Low Speed AC Fan Dehumidification - Honeywell Prestige 2.0 Thermostat and Evergreen IM ECM Variable Speed Fan Motor

Now that summer is just about here - I've been working on fine tuning my summer dehumidification as part of my indoor air quality project. One of the issues that I'm working on resolving has been a faint musty odour in the house particularly in the summertime. Last summer I was dealing with 60% relative humidity (RH) levels in the house - even with the air conditioning running, and a small 18 litre/day humidifier running in the basement.
Honeywell Prestige 2.0 IAQ Thermostat - With low speed fan AC dehumidification - and the RH right at the setpoint - 45%
I'm running a 4 ton thermopump with a large Lennox fan coil unit in the basement. The system heats well in the winter, cools well in the summer, but the single speed 3/4 hp blower was consuming a fair bit of energy running 24/7 so that I could keep the heat recovery ventilator (HRV) to try to deal with the musty odour. I've done a couple significant upgrades to the system since last summer; i) upgrading the 3/4 hp split capacitor single speed blower motor to an electronically commutated (ECM) permanent magnet varable speed Evergreern IM blower motor - which is much more efficient at full speed, and magnitudes more efficient when running in low speed circulation mode; and ii) installing a Honeywell Prestige 2.0 IAQ Thermostat and Equipment Interface Module (EIM) to manage all aspects of HVAC control - especially to control humidification and dehumidification in conjunction with the heating and cooling system.

The Honeywell Prestige 2.0 IAQ thermostat has a control option for dehumidification using the low speed fan option of an air handler / air conditioner with a multiple or variable speed motor. There are three user defined outputs on the EIM that can be set up and customized using the configuration tools on the Prestige 2.0 IAQ thermostat. One of these outputs controls my HRV. A second output controls my new Honeywell Truease Bypass Humidifier and Damper, and the third output was unused. Since my humidity levels were running a bit better than last summer, but still higher than I wanted (52 to 55% RH) I decided to try configuring the third output to slow the blower speed down when air conditioning - Low Speed AC Fan Dehumidification - as a way of getting the humidity level indoors between 45 and 50% RH. The theory behind low speed AC fan dehumidification is pretty simple - by slowing the airspeed across the evaporator coil - this allows the evaporator coil to run a little bit colder, increasing the temperature differential of the return air and the evaporator coil, and improving the quantity of condensate on the evaporator coil.

So - the Evergreen IM motor uses speed "taps" that sense control voltage (24VAC) for the various HVAC functions - such as 1st, 2nd, 3rd stage heating / cooling, emergency heating, etc. and allows you to configure the motor to run different speed ranges for different heating and cooling functions to optimize the performance of your fan coil / thermopump combination. My configuration is described at the followling link - but simply, I had thermopump heating or cooling running at high blower speed, electric backup heat running at medium high speed, and HRV / circulation running at low blower speed. The control connection is simple - the Evergreen IM high speed "tap" which is the yellow control wire is connected to the first stage thermopump compressor control wire ("Y") from the thermostat (or EIM with the Prestige 2.0 IAQ). Since the Evergreen IM speed always defaults to low speed whenever it doesn't receive a higher speed control signal from any of its speed taps - all that is required to switch the fan to low speed when the thermopump compressor is running is to interrupt the speed tap on the Evergreen IM motor connected to the "Y" control wire. I thought I would have to install a small control relay to interrupt the speed tap, but the Honeywell EIM user outputs for low speed fan dehumidification is software configurable as a normally open or a normally closed relay. So - run the speed trap through the EIM user input - configure the relay as normally closed - and when the thermostat calls for dehumidification - it will interrupt the speed tap on the first stage compressor and force the fan to low speed operation. Since I already had a pair of unused wires running between my fan coil control box and the EIM - I didn't have to run any new wire, just make the user output connections at the EIM, and wire the pair in between the Evergreen IM speed tap, and the "Y" connections on my furnace. Simple.

One of the nice features of the Honeywell Prestige is the Equipment Interface Module (EIM) - Instead of all connections being made at the Thermostat in the living area of the home - all the connections are made at the EIM installed at the furnace or air handling equipment - and the Thermostat only requires power - control is all wireless between the thermostat and the EIM. 


So - how does it work? In a word - excellent. It took about a day for the average RH to drop from the 55% range to 49 / 50%, and then after the second day it's been running at my setpoint - 45% - during the day (when there is enough solar energy hitting the house that the air conditioning is running during the day). At night, the RH will drift up towards 49% because the outdoor temperatures are just a bit higher than the indoor temperatures, but this will improve as the summer gets hotter in the evenings. I shouldn't have any trouble now keeping my indoor humidity level below 50%, and I didn't have to resort to purchasing a larger dehumidifier, or a whole house dehumidifier.

I have to say that I still really appreciate the Evergreen IM variable speed blower motor - the HVAC system in the house runs silently most of the time now, and I still really like the Honeywell Prestige 2.0 IAQ thermostat - I like the display and integrated humidity control, I like that it controls my HRV and I don't need a separate, uncoordinated HRV control on the wall, and I like the continuous outdoor temperature and humidity display, along with the indoor temperature and humidity display. I also appreciate the ability to monitor and change the thermostat settings from anywhere - inside or away from the home,  using my smartphone and the Honeywell Total Connect Comfort app. (I still intent to write a post about this app.)

Now that the house has been below 50% RH for a few days since making this upgrade - I've noticed a reduction in the musty odour. It's not completely gone yet - we'll see how this evolves, but I'm thinking of adding a UV lamp to the ductwork to further improve the indoor air quality.

Sources and Links

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