gas furnace - variable speed or not?

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Old 12-17-07, 12:33 PM
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gas furnace - variable speed or not?

Hi there,

I live in West Washington and have a 20-year old gas furnace that I would like to replace. Got conflicting recommendations from the technicians and would like to hear your advice. Thanks in advance! All suggestions and comments are much appreciated.

Situation:
- 3000 square feet, about 2100 on the first floor and 900 on the second (half of the house doesn't have a second floor)
- furnace on one side of the house, there is a long duct going to the other side
- 18 hot air registers, 3 cold air returns

Questions:
1. Variable speed or not?
Two contractors (Trane and Bryant) suggested variable speed but one (Rheem/Rudd) said he would not install a variable speed furnace due to the long ductwork. He said that it would be hard to get air to the far side of the house when the furnace is operating in the first stage; but once the near side of the house is warm enough the furnace will be turned off. So the other side would not be warm enough. Is this correct, or do you think it's just a sales pitch (I couldn't find a variable speed furnace on Rheem's website)? Is it possible that the Trane and Bryant contractor are simply trying to sell me a more expensive system (the Bryant contractor said that he doesn't install the single-speed system)?
On the other hand we are looking to sell the house in a year or two so a single-speed furnace may be a better investment. What do you think?

2. BTU rating
One contractor (Rheem) said since our ducts are big enough he would put in a 120,000 BTU unit while the other two recommended around 100,000 BTU. Is it better to go with a bigger one if the ducts can support it?

3. Base can installation
One contractor (Bryant) said that we need a 5-ton unit and he must install a base can for that. This raises the height of the whole system, and consequently some of the ducts need to be reconfigured. Another contractor (Rheem) will also install a 5-ton unit but says base can is not necessary (it's just one way to do it). Which one is correct regarding the base can (i.e. is it proper to install the 5-ton unit without a base can)?

Again, many thanks!
 
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Old 12-17-07, 07:16 PM
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Originally Posted by BillyZ View Post
- furnace on one side of the house, there is a long duct going to the other side
can it be moved more centered?

- 18 hot air registers, 3 cold air returns
What size are these returns?


he would not install a variable speed furnace due to the long ductwork.
Depends on if the ductwork is sized right for the air flow requirement.

He said that it would be hard to get air to the far side of the house when the furnace is operating in the first stage; but once the near side of the house is warm enough the furnace will be turned off.
If a two stage t-stat is used, it will have a longer run time, s the warm air will have a chance to get to the other end.
Also, the blower speed can be bumped up if needed.


On the other hand we are looking to sell the house in a year or two so a single-speed furnace may be a better investment. What do you think?
YOu could do that if you want... Reason for replacement?

One contractor (Rheem) said since our ducts are big enough he would put in a 120,000 BTU unit while the other two recommended around 100,000 BTU. Is it better to go with a bigger one if the ducts can support it?
How old is the house? This 90 or 80% furnace? Was a Manual-J (load Calc) been done?


One contractor (Bryant) said that we need a 5-ton unit and he must install a base can for that. This raises the height of the whole system, and consequently some of the ducts need to be reconfigured.
A system that is going to be moving 5 tons of air should have return on the bottom and side, or both sides. (Depends on the brad).. But for sure NOT only one side.
 
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Old 12-18-07, 09:33 AM
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Hi Jay,

Thanks for replying, and my apologies for not having those details in my first post.

- the furnace cannot be moved to the center
- the cold air returns are about 30in x 9 in (not precise but close)
- the Rheem contractor said that the ductwork is large enough to support a 120,000 BTU unit
- reason for replacement: 1) the unit is about 20 years old and not very efficient 2) in some rooms the air is not very warm
- the house was built around 86-87 and the contractors suggest the same BTU regardless of the efficiency (80 or higher)
- one contractor (Trane) did the heat loss calculation and came up with 100K, the Rheem contractor said that when the house was built a Manual J was done so he only had to go from the original size (about 90K), and the Bryant contractor didn't do the Manual J (so I suppose he also estimated from the original size of the furnace)

Thank you so much for the pointers on the t-stat and 5-ton returns!
 
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Old 12-18-07, 11:46 AM
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Originally Posted by BillyZ View Post
- the cold air returns are about 30in x 9 in (not precise but close)
That sure don't seem to be enough returns.... I would put more in if you can.


- reason for replacement: 1) the unit is about 20 years old and not very efficient 2) in some rooms the air is not very warm
What is the avg run time on a cold winter days, say in the teens? (Don't know where you are out of)

- the house was built around 86-87 and the contractors suggest the same BTU regardless of the efficiency (80 or higher)
Uhhh.. The better not go by that.. If we know for a fact that say, The calc came back at 90 to 100k, Then if you put in a 90%, you are going to need a 100k model, if you went with a 80%, you would then need a 120k.
 
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Old 12-18-07, 09:02 PM
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I also live in western Washington and I had my furnace replaced about two years ago. My house was also built in 1987 although it is one floor and about 1,550 square feet.

I had five or six bids and nobody offered to do a heat loss (manual J) calculation. I finally asked why and the answer I got was that unless I complained about the furnace not keeping the house comfortable they assumed that the current furnace was correctly sized.

This is the wrong approach.

If you have made ANY energy improvements such as more insulation or better windows you probably need a smaller furnace. Since furnaces are rated according to their BTU input, if you go with a higher efficiency unit you need a lower BTU rated unit.

A "quick and dirty" method of determining if your present furnace is oversized (it probably is) is to compare the "on" time to the "off" time during the coldest weather. If you are in the Puget Sound area then last year about this time is a good check. If your furnace was running less than about 80 to 90 percent of the time in the coldest weather then it is oversized.

Your return air ducts should have an aggregate area of 1-1/4 to 1-1/2 times the area of the supply ducts. Does your main supply duct "taper down" as it travels away from the furnace? Do you have "intermediate" ducts from the main that supply two or more individual ducts to the room registers? Do you have "balancing dampers" installed near where the individual ducts connect to the main (or intermediate) ducts? Is your ductwork insulated, either internally or externally?

I strongly recommend getting a two-stage furnace (or better, a modulating furnace) and the variable speed blower. You will need to balance the airflow to the various rooms regardless.

You may also need to relocate the thermostat (get a two-stage thermostat with the two-stage furnace) or possibly a thermostat that has remote sensors with an averaging function to properly maintain even temperatures. The two-stage furnace WILL help in maintaining even temperatures throughout the house.
 
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