01-05-12, 03:23 PM | #11 |
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A duct booster fan added to a system under high pressure due to having too many dampers closed isn't going to help because the velocity in the open ductwork is going to be high enough to where it will be a challenge to increase the flow any more. Ideally, to balance the system you would want to add more return and supply ductwork but to have it done right you would need the help of a pro unless you just added a flexduct run of supply and return to the coldest room so you could open some of your other dampers but that is a half-assed solution.
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01-05-12, 04:13 PM | #12 |
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Well I don't see the ductwork getting upgraded any time soon. The house is at least a century old and the 6" supply ducts all kind of spider out from the furnace in the basement, with a short plenum in either direction. The return ducts are made from metal sheeting between two large boards, just below the floor joists. Many of the ducts feed two registers, one downstairs and one upstairs. Some, however, run straight from the supply plenum to the main floor. Those are mostly the ones that I have reduced or closed the dampers on.
There are four main rooms on the east side of the center wall on the main level (2 living rooms, dining room, family room), each with at least one supply on the inside wall, and one large return on the outside wall, with the exception of the front room which has a non-functional (decommissioned) return. Each downstairs room except for the dining room has an adjoining room on the west side of the center wall (kitchen, bath, study) which each have a supply register but no return. Basically, all of the returns are on the east side of the house, and all of the supply registers are on the west center wall, or west outside wall. The upstairs has no returns, but there are supply registers in each of the 5 bedrooms and 1 bath. In the basement there are 3 return ducts measuring roughly 8" x 24" each. I'm no HVAC contractor, but it seems like these should provide adequate return airflow for a 100k furnace. The fact that there are no returns upstairs, however, makes for some unbalanced airflow issues. I think you can understand why I would have to close the damper on a short duct that feeds a register on the main floor, and open the damper on a long duct that feeds a downstairs register and an upstairs register. So basically I have been doing this careful balancing act of dampers trying to cut airflow to rooms that get too much air, and boost airflow to rooms that don't get enough air. I guess I always assumed that the blower was going to put out the same amount of air regardless of how much came out of each register. The way I have it set now, 5 of the downstairs ducts that lead to main floor register vents are either fully or mostly damped off, or closed at the register. Each room seems to maintain approximately the set temperature, when all doors are open. Last edited by abogart; 01-05-12 at 04:20 PM.. |
01-05-12, 08:18 PM | #13 |
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Update: I followed the instructions in the furnace manual for calculating the BTU/h input rating. This involves watching the gas meter and determining the amount of time that it takes to use 1 cubic foot of gas, then comparing that time to a chart in the manual. 100k BTU should take 36 seconds according to the chart. The time was taken with the water heater in pilot only.
The first reading after my little adjustment yesterday was 42 seconds. Extrapolated from the data in the chart, this works out to 87,500 BTU. So I adjusted the gas regulator on the furnace until I got exactly 36 seconds for 1 cubic foot. I can tell that it was overfired before I adjusted it yesterday because the regulator was maxed out, which gave me a time of 30 seconds today on the same setting. So now, at least by my home DIY methods, the burner should be set to the 100k that it's rated for. I measured a consistent 68 on the return and 135 on the supply, giving me a rise of 67 degrees. That's 10 degrees lower than it was before I messed with it. I also tried setting the thermostat to the recommended warm air setting. I didn't record how many times it cycled in a period, but it was more than at the hot water setting. I changed it back to the hot water setting and it does seem to go a little longer between cycles, still not quite long enough for my liking though. I'm wondering if I might get a few efficiency points by adding a combustion air supply pipe. The manual says that it can be done, but there is no cutout in cabinet for one. I'm thinking that I could cut a hole and run some PVC pipe straight in to the cabinet. Of course I would want to cover the existing vent slits in the cabinet and access panel so that it only draws fresh air from outside (I can already see the dirty looks from the HVAC guys out there ). ... Which raises the question: Would preheating the combustion air with heat from the exhaust vent (HVAC guys really glaring now) increase the overall efficiency of the system? My idea is to revamp one section of the galvanized exhaust vent with a double-walled heat exchanger can, similar to the heaters used on the mufflers of small aircraft (and old Volkswagens). Basically adding metal strips or spikes to the outside of the galvanized duct for heat transfer, then covering it with a larger diameter duct and sealing it with a cool fresh air inlet on one side, and a heated fresh air outlet on the other. I have worked with industrial furnaces that had combustion air recuperators on the exhaust vents and they seem to work quite well. I doubt that it would cool the exhaust enough to condense before exiting the house, and I don't really see it adding enough heat to the heat exchanger or cabinet controls to damage them. Just a thought... |
01-05-12, 10:21 PM | #14 | |
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01-06-12, 01:21 AM | #15 | |
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Of course, the good side of adding a fresh air intake duct is that your furnace won't be sucking air from the rest of the house, which must be made up for somehow.
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01-06-12, 05:23 PM | #16 | |
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Another excellent point. However, if I seal the cabinet, the only airflow in when the furnace is off will be due to stack effect from the warm heat exchanger and exhaust vent. Cold air would flow in through the intake (running over the warm heat scavenger in the process), in to the heat exchanger, and out the exhaust, which is being cooled by the same intake air. No outside air should enter the house at all if I seal it up well enough, whether the furnace is running or not. |
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01-09-12, 03:23 PM | #17 | |
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There is a very good chance from running the unit with severely plugged air filters, that alot of dirt has ended up on the Cooling Coil which has made a restriction to airflow thru the system. If so, your Cooling Coil will need cleaning which is best left to a professional since it may need to be completely removed and taken outside for a pressure wash procedure , then reinstalled . Other possibilities are : 1. The furnace wasnt properly setup when installed. The manifold gas pressure should be close to 3.5" w.c. using a pressure gauge hooked up to the leaving side of the gas valve. 2. The system should have been air balanced using duct dampers . It could be some or most of the duct dampers are closed which would also cause a restriction to airflow. 3. Your space thermostat's heat anticipator setting may not be set in accordance to the type of furnace you have , which will cause short cycling . |
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01-10-12, 06:48 AM | #18 |
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Thanks for the great information, it's always nice to get advice from people that work in the trade. As I said, I don't have a manometer, but I would imagine that setting the gas flow by the meter would have brought it pretty close to the optimal setting. As far as dust buildup on the evaporator coil goes, I never thought about that. I'll have to stick my head in the blower cabinet and see what's going on in there. It's due to be vacuumed out anyway.
Since I adjusted it its been cycling about 4-5 times per hour, about normal for the 30 degree nights we've been having lately. Still a little too sensitive for my taste but that will have to wait until I can get a new t-stat. |
01-10-12, 08:38 AM | #19 |
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4-5 times per hour still seems like an aweful lot. Mine comes on roughly 1x per hour or less I'd say.
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01-10-12, 10:10 AM | #20 | |
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