and use that as the design temperature.
Being a degree or two out will not make a huge difference in the
calculation.
The example below uses a design temperature of -35°C. At that
temperature, the maximum HDD per day is equal to 53, which is the difference
between 18°C and
-35°C. Calculating the
size of the furnace necessary on the coldest day of the year will mean that the
furnace has the capacity to handle any expected local temperature. You can find
a furnace’s efficiency rating on its EnerGuide label or in the product
documentation.
ExampleTotal gas consumption from
December to March = 1,320 m
3Estimated consumption for other gas
appliances (data from utility) = 306 m
3Therefore, gas consumption
during the period for heating = 1,320 - 306 = 1,014 m
3Heating
degree days for that period (from Environment Canada data) = 2,840
HDD
Heating consumption by degree day = 1,014 m
3/ 2,840 HDD =
0.3570 m
3/HDD
Heating consumption at 53 HDD/day = (53
HDD/day)(0.3570 m
3/HDD) = 18.9 m
3/day
Where gas has an
energy content of 37.5 MJ/m
3, and the existing furnace has an
efficiency of 72 per cent, then:
Heat loss
at 53 HDD/day = (18.9 m3/day) (37.5 MJ/m3)(0.72) = 510
MJ/day or 21.3 MJ/h*According to
the energy content of electricity, 3.6 MJ/h = 1 kW, then 21.3 MJ/h = 5.9
kWThis heat loss would require a furnace that produces an output of
5.9 kW or about 20,100 Btu/h (1 kW is approximately 3,412 Btu/h).
If we allow
the CAN/CSA F280 permissible oversizing of 40 per cent, then the proper furnace
sizing would be (1.4)(20,100 Btu/h) = approximately 28,100 Btu/h.
If you are
calculating for an oil furnace, heating oil has an energy content of 38.2
MJ/litre.
* Note: This calculation is correct,
although many people think the efficiency factor is in the wrong place. It is
not. We are calculating the house heat loss based on fuel used and furnace
efficiency. A more efficient furnace will have delivered more heat to the house,
and the heat loss will be higher.
Furnace Efficiency
There is a wide range of furnace efficiencies,
although only high-efficiency gas furnaces are sold in Canada as of 2010. The
range of efficiency will vary by fuel.
Electric furnaces work on electric
resistance. The full 100 per cent of the energy consumed goes towards the
heating of the house. The inefficiencies with electric heating happen before the
electricity reaches your house. If the electricity is created by burning fuels,
there are inefficiencies in that process plus losses as the electricity moves
through the lines.
Oil furnaces have become far more efficient since the
height of their popularity in the mid-twentieth century. Efficiencies have risen
from roughly 60 per cent to well over 80 per cent as a result of advanced
technologies — first to flame retention head burners and then to high static
pressure burners. The more efficient oil furnaces require a better chimney than
their conventional counterparts, so you will probably need to upgrade the
chimney with a stainless steel liner inside the old clay tile. Make sure this is
included in the quote.
Failure to have a properly sized chimney will result
in excessive chimney condensation and eventual destruction in the case of
masonry chimneys. There are high-efficiency, condensing oil furnaces as well.
Earlier versions had reliability problems. The new generation, launched in 2003,
may have resolved these difficulties.
New gas furnaces in Canada are
high-efficiency (89 – 96 per cent) condensing furnaces. The high-efficiency
furnaces use a plastic vent and are most often vented out the side wall. Propane
furnaces are usually modified natural gas equipment.
So… What Do I Buy?
Here are the most common questions about furnace
replacements to CMHC staff from Canadians, and our usual
answers:
Should I switch my heating fuel?In most parts
of Canada, it will be more expensive to heat with an electric furnace than one
using oil or gas. An exception would be if you heat primarily with a wood stove
and use the furnace only infrequently as backup. In this case, the low cost and
low maintenance requirements of an electric furnace may be a major advantage.
Deciding between oil and gas furnaces is a matter of choice. Make the
calculation to see if it is significantly cheaper to use one fuel or another
based on current prices in your area. Oil furnaces require a tank and usually a
chimney. There may be additional costs for chimney modification or oil storage
tanks when purchasing an oil furnace. Some home insurance companies require
periodic oil tank replacements. Check if a new gas furnace would also require
relining the chimney. Consult with your contractor and make sure that these
costs are included in your estimates.
