If we stay on topic, the issue is the shoulder months. Here in Minneapolis we experience the worst weather anywhere, summer and winter with moisture, in all it’s forms, ever present.

The new ECM fans typically draw 80 watts on low-fan mode. You must move air to condition it either actively or for sensor controls. You may use humidity, temperature, CO2 or some combination thereof to control IAQ.

In most of my hydronic based designed we use dedicated bath exhaust with push button 20 minute timers and minutes-per-hour exhaust to properly expel humid shower and cooking loads while maintaining a healthy home.

Each strategy is determined by climate and construction but using uncontrolled (cracks and holes) fresh air is too old-school to be seriously considered except in the mildest climates.

As for duct loss. With the new energy code it blower test, duct loss is a thing of the past.

I disagree vehemently! My family has had the good fortune to enjoy mild New England weather for the past couple weeks and windows are and have been wide open – no heating, no cooling, and no mechanical ventilation needed.

In Florida that strategy works for a month or more each spring and fall.

We should encourage our owners / clients to be mindful of outdoor temp and humidity and take advantage when appropriate.

The fact is, most CFIS systems are adjusted to a flow rate that’s close to the target ventilation rate. In that case, a cycle rate of 10 or 15 minutes per hour will seriously under-ventilate much of the time. Note that even during mid-winter, a properly sized furnace will only operate continuously during the coldest for part of the day. Same in mid-summer. Over the course of a year, the home receives far less than the prescribed ventilation rate.

Running the blower 24/7 would provide the correct ventilation, but that’s a stupid strategy for the reasons cited by Curt and John Proctor. If you live in a humid climate, your blower will re-evaporate moisture from the coil back into the airstream. This not only risks raising RH too high, but wastes the energy your system expended to condense the moisture in the first place. And it wastes a *lot* of energy to run the comparatively large central blower 24/7. Even an efficient ECM blower may consume on the order of 1,000 kWh extra per year if set to continous (depends on several factors). Variable speed ECM blowers typically operates at 50% of fan flow during off cycles. That would reduce the penalty (although not by half), but then you end up with the same issue as a cycled CFIS — either the CFIS will seriously over-ventilate during hottest and coldest hours, or it will seriously under-ventilate during the off-cycles. And if your furnace and/or A/C is multi-stage, this on-versus-off cycle variation can get even worse.

John, you didn’t say where you live. If you’re in a humid climate zone, see the above comments by Skye and myself for a much better supply-only ventilation strategy (install a separate SMALL, low-wattage supply fan and filter). If you live in a cold or dry climate zone, consider going with an exhaust only ventilation system. The best solution though would be a recovery ventilation system.

Finally, we can agree that we want to do all this at a reasonable installation and operating cost.

If you are in the house you need fresh air, all the time. This air must be “conditioned” unless, in the unlikely event, that the outdoor air is perfectly comfortable. Further the air brought into the house must displace air being exhausted.

Since we create excessive CO2 and moisture by breathing, bathing and cooking exhaust fans will be in order. Incidental exhaust loads such as showering are not a concern but an HRV or ERV integrated with a central heating/cooling system is the logical choice for exhausting stale air and delivering fresh air.

Should the “fresh” air not meet our IAQ standards for filtration, ambient temperature or humidity we have the perfect tool to condition it. The ERV transfers valuable conditioned air (temperature) and humidity (sensible) so that all is not lost but (recovered) or transferred from the exhaust air stream to the fresh air supply.

Depending on the operating conditions, climate and appliance about 50% of the energy in the conditioned air is captured and returned to the home before it gets outside.
I have had my old-school American Standard condensing furnace operating with “fan on” for over 10 years. Electric motors want to run. A fan will use more energy and wear out faster stopping and starting than they will in full-time operation. The new ECM motor are no exception except in the fact that they will last longer and use less energy do it.

As for fresh air. The ERV transfers sensible and latent heat. The latter is moisture not always addressed by a standard DX cooling system. More especially if the cooling system was not designed correctly. Even if this is the case, in certain climates and construction a whole house dehumidifier may be in order.

In any case the careful control of air movement through the envelope is essential to indoor air quality and occupant comfort. The use of an HRV or ERV dependent on climate is a useful tool to save energy and assure IAQ.

I insist that the water on the air handler coil be allowed to drain away, undisturbed by passing air, until the next call for cooling occurs. In round numbers, a modern high efficiency air handler coil has about 100 square feet of area, all soaking wet, per ton of installed system capacity. That’s a lotta water.

Other than that, I’m open to whatever ideas anyone has as to efficient, cost, and comfort-effective mechanical ventilation. I tend toward having two or more two speed Panasonic bath fans run in low speed speed all the time. It’s far from perfect but pretty good.

John, is the contractor
John, is the contractor installing a motorized damper and controller with the CFIS ventilation system? If so, it’s fine. My complaint in this article was about the simpler CFIS system that leaves those components out.