Thermostat for Variable-Speed Blower

[t527ed]

not real familiar with the older v drive systems but what you are describing pretty much sounds like the newer ones.

we can now pick
low speed heat cfm's
high speed heat cfm's
low spped cool cfm's
high speed cool cfm's
fan on cfm's

the blower will ramp up and down to and from the selected speeds depending on what ramping profile is selected by the installer.

blower will vary the actual speed to deliver the cfm's selected to allow for a filter getting dirty.

its not really an infinately variable motor.

3 main reasons we use the iaq thermostats are #1 we only need 3 wires between stat and module mounted at furnace, means we don't have to run new wire. #2 the relays for the iaq are in the module instead of the stat so the customer will never complain about the stat "clicking" every time it goes on and off.#3 i have always liked honeywell stats over lennox's "who's gonna make it this time" stats. when they do use rebranded honeywell stats thats what we use.

[wtw]

...the blower will ramp up and down to and from the selected speeds depending on what ramping profile is selected by the installer. blower will vary the actual speed to deliver the cfm's selected to allow for a filter getting dirty. its not really an infinately variable motor.

3 main reasons we use the iaq thermostats are #1 we only need 3 wires between stat and module mounted at furnace, means we don't have to run new wire. #2 the relays for the iaq are in the module instead of the stat so the customer will never complain about the stat "clicking" every time it goes on and off...

Thanks for the clarification...

So, in summary, it seems that the motor itself is variable-speed, but the thermostat does not control that speed directly. I wonder if other manufacturers "variable-speed" blowers operate in a similar fashion, or if their speed is under the direct control of the thermostat.

[beenthere]

Most VS motors operate that way.
With one or 2 exceptions.

[wtw]

Most VS motors operate that way.
With one or 2 exceptions.

Do you mean that the speed of most VS motors is under direct control of the thermostat/humidistat, or that their speed is not under direct control of the thermostat/humidistat?

What are the one or two exceptions?

[beenthere]

The thermostat can only slow it down through teh dehum terminal, what ever that brand allows in percent. The stat has no other blower speed control.

Carrier Infinity control, on the Infinity line.
And I think Lennox has a model furnace and special stat that has more blower control.

[wtw]

The thermostat can only slow it down through teh dehum terminal, what ever that brand allows in percent. The stat has no other blower speed control.

Carrier Infinity control, on the Infinity line.
And I think Lennox has a model furnace and special stat that has more blower control.

So (other than the two cases you mentioned) the VisionPro IAQ dehumidifies by slowing the blower down to whatever speed has been field-selected on the control module for that blower motor?

[bmathews]

Since there seems to be some confusion over what you have exactly. I would call a service company out to take a look at what you have. It sounds as if you have a fixed speed that is adjusted by moving motor speed taps to where you want the speed to be. If so, purchasing a thermostat to dehumidify will be a complete waste of money, it won't do anything except cost more money. If you have a true variable speed, which runs at a range of speeds and has a control module piggy backed on the actual motor, then have your service company check into what you need to make it work properly. I will say at 14 years old, I would also make sure it has the capability to run your new system. You could develop a coil leak or failed motor which fixing would be fairly expensive and a waste of money.

[a0128958]

Actually, if it turns out to be a PSC blower, then this would make it straight forward for a technician to install a relay to switch between a normal speed and a slower speed.

I remember this was how my Lennox 2-speed unit, circa-1992, was installed. The installer added a relay to switch the PSC blower from one speed for 1st stage cooling to a faster speed for 2nd stage cooling.

I think it's the variable-speed blowers that don't already have a set of external contacts, or don't support a field-installable module, for slower speed that are the hardest to adapt to the IAQ's dehumid 'port' - in fact, maybe just not possible.

Best regards,

Bill

[beenthere]

On most VS blowers. Fan on speed is fixed at 50% of the selected high cool speed.

[paul42]

Are most modern "variable-speed" blower motors continuously variable, and if so, how is this accomplished?

Yes and no. Through jumpers and connections to the control board, a certain CFM is requested. The control board varies the speed of the blower motor attempting to maintain the requested CFM. If the duct work isn't too far off or the filter too clogged, it will supply the requested CFM.

Common variable speed blower controls have at least 12 fixed CFM settings available.

So, the blower motor is continuosly variable, but there are discrete CFM steps that can be set through the controller.

[paul42]

For more information - embedded in a sales pitch

http://hcsupplyco.com/Product%20Pres...pt#377,1,Slide 1