Some dealers recommend a
furnace of 100,000 Btu/h, and some say 80,000 Btu/h will be fine. How do I
choose?See the previous discussion on
sizing.
If you are buying an oil furnace, proper sizing will affect the durability and
efficient operation of your appliance. Your choices are either to pay for a
proper heat loss analysis, to calculate house heat loss or to accept the
dealer's estimate. Sometimes government or utility programs subsidize house
testing. If such a program is in effect in your vicinity, this can be an
economical way to have your house heating load established.
Are there
any advantages to multi-stage, multi-speed furnaces?Multi-stage
furnaces have become more popular lately, although they are more expensive than
the single stage furnaces that have been sold for decades. Multi-stage furnaces
have two or three levels of burner function, and an efficient, modulating
circulation fan to move the heat into the house. They can provide additional
heat when a quick temperature rise is required, such as in the morning when a
house with a setback thermostat is being heated from 15°C to 21°C (59°F to
70°F). A traditional single speed furnace
would take longer to get up to temperature. The multi-stage furnaces are no more
efficient than single-stage furnaces; they offer more flexibility and perhaps
more comfort.
Is Furnace "A" better than Furnace "B"?
How can I find that out?There is little or no available
data to show that one manufacturer's furnace will operate longer and with less
trouble than a furnace from another manufacturer. This is frustrating for
consumers. We are used to being able to read ratings of one product versus
another product and to make a choice based on those ratings. However, a good
furnace will last 25 years. A poor one may break down prematurely at 15 years.
With lifetimes of this length, and with furnace design and model changes, it is
hard to predict which furnace will provide the best service.
There are two
factors to help you in your choice. Pick a furnace with a long heat exchanger
warranty, 20 years or more. If manufacturers are willing to back the most
expensive part of their appliance for a long time, this should inspire some
confidence. Also, pick a furnace manufacturer and a dealer that have been in
business for a significant period of time. A furnace with a lifetime warranty
offered by a company that has been in operation for only three years may not be
the best deal. One would expect to pay less for this level of uncertainty. Look
for contractors with memberships in trade organizations such as HRAI, which
would indicate an interest in professional qualifications.
The Hot Water Heater Conundrum
There are very few high-efficiency hot
water heaters available. Changing your furnace may lead to having to think about
your hot water heater. Existing hot water heaters are often located vertically
below the kitchen and bathrooms, where the water is used. If you are changing
from an electric to a conventional gas hot water tank, and the new gas appliance
has to be installed across the basement to be near the chimney, you will be
waiting longer for the hot water at the tap. Consider a gas hot water tank that
has side-wall venting and does not require a chimney. This way, it can stay
close to the plumbing appliances that use it.
Another hot water tank issue
can occur when you switch from a conventional gas furnace and hot water tank to
a new, high-efficiency side-wall vented furnace. Now the hot water tank has to
heat up that big chimney all by itself, and you probably will have to pay for
chimney relining. It is often better, when choosing a chimneyless furnace, to
switch your hot water tank to side-wall venting at the same time and seal the
old chimney closed. However, side-wall vented hot water heaters are more
expensive than conventional hot water heaters and can be
noisier.
Instantaneous hot water heaters, which do not use a storage tank,
are becoming more common. They may be more economical to operate.
Furnace Circulating Fan Choices
Most furnace circulating fans consume
high amounts of electricity (300 – 700 watts). If you will be using your furnace
circulating fan to move ventilation air around the house (for instance, if you
have a heat recovery ventilator connected to it, or a high-efficiency air
cleaner on the furnace), then look at upgrading the circulating fan to a
high-efficiency DC motor. The best furnace fans now will use less than 100 W on
low speed. This will result in considerable electrical savings over the life of
the furnace.
Other Choices
When replacing the furnace, you may want to look at
integrated systems that heat your house and your water and also provide
ventilation. Devices known as “combo” units provide house and water heating. New
appliances with advanced, integrated systems will provide ventilation as well as
space and water heating. For some replacements, these integrated appliances will
be your best choice.
Additional Resources
For further detailed information on all heating
appliances, there are excellent booklets published by Natural Resources Canada
in the
Heating
and Cooling Series.