This year will be the last that mid-efficiency forced air furnaces will be available. After Dec. 31 manufacturers will no longer be allowed to produce forced air furnaces with an efficiency of less than 90 percent AFUE. Add to this the recent changes in venting materials and the contractor with little high efficiency installation experience faces a learning curve.

Mid-efficiency units have always had a place. In a retrofit furnace changeout it is not always possible to upgrade to a 90 percent-plus product for one reason or another. There are many considerations during the initial sales visit, such as, can I vent it, and where can I drain it?

As a result, some homework must be completed before the sale. Sizing high efficiency units is different. An existing 100,000 Btu/h furnace is not necessarily replaced with a unit of the same size. The existing furnace may have been in operation for the last 20- plus years, running with a temperature rise in the 90oF range and a duct system designed from whatever was in the truck on the day it was installed.

Today’s equipment requires a temperature rise in the low to mid 50oF range. Airflow is increased by about 40 percent and could result in noise complaints were the furnace replaced size for size.

Back in the day, when a 100k furnace was the norm, fuels were inexpensive and home insulation was limited. High temperature rise and inefficient products were acceptable and “bigger is better” was part of the design thought.

A “shell” heat loss calculation will quickly determine the needed equipment sizing. Usually the resulting reduction in furnace size may be found compatible with the existing duct system and require minimal changes. It is also advisable to suggest a duct cleaning as well! A twenty-year-old dust bunny will make a major cleanup of the dwelling inevitable!

Careful setup
If you have ever been to a night class or upgrade heating class, the instructor always stresses the importance of the setup of any new equipment. The instructor will also encourage technicians to fill out a start-up sheet, which is usually included in the equipment’s installation manual.

This is primarily for future reference, to be viewed at a later date during a service call. If you are an Ontario contractor, you may be asked to produce this document during an onsite audit by that province’s Technical Standards and Safety Authority (TSSA).

Commissioning a new forced air furnace starts with checking the gas pressure into the home, and then the outlet manifold pressure to the burners. Most manufactures are looking for between 3.2 inches water column to 3.5 inches water column in a single-stage furnace.

If it is a two-stage furnace, low fire pressure setup is typically about 1.7 inches water column, (consult the manufacture’s guide for the exact settings.)

Remember, if you have a two-pipe direct vent appliance the doors must be on when setting the manifold pressure because your combustion air blower will reduce the ambient pressures in the combustion chamber. Setting the manifold pressures with the doors off may result in over-firing the appliance.

Once the gas pressure has been verified, it is time to start the temperature rise setup. Temperature rise ensures that there is sufficient airflow through the duct system. This rise is measured over a ten-minute run time for a single-stage unit and fifteen minutes for a two-stage furnace.

The temperature is measured on the return air side of a system, before the air cleaner and below the humidifier if one is present. A second temperature probe is located on the supply side of a system after the plenum take-off (PTO). If a second PTO is present it is important to take a second temperature reading of the supply and average the two temperatures.

Once a cold air temperature reading is taken and a supply temperature is found, the return temperature is subtracted from the supply temperature, equalling the temperature rise. The temperature rise is best set, if possible, in the centre of a manufacturer’s recommended range. For example, if the setup range was 40 to 70 degrees, the fan speed adjustment should be as close to the centre of rise at 55 degrees as possible.

Verify gas inputs
A dial test should be performed and recorded on the start-up sheets to verify proper gas inputs. The high limit, as well as any alternative limits, must be tested for proper operation. Record the actual temperatures of each switch. This will insure the safeties are all in working order.

Static pressures should be recorded on both the supply and return sides of the system. The technician must consult the blower data sheets provided in the manual to ensure the safe running amperages of the main blower.

Approved venting tables are included in the installation manuals provided with every appliance and are to be followed without exceeding the guidelines. Not only will they specify the materials to be used, but also the maximum vent pipe length, equivalent length of elbows and tees that can be included in the system and total length including any termination fittings. It is best to record this information of the venting system for further reference after the ceiling has been closed in.

The manufacturer provides a start-up sheet to ensure the product is properly commissioned for a long life span.

Today’s furnaces are manufactured specifically for the North American market. Some may have been shipped several thousand miles, from factory to warehouse, to distributor and finally to you and your customer. They are generally set up for the worst possible conditions by the factory. The manufacturer doesn’t know whether the unit will be installed in Toronto, Vancouver, or wherever, nor do they know the characteristics of the system.

It is up to the installer to perform a setup on every install to ensure maximum efficiency and comfort for local conditions and to record the setup for the future servicing of that furnace.

Robert (Bob) Bettles is lead technical service adviser and trainer for Wolseley HVAC/R Group Canada. Brian Guttormson is technical service advisor for Trent Metals Ltd. (Supply). Bob can be reached at Bob.Bettles@wolseleyinc.ca while Brian is at techsupport@tmlsupply.com.