Variable speed?

[APBT]

Considering a Trane XV95 2 stage variable speed furnace with a Honeywell VisionPro stat. Installer claims that the variable speed feature looks at duct pressure and that the XV95 really is not a suitable choice for a 25 yr old house with ducting which may not be sealing perfectly at all points- thus affecting the duct pressure readings.
Another installer is offering the Rheem RGRK with the same stat. When asked about the RGFD series, he claims it's not worth the extra money for the modulating furnace. Opinions please.

[joepuwi]

I had a variable speed Carrier system installed last September and so far I'm quite happy with it.

I live in a 20 yr old 3 story townhouse. With my old system the 3rd floor was never cool enough in summer or too hot in winter. The way it works with the VS blower, as far as I can tell, is it starts slow then ramps up till it settles on a speed where you get good airflow. For cooling I close all the first floor vents and open all the 3rd floor vents. It runs at high speed to get the cold air up. For heating I open the first floor vents and close half the 3rd floor vents and it runs at a slower speed. I get more even temperature now throughout the floors.

I talked to different contractors and I find that some generally just don't like 2-stage VS. They won't mention it as an option and if you ask, they'll say it won't do anything for you. When I pressed one on it, he told me he didn't like it because they were harder to maintain. Personally, I think that they don't like it because they aren't familiar with the technology. As such, it's more troublesome for them to maintain if something goes wrong while you're in the warranty period.

My advice is, if the contractor doesn't recommend it then don't press him on it as he can easily put the blame on you if later on it doesn't heat or cool properly. Find another one who likes it.

[dash]

The variable speed indoor fan is well worth having,IMHO.

Leaking ducts might leak a little more,they should be fixed either way.

They are probably concerned about undersized ducts ,which can cause issues.Water leaks,noise ,motor failure,etc..

The VS is going to try to deliver the correct amount of air ,even with undersized ducts,a standard motor will just undeliver on air flow.Low air flow reduces equipment capacity,damages the equipment,doesn't keep you comfortable in the rooms farthest from the furnace,etc.,etc..

If you have undersized ducts and your comfort level is okay in most of the home,you have oversized equipment.

A load calc,Manual J from www.acca.com ,by your local guy,is a good thing to do first.

[APBT]

Thanks for the info. I've had 4 contractors here and NONE of them has offered to do a Manual J, or any other type of load calculation! Currently have a 20 yr old Carrier 394GAW024075, 75000 BTU unit in the 1600 sq feet home. 10 ducts, single zone. Where exactly in the ducting does the XV95 measure the pressure? Near the furnace? A fixed distance away from the furnace? A computed distance away? Thanks for your help.

[beenthere]

The blower motor module senses the amp draw of the motor.(simplified explanation)
Their is no seperate sensor.

[Airmechanical]

The blower motor module senses the amp draw of the motor.(simplified explanation)
Their is no seperate sensor.

(unsimplified) the motor module controller monitors the cfm's (counts the rpm's) and then

makes the adjustments to supply the cfm which was selected at the dip switches or control panel!

an even more unsimplified explanation can probably be given by mark b.



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