Thursday, December 24, 2009


Recently I went through the process of shopping for a new furnace and A/C because the existing one was more than 35 years old. The old one still works, but the service technician said the heat exchanger is showing signs of cracking and there is danger of the exhaust gas mixing with indoor air. The old furnace is rated to output 100k BTU while using 125k BTU, which gives it a 80% efficiency. Given its age, the efficiency is probably lower than that. The chimney pipe gets very hot when it is running, indicating a lot of heat is wasted through the exhaust. It is loud when it is running the blower fan, which is a belt driven fan. The old electronic air cleaner creates zapping sounds when the air is flowing.

I got estimates from 8 different installers. Each one presents to me a proposal for a particular brand and model of furnace. Some sell Lennox, some sell Carrier, some sell Trane, some sell Rheem, and some sell Bryant. I quickly found out that Carrier and Bryant means the same equipment just with different labels. Trane is the same as American Standard, and Lennox owns Service Experts, which has 2 branches in my area. Interestingly, the 2 companies under Service Experts seem to be competitors of each other and they don't use the same system. Rheem has good reputation and it is physically shorter than other brands.

From reading online, I got the idea that good furnace installers do Manual J load calculations to know the house's heat gain and heat loss for sizing the system's heating and cooling capacities. Of the companies I called, only 3 would do it. Some installers just look at the existing furnace and see what capacity it is rated and go by that. Most of them just decide the furnace size by experience or by square footage.

After I got some initial choices of equipment, I started reading more about them and came to understand that Lennox furnace can modulate its heating output from 60% to 100%, but it does not use a communicating thermostat and so it does not know how far it is from the set point temperature. It decides its heating output based on history data and tries to guess what the appropriate heating output should be. Rheem makes a modulating furnace that uses a communicating thermostat, but the efficiency is 92%, which does not qualify for the federal tax credit. Rheem has 95% efficient models but they are not modulating. Bryant/Carrier has a 3 stage furnace that uses communicating thermostat, which is what I eventually installed. The communicating thermostat talks to the furnace and other connected equipment using a series bus, so it only requires 4 wires: 24V supply, ground, data A and data B. I am not familiar with what data A and data B are, but my guess is that one of them is a clock signal and the other is the actual data line. The thermostat talks to the furnace and automatically discovers what equipment it is connected to and automatically adjust its settings to fit the house. For example, it tests the static pressure of the duct system and finds the appropriate motor RPM to create the appropriate CFM of air flow. The thermostat knows how far the room temperature is from the set point and see if it is moving towards or away from the set point. The furnace would change its heating output accordingly to drive the room temperature towards the set point.

Another thing I liked about Bryant is that their technical literature such as install manual and product information is online and not too difficult to find, whereas the Lennox documents were almost impossible to find.

I also looked into options for filtering. There are media filters, electronic air cleaners, and HEPA systems. The media filters come in different sizes, but the one I am looking for is the 4 7/8" thick kind that comes with MERV rating between 8 and 16. The issue with electronic air cleaners is that they require electricity, create ozone, and need expensive replacement UV light bulbs every year. The HEPA filters are typically by-pass type filters that cleans a portion of the air going through the furnace, and the install cost is high. I think the media filter with a high MERV rating will be sufficient for me.

For humidifier, I gathered that there are 3 types: by-pass, fan powered, and steam. The by-pass one takes air from the return duct, passes through the humidifier, and goes into the supply duct. The fan powered models mount on the supply duct and blows air over the water in the humidifier and into the duct. The steam ones also just mount on the supply duct and it makes steam which goes into the supply duct. I didn't get a humidifier, but I think I will get the steam type if I decide that I need a humidifier.