My apartment's mid-90s heating system consists of a gas-fired hot water heater feeding a First Co TAQ1824 air handler. The air handler has a fan and blows hot air through air ducts. The system is described in detail in my previous post.
I recently swapped out the old Honeywell basic digital thermostat for a Nest. With the Honeywell, anytime the system was heating or cooling, the fan ran full blast and you could hear it loud and clear and feel a strong rush of air from the vents throughout the apartment, including cool air in the first 1-2 minutes it takes the air handler radiator to heat up.
With the Nest, when it calls for heat, the air handler's fan runs at a different speed – specifically, VERY SLOW. Like, inaudible unless you're right by the air handler, and you can barely feel air coming out of the vents.
After some research, I learned that typically, for gas fired furnaces, the thermostat only calls for heat (connects +24V to W), leaving it up to the furnace to decide when to turn on the fan. For electric furnaces, the thermostat calls Heat and Fan (W+G). After reconfiguring the Nest for electric heat, it now behaves like the Honeywell: anytime it's trying to heat, the fan is going full blast. I also found that the Honeywell could be configured to run in 'gas' mode the same way.
When the Nest was in 'gas' mode and the fan was running slowly, it did still heat the apartment in about the same amount of time. There was less air blowing around (more pleasant, really), but the air slowly coming out of the vents was much hotter (spending more time in the radiator), so it kind of balanced out.
Now I've got a handful of questions about this:
- Does it makes sense that the First Co air handler has two fan speeds: one for gently pushing hotter air out, and one for more forcibly circulating air? I thought multi-speed fans in HVACs was a recent higher-end feature.
- If #1 doesn't make sense, what explain the slow feed of hot air when the Nest is only connecting W? Is there some electrical leakage path that makes the fan spin slightly? If that's the case, I'm assuming it's unsafe to run in 'gas' mode.
- Ultimately, how should I have this system set up? As 'gas' so the air handler determines what speed to run the fan, i.e. very slow as described above? Or should I configure it as electric so it's always running the fan full blast? I don't have any particular reason to think the Honeywell's previous setup should be authoritative.
Note that although the hot water is gas-fired, the air handler does not use gas and doesn't really need time to heat up. It's always got hot water on tap right next to it.
Also I have not been able to find an owner's manual for the First Co air handler.
Thanks.
Best Answer
I'm going to try to explain this as simply as possible. Most air handling units having several speeds available on the motor usually at least 4. Normally the board will have at least two speeds, one for heat and one for cool. In the old days the lower heat speed was to eliminate the possibility of flue gas condensation as the units, be it oil or gas were not designed to handle the slightly acidic condensate. Nowadays most units are high efficiency condensing units. Everything is still designed around the lower fan speed, and that's a good thing. It's quieter, allows for a longer and thus more efficient heat cycle. All the heat loss calculations, duct sizes, unit capacities and fan speeds are set up with this lower fan speed in mind. Electric resistive elements generate alot of heat for the resistive metal handle and need the higher fan speed to remove the heat before burning themselves up.
Air-Conditioning has a very specific cfm(cubic feet per minute) per ton of cooling. 400 cfm per ton of air across the evaporator. So if you have a 2 ton air-conditioner you need 800cfm.
The thermostat on cooling energizes R(24v) to Y(compressor) and G (fan). On heat with gas selected, as you know energizes R to W(heat).
In your case, running a hydronic coil it's a balance between comfortable noise level, capacity and efficiency. Being connected to a water heater and not a boiler, return temperature is irrelevant. So that rules out efficiency concerns. If it was connected to a boiler then the lowest return temperature possible is ideal. So that leaves comfort vs capacity. At lower fan speeds capacity is diminished. As long as your house is comfortable at the lowest fan speed then that's the way to go.
On modern units there are sometimes dozens of available speeds. By setting dip switches you can select the desired speed in cfm for the application you can even set a baseline continuous fan at say 200 cfm. Then have a separate speed when the board senses power on G(fan switch on the stat set to ON) say 400 cfm. Then for your 2 ton air-conditioner when the board senses power on Y it goes to 800 cfm. Units with this capability are often modulating and the heat speed is set by an algorithm in conjunction with the burner speed. It will start on a very low setting at the start of the heat cycle and ramp up over time or based on indoor temperature readings.
Conclusion
Yes it makes sense
Gas is a good setting
Run the heat as low as possible to maintain a satisfactory level of comfort balanced with noise. And don't touch the air-conditioning speed on the board.