When I decided to tackle my forced air, oil furnace problems (see my "Replace heat exchanger or entire furnace? ", thread) I got to thinking about my HVAC system as a means to not only condition the air inside my home, but to enhance the air quality if I could. So I did three enhancements that I thought were a good idea, and I'm considering a forth that I'd welcome comments on. 1. Humidifier thermostat. For years I put up with the reservoir pump in the excess water collection reservoir for the un-evaporated water for my AprilAire humidifier unit, kicking on after almost every heating cycle. I tried valving-down the stop-cock I had, but that required a lot of fiddling and experimentation at the beginning of every heating season, since I'd normally close the water stop-cock at the end of every season. So I bought an electronic thermostatic control unit off of eBay for about $12, and wired it in as an interupt on the 24VAC line that energized the water control solenoid for the AprilAire. Normally this solenoid energizes upon the flag switch in the return duct, sensing air flow, and as long as the humidistat is calling for greater humidity. But this setup started water flowing before the air was hot enough to evaporate the water, and kept water running into the AprilAire much longer than it could be evaporated, since, at the end of the heating cycle, the temperature would have to drop to about 90F before the blower turned off. So I put the electronic thermostatic switch probe in the air handler, just above the heat-exchanger, and just before the A-coil in the air path. I have the thermostat set to close the water valve solenoid circuit above 124F, so the furnace usually takes about a minute and a half to get the air at the probe to that temperature, and it takes about the same amount of time when the burner has cut-out, for the temperature to drop below 125F; this leaves the furnace another two minutes to move air through the AprilAire unit, drying it out, until the blower cuts-out at about 90F. This upgrade has kept the over-flow reservoir pump motor, running to about once a week. One other thing I had done before installing the electronic thermostat, to try to limit the running of the reservoir pump was to install a check-valve on the outlet of the pump, to prevent water flowing back into the reservoir from the output tube that went to an outside drain-pipe, that was 6' above the pump. That upgrade helped, but it wasn't enough to keep the pump from running every heating cycle. 2. UV Lamps in the air handler. I bought a two UV lamp setup and installed it in my return duct, above the filter. While what I've read seems to indicate that the split second a bacterium may be exposed to the light, won't be enough to kill it, any organic action that could occur on the filter, should be severely limited, since the UV light is on it constantly. The manufacturer recommends the unit be on 24/7, and replacing the bulbs annually. 3. Intermittent air movement timing cycle. When thinking about how many hours the HVAC system is not running for temperature purposes, I realized their were times over-night, when we were sleeping in bedrooms with our doors shut, that there was no air exchanging or replacement going on. So I built a circuit using a 24 hour lamp-timer that have 15 minute on-off segmentation, and a DPDT relay. The blower motor is 3 speeds, it was originally installed with the black (high speed) wire, wired to the mechanical thermostatic control on the heat exchanger. I decided to make the Red wire (low speed), the wire that would be energized by the lamp timer. But to do this I had to make sure that there was no way both wires could ever be energized at the same time; to do this I installed a 120VAC coil, DPDT relay. So when the relay is resting (no power to relay coil) the hot side of the lamp timer outlet goes to the closed contacts on one of the relay's throws, and when the normal blower motor line from the mechanical thermostat gets energized, that 120VAC sits on the other throw of the relay, and is used to 1) energize the relay coil, and 2) send current through the contacts now closed by the energized coil (again, on the opposite throw from the low speed throw). This arrangement keeps the voltage from being applied to both the low-speed, and high speed motor coils, at the same time. The relay is inside the plenum, near the blower motor, and the relay coil return, is spliced into the neutral side of the blower motor wire. I have the timer set during evening hours to turn on once an hour (on 15 minute, off 45 minutes. If I could find a timer with higher resolution, I'd probably set it for "on" for 5 only minutes every 20 minutes or so, but so be it, this is simple and easily replaced when the timer goes. The switch next to the timer is just a heavy duty two-way switch that I wired so that I could select where the current goes (high speed or medium speed blower motor wires) when the relay is energized; I did this because the new blower motor had a high RPM rating than the old motor, and while it wasn't more than 10% higher, I wanted the option to switch the blower from high to medium if I had a good reason...so far I don't have a good reason. Finally, I'm thinking about designing a passive, "Venturi-principle", fresh air intake, with a counter current, heat exchanger, that would be installed on the return side of the duct-work, so as to infuse fresh air into the house, while recovering some of the conditioned air temperature from the air being "pushed-out". I have some design work to do, as I want the whole operation to be completely passive, but I think it is important to get fresh air into the living space, since the house is fairly tight, with modern windows and doors, and just a 3/4" gap under the basement/garage door, that I don't want to restrict any further, since the oil burner does need to pull air from somewhere when running. Comments?