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Fan Cycling Frequently Asked Questions
Q. If my house has hydronic baseboard heat and forced air central cooling, what do I do with the fan cycling control Wt terminal (which is normally wired in parallel with the heat line at the air handler). A. If the heating system is other than forced air, simply ignore the Wt terminal, do not connect the heat line to the fan cycling control. Since the hydronic heating system does not operate the central fan, it does not need to be connected to the fan recycling control. The control will operate the fan periodically throughout the year as set up with the Fan OFF and Fan ON times.
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 are not comfortable throughout the house. Can the fan cycling control help me with that problem? A. Yes. The fan cycling control 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 fan cycling control will give you the same performance at reduced operating cost. You can adjust the fan cycling control 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 fan cycling control solve that problem? A. In most cases, yes. Similar to the question above, fan cycling will smooth out temperature variations from room to room by periodically mixing air from all parts of the house. The fan cycling 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 fan cycling controls or can I make one to operate both systems? A. If you want fan cycling capability on both units, then you need two controls. The control wiring for each air handler unit needs to be isolated from the other.
Q. I am using central-fan-integrated ventilation with fan cycling 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 is concerned that an outside air duct into the return side of my gas furnace may cause the furnace heat exchanger to fail. Can you comment on that? A. Furnace manufacturers put a lower limit on the air temperature that their heat exchangers are exposed to. This is to reduce condensing of combustion products in heat exchangers that were not designed to be condensing heat exchangers, and to reduce expansion and contraction of heat exchanger parts at startup and shutdown which could lead to cracking. Usually the lower limit is about 55 F, which incidentally suggests that simply setting the thermostat back below that level (even without a fresh air duct) while the house is not occupied could violate the manufacturers warranty. All gas combustion heat exchangers get wet inside upon startup. This is because the rate of water production is greater than the rate of evaporation until the heat exchanger gets hot. The percentage of cold outside air must be limited so that the heat exchanger can get hot. Properly sized systems will run long enough to keep the heat exchanger dry. The first and only time this issue formally arose with central-fan-integrated ventilation was with a contractor who said his Carrier distributor would void the warranty if the air temperature at the heat exchanger was below 60 F. After a brief discussion with the equipment distributor and sending a mixed temperature chart to them, the issue was over. The chart assumed an outside air fraction of 7% (which is what the ventilation system was designed for), and showed that for a -25 F outdoor temperature and a low 66 F inside temperature, the mixed air temperature at the furnace was 60 F. While this met the manufacturers criteria, the questions to them remained unanswered??Do you have test data to support the 60 F criteria, and do you void your customer?s warranty if they set the thermostat below 60 F when they leave the house for a time?? I personally have never had first hand experience with a heat exchanger failure due to low return air temperature. An engineering colleague and practitioner, David Hill, of Vancouver British Columbia has, however, upon investigation, those systems had outside air fractions of about 20%. I recommend outside air fractions as follows:
Actual vs. Real?Q. I started to construct a spreadsheet to see what the sensitivity of average ventilation run time was, based on fan on/off times and vent damper open/closed times. I want to look at this because it's not intuitively obvious what average ventilation rates will be as outdoor temperatures fluctuate. As I got into it, I realized that it gets pretty complicated, because fan run times based on T-stat calls will vary depending on equipment sizing related to the loads, thermostat deadband, and other factors. (I was intending to look at this from a BIN perspective to see what the effects would be at various temperatures). It occurred to me that you may have done some of this analysis on your own, I'm curious if you could share your experience and any analysis or monitoring you've done, to help save my re-inventing something that's already been done. A. In the early to mid 1990's, after thinking of, and working with many different ways to control outside air intake using a ventilation system integrated with the central system air handler, I came to the conclusion that what I call a "simply smart" approach was best (especially for our production builder clients). Since my desired approach was not available in the market, I developed a fan cycling and outside air damper cycling controls. As your question illuminates, "simple" usually requires some compromises that move away from precision. The important question then becomes, how precise do you need to be to get the desired result? I agree, trying to simulate actual performance is very complicated because there are so many variables and assumptions required. So to get at the answer more believably I have tested and monitored the controls in many houses to establish how much under- or over-ventilation would normally occur due to the logic of the control and the varying cycle periods of the heating and cooling equipment. I have found that over daily, weekly, and monthly periods, the variation is less than 5% of the ventilation design. That is, if you were shooting for a 33% ventilation duty cycle (for example Fan OFF time=20 minutes and Fan ON time=10) then the actual ventilation duty cycle would be within 28% and 38%. Colleagues at an engineering and architectural consulting firm IBACoS Inc., in Pittsburgh, PA in collaboration with the National Renewable Energy Laboratory have used and evaluated the controls in homes and came to the same general conclusion, that, over daily periods or greater, the control provided the desired ventilation operation time within a few percent. The Canadian Housing and Mortgage Corporation also evaluated the fan cycling control as part of a retrofit ventilation program and had only good things to say about it. Please see a condensed version of their Research Highlights Report from that study. Cases #6 and #10 used the FanRecycler. If you use a fan cycling and outside air damper cycling control (fan recycling plus motorized outside air damper recycling), the variation is only about 2%. The fan cycling control assures a minimum amount of ventilation, while the outside air damper cycling control limits ventilation to a maximum by closing the outside air damper after the air handler blower has operated long enough to satisfy the ventilation design. The damper also spring returns closed when the blower is de-energized, closing the intentional hole to outside. This is the logical upgrade to the fan cycling system. At this point, few of our production builders have gone with the upgrade, since they are happy with the performance of the fan cycling system. However, I think it is well worth the extra cost (about $100 total cost difference for the control and installed damper), especially for very cold and hot-humid climates. I hope this has helped you. For additional information, look for the May/June 2001 issue of Home Energy Magazine which will have an article on ventilation and presents some monitored data from one of our clients houses in Chicago. |
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