What CFM in first stage

[mchild]

I have read on here that some two stage condensing units have a first stage capacity that is about 50% of the total and some higher at around 70%. Since not all condensing units split the capacity between the first and second stage at the same point, how does the air handler know what cfm to supply when on first stage?

I am interested in units that have the 50% split for somewhat lower operating cost while in the first stage, better dehumidification, and less over cooling during dehumidification. But, if the first stage airflow is based on a default split of 70% then it seems like it will not be nearly as effective.

[Shophound]

The answer to your question is likely manufacturer specific, since different OEM's of residential equipment likely have different staging strategies between them.

One way the air handler blower could shift speed is for the fan speed changeover to be concurrent with staging of the compressor. IOW when second stage is called for, both the compressor and the indoor blower kick into high speed. Or in the case of two compressor units, such as Trane's, the lower capacity compressor shuts off and the higher capacity compressor energizes, and the indoor blower ramps up to high speed.

If the overall goal is to have the system's capacity match the load as closely as possible at all times (a basic motivation behind staging strategies), then the inverter driven compressors, condenser fans, and blower motors are the ticket. For further sophistication, include discharge air and return air sensors for temperature and wet bulb that constantly monitor the air stream. Feed that into a controller that monitors all conditions and maintains blower speed, discharge air temperature, metering device, condenser fan, and compressor capacity at the right levels for the given heat load.

You get pretty darn close to that with the Nordyne 23 SEER system and the inverter driven multi-splits. The Nordyne can self monitor superheat and subcooling, among other things, even give these readings along with system pressures to the service tech.

As for more conventional staging strategies, your aim seems to be toward more optimal dehumidification. The best way to increase dehumidifying performance in a house is to reduce the overall latent load via infiltration. IOW, tighten up the house. This would not eliminate a desire to stage a system for dehumidifying, but it would make the chore much easier on the system, and lower operating costs.

If you've ever run a Manual J calculation, it's interesting to note the percentage of both sensible and latent heat gain is due to infiltration. It's not a small number, unless the house is exceptionally tight.

[mchild]

shophound,

Thanks for the response. I agree that the t-stat/controller is what will call for the second stage to come in and it would then signal for the fan to ramp up to that level. But my question deals with how the air handler knows what CFM to provide for the first stage when the CFM needed can be significantly different between condensing units.

As an example, in the case of Trane, while the 19i has two compressors for the two stages the 16i only has one with about a 70% split while the 19i has about a 50% split. The air handler only has DIP switches which are used to set the total size of the condenser, but there is nothing that allows for selection of what capacity the first stage is of the total. Same thing for Carrier, you can only select the total condenser size.

In my last home I had a Carrier system installed, which had a VS air handler. I watched as the installer set everything up. He was quite helpful in answering all of my questions and left for me all the installation manuals including for the controller. I can’t remember, in any of that material, there being a selection for the capacity of the first stage – only the total capacity.

[dash]

Infinity reads the model and even the serial of the units and selects the correct cfms for both stages ,for both the 50% and 70% first stage outdoor unit.

Can't sat for a Trane ,but I'd assume the dip switch setting determne it.

[dash]

Infinity reads the model and even the serial of the units and selects the correct cfms for both stages ,for both the 50% and 70% first stage outdoor unit.

Can't sat for a Trane ,but I'd assume the dip switch setting determne it,or select it on the fan board.

[mchild]

I'm not surprised that the Infinity system knows what fan CFM to put out based on knowing which condenser is outside. It's all the other systems that have two stage that I'm wondering about.

[dash]

I'm not surprised that the Infinity system knows what fan CFM to put out based on knowing which condenser is outside. It's all the other systems that have two stage that I'm wondering about.

Most probably require a combination of dip swith settings and pins or jumopers correctly selected on the fan board.That's what Carriers previous VS required.

[greggg1]

wouldnt the call from t/stat-y1 or y2 energize the proper controls on icm board, telling fan module which cfm is needed.

[dan sw fl]

As an example, in the case of Trane, while the 19i has two compressors for the two stages the 16i only has one with about a 70% split while the 19i has about a 50% split. The air handler only has DIP switches which are used to set the total size of the condenser, but there is nothing that allows for selection of what capacity the first stage is of the total.


With ~20 DIP switch settings and multiple size Variable Speed air handlers, E.S.P. adjustment with damper setting, zoning, the near infinite capability to customize should not be a concern.

[mchild]

wouldnt the call from t/stat-y1 or y2 energize the proper controls on icm board, telling fan module which cfm is needed.

I would agree that the t-stat call is what tells the fan module to run in the lower cfm for first stage. But, how does it know what outdoor unit is operating and whether that CFM is based on 50% or 70% of the total capicity of the condenser? I suspect that it is preprogramed into the board and is a set value and is probably a little low for the 70% and a little high for the 50%.