Got called in recently to address thermostat and comfort issues for a local builder we work with. In reconfiguring the thermostat (Ecobee Smart Si), I noticed that it was wired up as a single stage for heating, although the furnace (Carrier 59TP6) is a two-stage model - that is, furnace staging decisions are made at the furnace control board, not by the thermostat. I've tried contacting Carrier, but haven't found anyone who can explain the furnace control board logic w/ respect to staging.
In examining several other recently finished houses for this builder, I've seen the same thermostat wiring setup (had never thought to check it prior to the last month). I'd expect better comfort and efficiency when furnace staging was controlled by the thermostat, particularly a "smart" thermostat. Just wondering if anyone has any thoughts on this before I talk with the builder & HVAC subs about this.
I'm not sure about the Ecobee Smart Si (I couldn't immediately find the install manual) but this FAQ (http://bit.ly/2Cf2UsR) describes Ecobee's staging logic -- it's selectable based on either elapsed time or temperature 'droop'.
Most modern thermostats use a PID algorithm for staging -- essentially this looks at the rate and direction of the temperature change. This is especially important for supplemental heat staging on a heat pump, not so much for a furnace. With a furnace, premature upstaging could have a slight to moderate impact on comfort (depending on how badly the furnace is oversized) but will have little impact on efficiency.
I'm not familiar with that furnace's internal staging logic. You should be able to find the install manual via Google search. But since it doesn't know the zone temperature setpoint, it's almost certainly based on time, and may be selectable.
Without the option of PID staging, I would prefer 1F droop over a fixed time delay. If you confirm the Ecobee Si works as described in the FAQ, it could easily be wired to stage the furnace based on a 1F temperature difference.
Is the A/C single stage?
David's comment is spot on. I would always prefer thermostat staging control over furnace staging control. (I am assuming the thermostat is making and breaking the control call for heating - not communicating the temperature back to the furnace controls. This may be a bad assumption with communicating system. It is a good assumption with a standard red =24V / yellow = cooling / green =blower / white = 1st stage heat)
Unless the furnace controls know the temperature and set point, the only thing it can use to bring on second stage heat is run time of 1 stage heat. So it may turn on second stage just as the set point is reached (causing larger temperature swing) or wait a long time in first stage before bringing on second stage (delaying reaching set point). Either way, the customer is not getting the comfort they have paid extra for.
Thanks for the quick feedback, David. I did find the furnace install manual earlier; there's no detail (that I was able to find) on staging logic or adjusting staging timing. Glad to understand the typical thermostat staging in a bit more detail - I'll try to track down the manual for this model.
The A/C is single stage, so no accompanying concerns about humidity management w/ respect to staging.
I hate to be the one to bring this up, but two stage furnaces are quite capable of having lower installed efficiency than single stage furnaces with the same max input. This comes from two sources: 1) the fuel air mixture on low fire often has too much air. 2) the increased resident time of the conditioned air in the ducts increases duct losses.
Similar issues exist for two speed ACs
Thanks, John. The HVAC installers have actually responded to incentives and moved ducts within the building envelope on the vast majority of the furnaces we see, so losses to unconditioned space are less of a concern for us. Thanks for bringing up the issue of fuel air mixture.
Set up for 1st stage to run at highest eff. With my set up will run on 1st stage 93% of time. Use a high end flue gas meter and set up for max eff.
@Tray, the staging logic is described on pg 85 of the furnace installation manual. In particular, here's what it says:
"This furnace can operate as a two--stage furnace with a single-stage thermostat because the furnace control CPU includes a programmed adaptive sequence of controlled operation, which selects low-heat or high-heat operation. This selection is based upon the stored history of the length of previous gas-heating periods of the single--stage thermostat.
"The furnace will start up in either low-- or high--heat. If the furnace starts up in low--heat, the control CPU determines the low--heat on--time (from 0 to 16 minutes) which is permitted before switching to high-heat."
This is better than straight timing option in the Ecobee. I'm not sure if it's better than setting a 1F degree staging temperature differential (droop). At least with the latter, the homeowner can get their mind around what it's supposed to do.
As a side note, I find that most furnaces are seriously oversized. If you suspect that might be the case, you could recommend to the homeowner to disable the high stage (SW1, position 2 ON, W2 not connected). If low stage can handle the load, great. If not, SW1-2 simply needs to be flipped back to OFF to enable the furnace staging logic (or, alternatively, connect Ecobee W2 to furnace W2 and set thermostat for 2-stage ops, leaving SW1-2 ON).
If ducts are in unconditioned space, I agree with John that low stage ops imposes an efficiency hit (low velocity in a duct system sized for cooling capacity increases conducted losses). Operating at high stage becomes a trade-off between efficiency and comfort. On the other hand, if ducts are in conditioned space, it's simple enough to have the tech verify optimal mixture in low stage, as suggested by Eric. Have your cake and eat it, too.