FanCycler.com - Frequently Asked Questions

Q. I have a two story house with the heating and cooling thermostat on the first level. The temperature in upstairs rooms is inconsistent with the downstairs and we not comfortable throughout the house. Can the FanCycler^® help me with that problem?

A. Yes. The FanCycler will periodically operate the central system fan to re-average the indoor environmental conditions (temperature, humidity, air quality) throughout the zone it serves. In your case, that means that the upstairs air will mix with the downstairs air to smooth out variations and give the thermostat better feedback on the true average temperature of the house. The same holds true for single story houses with open or closed rooms that are not near the thermostat.

If you are still unsure, a good test you can try is to turn your central blower from AUTO to ON for awhile. If you are satisfied with continuous blower operation after a day or two, then the FanCycler will give you the same performance at reduced operating cost. You can adjust the FanCycler settings to optimize for your situation (i.e. find the best Fan OFF and Fan ON times for you).

Q. Some rooms in our house can be very uncomfortable depending on where the sun is, can the FanCycler solve that problem?

A. In most cases, yes. Similar to the question above, the FanCycler will smooth out temperature variations from room to room by periodically mixing air from all parts of the house. The FanCycler’s patented fan recycling strategy operates the central blower if it has been inactive for a period of time. This is effective in making rooms throughout the house more comfortable whether the discomfort be due to variations in solar heat gains as a function of time-of-day, or due to door closure, distance from the thermostat, inadequate insulation, or poor duct design. Of course, there are limitations, if the insulation is severely inadequate, or worse, the duct design doesn’t allow much air flow to the uncomfortable space, then the effectiveness will be diminished.

Q. My house has two central air handlers. Do I need to use two FanCyclers or can I make one FanCycler operate both systems?

A. If you want fan recycling capability on both units, then you need two FanCyclers. The control wiring for each air handler unit needs to be isolated from the other.

Q. I am using central-fan-integrated ventilation with the FanCycler for my house that has two air handlers. Should I install an outside air duct to the return of both systems or just one?

A. It would be best to install a smaller outside air duct to the return of both air handlers. Splitting the heating and cooling load for ventilation air between the two systems will give better performance and will give better distribution of ventilation air throughout the house. If you have to choose only one system, choose the one that serves the bedrooms since that is where people will be for the longest time.

Q. I live in a humid cooling dominated climate. My HVAC contractor tells me that drawing fresh air to the return side off my cooling system blower will be too hard on my cooling system. Is that true?

A. No, as long as the following is true. The air distribution system (blower and ducts) should either be inside the conditioned space or they should be well sealed with mastic or foil tape so that the air leakage from the ducts to outside is less than 5% of the air handler flow rate. The house should also be constructed to reduce the random and unknown quantity of air leakage from outdoors, while allowing the mechanical ventilation system to provide and distribute a known amount of fresh air, from a known source, on a regular basis.

Q. I live in a cold climate. My heating equipment supplier has raised a concern about cold outside air affecting my gas furnace. Can you comment on that?

A. Furnace manufacturers put a lower limit on the return air temperature that their gas furnaces are exposed to. Based on a survey of manufacturers, summarized in Table 1, the lower limit is between 50oF and 55oF for intermittent operation and 60oF for continuous blower operation. This means that when cold outside air is mixed with recirculated house air, the resulting mixed return air temperature should not be less than the minimum specified by the furnace manufacturer. The mixed return air temperature is calculated as the fraction of outside air times the outside air temperature plus the fraction of inside air times the inside air temperature. Figure 1 shows the results of that calculation for a range of outside air temperatures and outside air fractions. Referring to Figure 1, for an outside air fraction of 15% and an outside air temperature of -30oF the mixed return air temperature is 55oF. From that, you can see that the outdoor temperature conditions have to be pretty severe before this becomes a concern. I recommend outside air fractions as shown in Table 2.

Table 1. Summary of survey of furnace manufacturers regarding minimum return air temperature

Figure 1. Mixed return air temperature for a range of outside air temperatures and outside air fractions.

Q. Under what circumstances would fan cycling be used without motorized damper cycling?

A. Fan cycling without motorized damper cycling would be used in the following cases:

Q. How much can I expect to pay for electricity to operate my central fan with the fan cycler controls for ventilation and whole-house mixing?

A. As a reality check, based on my monitoring in many U.S. climate regions, it has mostly worked out to about half the year needing fan cycling. At 33% duty cycle, that means that the fan operation charged to ventilation and whole-house mixing would be about 15% to 20% of the total hours in year (0.5/3=0.17). That's about 1500 hours per year, which, for a 500 W fan, is 750 kWh/yr, or $75/yr at $0.10/kWh. If an electronically commutated motor (ECM) is used with the blower instead of the more common permanent split capacitor motor, 1/2 to 2/3 less energy consumption can be expected.

Results came out in the same range in a 2001 study conducted by the National Association of Home Builders Research Center, entitled, "Field Investigation of Mechanical Ventilation Strategies in Residential Construction," prepared for the U.S. Environmental Protection Agency.

Q. I have a new home in Waco, Texas. I am in search of an Energy Recovery Ventilator (ERV) to install that will have a reasonable acquisition cost and service life. Can you send me some guidance?

A. There are no inexpensive ERV's that I know of--mostly in the range of $750 contractor price to $1200 retail price. Unless you need a lot of outside air (more than ASHRAE Standard 62.2 requires), the energy savings of an ERV will not pay for the more costly equipment for a long time (10+ years). Neither will an ERV provide year-around humidity control because it cannot dehumidify house air, it can only lessen the incoming load. You also have to consider the cost of ducting to get distribution of the ventilation air (at least $75 for each intake or outlet in new construction). If you still want an ERV, there are number of fine units on the market. Venmar makes good units, and Shelter Supply in Minnesota is one good place to buy them.

I would recommend a less expensive but very effective and practical system for you. We call it central-fan-integrated supply ventilation. It involves a 6" outside air duct connected to the return side of your air handling unit with a controller that makes sure the fan operates enough if it already hasn't, and makes sure that a damper closes off the outside air if the fan operates longer than needed. For more information, see the Products page.

If you want to control humidity year-around, even when it is not hot outside and the thermostat is not calling for cooling, you should add a dehumidifier separate from your cooling system. You can see a number of successful configurations by exploring www.buildingscience.com. Aprilaire sells an Energy Star® dehumidifier package (model 1700) with the ventilation controller already built in. It should cost about $1000 for the equipment including a motorized outside air damper, which is no more than the ERV and a far better system for your climate. See the Aprilaire Dehumidifier.

Q. My preferred ventilation method is an outside air duct to the central system return without any extra fan cycling. Can I limit the outside air damper open time without having to have fan cycling also?

A. To paraphrase, it seems that you want central-fan-integrated supply ventilation only during normal calls for cooling and heating, and you want to limit over-ventilation during either long equipment runtimes (peak conditions or recovery from setup/setback) or constant fan selection by the occupant. Both the AirCycler^® FR-V and the SCI ERV24 thermostat can be set to cycle the motorized outside air damper to limit open time without initiating any fan cycling. The FAN ON time would be set to zero, and the VENT ON/DAMPER OPEN and VENT OFF/DAMPER CLOSED times would be set to some limiting values that suited you. For example, you can set the controls so that the damper would be open for a maximum of 20 minutes, then be closed for at least 40 minutes.

Q. I have a 10,000 square foot house in northern Illinois. We built it with 2x6 framing sheathed with 1” rigid foam board, and all wall and ceiling cavities are sprayed full with Icynene®. The basement walls have at least 6”of sprayed foam on the interior, and we installed a full-height insulation/drainage/waterproofing system on the exterior. The house has been tested with a blower door and found to be exceptionally airtight. There are five furnaces. I need a better way of being sure how much ventilation air is needed. What are your thoughts?

A. Sounds like a great house! Here's my advice on "enough ventilation." We think the house should be delivered capable of meeting the ASHRAE 62.2-2003 Standard. That means that you should have ventilation capacity capable of at least (7.5)(number of bedrooms+1) + (0.01)(conditioned floor area). Given that the house has 10,000 ft² floor area, and assuming it has five bedrooms, the ventilation requirement would be (7.5)(6) + (0.01)(10,000) = 45 + 100 = 145 cfm. That 145 cfm would be the continuous rate, but you can do it all with an intermittent system as long as you factor in the duty cycle rate.

Having two 80 cfm, low sone bathroom exhaust fans with a standard wall switch would meet the 62.2 Standard all by itself. Technically, the switch for those fans should be marked "Ventilation." The occupants could run them all of the time, part of the time, or none of the time, and the Standard would still be met.

You might be thinking, how would two bathroom exhaust fans running somewhere in a 10,000 square foot house provide any confidence of ventilation air being distributed throughout the house? It doesn't. That is why we believe the Standard should have requirements for ventilation air distribution, which it may, someday. That is also why we always add outside air to the central system return and use fan cycling to distribute ventilation air and provide whole-house mixing -- smoothing out differing air conditions throughout the house.

So what would I recommend for your house? Without having seen the plans, I would add 50 cfm of outside air to at least 4 of the 5 air handlers, via a 6" outside air duct to each, and set the individual fan cycling controls to make sure the air handlers run 20 minutes per hour. I would also make sure that enough bathroom exhaust fans, summing to 145 cfm, were 1.0 sone or less to meet the 62.2 Standard.

Q. How do you test to know or estimate how much outside air (OA) is coming into a home with a Central Fan Integrated Supply (CFIS) ventilation system?

While this level of testing accuracy can be laborious, after you have performed enough tests of this nature to establish a good relationship between duct pressure measurements and duct air flow, and you have gained confidence over your range of systems and applications, you can simply take pressure measurements and correlate them to air flow based on the chart below and/or your own field experience.

		Flow rate in outside air duct (cfm) (with 25 ft of flex and 6" wall cap)
OA duct pressure (Pa)	OA duct pressure (inch wc)	Flex Duct Diameter (Inch)
OA duct pressure (Pa)	OA duct pressure (inch wc)	5	6	7	8	9
-5.0	-0.0201	32	37	53	63	66
-7.5	-0.0301	39	45	65	76	80
-10.0	-0.0402	45	52	75	87	91
-12.5	-0.0502	50	59	83	97	101
015.0	-0.0603	55	64	91	105	110
-17.5	-0.0703	59	69	99	113	119
-20.0	-0.0804	63	74	105	121	126
-22.5	-0.0904	67	79	112	127	133
-25.0	-0.1005	71	83	118	134	140
-27.5	-0.1105	74	87	123	140	146
-30.0	-0.1206	78	91	129	146	152
-35.0	-0.1407	84	98	139	157	164
-40.0	-0.1607	89	105	149	167	174

Note that these are all tests that an HVAC technician would perform. FanCycler.com recommends a testing regimen similar to the one used by the EPA Energy Star® program: test all of the first models of any product line, and then test at least every 1 in 7 homes after that.

Q. We have considered using central-fan-integrated supply (CFIS) ventilation to add fresh air to the homes we build. We have heard about a duct mold problem with ventilation into the central return, but that was with a different system. I am wondering what risk there is for mold in the ducts using the CFIS system? My location is in a mixed-humid climate.

A. By the fundamental principles, CFIS ventilation cannot be a cause of mold in the ducts because the high percentage of conditioned air from indoors (say 90%) mixed with the low percentage of outdoor air (say 10%) eliminates that problem. We have never heard of mold in ducts caused by CFIS ventilation after over 100,000 installations that we have been connected with over the last ten years (1996 to 2006). These were in all U.S. climates including mixed-humid.

My guess is that the system you heard about was injection of outside air into the central return by a separate fan without the central fan running at the same time. That is a mold disaster in any humid climate under cooling conditions. Humid air contacts cool supply ducts and condenses after the cooling shuts off.

Q. A motorized damper in an outside air duct that closes when the central air handler fan goes off won’t allow make up air to enter, for instance, when the dryer or another exhaust is on. Are there solutions to this?

A. Unless natural draft combustion appliances are located inside the conditioned space (which they should not be), make-up air should not be needed due to intermittent exhaust up to about 175 cfm. If you still want the outside air duct to also serve as a passive make-up air duct then here are two possible solutions: